Solid-phase extraction of food azodyes on magnetite nanoparticles and electrospun nanofibers: A comparative study

Solid-phase extraction of food azodyes on magnetite nanoparticles and electrospun nanofibers: A comparative study | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Solid-phase extraction of food azodyes on magnetite nanoparticles and electrospun nanofibers: A comparative study Alexandra I. Arzhanukhina, Kseniya O. Andreeva, Alina M. Kerimova, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6796990/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 8 You are reading this latest preprint version Abstract In this work, we consider the preparation of magnetic magnetite nanoparticles (MNPs, Fe 3 O 4 )) by chemical precipitation method, modified with biocompatible polymers as polyethyleneimine (PEI) and chitosan (CS) to get Fe 3 O 4 @PEI and Fe 3 O 4 @CS respectively, alongside electrospun polyamide (PA) nanofibers, and their sorption properties towards some of the synthetic food azo dyes (Tartrazine (TRT), Sunset Yellow FCF (SY), Azorubine (AR), Ponceau 4R (P-4R)). Spherical MNPs (7.5 ± 0.2 nm, TEM) and PA electrospun nanofibers (52 ± 3 nm - 104 ± 11 nm, SEM) were prepared, with specific surface area of 101 m 2 g -1 (MNPs) and 44 m 2 g -1 (PA) . The pore space volume of PA nanofibers was 0.024 cm 3 g -1 which is much less than for MNPs (Fe 3 O 4 – 0.282 cm 3 g -1 ; Fe 3 O 4 @CS – 0.218 cm 3 g -1 and Fe 3 O 4 @PEI – 0.154 cm 3 g -1 ). The saturation magnetization of Fe 3 O 4 @CS (40 emu∙g -1 ) and Fe 3 O 4 @PEI (43 emu∙g -1 ) was slightly lower than that of Fe 3 O 4 (48 emu∙g -1 ). Sorption studies under optimized conditions (pH, time, sorbent mass) achieved 95–99% dye recovery. The kinetics of dye sorption was studied and pseudo-second order of sorption was preferable. For Fe 3 O 4 @PEI the maximum of sorption capacity (q max , mg∙g -1 ) values increased 111 (AR), 190 (SY), 274 (P-4R) and 376 (TRT), for Fe 3 O 4 @CS: 57 (SY), 139 (AR), 290 (TRT) and 395 (P-4R); for PA: 27.8 (Ar), 30.6 (TRT), 33.4 (P-4R) and 38.7 (SY) within the Langmuir model. The difference in sorption capacity and recovery may be attributed to steric factors and chemical structure of the azo dye molecule. food azo dyes sorption preconcentration magnetic nanoparticles electrospun nanofibers Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction The determination of synthetic, which are widely used to provide color to food products (for example beverages, sweets, etc.) or dosage forms, is of interest in analytical chemistry. On the one hand, the use of the synthetic food dyes which are reliable and cost-effective is the attractive alternative to the natural dyes which in their turn are unstable and can easily degrade under production conditions. On the other hand, it is necessary to control the qualitative and quantitative content of synthetic dyes for instance in food products, which is associated with their negative impact on the human body and taking into account that they can be falsified. Some food dyes are not harmless and have varying degrees of toxicity (allergens, carcinogens, mutagens, etc.), so such colorings are banned in many countries. The list of permitted synthetic dyes varies in each country, and their content in food products is strictly regulated, which requires their constant monitoring. The Joint Expert Committee on Food Additives (JECFA) and the European Food Safety Authority (EFSA) have established the acceptable daily intake (ADI) standard for TRT, SY, AR and P-4R as 0–7.5 [ 1 ], 0–4 [ 1 ], 0–4 [ 2 ], 0–0.7 [ 3 ] mg/kg body-weight/day, respectively. In addition, food dyes can enter the environment when wastewater is discharged from the food manufacturing. The high resistance of some synthetic food dyes to photo- and biodegradation in the natural environment leads to their accumulation in natural waters. The negative impact of synthetic food dyes or their products of destruction on living organisms, and primarily humans, raises the challenge of developing simple express methods for their determination. The key stage preceding the determination of dyes is a sample preparation. The analyte should be pre-concentrated without degradation, and the matrix components must be removed. Depending on the type of object, different methods of dyes extraction are used. Thus, solid phase extraction (SPE) [ 4 – 6 ] or liquid-liquid extraction using various solvents [ 7 ], cloud point extraction, ultrasound assisted solvent extraction are used [ 8 , 9 ], and in some cases a combination of these methods is utilized. After SPE dyes are usually desorbed and determined using various analytical methods, for example, UV–Vis spectrophotometry [ 10 ] or colorimetry (alone [ 11 ] or coupled [ 12 ] with the chemometrics methods for analysis of dye mixtures), high performance liquid chromatography (HPLC) with UV-visible or MS detection, electrochemical methods, capillary electrophoresis, etc. [ 13 ]. Desorption in the case of SPE is often difficult and is carried out under harsh conditions when heated in aggressive environments, which can lead to the destruction of dyes. In this regard, it is relevant to develop new methods for the extraction of synthetic food dyes for their further determination. Currently, much attention in SPE is being paid to nanosorbents, with magnetic nanoparticles (MNPs) [ 14 – 16 ] and nanofibers (NFs) with a wide range of fields in their applications [ 17 – 23 ] dominating. It is noted that their common properties are a high surface area to volume ratio, high porosity, stability, and the possibility of reuse, which promises much greater sorption capacity, efficiency, and cost-effectiveness of use compared to traditional SPE macrosorbents. Another common property is the integration of various sample preparation steps, including sampling, extraction, purification, and pre-concentration, into a single step [ 24 ]. Magnetic nanosorbents usually have a shape close to spherical, a size of about 5–12 nm, they aggregate in solution, therefore their surface is modified with surfactants or polyelectrolyte molecules, which impart a charge to the nanoparticles [ 14 – 16 , 25 – 28 ]. The main advantage of MNPs is that separation from the matrix solution and washing of the sorbent take several seconds due to the phenomenon of superparamagnetism. The main disadvantage is the need for desorption of the analyte, since direct photometric determination is almost impossible. Nanofiber, on the contrary, allows for direct photometric determination of the analyte without it’s desorption. The diameter of fibers varies from several tens to hundreds of nanometers; they contain a large number of different functional groups, and can separate chemicals and impurities by different capture mechanisms including electrostatic, affinity, covalent and H-bonding, chelation, etc. [ 17 – 21 ]. It follows from many publications that both types of the sorbents under consideration are used for the sorption and preconcentration of various drugs, pesticides, polycyclic aromatic hydrocarbons, enzymes, metal ions and other pollutants [ 14 – 20 , 29 ]. At the same time, the sorption and preconcentration of food dyes using MNPs and NFs is described in only a few papers. To our knowledge, there are no publications at all comparing the sorption and preconcentrating capabilities of MNPs and NFs for one series of azo dyes, which could be of interest to specialists working with dyes in various fields, including the food industry and ecology. In this regard, the aim of our work was to compare the degree of extraction, sorption capacity, mechanism and kinetics of sorption of four food azo dyes TRT, SY, AR, P-4R on two types of sorbents: nanomagnetite Fe 3 O 4 and nanofiber based on polyamide. Since all azo dyes contained sulfo groups and were anions in the 2–12 pH range, two biocompatible cationic polyelectrolytes were chosen for the functionalization of MNPs: PEI and CS. The material for obtaining the nanofibers was polyamide (PA). The structure of the units of all three polymers is shown in Fig. 1 . The choice of MNPs modifiers, as well as the polymer for electrospun nanofibers, is determined by the presence in their structures a large number of primary, secondary, tertiary amino groups and quaternary ammonium, which participate in the sorption of azo dyes in a wide pH range. Thus, all sorbents depending on the pH of the solution could interact with azo dyes both due to electrostatic forces, formation of H-bonds as well as hydrophobic interactions. 2. Materials and methods 1.1 Chemicals and reagents Iron (II) chloride tetrahydrate (FeCl 2 ·4H 2 O) (≥ 98%), iron (III) chloride hexahydrate (FeCl 3 ·6H 2 O) (≥ 99%) were purchased from Acros Organics (Germany). Sodium hydroxide (99.8%), ammonium hydroxide (25%), acetic acid, formic acid (98%), citric acid monohydrate (99.5%), hydrochloric acid (38%), ethanol, all of analytical grade were obtained from LLC “NPO ECROS” (Russian Federation). Chitosan (CS), 90 кDa, deacetylated by 85%, was supplied by LLC “Bioprogress” (Russian Federation). Branched polyethyleneimine (PEI), 25 кDa, 50% solution, was obtained from Aldrich (Germany). Polyamide 6 (PA 6, Ultramid® brand, number average molecular weight (M n ) of 21.100 g/mol and a weight average molecular weight (M w ) of 55.600 g/mol) was obtained from BASF (Germany). TRT (E102) ≥ 99%, SY (Е110) ≥ 85%, AR (E122) ≥ 98%, P-4R (E124) ≥ 85%, were obtained from Sigma-Aldrich (USA). Hydrochloric acid and sodium hydroxide were used for pH adjustments. All chemicals were of analytical reagent grade and were used as received. Milli-Q water was used in all the experiments (Milli-Q Purification System, Millipore, Merck, USA). Stock solutions of dyes (1.0 gL − 1 ) were prepared by dissolving the proper amounts of dyes in Milli-Q water. 1.2 Synthesis of Fe 3 O 4 nanoparticles (MNPs) Fe 3 O 4 nanoparticles were prepared by chemical precipitation method. For the synthesis of Fe 3 O 4 MNPs, FeCl 3 ∙6H 2 O (1.300 g, 0.02 mol) and FeCl 2∙ 4H 2 O (0.478 g, 0.01 mol) were dissolved in 35 mL of deionized water under magnetic stirring for 10 min at room temperature. Then the solution was heated up to 40°C, under nitrogen atmosphere in 150 mL of DI. Subsequently, the ammonium hydroxide solution (25 mL, 4 v/v%) was added dropwise to the reaction mixture and was allowed to continue about 30 min. The reaction mixture was cooled to room temperature and then, the black precipitates were separated in a magnetic field from the reaction mixture, repeatedly washed with deionized water for several times to remove the impurities and unreacted materials [ 25 ]. 1.3 Surface modification of Fe 3 O 4 nanoparticles The synthesis of magnetite nanoparticles was combined with the stage of surface modification. Modification of MNPs was carried out with solutions of CS and PEI. The colloidal solution of un-stabilized MNPs (150 mL) was placed in a 250 mL glass laboratory conical flask. In the first case 60 mL of CS solution (2 wt.%) in acetic acid solution was added to MNPs and mixed at a temperature of 60°C. In the second case 13.5 mL of PEI aqueous solution with a concentration of 40 mg/mL was added to MNPs and the contents of the flask were mixed for 15 minutes. In both cases the modification was carried out in a nitrogen atmosphere for 30 minutes at a rotation speed of ≈ 1800 rpm. The resulting black precipitate of modified MNPs was separated using a Nd–Fe–B permanent magnet and the supernatant liquid was decanted. Then, while stirring, 150 ml of deionized water was added to the precipitate, and mixed again for 5–7 minutes. This procedure was repeated 3–4 times. 1.4 Fabrication of polyamide electrospun nanofibers Polyamide (PA) electrospun nanofibers were obtained using a Nanospider NS LAB 200 S (Elmarco, Czech Republic) laboratory machine with a string electrode. PA was dissolved in solution of formic and acetic acids (1:2) while stirring to obtain a homogenous solution with a concentration of 5–15 wt.%. The solution was stirred for 24 hours at 50 ± 5°C. The electrospinning process was carried out under the following conditions: humidity of 14 ± 5%, temperature of 25 ± 2°C, distance between the electrodes was fixed to 160 mm, voltage of 70 ± 1 kV, electrospinning time of 10–30 minutes, and polypropylene spunbonded material as a collector. The PA nanofibers dried in the air for 24 h at room temperature. 