Modification of Biochar by Iron Containing Adsorption Centers as a Method to Enhance the Remediation of Perfluorooctanoic (PFOA) and Perfluorooctanesulphonic (PFOS) Acids from Water and Soil: A Density Functional Theory Study

Modification of Biochar by Iron Containing Adsorption Centers as a Method to Enhance the Remediation of Perfluorooctanoic (PFOA) and Perfluorooctanesulphonic (PFOS) Acids from Water and Soil: A Density Functional Theory 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 Modification of Biochar by Iron Containing Adsorption Centers as a Method to Enhance the Remediation of Perfluorooctanoic (PFOA) and Perfluorooctanesulphonic (PFOS) Acids from Water and Soil: A Density Functional Theory Study Leonid Gorb, Anita Sosnowska, Natalia Bulawska, Danuta Leszczynska, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7111972/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 13 Sep, 2025 Read the published version in Journal of Molecular Modeling → Version 1 posted 9 You are reading this latest preprint version Abstract Context Per- and polyfluoroalkyl substances (PFAS), with over 15,000 types listed in the US EPA’s CompTox database, are found in everyday items like textiles, packaging, firefighting foams, and medical devices. Their widespread use has led to concerning health effects—including cancers, elevated cholesterol, and fertility issues—with detectable levels present in 98% of Americans. While PFOA and PFOS are among the most studied, their environmental behavior and ecological interactions remain poorly understood. Advances in computer-based methods, including chemoinformatics and quantum modeling, now aid in predicting properties and simulating PFAS dynamics. Biochar (BC), produced via biomass pyrolysis under limited oxygen, is known for its porosity and adsorption capabilities. Magnetic biochar (MBC), enhanced with iron-based compounds, adds the benefit of magnetic separation, making it ideal for water decontamination. This paper explores the use of MBC to remove PFOA and PFOS from the environment, leveraging computational tools to investigate molecular interactions and adsorption properties. Methods A doubled crystallographic unit of hematite (Fe₂₄O₃₆) was constructed and fully optimized using density functional theory (DFT) with the M06-2X functional. Geometry optimization used the 6-31G(d,p) basis set, while single-point energies were calculated with 6-311 + + G(d,p). Antiferromagnetic conditions were ensured by setting the total spin to zero (Sz = 0), and triplet instability analysis was performed to evaluate ferromagnetic potential. To simulate bulk water effects on adsorption, the CPCM solvation model (ε = 78.3) was applied. Harmonic frequency analysis confirmed structural minima, and Gibbs free energies were calculated using Gaussian 16. PFOA and PFOS, with highly negative pKa values (~–0.1 and < Quadratic SCF convergence (scf = qc) addressed numerical challenges, and interaction energies were corrected for basis set superposition error using the counterpoise method. Calculated IR spectra and molecular visualizations were generated with Chemcraft, without applying scaling factors. Magnetic biochar poly-fluoroalkyl substances perfluorooctanoic acid perfluorooctanesulphonic acid PFAS remediation density functional theory (DFT) Figures Figure 1 Figure 2 Figure 3 1. Introduction Per- and polyfluoroalkyl substances (PFAS) are a class of chemicals characterized by having at least one fully fluorinated carbon atom in the structure. The history behind their discovery dates back approximately 90 years. In 1934, two scientists from IG Farben, a German company, discovered polychlorotrifluoroethylene (PCTFE). In 1938, Dr. Roy J. Plunkett and his group synthesized polytetrafluoroethylene (PTFE) that we know as Teflon™[ 1 ]. Today, the US EPA's database, CompTox [ 2 ], lists approximately 15,000 different types of PFAS. The size of this group is a consequence of the desired characteristics of stability and resilience. Due to this, PFAS are used in hundreds of products, including stain-resistant textiles, food-handling materials, firefighting foams, construction materials, personal care products, medical devices, and more. The enormous commercial value of added/used PFAS has brought dire side effects, such as detrimental health problems, such as various cancers, obesity/increased cholesterol, decreased fertility, etc. The common exposure