1.5 Batch sorption experiment Food dye sorption study was carried out using modified MNPs (Fe 3 O 4 @CS and Fe 3 O 4 @PEI) and PA electrospun nanofibers as sorbents. MNPs were added to the solutions of TRT, SY, AR, and P-4R within the concentration range of 0–80 mg.L − 1 while stirring from 0 to 40 min in 5 mL Eppendorf tube. pH (3–11) of the solutions was adjusted using hydrochloric acid and sodium hydroxide. After dye sorption Fe 3 O 4 @CS and Fe 3 O 4 @PEI MNPs were quickly separated (10 sec) from sample solutions using a permanent Nd–Fe–B magnet. Electrospun PA nanofiber pieces of 3 x 3 cm in size were placed in the conical flasks (100 ml), then dye solutions were added in the concentration range of 0–10 mg L − 1 , and pH in the range from 1.5 to 7. The flasks were continuously stirred on a horizontal shaker for 10–120 minutes to establish sorption equilibrium. Consequently, dye concentrations in the aqueous transparent supernatant phases were determined by UV–Vis spectroscopy based on calibration curves at maximum absorbance (λ TRT = 427 nm, λ SY = 484 nm, λ P−4R = 506 nm, λ AR = 516 nm). Each experiment was repeated three times and the results were given as averages. The recovery (R, %) and specific sorption (q, µg g − 1 ) are calculated as follows (Eq. ( 1 )) and (Eq. ( 2 )): $$\:R,\:\left(\%\right)=\frac{{С}_{0}-{С}_{1}}{{С}_{0}}\times\:100$$ 1 $$\:q=\frac{({С}_{0}-{С}_{1})\times\:V}{m}$$ 2 where C o and C are the initial and equilibrium concentrations of dye solution in µg L − 1 after sorption respectively; V is the volume of the solution in L; m is the mass of the sorbent in g. 1.6 Instrumentation Spectrophotometer Shimadzu UV-1800 (Japan) was applied for UV-visible absorbance measurements. The size and shape of nanoparticles were measured using a transmission electron microscope (TEM) Carl Zeiss AG - LIBRA 120 (Germany). The morphology of electrospun nanofibers was studied using scanning electron microscope (SEM) Mira II LMU, Tescan (Czech Republic). Average hydrodynamic size and zeta-potential (ζ) were evaluated on a Zetasizer Nano-Z analyzer, model ZEN3600 Malvern (UK). Magnetization curves were monitored using a 7407 vibrating sample magnetometer Lake Shore Cryotronics Inc. (USA) at room temperature. For magnetic separations, a Nd–Fe–B permanent magnet with (BH) max = 40 MGOe (China) was used. NOVA 1200e analyzer Quantachrome (USА) with accompanying software was used for specific surface area measurements by Brunauer–Emmett–Teller (BET) method via physical sorption of nitrogen molecules and consequent analysis of sorption isotherms. Pore size distribution, total pore space volume and pore specific surface area was evaluated by Barrett–Joyner–Halenda (BJH) method. 3. Results and discussion 3.1 Characterization of MNPs and nanofibers The morphology, average sizes ( d ) and size distribution of the synthesized unmodified (Fig. 2 a,d) and polyelectrolyte-modified (Fig. 2 b,c and e,f ) MNPs were assessed by TEM; information on similar parameters for nanofibers with different polyamide content (Fig. 2 g, j, w, a, k, l) was obtained by SEM. It can be seen that the parent and modified Fe 3 O 4 MNPs have a shape close to spherical, with smooth edges and an average size of (7.5 ± 0.2) nm. It can also be seen that modification of the MNP surface contributes to a decrease in the size of the MNP core for both polyelectrolytes to 5.3 ± 0.3 nm. This decrease in the size of MNPs after modification with CS or PEI can be associated with stabilization of MNPs and prevention of aggregation with the production of more dispersed and stable nanoparticles of smaller size, which is consistent with previously known data [ 28 , 30 ]. The polymer concentration during the production of fibers was varied from 5 to 15 wt. %. This range of concentrations is optimal for further study of the nanofiber’s morphology, since below a concentration of 5 wt. %, nanofibers are obtained with beads and defects, while at a high polymer concentration the solution becomes very viscous, which makes the electrospinning process extremely difficult. As can be seen from Fig. 2 ( j-l ), PA nanofibers are uniform and have a rounded shape in cross section, a smooth surface with no beads or any visible defects. With an increase in the polymer concentration from 5 to 15 wt.%, the average sizes of nanofibers increased from 52 ± 3 nm to 104 ± 11 nm, respectively. The choice of polymer concentration within the range from 5 to 15 wt. % is determined not only by the morphology of the resulting fiber, but also by the material strength and the electrospinning process time (which is the speed of the nanofiber material obtaining). Therefore, it is desirable to minimize the spinning time while obtaining uniform nanofibers. Polymer concentration of 5 and 10 wt. % and spinning time less than 30 minutes lead to nanofiber material becoming brittle and rapidly undergo mechanical destruction over time, therefore, 15 wt. % is chosen as the optimal concentration of PA. The specific surface area of MNPs was determined to be 101 m 2 g − 1 (Table 1 ). The average pore size in the aggregates of Fe 3 O 4 NPs before modification as well as Fe 3 O 4 @CS and Fe 3 O 4 @PEI is 10 nm, 6.7 nm, and 3.8 nm respectively. These results show that the Fe 3 O 4 pore structure before and after CS and PEI modification is in the mesopore range. The pore space volume and pore size of MNPs decreased after coating with polyelectrolyte molecules, which can be explained by formation of a polyelectrolyte layer on the surface of nanomagnetite leads to the appearance of aggregates with an interconnected porous network of H-bonds around individual particles. Table 1 Specific surface area, pore space volume, pore surface area and average pore radius of MNPs and nanofibers Sample Specific surface area, m 2 g − 1 Pore ​​space volume, cm 3 g − 1 Pore specific surface area, m 2 g − 1 Average pore ​radius, nm Fe 3 O 4 101 0.282 75.2 10.0 Fe 3 O 4 @CS 119 0.218 88.5 6.7 Fe 3 O 4 @PEI 106 0.154 77.8 3.8 PA 15% 44.0 0.024 68.4 4.3 The specific surface area of PA nanofibers is 44 m 2 g − 1 and the pore space volume is 0.024 cm 3 g − 1 which is much less then for MNPs (Fe 3 O 4 – 0.282 cm 3 g − 1 ; Fe 3 O 4 @CS – 0.218 cm 3 g − 1 and Fe 3 O 4 @PEI – 0.154 cm 3 g − 1 ). The average pore radius (in this case, the inter-fiber distance) is 4.2 nm which is also in the mesopore range. This can be associated with dense packing of nanofibers during electrospinning process. Spinning time affects the thickness of the nanofiber material (and doesn't affect the nanofiber diameter) and the tightness of the fibers to each other: the longer the electrospinning takes, the higher the density, but if the spinning time is too short, the material is brittle and inconvenient to use, therefore in this work 30 minutes of electrospinning was selected as sufficient time. 3.2 Magnetization of MNPs Changes in magnetic saturation (MS) are also useful to verify surface modification. The magnetization curves for Fe 3 O 4 , Fe 3 O 4 @PEI and Fe 3 O 4 @CS nanoparticles are shown in Fig. 3 . The saturation magnetization of Fe 3 O 4 @CS (40 emu∙g − 1 ) and Fe 3 O 4 @PEI (43 emu∙g − 1 ) was slightly lower than for Fe 3 O 4 (48 emu∙g − 1 ). This may be due to the formation of a non-magnetic polymer layer (shell) on the MNPs surface, which thickness can be up to 10–20 nm [ 31 , 32 ]. It is evident that the addition of modifiers slightly reduces magnetization (about 10%) compared to the original magnetite. Small differences between the nature of the polyelectrolyte modifier may be due to a small difference in the thickness of the modifier layer on the magnetite surface. Thus, magnetic properties of the samples change slightly and do not affect the magnetic separation of particles. 3.3 Sorption study In this work, we studied the sorption of food azo dyes, which have an acidic nature due to the presence of sulfo groups in their structure. (Fig. 4 ). In this section, the influence of the acidity of the medium and the sorbent mass on the dye’s sorption by magnetic nanoparticles and polymer nanofibers is considered. The sorption kinetics and sorption isotherms are also presented. 3.3.1 Effect of pH Since all azo dyes contained sulfo groups, they were only in anionic form in solution at pH 2–12 [ 33 ]. Therefore, changing the pH of the solution will only affect the acid-base state of the MNP modifiers, i.e. PEI, CS, as well as the state of the polyamide that forms the nanofiber. This is due to the presence in their structures of a large number of primary, secondary, and tertiary amino groups [ 34 ], which are protonated and participate in electrostatic interaction with azo dye anions in a certain pH range during sorption on nano-objects. Thus, the main driving force of sorption is the electrostatic interaction between the negatively charged sulfo groups and the nitrogen-containing cationic centers of the polyelectrolytes or the polyamide polymer. Such interaction should depend on the pK values ​​of the amino groups of the polymers and, consequently, the pH of the solution: R-NH 3 + + Dye-SO 3 − ↔ R-NH 3 + ... − SO 3 -Dye (or R-NH 2 + ... − SO 3 -Dye ) Magnetic nanoparticles . First of all, let us consider sorption of dyes by modified magnetic nanoparticles. Figure 5 a illustrates possible types of interaction between the surface of Fe 3 O 4 @CS nanoparticles and the dye, which is similar to the sorption of acid dyes on other sorbents with the same modifier. A similar scheme for the interaction of the dye with polyamide is shown in Fig. 5 b. When varying the pH in the range from 3 to 11, it was found that below pH 3, the sorption is decreased. One of the reasons may be the instability of magnetite nanoparticles in an acidic environment due to the transition into solution and oxidation of Fe (II), and the formation of maghemite on the surface [ 35 ]. From a comparison of the results of sorption on the surface of MNPs modified with СS and PEI (Fig. 6 a, b), it is evident that in the first case the degree of dye extraction decreases after pH 8–9, while in the case of PEI it is effective up to pH 11. The wider pH range for dye adsorption on the Fe 3 O 4 @PEI adsorbent is consistent with the pH dependence of the zeta-potential for these magnetic nanoparticles [ 26 ] and is associated with the presence of a large number of amino groups, including primary amino groups, in the polyethyleneimine molecule [ 34 ]. The dependences of the zeta-potential on the pH of the solution for nanoparticles Fe 3 O 4 , Fe 3 O 4 @CS and Fe 3 O 4 @PEI, obtained by the dynamic light scattering method, is presented in Table 2 . Table 2 Dependence of the ζ-potential (mV) of Fe 3 O 4 , Fe 3 O 4 @CS and Fe 3 O 4 @PEI on pH (T = 298 K, n = 3, P = 0.95) рН 3 4 5 6 7 8 9 10 11 Fe 3 O 4 26 20 15 7 -15 -18 -19 -20 -22 Fe 3 O 4 @CS 43 40 38 27 -7 -15 -20 -25 -25 Fe 3 O 4 @PEI 36 36 35 32 28 22 17 12 -5 We assume that interactions of Fe 3 O 4 @PEI with the dyes in alkaline media also occur through hydrogen bonding between the protons of the amino and imino groups of PEI and oxygen atoms of the dyes. One unit of chitosan contains one amino group and two hydroxyls, whereas the unit of branched PEI contains 11 nitrogen-containing groups, which results in fewer possible hydrogen bonds with chitosan and significantly lower dye recovery in alkaline solutions. A similar pH dependence for the sorption of negative charged dyes was also observed in previous studies [ 26 – 28 , 32 ]. As follows from Fig. 6 a and b , quantitative extraction of SY, AR, TRT, P-4R dyes by Fe 3 O 4 @CS sorbent is observed in the pH range of 5–9; 5–7; 5–8 and 4–8, respectively, and on Fe 3 O 4 @PEI sorbent at pH 3–6; 4–10; 5–11 and 4–10, respectively. These optimal pH ranges were selected further to study the effect of other factors on dye sorption. Extraction for all azo dyes in optimal pH ranges is within 98–100%. Comparison of Fig. 6 a and b shows that the pH ranges, at which the sorption of azo dyes is effective, are affected by the nature of the MNP modifier. Thus, the pH range of quantitative sorption for all four dyes on the Fe 3 O 4 @CS sorbent is practically the same pH 5–8, i.e. the nature of the sorbate, if it affects, then insignificantly. Electrostatic interaction in an acidic medium (pH 3–4) and H-bond in an alkaline medium (pH 9–11) are weak. On the Fe 3 O 4 @PEI sorbent, the influence of the nature of the azo dye is clearly visible: photo of the cuvettes with solutions of SY (4.52 mgL − 1 ) and AR (5.02 mgL − 1 ) before and after their sorption by Fe 3 O 4 @PEI are presented on Fig. 6 d and e . Dye SY, which has fewer acid groups and aromatic rings, is sorbed only in the acidic region (pH 3–6), i.e., the electrostatic factor is apparently mainly at work. The other three dyes are actively sorbed in both acidic and alkaline media. Sorption in an alkaline medium is facilitated by a greater number of acid groups participating in the formation of H-bonds, as well as a greater number of aromatic rings that can participate in hydrophobic interaction. Thus, Fe 3 O 4 @CS and Fe 3 O 4 @PEI sorbents depending on the pH of the solution could interact with azo dyes both due to electrostatic forces, formation of H-bonds as well as hydrophobic interactions. PA nanofibers . The effect of pH on the sorption of azo dyes by electrospun PA nanofibers (Fig. 6 c) is very different from the sorption of MNPs. Firstly, the nature of the dye does not affect the pH range, and secondly, sorption is possible only in a narrow pH range of 1.5–3.0. It is interesting that the sorption of another azo dye on polyamide is also most effective in the acidic region at pH 2.6 [ 36 ]. Moreover, the curve of the degree of extraction of azo dyes on pH has the same shape as the dependence of the zeta-potential of the polyamide film calculated from the contact angle data [ 37 ]. From Fig. 3 a in Ref. 35 it is evident that after pH of approximately 4.2–4.3 the zeta-potential changes its charge sign from positive to negative due to the presence of an electron pair on the secondary amine groups of PA; therefore, azo dye anions are repelled from the surface of the nanofibers. The same change in the sign of the zeta potential charge from positive to negative in the pH range of 2.8–3.7 was noted by the authors [ 38 ]. It is possible that after pH 3 the main type of interaction becomes the H-bond, which is significantly weaker than the electrostatic interaction between the sulfo groups of the dyes and the secondary amino groups of the PA, protonated in an acidic medium. On the other hand, changing the fiber charge can contribute to changing its spatial structure, expanding the network and increasing interfiber distances. Thus, dye molecules can penetrate into the interfiber space and interact with the surface due to physical sorption. Samples of polyamide fiber dyed with azo dyes are shown in Fig. 6 f. 3.3.2 Effect of adsorbent mass To select the optimal amount of magnetic adsorbent required for quantitative extraction of azo dyes, various masses of MNPs in the range from 2.5 to 11.8 mg were tested (Fig. 6 ). According to the results, the maximum extraction (95–99%) was obtained with 2.9 mg Fe 3 O 4 @CS and 2.6 mg Fe 3 O 4 @PEI, respectively. A further increase in the mass of the sorbent changes the efficiency of sorption of synthetic food azo dyes very slightly. Therefore, 2.9 and 2.6 mg of Fe 3 O 4 @CS and Fe 3 O 4 @PEI respectively are chosen as optimal amounts of sorbents for the following studies. The main idea of using nanofibers is to further develop methods for the colorimetric determination of dyes; therefore, in this case, one piece of material is needed that would be uniformly colored. The amount of nanofiber was varied from 3 to 20 mg (here 1 piece of nanofiber material of 3 x 3 cm was used, and this difference in mass was achieved by varying the electrospinning time from 10 to 40 minutes). The mixtures containing dye solutions of 2.00 mg L-1 and samples of nanofibers of various weights were stirred at room temperature for 60 min. Figure 7 shows the relationship between nanofiber mass and dye recovery. By increasing the nanofiber mass from 3 to 20 mg (10–40 min of spinning time), the sorption efficiency increased from 83–95%, which associated with an increase in active centers and exposed area for efficient removal of the dye [ 39 ]. 3.3.3 Adsorption Kinetics The kinetic modeling was performed based on the experimental data on sorption at different time points [ 40 , 41 ]. Pseudo-first-order and pseudo-second-order kinetic models were analyzed and compared (Eq. ( 3 ) and Eq. ( 4 )). The equation of the pseudo-first order kinetic model can be represented in a linear form: $$\:ln\frac{({q}_{e}-{q}_{t})}{{q}_{e}}=\frac{{К}_{1}t}{2.303}$$ 3 $$\:\frac{1}{{q}_{e}-{q}_{t}}=\frac{1}{{q}_{e}}+{К}_{2}t$$ 4 where q t is the amount of adsorbed dye at the time t per 1 g of sorbent (mg·g –1 ), q e is the equilibrium amount of adsorbed dye per 1 g of sorbent (mg·g –1 ), k 1 (min –1 ) and k 2 (g·mg –1 ·min –1 ) are the pseudo-first and pseudo-second order constants, respectively. The sorption capacities for MNPs (Fig. 8 a) and PA electrospun nanofibers (Fig. 8 b) were determined from the curves. The resulting half-load time for MNPs is less than 10 min for all studied dyes in their concentration range of 0.7–50 µM. The fast sorption time is most likely the result of the high affinity between the dye anion and the positive surface of modified Fe 3 O 4 @CS at slightly acidic pH. In addition, such a high sorption rate can be associated with the formation of nanosized particles of the magnetic sorbent during preparation, which leads to an increase in the surface area, more favorable for the availability of functional groups on the surface of the modified MNPs. 3.3.4 Sorption Isotherms The Langmuir and Freindlich isotherm models were used to get an insight into the sorption mechanism [ 40 , 41 ]. The Langmuir model is based on the assumption that a single layer of a substance exists on a homogeneous surface, where the sorption centers are identical and energetically equivalent. The obtained data were presented in coordinates 1/ q = f(1/ C ), describing the Langmuir equation in a linear form: $$\:\frac{С}{q}=\frac{1}{{q}_{max}\:{K}_{L}}+\frac{C}{{q}_{max}}$$ 7 where q (mg , g − 1 ) and C (mg , L − 1 ) are the capacity and dyes concentration at an equilibrium state, respectively, and K L (L mg − 1 ) is the Langmuir constant. For comparison, similar experimental data were processed taking into account the possible sorption mechanism described by the Freundlich equation, which assumes the heterogeneous surface sorption with interaction between adsorbed molecules (multilayer sorption) and is characterized by a linear bilogarithmic dependence between С / q and С : $$\:lg\frac{С}{q}=\frac{1}{n}lg{C}_{.}+lg{K}_{F}$$ 8 where q is the solid-phase dye concentration at equilibrium (mg∙g − 1 ), C is the liquid-phase dye concentration at equilibrium (mg∙L 1 ), K F is the Freundlich constant and 1/n is the heterogeneity factor. A plot of log q e versus log C enables the constant K F and the exponent 1/n to be determined. The parameters values of models were calculated by linearization method, and the calculated values were compared with the experimental data. Table 3 shows that the Langmuir model is in good agreement with the experimental data for all synthetic food azo dyes in accordance with the coefficients of determination (R 2 ). The difference in K L values ​​for synthetic food azo dyes refers to the different binding strength and sorption capacity of the dyes to the surface of the modified magnetic nanoparticles. The maximum sorption capacity in the Langmuir model on the surface of Fe 3 O 4 @PEI increases in the series AR, SY, P-4R, TRT (q max 111; 190; 274; 376 mg∙g − 1 ), while on Fe 3 O 4 @CS it increases in the series SY, AR, TRT, P-4R (q max 57; 139; 290; 395 mg∙g − 1 ) respectively (Fig. 9 a,b, Table 3 ). The maximum sorption capacity in the Langmuir model for PA nanofibers increases in the series AR, TRT, P-4R, SY (q max 27.8; 30.6; 33.4; 38.7 mg∙g − 1 ) which is presented in Fig. 9 c. The difference in the degree of sorption may be due to the steric factor (the size and chemical structure of the azo dye molecule). For example, both dyes SY and AR have two sulfonic acid groups but the molecule AR and P-4R have two naphthalene rings, while TRT and SY have one. Presumably, the SY molecule has a smaller size (due to the steric factor), its methyl group is in the ortho position of the benzene ring, which not only increases the concentration of the azo dye on the surface of the chitosan particle, but also does not prevent deeper penetration of the dye molecules into the internal porous structure of Fe 3 O 4 @CS. The bivalent nature of SY and AR dye molecules provides more accessible ‒SO 3 groups for the sorption of food azo dye molecules on protonated ‒NH 2 on the CS surface. This allows the dye molecules to pack more tightly on the surface of the sorbent. The high sorption capacity of Fe 3 O 4 @CS with respect to azo dyes can reasonably be attributed to the high specific surface area of ​​magnetic chitosan nanoparticles with a much smaller diameter, which leads to almost all available active sites. And the sorption capacity of Fe 3 O 4 @PEI is much higher than that of Fe 3 O 4 @CS, this may be due to the presence of a large number of primary, secondary, tertiary –NH 2 groups in the branched structure of PEI. Table 3 Parameters of Langmuir and Freundlich isotherm models of azo dyes sorption by Fe 3 O 4 @CS (τ = 10 min), Fe 3 O 4 @PEI (τ = 5 min) and PA (15 wt.%, τ = 30 min) at 298 K. Dye Sorbent \(\:{\mathbf{q}}_{\mathbf{e}}^{\mathbf{e}\mathbf{x}\mathbf{p}}\) µg g − 1 R L Langmuir Freundlich \(\:{\mathbf{q}}_{\mathbf{m}\mathbf{a}\mathbf{x}}^{\mathbf{e}\mathbf{x}\mathbf{p}}\) , µg g − 1 К L, L mg − 1 R 2 logК F, mg g − 1 (L mg − 1 ) 1/n n R 2 SY Fe 3 O 4 @CS 55 0.82 57 0.958 ± 0.008 0.993 1.34 ± 0.03 4.17 ± 0.01 0.971 Fe 3 O 4 @ PEI 123 0.96 150 0.656 ± 0.003 0.996 1.66 ± 0.05 2.68 ± 0.02 0.964 PA 17.4 0.72 38.7 0.019 ± 0.004 0.999 0.33 ± 0.04 0.84 ± 0.05 0.981 AR Fe 3 O 4 @CS 106 0.89 142 0.781 ± 0.004 0.996 1.57 ± 0.06 3.09 ± 0.02 0.955 Fe 3 O 4 @ PEI 145 0.88 151 1.317 ± 0.004 0.986 1.67 ± 0.03 3.11 ± 0.01 0.988 PA 20.9 0.83 27.8 0.010 ± 0.004 0.999 0.35 ± 0.04 0.89 ± 0.05 0.984 TRT Fe 3 O 4 @CS 94 0.98 104 0.332 ± 0.001 0.995 1.62 ± 0.03 3.22 ± 0.01 0.985 Fe 3 O 4 @ PEI 148 0.98 176 0.3964 ± 0.0004 0.999 1.77 ± 0.05 3.02 ± 0.02 0.966 PA 18.5 0.83 30.6 0.010 ± 0.005 0.999 0.34 ± 0.04 0.86 ± 0.05 0.979 P-4R Fe 3 O 4 @CS 98 0.99 139 0.2744 ± 0.0003 0.999 1.68 ± 0.04 3.69 ± 0.02 0.973 Fe 3 O 4 @ PEI 190 0.98 204 0.677 ± 0.002 0.992 1.87 ± 0.04 3.28 ± 0.01 0.981 PA 19.2 0.75 33.4 0.016 ± 0.005 0.999 0.34 ± 0.04 0.87 ± 0.05 0.984 The shape of the isotherm can make it possible to evaluate by using dimensionless factor R L = 1/(1 + C 0 K L ) whether the sorption process is "favorable" (0 < R L 1), linear ( R L = 1) or irreversible ( R L = 0). In our case, all the values of R L are more than 0 and less than 1 for Fe 3 O 4 @CS, Fe 3 O 4 @PEI and PA nanofibers at 298 K, indicating that sorption of SY, AR, TRT and P-4R on is favorable. Conclusion In this paper, using four acidic monoazo dyes as an example, we compared solid-phase micro- extraction on magnetite nanoparticles modified with cationic polyelectrolytes polyethyleneimine and chitosan with sorption on polyamide nanofibers. All three nanosorbents had primary and secondary amino groups, which imparted a cationic charge to the nanoparticles in various pH ranges. It was found that the widest pH range of sorption 4–10 is provided by PEI, which has a large number of primary, secondary and tertiary amino groups in the molecule, a narrower pH range of 4–8 is observed for MNPs coated with chitosan, and effective sorption of azo dyes on polyamide is possible only at pH 2–3. It is shown that the reason for this effect of pH is the region of existence of a positive zeta potential of sorbents. The nature of the azo dye has a greater effect on sorption when modifying MNPs with polyethyleneimine, and to a lesser extent on polyamide nanofibers. A comparison of average pore radius, pore specific surface area, pore ​​space volume and specific surface area is given. It is shown that the sorption capacity with respect to azo dyes on polyamide is 5–10 times less than on magnetic nanoparticles, and the effect of the nature of the MNP modifier is not so significant. It is found that the Langmuir model agrees with the experiment better than the Freundlich model. Four kinetic models of sorption are considered and it is shown that sorption corresponds to pseudo-second order. The advantage of MNP is the rapid separation of the sorbent from the matrix by the action of a permanent magnet, very small masses of the sorbent, the advantage of polyamide is the ability to determine dyes without desorption. In summary, modified MNPs and nanofibers are promising sorbents of synthetic food azo dyes offering potential applications in wastewater or food analysis. Declarations CRediT authorship contribution statement Alexandra I. Arzhanukhina: Writing – original draft, Visualization, Conceptualization, Supervision, Methodology. Kseniya O. Andreeva: Methodology, Investigation, Writing - original draft, Visualization, Data curation, Formal analysis. Alina M. Kerimova: Methodology, Investigation, Writing - original draft, Visualization, Data curation, Formal analysis. Тatiana Yu. Rusanova: Writing – review & editing, Supervision, Funding acquisition. Sergei N. Shtykov: Writing – review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author Contribution A.I. A.: writing – original draft, visualization, conceptualization, supervision, methodology. K.O.A.: methodology, investigation, writing - original draft, visualization, data curation, formal analysis. A.M.K.: methodology, investigation, writing - original draft, visualization, data curation, formal analysis. Т.Yu.R.: writing – review & editing, supervision, funding acquisition. S.N.S.: writing – review & editing, supervision. Acknowledgement The work was supported by the Russian Science Foundation grant number 24-23-00519. Data availability The authors do not have permission to share data. References Lehmkuhler A, Miller MD, Bradman A, Castorina R, Chen MA, Xie T, Mitchell AE (2022) Levels of FD&C certified food dyes in foods commonly consumed by children. 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Supplementary Files floatimage1.jpeg Graphical Abstract Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 04 Jul, 2025 Reviewers agreed at journal 23 Jun, 2025 Reviews received at journal 20 Jun, 2025 Reviewers agreed at journal 20 Jun, 2025 Reviewers invited by journal 19 Jun, 2025 Editor assigned by journal 04 Jun, 2025 Submission checks completed at journal 04 Jun, 2025 First submitted to journal 01 Jun, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Andreeva","email":"","orcid":"","institution":"Saratov State University","correspondingAuthor":false,"prefix":"","firstName":"Kseniya","middleName":"O.","lastName":"Andreeva","suffix":""},{"id":475362376,"identity":"5997a58a-537f-475b-9632-16f9cc57f48b","order_by":2,"name":"Alina M. Kerimova","email":"","orcid":"","institution":"Saratov State University","correspondingAuthor":false,"prefix":"","firstName":"Alina","middleName":"M.","lastName":"Kerimova","suffix":""},{"id":475362378,"identity":"09e6883d-52fb-4731-acb4-920634c21a76","order_by":3,"name":"Тatiana Yu. Rusanova","email":"","orcid":"","institution":"Saratov State University","correspondingAuthor":false,"prefix":"","firstName":"Тatiana","middleName":"Yu.","lastName":"Rusanova","suffix":""},{"id":475362379,"identity":"b8fdfa53-ab48-468e-825f-aafb10b638a5","order_by":4,"name":"Sergei N. Shtykov","email":"","orcid":"","institution":"Saratov State University","correspondingAuthor":false,"prefix":"","firstName":"Sergei","middleName":"N.","lastName":"Shtykov","suffix":""}],"badges":[],"createdAt":"2025-06-01 17:23:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6796990/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6796990/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":85308511,"identity":"1314709d-95b7-4487-acec-f2022f705532","added_by":"auto","created_at":"2025-06-24 13:10:20","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":144051,"visible":true,"origin":"","legend":"\u003cp\u003eThe structure of the units of polymers.