occurs through the direct use of commercial products containing PFAS and indirectly through environmental contamination. The typical indirect pathways include drinking water (without removing PFAS), food grown on contaminated soil or in contaminated water, and insufficient water and wastewater treatment (not adjusted for PFAS removal), among others. The research on health effects related to PFAS concurs with the results of a study by the Centres for Disease Control and Prevention (CDC, conducted between 2000 and 2014. It was found that 98% of Americans have various detectable levels of PFAS in their blood. Currently, investigations and database updates are routinely conducted through community-wide blood testing. [ 3 , 4 ] Finally, on April 10, 2024, the U.S. EPA announced the National Primary Drinking Water Regulation (NPDWR) for six PFAS, including enforceable Maximum Contaminant Levels (MCLs). The regulated six PFAS in drinking water include perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorohexane sulfonic acid (PFHxS), perfluorononanoic acid (PFNA), and 2,3,3,3-Tetrafluoro-2-(heptafluoropropoxy)propanoic acid (HFPO-DA) with individual MCLs, and PFAS mixtures containing at least two or more of PFHxS, PFNA, HFPO-DA, and also perfluorobutanesulfonic acid (PFBS) using a Hazard Index MCL for the combined and co-occurring levels of these PFAS. In addition, the EPA finalized health-based, non-enforceable Maximum Contaminant Level Goals (MCLGs) for these PFAS. The final rules include the completion of initial monitoring (by 2027) of public water systems, implementation (by 2029) of solutions to decrease excessive levels of PFAS, and, after 2029, mandatory notification to the public when violations persist. [ 4 ] Within the EU Total PFAS are limited to 0.5 µg/l and levels of 20 individual PFAS to 0.1 µg/l in drinking water under the revised Drinking Water Directive set by European Environmental Agency. While PFOA and PFOS have been the subject of extensive research, their fate, behavior, and interaction in natural ecosystems remain inadequately understood. Therefore, further research is needed to enhance our understanding of PFAS and develop effective strategies for managing its environmental impact.[ 5 ] In light of this statement, computer methods are becoming increasingly helpful [ 6 ] (both chemoinformatic (e.g., machine learning) methods - for predicting physicochemical properties important in assessing the environmental fate of PFAS as well as computer chemistry methods allowing for modeling the processes). [ 7 ] Several studies have investigated diverse approaches for PFOA and PFOS remediation. [ 8 – 10 ] Such adsorbents as nanomaterials, clay, biochar (BC), ion exchange resins, polymers, graphene, carbon nanotubes, and minerals have been recognized as effective agents for the removal of PFOA and PFOS from wastewater.[ 10 – 13 ] However, the current technological demands for adsorbents are multifaceted and encompass attributes beyond those provided by mostly natural materials. In essence, modern adsorbents must demonstrate adsorption properties that surpass those of their natural counterparts. BC represents a carbon-containing product obtained by biomass pyrolysis (e.g., wood waste, agricultural waste, etc.) in conditions of limited oxygen availability (see for example [ 14 , 15 ]). This material is widely used to improve soil fertility, as a carbon storage agent, or as a filtration medium for purifying water due to its high porosity and ability to adsorb various substances. It is possible to distinguish five potential parameters of modification that affect the BC adsorption capacity of PFAS, which are (i) temperature, (ii) pH, (iii) coexisting contaminants, (iv) contact time, and (v) ionic strength. Magnetic biochar (MBC) has been modified to exhibit the magnetic properties of BC, thereby expanding the scope of its applications. MBC is made by adding to biochar magnetic materials, such as iron or iron oxides [ 16 – 18 ], during or after its production process. One of the key advantages of an MBC is its ability to effectively remove contaminants from water. In addition, after the process, MBC can be easily separated from the water using a magnet. In this paper, we continue our investigations into the adsorption ability of coal-like materials and their derivatives towards interactions with various environmental contaminants. In initial studies, we developed computational protocols that enable the more accurate prediction of the Gibbs free energy of adsorption than was previously feasible using routine density functional theory (DFT) approximations.