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/c1e105f750ab222a43ee7864.png"},{"id":85308515,"identity":"279357ba-2262-409b-9e4e-989d85665d66","added_by":"auto","created_at":"2025-06-24 13:10:20","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":697186,"visible":true,"origin":"","legend":"\u003cp\u003eTEM (a-f) and SEM (g-l) images of MNPs and nanofibers with their particle size and fiber diameter distributions: Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (a, d); Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS (b, e); Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI (c, f); 5 wt.% PA nanofibers (g, j); 10 wt.% PA nanofibers (h, k); 15 wt. % PA nanofibers (i, l).\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/e28208f251786060c6fb550b.jpeg"},{"id":85308824,"identity":"a30ee08b-11e4-484d-8f6f-4d0bbaee97c3","added_by":"auto","created_at":"2025-06-24 13:18:20","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":80390,"visible":true,"origin":"","legend":"\u003cp\u003eMagnetization curves of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS nanoparticles.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/5d6377452a1e9ac9e7fde41a.jpeg"},{"id":85308517,"identity":"e9bd0e4d-a6e7-46e8-b7bd-04bf14b8209b","added_by":"auto","created_at":"2025-06-24 13:10:20","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":131614,"visible":true,"origin":"","legend":"\u003cp\u003eStructural formulas of food azo dyes.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/c988387d58d5b6e43e2e4b1c.jpeg"},{"id":85308513,"identity":"ccaa4d84-2184-47ef-9248-fa62bb563208","added_by":"auto","created_at":"2025-06-24 13:10:20","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":271867,"visible":true,"origin":"","legend":"\u003cp\u003eProposed mechanism of sorption of synthetic food azo dyes on modified MNPs (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS as an example) in pH = 6 (a) and PA electrospun nanofibers in pH = 3 (b) (SY as an example).\u003cbr\u003e\n\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/4599638dba38c4b1aeaca1ab.jpeg"},{"id":85308825,"identity":"a916b88c-cdf6-449a-8ded-926cbba0a151","added_by":"auto","created_at":"2025-06-24 13:18:20","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1454952,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of pH on the recovery (%) of TRT, SY, AR and P-4R by (a) Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS, 2.9 mg; (b) Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI, 2.6 mg. (c) PA nanofibers, 0.020 ± 0.005 g. Dyes concentrations: 10 µM (TRT – 5.34 mg L\u003csup\u003e-1\u003c/sup\u003e; SY – 4.52 mgL\u003csup\u003e-1\u003c/sup\u003e; AR – 5.02 mgL\u003csup\u003e-1\u003c/sup\u003e; P-4R – 6.04 mgL\u003csup\u003e-1\u003c/sup\u003e). Photo of cuvettes with solutions of SY (4.52 mgL\u003csup\u003e-1\u003c/sup\u003e) (d) and AR (5.02 mgL\u003csup\u003e-1\u003c/sup\u003e) (e) before and after sorption by Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI at pH 7, as an example, where 1, 2, 3 is for 0, 5, and 10 min (by Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS) of sorption respectively. (f) Photo of PA (15 wt.%) electrospun nanofibers before (1) and after sorption of TRT (2), SY (3), P-4R (4) and AR (5) at pH 3. Dyes concentrations: 10 mgL\u003csup\u003e-1\u003c/sup\u003e; V\u003cem\u003e \u003c/em\u003e– 25 mL.\u003cbr\u003e\n\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/1548d7b5cbe0777403864f3a.png"},{"id":85308827,"identity":"6872bf50-fb28-4486-8da1-d72908dfa906","added_by":"auto","created_at":"2025-06-24 13:18:20","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":108373,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of sorbent mass on azo dyes recovery: (a) MNPs modified with PEI (pH = 5 for TRT, pH = 7 for SY, AR, P-4R) and CS (pH = 6 for TRT, SY, AR, P-4R). Dyes concentrations: 10 µM (TRT – 5.34 mgL\u003csup\u003e-1\u003c/sup\u003e; SY – 4.52 mgL\u003csup\u003e-1\u003c/sup\u003e; AR – 5.02 mgL\u003csup\u003e-1\u003c/sup\u003e; P-4R – 6.04 mgL\u003csup\u003e-1\u003c/sup\u003e); V = 4 mL.\u003cbr\u003e\n\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/d31ee5d5c2262ce06b127715.jpeg"},{"id":85308522,"identity":"d06f1bdc-20c8-451d-bdba-816d10a8d58a","added_by":"auto","created_at":"2025-06-24 13:10:20","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":576875,"visible":true,"origin":"","legend":"\u003cp\u003eKinetic curves of azo dyes sorption (dependence of dyes recovery (%) on time). (a): Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI (m = 2.6 mg; pH 5 for TRT, pH 7 for SY, AR and P-4R), Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS (m\u003csub\u003e \u003c/sub\u003e= 2.9 mg; pH 6 for TRT, SY, AR and P-4R). Dyes concentration: 10 µM (TRT – 5.34 mg L\u003csup\u003e-1\u003c/sup\u003e; SY – 4.52 mgL\u003csup\u003e-1\u003c/sup\u003e; AR – 5.02 mgL\u003csup\u003e-1\u003c/sup\u003e; P-4R – 6.04 mgL\u003csup\u003e-1\u003c/sup\u003e); \u003cem\u003eV = \u003c/em\u003e4 mL. (b): PA (15 wt.%) electrospun nanofibers (m = 0.020 ± 0.005 g, pH 3). Dyes concentration: 2 mgL\u003csup\u003e-1\u003c/sup\u003e; V\u003cem\u003e \u003c/em\u003e– 25 mL.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/bbba23b094e58c8c752b214e.png"},{"id":85308516,"identity":"eb501d7e-92c0-4485-bed1-3d83d7954cc5","added_by":"auto","created_at":"2025-06-24 13:10:20","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":902424,"visible":true,"origin":"","legend":"\u003cp\u003eSorption isotherms of azo dye by (a) Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI (m = 2.6 mg; pH 5 for TRT; pH 7 for SY, AR, P-4R); (b) Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS (m = 2.9 mg; pH 6 for TR, SY, AR and P-4R), V = 4 mL; (c) PA (15 wt.%) electrospun nanofibers (m = 0.01 mg; pH 3; V = 25 mL).\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/5b187eb9a8a5ff3decd107ee.png"},{"id":85309930,"identity":"af744808-1e10-4814-a22c-9821c8aa36eb","added_by":"auto","created_at":"2025-06-24 13:26:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5592435,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/0bdd4942-4231-4e28-9243-83e9ec399cd2.pdf"},{"id":85308826,"identity":"d04a44bf-486b-42d8-b659-3216430bb7ec","added_by":"auto","created_at":"2025-06-24 13:18:20","extension":"jpeg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":333886,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGraphical Abstract\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6796990/v1/318a2ac25b3a559d77e44f9e.jpeg"}],"financialInterests":"No competing interests reported.","formattedTitle":"Solid-phase extraction of food azodyes on magnetite nanoparticles and electrospun nanofibers: A comparative study","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe determination of synthetic, which are widely used to provide color to food products (for example beverages, sweets, etc.) or dosage forms, is of interest in analytical chemistry. On the one hand, the use of the synthetic food dyes which are reliable and cost-effective is the attractive alternative to the natural dyes which in their turn are unstable and can easily degrade under production conditions. On the other hand, it is necessary to control the qualitative and quantitative content of synthetic dyes for instance in food products, which is associated with their negative impact on the human body and taking into account that they can be falsified. Some food dyes are not harmless and have varying degrees of toxicity (allergens, carcinogens, mutagens, etc.), so such colorings are banned in many countries. The list of permitted synthetic dyes varies in each country, and their content in food products is strictly regulated, which requires their constant monitoring.\u003c/p\u003e \u003cp\u003eThe Joint Expert Committee on Food Additives (JECFA) and the European Food Safety Authority (EFSA) have established the acceptable daily intake (ADI) standard for TRT, SY, AR and P-4R as 0\u0026ndash;7.5 [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e], 0\u0026ndash;4 [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e], 0\u0026ndash;4 [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], 0\u0026ndash;0.7 [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] mg/kg body-weight/day, respectively. In addition, food dyes can enter the environment when wastewater is discharged from the food manufacturing. The high resistance of some synthetic food dyes to photo- and biodegradation in the natural environment leads to their accumulation in natural waters. The negative impact of synthetic food dyes or their products of destruction on living organisms, and primarily humans, raises the challenge of developing simple express methods for their determination.\u003c/p\u003e \u003cp\u003eThe key stage preceding the determination of dyes is a sample preparation. The analyte should be pre-concentrated without degradation, and the matrix components must be removed. Depending on the type of object, different methods of dyes extraction are used. Thus, solid phase extraction (SPE) [\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] or liquid-liquid extraction using various solvents [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], cloud point extraction, ultrasound assisted solvent extraction are used [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], and in some cases a combination of these methods is utilized. After SPE dyes are usually desorbed and determined using various analytical methods, for example, UV\u0026ndash;Vis spectrophotometry [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] or colorimetry (alone [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] or coupled [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] with the chemometrics methods for analysis of dye mixtures), high performance liquid chromatography (HPLC) with UV-visible or MS detection, electrochemical methods, capillary electrophoresis, etc. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Desorption in the case of SPE is often difficult and is carried out under harsh conditions when heated in aggressive environments, which can lead to the destruction of dyes. In this regard, it is relevant to develop new methods for the extraction of synthetic food dyes for their further determination.\u003c/p\u003e \u003cp\u003eCurrently, much attention in SPE is being paid to nanosorbents, with magnetic nanoparticles (MNPs) [\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] and nanofibers (NFs) with a wide range of fields in their applications [\u003cspan additionalcitationids=\"CR18 CR19 CR20 CR21 CR22\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] dominating. It is noted that their common properties are a high surface area to volume ratio, high porosity, stability, and the possibility of reuse, which promises much greater sorption capacity, efficiency, and cost-effectiveness of use compared to traditional SPE macrosorbents. Another common property is the integration of various sample preparation steps, including sampling, extraction, purification, and pre-concentration, into a single step [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMagnetic nanosorbents usually have a shape close to spherical, a size of about 5\u0026ndash;12 nm, they aggregate in solution, therefore their surface is modified with surfactants or polyelectrolyte molecules, which impart a charge to the nanoparticles [\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan additionalcitationids=\"CR26 CR27\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The main advantage of MNPs is that separation from the matrix solution and washing of the sorbent take several seconds due to the phenomenon of superparamagnetism. The main disadvantage is the need for desorption of the analyte, since direct photometric determination is almost impossible.\u003c/p\u003e \u003cp\u003eNanofiber, on the contrary, allows for direct photometric determination of the analyte without it\u0026rsquo;s desorption. The diameter of fibers varies from several tens to hundreds of nanometers; they contain a large number of different functional groups, and can separate chemicals and impurities by different capture mechanisms including electrostatic, affinity, covalent and H-bonding, chelation, etc. [\u003cspan additionalcitationids=\"CR18 CR19 CR20\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIt follows from many publications that both types of the sorbents under consideration are used for the sorption and preconcentration of various drugs, pesticides, polycyclic aromatic hydrocarbons, enzymes, metal ions and other pollutants [\u003cspan additionalcitationids=\"CR15 CR16 CR17 CR18 CR19\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. At the same time, the sorption and preconcentration of food dyes using MNPs and NFs is described in only a few papers. To our knowledge, there are no publications at all comparing the sorption and preconcentrating capabilities of MNPs and NFs for one series of azo dyes, which could be of interest to specialists working with dyes in various fields, including the food industry and ecology.\u003c/p\u003e \u003cp\u003eIn this regard, the aim of our work was to compare the degree of extraction, sorption capacity, mechanism and kinetics of sorption of four food azo dyes TRT, SY, AR, P-4R on two types of sorbents: nanomagnetite Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and nanofiber based on polyamide. Since all azo dyes contained sulfo groups and were anions in the 2\u0026ndash;12 pH range, two biocompatible cationic polyelectrolytes were chosen for the functionalization of MNPs: PEI and CS. The material for obtaining the nanofibers was polyamide (PA). The structure of the units of all three polymers is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The choice of MNPs modifiers, as well as the polymer for electrospun nanofibers, is determined by the presence in their structures a large number of primary, secondary, tertiary amino groups and quaternary ammonium, which participate in the sorption of azo dyes in a wide pH range. Thus, all sorbents depending on the pH of the solution could interact with azo dyes both due to electrostatic forces, formation of H-bonds as well as hydrophobic interactions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e1.1 Chemicals and reagents\u003c/h2\u003e \u003cp\u003eIron (II) chloride tetrahydrate (FeCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;4H\u003csub\u003e2\u003c/sub\u003eO) (\u0026ge;\u0026thinsp;98%), iron (III) chloride hexahydrate (FeCl\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO) (\u0026ge;\u0026thinsp;99%) were purchased from Acros Organics (Germany). Sodium hydroxide (99.8%), ammonium hydroxide (25%), acetic acid, formic acid (98%), citric acid monohydrate (99.5%), hydrochloric acid (38%), ethanol, all of analytical grade were obtained from LLC \u0026ldquo;NPO ECROS\u0026rdquo; (Russian Federation). Chitosan (CS), 90 кDa, deacetylated by 85%, was supplied by LLC \u0026ldquo;Bioprogress\u0026rdquo; (Russian Federation). Branched polyethyleneimine (PEI), 25 кDa, 50% solution, was obtained from Aldrich (Germany). Polyamide 6 (PA 6, Ultramid\u0026reg; brand, number average molecular weight (M\u003csub\u003en\u003c/sub\u003e) of 21.100 g/mol and a weight average molecular weight (M\u003csub\u003ew\u003c/sub\u003e) of 55.600 g/mol) was obtained from BASF (Germany). TRT (E102)\u0026thinsp;\u0026ge;\u0026thinsp;99%, SY (Е110)\u0026thinsp;\u0026ge;\u0026thinsp;85%, AR (E122)\u0026thinsp;\u0026ge;\u0026thinsp;98%, P-4R (E124)\u0026thinsp;\u0026ge;\u0026thinsp;85%, were obtained from Sigma-Aldrich (USA). Hydrochloric acid and sodium hydroxide were used for pH adjustments. All chemicals were of analytical reagent grade and were used as received. Milli-Q water was used in all the experiments (Milli-Q Purification System, Millipore, Merck, USA). Stock solutions of dyes (1.0 gL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) were prepared by dissolving the proper amounts of dyes in Milli-Q water.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e1.2 Synthesis of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles (MNPs)\u003c/h2\u003e \u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles were prepared by chemical precipitation method. For the synthesis of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e MNPs, FeCl\u003csub\u003e3\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO (1.300 g, 0.02 mol) and FeCl\u003csub\u003e2∙\u003c/sub\u003e4H\u003csub\u003e2\u003c/sub\u003eO (0.478 g, 0.01 mol) were dissolved in 35 mL of deionized water under magnetic stirring for 10 min at room temperature. Then the solution was heated up to 40\u0026deg;C, under nitrogen atmosphere in 150 mL of DI. Subsequently, the ammonium hydroxide solution (25 mL, 4 v/v%) was added dropwise to the reaction mixture and was allowed to continue about 30 min. The reaction mixture was cooled to room temperature and then, the black precipitates were separated in a magnetic field from the reaction mixture, repeatedly washed with deionized water for several times to remove the impurities and unreacted materials [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e1.3 Surface modification of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles\u003c/h2\u003e \u003cp\u003eThe synthesis of magnetite nanoparticles was combined with the stage of surface modification. Modification of MNPs was carried out with solutions of CS and PEI. The colloidal solution of un-stabilized MNPs (150 mL) was placed in a 250 mL glass laboratory conical flask. In the first case 60 mL of CS solution (2 wt.%) in acetic acid solution was added to MNPs and mixed at a temperature of 60\u0026deg;C. In the second case 13.5 mL of PEI aqueous solution with a concentration of 40 mg/mL was added to MNPs and the contents of the flask were mixed for 15 minutes. In both cases the modification was carried out in a nitrogen atmosphere for 30 minutes at a rotation speed of \u0026asymp;\u0026thinsp;1800 rpm. The resulting black precipitate of modified MNPs was separated using a Nd\u0026ndash;Fe\u0026ndash;B permanent magnet and the supernatant liquid was decanted. Then, while stirring, 150 ml of deionized water was added to the precipitate, and mixed again for 5\u0026ndash;7 minutes. This procedure was repeated 3\u0026ndash;4 times.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e1.4 Fabrication of polyamide electrospun nanofibers\u003c/h2\u003e \u003cp\u003ePolyamide (PA) electrospun nanofibers were obtained using a Nanospider NS LAB 200 S (Elmarco, Czech Republic) laboratory machine with a string electrode. PA was dissolved in solution of formic and acetic acids (1:2) while stirring to obtain a homogenous solution with a concentration of 5\u0026ndash;15 wt.%. The solution was stirred for 24 hours at 50\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u0026deg;C. The electrospinning process was carried out under the following conditions: humidity of 14\u0026thinsp;\u0026plusmn;\u0026thinsp;5%, temperature of 25\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C, distance between the electrodes was fixed to 160 mm, voltage of 70\u0026thinsp;\u0026plusmn;\u0026thinsp;1 kV, electrospinning time of 10\u0026ndash;30 minutes, and polypropylene spunbonded material as a collector. The PA nanofibers dried in the air for 24 h at room temperature.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e1.5 Batch sorption experiment\u003c/h2\u003e \u003cp\u003eFood dye sorption study was carried out using modified MNPs (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI) and PA electrospun nanofibers as sorbents. MNPs were added to the solutions of TRT, SY, AR, and P-4R within the concentration range of 0\u0026ndash;80 mg.L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e while stirring from 0 to 40 min in 5 mL Eppendorf tube. pH (3\u0026ndash;11) of the solutions was adjusted using hydrochloric acid and sodium hydroxide. After dye sorption Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI MNPs were quickly separated (10 sec) from sample solutions using a permanent Nd\u0026ndash;Fe\u0026ndash;B magnet.\u003c/p\u003e \u003cp\u003eElectrospun PA nanofiber pieces of 3 x 3 cm in size were placed in the conical flasks (100 ml), then dye solutions were added in the concentration range of 0\u0026ndash;10 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and pH in the range from 1.5 to 7. The flasks were continuously stirred on a horizontal shaker for 10\u0026ndash;120 minutes to establish sorption equilibrium.\u003c/p\u003e \u003cp\u003eConsequently, dye concentrations in the aqueous transparent supernatant phases were determined by UV\u0026ndash;Vis spectroscopy based on calibration curves at maximum absorbance (λ\u003csub\u003eTRT\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;427 nm, λ\u003csub\u003eSY\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;484 nm, λ\u003csub\u003eP\u0026minus;4R\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;506 nm, λ\u003csub\u003eAR\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;516 nm). Each experiment was repeated three times and the results were given as averages. The recovery (R, %) and specific sorption (q, \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) are calculated as follows (Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e)) and (Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e)):\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:R,\\:\\left(\\%\\right)=\\frac{{С}_{0}-{С}_{1}}{{С}_{0}}\\times\\:100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:q=\\frac{({С}_{0}-{С}_{1})\\times\\:V}{m}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere C\u003csub\u003eo\u003c/sub\u003e and C are the initial and equilibrium concentrations of dye solution in \u0026micro;g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e after sorption respectively; V is the volume of the solution in L; m is the mass of the sorbent in g.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e1.6 Instrumentation\u003c/h2\u003e \u003cp\u003eSpectrophotometer Shimadzu UV-1800 (Japan) was applied for UV-visible absorbance measurements. The size and shape of nanoparticles were measured using a transmission electron microscope (TEM) Carl Zeiss AG - LIBRA 120 (Germany). The morphology of electrospun nanofibers was studied using scanning electron microscope (SEM) Mira II LMU, Tescan (Czech Republic). Average hydrodynamic size and zeta-potential (ζ) were evaluated on a Zetasizer Nano-Z analyzer, model ZEN3600 Malvern (UK). Magnetization curves were monitored using a 7407 vibrating sample magnetometer Lake Shore Cryotronics Inc. (USA) at room temperature. For magnetic separations, a Nd\u0026ndash;Fe\u0026ndash;B permanent magnet with (BH)\u003csub\u003emax\u003c/sub\u003e = 40 MGOe (China) was used. NOVA 1200e analyzer Quantachrome (USА) with accompanying software was used for specific surface area measurements by Brunauer\u0026ndash;Emmett\u0026ndash;Teller (BET) method via physical sorption of nitrogen molecules and consequent analysis of sorption isotherms. Pore size distribution, total pore space volume and pore specific surface area was evaluated by Barrett\u0026ndash;Joyner\u0026ndash;Halenda (BJH) method.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Characterization of MNPs and nanofibers\u003c/h2\u003e \u003cp\u003eThe morphology, average sizes (\u003cem\u003ed\u003c/em\u003e) and size distribution of the synthesized unmodified (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea,d) and polyelectrolyte-modified (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb,c and \u003cb\u003ee,f\u003c/b\u003e) MNPs were assessed by TEM; information on similar parameters for nanofibers with different polyamide content (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg, j, w, a, k, l) was obtained by SEM. It can be seen that the parent and modified Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e MNPs have a shape close to spherical, with smooth edges and an average size of (7.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2) nm. It can also be seen that modification of the MNP surface contributes to a decrease in the size of the MNP core for both polyelectrolytes to 5.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 nm. This decrease in the size of MNPs after modification with CS or PEI can be associated with stabilization of MNPs and prevention of aggregation with the production of more dispersed and stable nanoparticles of smaller size, which is consistent with previously known data [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe polymer concentration during the production of fibers was varied from 5 to 15 wt. %. This range of concentrations is optimal for further study of the nanofiber\u0026rsquo;s morphology, since below a concentration of 5 wt. %, nanofibers are obtained with beads and defects, while at a high polymer concentration the solution becomes very viscous, which makes the electrospinning process extremely difficult. As can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (\u003cb\u003ej-l\u003c/b\u003e), PA nanofibers are uniform and have a rounded shape in cross section, a smooth surface with no beads or any visible defects. With an increase in the polymer concentration from 5 to 15 wt.