[ 19 ] The following paper reports an investigation of the nanocomposites formed by graphene oxide and polyvinyl alcohol. [ 20 ] Then, we focused on addressing potential solutions to well-known environmental issues, such as the removal of PFOA and PFOS from the environmentю.[ 21 ] Several of our investigations are devoted to the interaction of iron-containing compounds with the species which are, in fact, environmental pollutants. An investigation of iron-containing compounds is known to be the most challenging task for quantum chemistry because of the number of theoretical and computational problems associated, in particular, with the open-shell electron structure of these systems.[ 22 , 23 ] The current paper extends the previous study and provides a novel approach to investigations of the interaction of PFOA and PFOS with the Fe 2 O 3 component in magnetic biochar. The use of MBC is a promising solution for the easy separation and regeneration of adsorbents after adsorption. To enhance the sorption performance of PFOA and PFOS and facilitate the separation of post-adsorbents by an external magnetic field, a feasible method was used to prepare MBC. The synthesis of MBC involves incorporating various nanoparticles, of approximately 100 nm in size, and micro-sized ferromagnetic metals, into the feedstock materials. The magnetization of biochar can be achieved either before pyrolysis, through pre-modification, or after pyrolysis, through post-modification. To facilitate the easy separation of adsorbents after post-adsorption, magnetic properties can be induced by co-precipitating iron nanoparticles in and around feedstock materials. Furthermore, the incorporation of metal nanoparticles, such as MgO, Fe 2 O 3 , CaO, La 2 O 3 , and Al 2 O 3 , in BC increases the adsorbent's positive charge, making them ideal for anion sorption. The synthesis and application of MBC have been used successfully to remediate various heavy metal(loid)s and organic contaminants. However, the adsorption of PFOA and PFOS by MBC and their interfacial interaction sorption mechanisms require further verification. 5 2. Materials and Methods As we already mentioned, due to the large size, we were not able to model the size of Fe 2 O 3 species observed experimentally at a reliable quantum-chemical level. Instead, the crystallographic unit of hematite taken from the previous study [ 24 ] was initially adopted and multiplied twice in the direction of th e crystallographic axis a. Obtained in this way, a structure having chemical composition Fe24O36 (see Fig. 1 A) has been fully optimized at the density functional level of theory (DFT). Specifically, we employed the M06-2X exchange-correlation functional, which has been demonstrated to be effective in our recent study.[ 23 ] Also, the 6-31G(d,p) basis set was used to optimize the geometry and 6-311 + + G(d,p) for single-point calculations. To keep the system to be antiferromagnetic, electronic spin has been assigned to zero (S z = 0). To verify the ability of the optimized Fe 24 O 26 structure to possess also a ferromagnetic state, the analysis of single determinant DFT wavefunction regarding triplet instability has been performed. Since the separation of the adsorbed substances mostly occurs from the bulk water, the CPCM model, which mimics the influence of the bulk water, was applied with a dielectric permittivity of 78.3.[ 25 ] The Cartesian coordinates of all investigated species are presented in the Supplemental Materials. Calculations of harmonic vibrational frequencies have verified all the structures. The Gibbs free energies have been calculated as implemented in the Gaussian 16 program package using all calculated parameters at 6-31G(d,p) level, except the total energy, which was calculated at the 6-311 + + G(d,p) level. It is essential to acknowledge that PFOA and PFOS have significantly acidic character, with pKa values of approximately − 0.1 [Values of PFOA and Other Highly Fluorinated Carboxylic [ 26 ] and much less than zero [ 27 ], respectively. Consequently, for PFOA, the amount of PFOA anions will exceed the concentration of its non-dissociated molecular form by over five times. Given the negative pKa values, the presence of the non-dissociated form is expected to be minimal. Therefore, for PFOA, both the anionic and the non-dissociated forms were considered for adsorption, whereas for PFOS, only the anionic form was accounted for in the adsorption analysis. Due to the numerical inconveniences of the routine SCF procedure implemented in Gaussian 16, quadratic convergence was applied (scf = qc). To correctly calculate interaction energy, a basis set superposition error has been considered as a counterpoise correction. 