%, the average sizes of nanofibers increased from 52\u0026thinsp;\u0026plusmn;\u0026thinsp;3 nm to 104\u0026thinsp;\u0026plusmn;\u0026thinsp;11 nm, respectively. The choice of polymer concentration within the range from 5 to 15 wt. % is determined not only by the morphology of the resulting fiber, but also by the material strength and the electrospinning process time (which is the speed of the nanofiber material obtaining). Therefore, it is desirable to minimize the spinning time while obtaining uniform nanofibers. Polymer concentration of 5 and 10 wt. % and spinning time less than 30 minutes lead to nanofiber material becoming brittle and rapidly undergo mechanical destruction over time, therefore, 15 wt. % is chosen as the optimal concentration of PA.\u003c/p\u003e \u003cp\u003eThe specific surface area of MNPs was determined to be 101 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The average pore size in the aggregates of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e NPs before modification as well as Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI is 10 nm, 6.7 nm, and 3.8 nm respectively. These results show that the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e pore structure before and after CS and PEI modification is in the mesopore range. The pore space volume and pore size of MNPs decreased after coating with polyelectrolyte molecules, which can be explained by formation of a polyelectrolyte layer on the surface of nanomagnetite leads to the appearance of aggregates with an interconnected porous network of H-bonds around individual particles.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSpecific surface area, pore space volume, pore surface area and average pore radius of MNPs and nanofibers\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSpecific surface area, m\u003csup\u003e2\u003c/sup\u003eg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePore ​​space volume, cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePore specific surface area, m\u003csup\u003e2\u003c/sup\u003eg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAverage\u003c/p\u003e \u003cp\u003epore ​radius, nm\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e101\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.282\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e75.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e@CS\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e119\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.218\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e88.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e@PEI\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e106\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.154\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e77.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e3.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003ePA 15%\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e44.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.024\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e68.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e4.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe specific surface area of PA nanofibers is 44 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the pore space volume is 0.024 cm\u003csup\u003e3\u003c/sup\u003eg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e which is much less then for MNPs (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e \u0026ndash; 0.282 cm\u003csup\u003e3\u003c/sup\u003eg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS \u0026ndash; 0.218 cm\u003csup\u003e3\u003c/sup\u003eg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI \u0026ndash; 0.154 cm\u003csup\u003e3\u003c/sup\u003eg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The average pore radius (in this case, the inter-fiber distance) is 4.2 nm which is also in the mesopore range. This can be associated with dense packing of nanofibers during electrospinning process. Spinning time affects the thickness of the nanofiber material (and doesn't affect the nanofiber diameter) and the tightness of the fibers to each other: the longer the electrospinning takes, the higher the density, but if the spinning time is too short, the material is brittle and inconvenient to use, therefore in this work 30 minutes of electrospinning was selected as sufficient time.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Magnetization of MNPs\u003c/h2\u003e \u003cp\u003eChanges in magnetic saturation (MS) are also useful to verify surface modification. The magnetization curves for Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS nanoparticles are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The saturation magnetization of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS (40 emu∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI (43 emu∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was slightly lower than for Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (48 emu∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). This may be due to the formation of a non-magnetic polymer layer (shell) on the MNPs surface, which thickness can be up to 10\u0026ndash;20 nm [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. It is evident that the addition of modifiers slightly reduces magnetization (about 10%) compared to the original magnetite. Small differences between the nature of the polyelectrolyte modifier may be due to a small difference in the thickness of the modifier layer on the magnetite surface. Thus, magnetic properties of the samples change slightly and do not affect the magnetic separation of particles.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Sorption study\u003c/h2\u003e \u003cp\u003eIn this work, we studied the sorption of food azo dyes, which have an acidic nature due to the presence of sulfo groups in their structure. (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In this section, the influence of the acidity of the medium and the sorbent mass on the dye\u0026rsquo;s sorption by magnetic nanoparticles and polymer nanofibers is considered. The sorption kinetics and sorption isotherms are also presented.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.3.1 Effect of pH\u003c/h2\u003e \u003cp\u003eSince all azo dyes contained sulfo groups, they were only in anionic form in solution at pH 2\u0026ndash;12 [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Therefore, changing the pH of the solution will only affect the acid-base state of the MNP modifiers, i.e. PEI, CS, as well as the state of the polyamide that forms the nanofiber. This is due to the presence in their structures of a large number of primary, secondary, and tertiary amino groups [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], which are protonated and participate in electrostatic interaction with azo dye anions in a certain pH range during sorption on nano-objects.\u003c/p\u003e \u003cp\u003eThus, the main driving force of sorption is the electrostatic interaction between the negatively charged sulfo groups and the nitrogen-containing cationic centers of the polyelectrolytes or the polyamide polymer. Such interaction should depend on the pK values ​​of the amino groups of the polymers and, consequently, the pH of the solution:\u003c/p\u003e \u003cp\u003e \u003cb\u003eR-NH\u003c/b\u003e \u003csub\u003e \u003cb\u003e3\u003c/b\u003e \u003c/sub\u003e \u003csup\u003e \u003cb\u003e+\u003c/b\u003e \u003c/sup\u003e \u003cb\u003e+ Dye-SO\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003csup\u003e\u003cb\u003e\u0026minus;\u003c/b\u003e\u003c/sup\u003e \u003cb\u003e\u0026harr; R-NH\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003csup\u003e\u003cb\u003e+\u003c/b\u003e\u003c/sup\u003e...\u003csup\u003e\u003cb\u003e\u0026minus;\u003c/b\u003e\u003c/sup\u003e\u003cb\u003eSO\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e-Dye\u003c/b\u003e (or \u003cb\u003eR-NH\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e\u003csup\u003e\u003cb\u003e+\u003c/b\u003e\u003c/sup\u003e...\u003csup\u003e\u003cb\u003e\u0026minus;\u003c/b\u003e\u003c/sup\u003e\u003cb\u003eSO\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e-Dye\u003c/b\u003e)\u003c/p\u003e \u003cp\u003e \u003cb\u003eMagnetic nanoparticles\u003c/b\u003e. First of all, let us consider sorption of dyes by modified magnetic nanoparticles. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea illustrates possible types of interaction between the surface of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS nanoparticles and the dye, which is similar to the sorption of acid dyes on other sorbents with the same modifier. A similar scheme for the interaction of the dye with polyamide is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWhen varying the pH in the range from 3 to 11, it was found that below pH 3, the sorption is decreased. One of the reasons may be the instability of magnetite nanoparticles in an acidic environment due to the transition into solution and oxidation of Fe (II), and the formation of maghemite on the surface [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. From a comparison of the results of sorption on the surface of MNPs modified with СS and PEI (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, b), it is evident that in the first case the degree of dye extraction decreases after pH 8\u0026ndash;9, while in the case of PEI it is effective up to pH 11. The wider pH range for dye adsorption on the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI adsorbent is consistent with the pH dependence of the zeta-potential for these magnetic nanoparticles [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] and is associated with the presence of a large number of amino groups, including primary amino groups, in the polyethyleneimine molecule [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The dependences of the zeta-potential on the pH of the solution for nanoparticles Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI, obtained by the dynamic light scattering method, is presented in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDependence of the ζ-potential (mV) of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI on pH (T\u0026thinsp;=\u0026thinsp;298 K, n\u0026thinsp;=\u0026thinsp;3, P\u0026thinsp;=\u0026thinsp;0.95)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"10\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eрН\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003e3\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003e4\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003e5\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003e6\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003e7\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cem\u003e8\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u003cem\u003e9\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003cem\u003e10\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003e\u003cem\u003e11\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e-22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e@CS\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e-25\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e@PEI\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e-5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eWe assume that interactions of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI with the dyes in alkaline media also occur through hydrogen bonding between the protons of the amino and imino groups of PEI and oxygen atoms of the dyes. One unit of chitosan contains one amino group and two hydroxyls, whereas the unit of branched PEI contains 11 nitrogen-containing groups, which results in fewer possible hydrogen bonds with chitosan and significantly lower dye recovery in alkaline solutions. A similar pH dependence for the sorption of negative charged dyes was also observed in previous studies [\u003cspan additionalcitationids=\"CR27\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAs follows from Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea and \u003cb\u003eb\u003c/b\u003e, quantitative extraction of SY, AR, TRT, P-4R dyes by Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS sorbent is observed in the pH range of 5\u0026ndash;9; 5\u0026ndash;7; 5\u0026ndash;8 and 4\u0026ndash;8, respectively, and on Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI sorbent at pH 3\u0026ndash;6; 4\u0026ndash;10; 5\u0026ndash;11 and 4\u0026ndash;10, respectively. These optimal pH ranges were selected further to study the effect of other factors on dye sorption. Extraction for all azo dyes in optimal pH ranges is within 98\u0026ndash;100%. Comparison of Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea and \u003cb\u003eb\u003c/b\u003e shows that the pH ranges, at which the sorption of azo dyes is effective, are affected by the nature of the MNP modifier. Thus, the pH range of quantitative sorption for all four dyes on the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS sorbent is practically the same pH 5\u0026ndash;8, i.e. the nature of the sorbate, if it affects, then insignificantly. Electrostatic interaction in an acidic medium (pH 3\u0026ndash;4) and H-bond in an alkaline medium (pH 9\u0026ndash;11) are weak. On the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI sorbent, the influence of the nature of the azo dye is clearly visible: photo of the cuvettes with solutions of SY (4.52 mgL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and AR (5.02 mgL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) before and after their sorption by Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI are presented on Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed and \u003cb\u003ee\u003c/b\u003e. Dye SY, which has fewer acid groups and aromatic rings, is sorbed only in the acidic region (pH 3\u0026ndash;6), i.e., the electrostatic factor is apparently mainly at work. The other three dyes are actively sorbed in both acidic and alkaline media. Sorption in an alkaline medium is facilitated by a greater number of acid groups participating in the formation of H-bonds, as well as a greater number of aromatic rings that can participate in hydrophobic interaction. Thus, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI sorbents depending on the pH of the solution could interact with azo dyes both due to electrostatic forces, formation of H-bonds as well as hydrophobic interactions.\u003c/p\u003e \u003cp\u003e \u003cb\u003ePA nanofibers\u003c/b\u003e. The effect of pH on the sorption of azo dyes by electrospun PA nanofibers (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec) is very different from the sorption of MNPs. Firstly, the nature of the dye does not affect the pH range, and secondly, sorption is possible only in a narrow pH range of 1.5\u0026ndash;3.0. It is interesting that the sorption of another azo dye on polyamide is also most effective in the acidic region at pH 2.6 [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Moreover, the curve of the degree of extraction of azo dyes on pH has the same shape as the dependence of the zeta-potential of the polyamide film calculated from the contact angle data [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. From Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea in Ref. 35 it is evident that after pH of approximately 4.2\u0026ndash;4.3 the zeta-potential changes its charge sign from positive to negative due to the presence of an electron pair on the secondary amine groups of PA; therefore, azo dye anions are repelled from the surface of the nanofibers. The same change in the sign of the zeta potential charge from positive to negative in the pH range of 2.8\u0026ndash;3.7 was noted by the authors [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIt is possible that after pH 3 the main type of interaction becomes the H-bond, which is significantly weaker than the electrostatic interaction between the sulfo groups of the dyes and the secondary amino groups of the PA, protonated in an acidic medium. On the other hand, changing the fiber charge can contribute to changing its spatial structure, expanding the network and increasing interfiber distances. Thus, dye molecules can penetrate into the interfiber space and interact with the surface due to physical sorption. Samples of polyamide fiber dyed with azo dyes are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e3.3.2 Effect of adsorbent mass\u003c/h2\u003e \u003cp\u003eTo select the optimal amount of magnetic adsorbent required for quantitative extraction of azo dyes, various masses of MNPs in the range from 2.5 to 11.8 mg were tested (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). According to the results, the maximum extraction (95\u0026ndash;99%) was obtained with 2.9 mg Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS and 2.6 mg Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI, respectively. A further increase in the mass of the sorbent changes the efficiency of sorption of synthetic food azo dyes very slightly. Therefore, 2.9 and 2.6 mg of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI respectively are chosen as optimal amounts of sorbents for the following studies.\u003c/p\u003e \u003cp\u003eThe main idea of using nanofibers is to further develop methods for the colorimetric determination of dyes; therefore, in this case, one piece of material is needed that would be uniformly colored. The amount of nanofiber was varied from 3 to 20 mg (here 1 piece of nanofiber material of 3 x 3 cm was used, and this difference in mass was achieved by varying the electrospinning time from 10 to 40 minutes). The mixtures containing dye solutions of 2.00 mg L-1 and samples of nanofibers of various weights were stirred at room temperature for 60 min. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the relationship between nanofiber mass and dye recovery. By increasing the nanofiber mass from 3 to 20 mg (10\u0026ndash;40 min of spinning time), the sorption efficiency increased from 83\u0026ndash;95%, which associated with an increase in active centers and exposed area for efficient removal of the dye [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e3.3.3 Adsorption Kinetics\u003c/h2\u003e \u003cp\u003eThe kinetic modeling was performed based on the experimental data on sorption at different time points [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Pseudo-first-order and pseudo-second-order kinetic models were analyzed and compared (Eq.\u0026nbsp;(\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) and Eq.\u0026nbsp;(\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e)). The equation of the pseudo-first order kinetic model can be represented in a linear form:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:ln\\frac{({q}_{e}-{q}_{t})}{{q}_{e}}=\\frac{{К}_{1}t}{2.303}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:\\frac{1}{{q}_{e}-{q}_{t}}=\\frac{1}{{q}_{e}}+{К}_{2}t$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003ewhere \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e is the amount of adsorbed dye at the time \u003cem\u003et\u003c/em\u003e per 1 g of sorbent (mg\u0026middot;g\u003csup\u003e\u0026ndash;1\u003c/sup\u003e), \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e is the equilibrium amount of adsorbed dye per 1 g of sorbent (mg\u0026middot;g\u003csup\u003e\u0026ndash;1\u003c/sup\u003e), \u003cem\u003ek\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e (min\u003csup\u003e\u0026ndash;1\u003c/sup\u003e) and \u003cem\u003ek\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e (g\u0026middot;mg\u003csup\u003e\u0026ndash;1\u003c/sup\u003e\u0026middot;min\u003csup\u003e\u0026ndash;1\u003c/sup\u003e) are the pseudo-first and pseudo-second order constants, respectively.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe sorption capacities for MNPs (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea) and PA electrospun nanofibers (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb) were determined from the curves. The resulting half-load time for MNPs is less than 10 min for all studied dyes in their concentration range of 0.7\u0026ndash;50 \u0026micro;M. The fast sorption time is most likely the result of the high affinity between the dye anion and the positive surface of modified Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS at slightly acidic pH. In addition, such a high sorption rate can be associated with the formation of nanosized particles of the magnetic sorbent during preparation, which leads to an increase in the surface area, more favorable for the availability of functional groups on the surface of the modified MNPs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e \u003ch2\u003e3.3.4 Sorption Isotherms\u003c/h2\u003e \u003cp\u003eThe Langmuir and Freindlich isotherm models were used to get an insight into the sorption mechanism [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. The Langmuir model is based on the assumption that a single layer of a substance exists on a homogeneous surface, where the sorption centers are identical and energetically equivalent. The obtained data were presented in coordinates 1/\u003cem\u003eq\u003c/em\u003e\u0026thinsp;=\u0026thinsp;f(1/\u003cem\u003eC\u003c/em\u003e), describing the Langmuir equation in a linear form:\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:\\frac{С}{q}=\\frac{1}{{q}_{max}\\:{K}_{L}}+\\frac{C}{{q}_{max}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eq\u003c/em\u003e (mg\u003csup\u003e,\u003c/sup\u003eg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and \u003cem\u003eC\u003c/em\u003e (mg\u003csup\u003e,\u003c/sup\u003eL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) are the capacity and dyes concentration at an equilibrium state, respectively, and \u003cem\u003eK\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e (L mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is the Langmuir constant.\u003c/p\u003e \u003cp\u003eFor comparison, similar experimental data were processed taking into account the possible sorption mechanism described by the Freundlich equation, which assumes the heterogeneous surface sorption with interaction between adsorbed molecules (multilayer sorption) and is characterized by a linear bilogarithmic dependence between \u003cem\u003eС\u003c/em\u003e/\u003cem\u003eq\u003c/em\u003e and \u003cem\u003eС\u003c/em\u003e:\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$$\\:lg\\frac{С}{q}=\\frac{1}{n}lg{C}_{.}+lg{K}_{F}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e8\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eq\u003c/em\u003e is the solid-phase dye concentration at equilibrium (mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), \u003cem\u003eC\u003c/em\u003e is the liquid-phase dye concentration at equilibrium (mg∙L\u003csup\u003e1\u003c/sup\u003e), \u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003eF\u003c/em\u003e\u003c/sub\u003e is the Freundlich constant and 1/n is the heterogeneity factor.\u003c/p\u003e \u003cp\u003eA plot of log \u003cem\u003eq\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e versus log \u003cem\u003eC\u003c/em\u003e enables the constant \u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003eF\u003c/em\u003e\u003c/sub\u003e and the exponent 1/n to be determined. The parameters values of models were calculated by linearization method, and the calculated values were compared with the experimental data.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows that the Langmuir model is in good agreement with the experimental data for all synthetic food azo dyes in accordance with the coefficients of determination (R\u003csup\u003e2\u003c/sup\u003e). The difference in K\u003csub\u003eL\u003c/sub\u003e values ​​for synthetic food azo dyes refers to the different binding strength and sorption capacity of the dyes to the surface of the modified magnetic nanoparticles. The maximum sorption capacity in the Langmuir model on the surface of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI increases in the series AR, SY, P-4R, TRT (q\u003csub\u003emax\u003c/sub\u003e 111; 190; 274; 376 mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), while on Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS it increases in the series SY, AR, TRT, P-4R (q\u003csub\u003emax\u003c/sub\u003e 57; 139; 290; 395 mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea,b, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The maximum sorption capacity in the Langmuir model for PA nanofibers increases in the series AR, TRT, P-4R, SY (q\u003csub\u003emax\u003c/sub\u003e 27.8; 30.6; 33.4; 38.7 mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) which is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec. The difference in the degree of sorption may be due to the steric factor (the size and chemical structure of the azo dye molecule). For example, both dyes SY and AR have two sulfonic acid groups but the molecule AR and P-4R have two naphthalene rings, while TRT and SY have one. Presumably, the SY molecule has a smaller size (due to the steric factor), its methyl group is in the ortho position of the benzene ring, which not only increases the concentration of the azo dye on the surface of the chitosan particle, but also does not prevent deeper penetration of the dye molecules into the internal porous structure of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS. The bivalent nature of SY and AR dye molecules provides more accessible ‒SO\u003csub\u003e3\u003c/sub\u003e groups for the sorption of food azo dye molecules on protonated ‒NH\u003csub\u003e2\u003c/sub\u003e on the CS surface. This allows the dye molecules to pack more tightly on the surface of the sorbent. The high sorption capacity of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS with respect to azo dyes can reasonably be attributed to the high specific surface area of ​​magnetic chitosan nanoparticles with a much smaller diameter, which leads to almost all available active sites. And the sorption capacity of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI is much higher than that of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS, this may be due to the presence of a large number of primary, secondary, tertiary \u0026ndash;NH\u003csub\u003e2\u003c/sub\u003e groups in the branched structure of PEI.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eParameters of Langmuir and Freundlich isotherm models of azo dyes sorption by Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS (τ\u0026thinsp;=\u0026thinsp;10 min), Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI (τ\u0026thinsp;=\u0026thinsp;5 min) and PA (15 wt.%, τ\u0026thinsp;=\u0026thinsp;30 min) at 298 K.