27 All the calculations were performed using the Gaussian 16 program package.[ 28 ] IR-vibrational spectrum, as well as obtained results, has been visualized using the Chemcraft program.[ 29 ] No scaling factors have been incorporated into computational results. 3. Results and Discussion There are two drawbacks that we can only be considered partially in this work. As mentioned above, the size of Fe 2 O 3 composites adsorbed or chemosorbed by the surface of biochar is near 100 nm. Previously, the largest fragment of iron(II) oxide that we were able to model computationally was Fe 13 O 13 .[23] The geometry of Fe 13 O 13 was frozen in this study. Currently, we have significantly extended the size of the iron oxide, investigating the Fe 24 O 36 species. The geometry of this compound has been fully optimized. The reason to consider the interaction of PFOA and PFOS with the optimized structure of Fe 24 O 36 is based on the obvious fact of geometrical relaxation of the Fe 24 O 36 surface from the initial geometry upon the complexation of the investigated species. Initial geometry was the simple superposition of two crystal unit cells of hematite. A relaxation of the surface during the complexation will change surface atoms' adsorption ability due to the change in their positions (see Fig. 1). Obviously, not exactly such adsorption centers will be formed during the interactions of experimentally studied much large, c.a. 100 nm species; however, we believe in similarities of their active structures. The next issue is related to choosing the realistic electronic and spin states for the considered species. Previously, to solve this problem for the Fe 13 O 13 nano-particle, we considered it as an associate of 13(FeO) molecules. This assumption allows to assign the initial spin state of Fe 13 O 13 to be equal to S z = 26. Then the dependence of the total energy of unoptimized geometry on S z in the vicinity of 26 value was studied, and the final value of S z = 28 was assigned. Unfortunately, the size of the considered here system and the necessity to optimize the geometry prevent us from applying the procedure described above. Therefore, we simply took into account the fact that hematite, the most common mineral possessing a composition of Fe 2 O 3 , is an antiferromagnetic compound. This is the reason to assign S z = 0 spin state to the considered Fe 24 O 36 species. We begin the discussion by considering the changes that have occurred in the initial geometric structure of the Fe 24 O 36 species after the optimization of the geometry (see Fig. 1). To do this, the initial structures (Fig. 1A and 1B) have to be compared with the optimized ones (Fig. 1C and 1D). The tendency to transform the initial shape of the parallelepiped into a form rather resembling a globule is clearly displayed. During this transformation, the molecular cavity formed during CPCM calculations lost 15% of its volume (from 919 Å 3 to 786 Å 3 ). The formed structure still exhibits certain features of the topology of the initial species, which is especially evident from the comparison of structures B and D (Fig. 1), but with the transition of iron atoms (36, 37, 38, 39 see, Fig. 1A) from the middle layer to the formed layers of the globule (see Fig. 1D). The values of the coordination numbers of iron and oxygen atoms in the initial and optimized structures are given in Table 1S. Analysis of the data displayed in this table showed that during the optimization (relaxation) of the Fe 24 O 36 structure, it is transformed from the parallellipid like to the globule (see Fig. 1). This is accompanied by an intuitively clear increase in the coordination of iron atoms by oxygen atoms with the dominance of iron atoms coordinated by five oxygen atoms. One may see that five-coordinated iron atoms are located in the centre of the globule, and lower-coordinated ones are at its boundaries. This arrangement of iron atoms prompted us to investigate the interaction of PFOA and PFOS with five-coordinated iron atoms, specifically those at positions 4, 31, and 34 (see Fig. 1). We would also like to mention that in the bulk of the hematite crystal, the coordination number of iron atoms is equal to six.