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"10\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eDye\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSorbent\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\mathbf{q}}_{\\mathbf{e}}^{\\mathbf{e}\\mathbf{x}\\mathbf{p}}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e\u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eR\u003csub\u003eL\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e \u003cp\u003eLangmuir\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c10\" namest=\"c8\"\u003e \u003cp\u003eFreundlich\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\mathbf{q}}_{\\mathbf{m}\\mathbf{a}\\mathbf{x}}^{\\mathbf{e}\\mathbf{x}\\mathbf{p}}\\)\u003c/span\u003e\u003c/span\u003e,\u003c/p\u003e \u003cp\u003e\u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eК\u003csub\u003eL,\u003c/sub\u003e\u003c/p\u003e \u003cp\u003eL mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003elogК\u003csub\u003eF,\u003c/sub\u003e\u003c/p\u003e \u003cp\u003emg g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e(L mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003csup\u003e1/n\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003en\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e\u003cb\u003eSY\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e@CS\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.958\u0026thinsp;\u0026plusmn;\u0026thinsp;0.008\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.993\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e4.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.971\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e 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align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.981\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e\u003cb\u003eAR\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e@CS\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e106\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e142\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.781\u0026thinsp;\u0026plusmn;\u0026thinsp;0.004\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.996\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.57\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e3.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.955\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e@ PEI\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e145\u003c/p\u003e \u003c/td\u003e 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align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.999\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.86\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.979\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e\u003cb\u003eP-4R\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e@CS\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e98\u003c/p\u003e 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\u003cp\u003e\u003cb\u003eFe\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003csub\u003e\u003cb\u003e4\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e@ PEI\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e190\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e204\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.677\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.992\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e3.28\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.981\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003ePA\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e19.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e33.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.016\u0026thinsp;\u0026plusmn;\u0026thinsp;0.005\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.999\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.984\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe shape of the isotherm can make it possible to evaluate by using dimensionless factor \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eL\u003c/em\u003e\u003c/sub\u003e = 1/(1\u0026thinsp;+\u0026thinsp;C\u003csub\u003e0\u003c/sub\u003e\u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003eL\u003c/em\u003e\u003c/sub\u003e) whether the sorption process is \"favorable\" (0\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eL\u003c/em\u003e\u003c/sub\u003e \u0026lt; 1), \"unfavorable\" (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eL\u003c/em\u003e\u003c/sub\u003e \u0026gt; 1), linear (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eL\u003c/em\u003e\u003c/sub\u003e = 1) or irreversible (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eL\u003c/em\u003e\u003c/sub\u003e = 0). In our case, all the values of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eL\u003c/em\u003e\u003c/sub\u003e are more than 0 and less than 1 for Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI and PA nanofibers at 298 K, indicating that sorption of SY, AR, TRT and P-4R on is favorable.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this paper, using four acidic monoazo dyes as an example, we compared solid-phase micro- extraction on magnetite nanoparticles modified with cationic polyelectrolytes polyethyleneimine and chitosan with sorption on polyamide nanofibers. All three nanosorbents had primary and secondary amino groups, which imparted a cationic charge to the nanoparticles in various pH ranges. It was found that the widest pH range of sorption 4\u0026ndash;10 is provided by PEI, which has a large number of primary, secondary and tertiary amino groups in the molecule, a narrower pH range of 4\u0026ndash;8 is observed for MNPs coated with chitosan, and effective sorption of azo dyes on polyamide is possible only at pH 2\u0026ndash;3. It is shown that the reason for this effect of pH is the region of existence of a positive zeta potential of sorbents. The nature of the azo dye has a greater effect on sorption when modifying MNPs with polyethyleneimine, and to a lesser extent on polyamide nanofibers. A comparison of average pore radius, pore specific surface area, pore ​​space volume and specific surface area is given. It is shown that the sorption capacity with respect to azo dyes on polyamide is 5\u0026ndash;10 times less than on magnetic nanoparticles, and the effect of the nature of the MNP modifier is not so significant. It is found that the Langmuir model agrees with the experiment better than the Freundlich model. Four kinetic models of sorption are considered and it is shown that sorption corresponds to pseudo-second order. The advantage of MNP is the rapid separation of the sorbent from the matrix by the action of a permanent magnet, very small masses of the sorbent, the advantage of polyamide is the ability to determine dyes without desorption. In summary, modified MNPs and nanofibers are promising sorbents of synthetic food azo dyes offering potential applications in wastewater or food analysis.\u003c/p\u003e"},{"header":"Declarations","content":" \u003cp\u003e \u003cb\u003eCRediT authorship contribution statement\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAlexandra I. Arzhanukhina: Writing \u0026ndash; original draft, Visualization, Conceptualization, Supervision, Methodology. Kseniya O. Andreeva: Methodology, Investigation, Writing - original draft, Visualization, Data curation, Formal analysis. Alina M. Kerimova: Methodology, Investigation, Writing - original draft, Visualization, Data curation, Formal analysis. Тatiana Yu. Rusanova: Writing \u0026ndash; review \u0026amp; editing, Supervision, Funding acquisition. Sergei N. Shtykov: Writing \u0026ndash; review \u0026amp; editing, Supervision.\u003c/p\u003e \u003cp\u003e \u003ch2\u003eDeclaration of Competing Interest\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eA.I. A.: writing \u0026ndash; original draft, visualization, conceptualization, supervision, methodology. K.O.A.: methodology, investigation, writing - original draft, visualization, data curation, formal analysis. A.M.K.: methodology, investigation, writing - original draft, visualization, data curation, formal analysis. Т.Yu.R.: writing \u0026ndash; review \u0026amp; editing, supervision, funding acquisition. S.N.S.: writing \u0026ndash; review \u0026amp; editing, supervision.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe work was supported by the Russian Science Foundation grant number 24-23-00519.\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eThe authors do not have permission to share data.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLehmkuhler A, Miller MD, Bradman A, Castorina R, Chen MA, Xie T, Mitchell AE (2022) Levels of FD\u0026amp;C certified food dyes in foods commonly consumed by children. 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Adv Colloid Interface Sci 152:2\u0026ndash;13. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.cis.2009.07.009\u003c/span\u003e\u003cspan address=\"10.1016/j.cis.2009.07.009\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"food azo dyes, sorption, preconcentration, magnetic nanoparticles, electrospun nanofibers","lastPublishedDoi":"10.21203/rs.3.rs-6796990/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6796990/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this work, we consider the preparation of magnetic magnetite nanoparticles (MNPs, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e)) by chemical precipitation method, modified with biocompatible polymers as polyethyleneimine (PEI) and chitosan (CS) to get Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS respectively, alongside electrospun polyamide (PA) nanofibers, and their sorption properties towards some of the synthetic food azo dyes (Tartrazine (TRT), Sunset Yellow FCF (SY), Azorubine (AR), Ponceau 4R (P-4R)). Spherical MNPs (7.5 ± 0.2 nm, TEM) and PA electrospun nanofibers (52 ± 3 nm - 104 ± 11 nm, SEM) were prepared, with specific surface area of 101 m\u003csup\u003e2\u003c/sup\u003eg\u003csup\u003e-1\u003c/sup\u003e (MNPs) and 44 m\u003csup\u003e2\u003c/sup\u003eg\u003csup\u003e-1 \u003c/sup\u003e(PA)\u003csub\u003e.\u003c/sub\u003e The pore space volume of PA nanofibers was 0.024 cm\u003csup\u003e3\u003c/sup\u003eg\u003csup\u003e-1\u003c/sup\u003e which is much less than for MNPs (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003e– 0.282 cm\u003csup\u003e3\u003c/sup\u003eg\u003csup\u003e-1\u003c/sup\u003e; Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS – 0.218 cm\u003csup\u003e3\u003c/sup\u003eg\u003csup\u003e-1\u003c/sup\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI – 0.154 cm\u003csup\u003e3\u003c/sup\u003eg\u003csup\u003e-1\u003c/sup\u003e). The saturation magnetization of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS (40 emu∙g\u003csup\u003e-1\u003c/sup\u003e) and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI (43 emu∙g\u003csup\u003e-1\u003c/sup\u003e) was slightly lower than that of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003e(48 emu∙g\u003csup\u003e-1\u003c/sup\u003e).\u0026nbsp; Sorption studies under optimized conditions (pH, time, sorbent mass) achieved 95–99% dye recovery. The kinetics of dye sorption was studied and pseudo-second order of sorption was preferable. For Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@PEI the maximum of sorption capacity (q\u003csub\u003emax\u003c/sub\u003e,\u003csub\u003e \u003c/sub\u003emg∙g\u003csup\u003e-1\u003c/sup\u003e) values increased 111 (AR), 190 (SY), 274 (P-4R) and 376 (TRT), for Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e@CS: 57 (SY), 139 (AR), 290 (TRT) and 395 (P-4R); for PA: 27.8 (Ar), 30.6 (TRT), 33.4 (P-4R) and 38.7 (SY) within the Langmuir model. The difference in sorption capacity and recovery may be attributed to steric factors and chemical structure of the azo dye molecule.\u003c/p\u003e","manuscriptTitle":"Solid-phase extraction of food azodyes on magnetite nanoparticles and electrospun nanofibers: A comparative study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-24 13:10:15","doi":"10.21203/rs.3.rs-6796990/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-07-04T05:47:05+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"105877923672431050892354236213694099354","date":"2025-06-23T19:03:26+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-06-20T22:46:21+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"309236951654967978251040884376864556192","date":"2025-06-20T09:18:28+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-20T03:06:41+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-04T05:25:32+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-04T05:24:26+00:00","index":"","fulltext":""},{"type":"submitted","content":"Colloid and Polymer Science","date":"2025-06-01T17:13:53+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"5fb37a53-05bb-49af-9391-30799839e249","owner":[],"postedDate":"June 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-08-04T05:38:17+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-24 13:10:15","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6796990","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6796990","identity":"rs-6796990","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}