[24] Figure 1 is here The last issue that we would like to discuss before analyzing the adsorption of PFOA and PFOS is the similarity in magnetic properties of hematite and the investigated structure of Fe 24 O 36 . As follows from numerous studies (see for example [30], hematite is antiferromagnetic below 260 K and exhibits weak ferromagnetic properties between 260 K and 950 K. This finding is confirmed by very accurate computations of (Fe 2 O 3 ) n (n = 1–5) clusters 31 . This investigation basically highlights two facts: Electronic configurations of the clusters (Fe 2 O 3 ) n (n = 1–5) reveal the appearance of antiferromagnetic and ferromagnetic states. The electronic configurations of the clusters has only small influence on their geometric structure. To study the possible emergence of ferromagnetic states, we have analyzed the obtained M06-2x/6-31G(d,p) DFT wavefunction on the stability with respect to the UHF solution (ability to form the electronic states containing unpaired electrons). Indeed, we found such instability concerning two double-occupied molecular orbitals of Fe 24 O 36 (see Fig. 1C and D). The consequence of such RHF wave function instability could be the appearance of so-called broken symmetry ferromagnetic electronic states. The geometric structure of the PFOA and PFOS adsorption complexes is shown in Fig. 2. We note that both PFOA and PFOS are adsorbed in the so-called skewed form. Such an orientation indicates a particular stabilizing contribution of the electrostatic and dispersion interaction between the surface and the adsorbed molecules. This is in contrast to the interaction of considered species with the surfaces of graphene, graphene oxide, and fluorinated graphene which exhibit the parallel orientation of PFOA and PFOS regarding the adsorbing surface 21 Therefore, we guess that the stabilization contribution is probably smaller than the one which characterizes the interaction of those species with the graphene surface of graphene oxide and fluorinated graphene surface. By making such comparisons, we imply that the structure of the surfaces under discussion can be a simplified model of biochar, which is considered as an effective adsorbent of PFOA and PFOS (see the Introduction). Table 1 presents the interaction energy and Gibbs free energy values for all three complexes. Although both values do not contain the correction associated with the basis set superposition error, the range of the analyzed values confidently indicates virtually 100% adsorption of PFOA and PFOS by five-coordinated Fe atoms. Biochar is one of the adsorbents to which hematite (Fe 2 O 3 ) is added. Therefore, it is interesting to compare the adsorption capacity of biochar adsorption centers with the adsorption capacity of five-coordinated Fe atoms generated by us computationally. To facilitate this comparison, we utilized results from our previous study [21] and transferred the calculated values of interaction energies and Gibbs free energies to Table 1. It can be seen that the interaction with the adsorption centres of "pristine" biochar provides almost complete absorption of PFOA and PFAS, but the additional interaction with the adsorption centres of hematite further enhances this adsorption property. This conclusion is in line with the experimental observation that the presence of iron oxides not only makes the easy extraction of biochar from the aqueous bulk more efficient, but also enhances its adsorption properties of modified in this way a biochar surface. [5] Concluding the section related to the discussion of the geometry and energy of adsorption, we would like to note a novel, interesting feature that is not typical for the adsorption of PFOA and PFOS by the surfaces of graphene and graphene oxide. By interacting with the five-coordinated Fe atom, the PFOA is not adsorbed in molecular form. Instead, a proton is transferred to the surface of the hematite to form an ion pair. (see Fig. 1A and B). Table 1 Adsorption energies. Adsorption complex ΔE int ΔG\(\:\frac{298}{\:}\) ΔE int (BC) 21 ΔG\(\:\frac{298}{\:}\)(BC) 21 CF3(CF 2 ) 6 COOH…Fe 24 O 36 -53.6 -36.5 -10.0 – -14.8 -6.3 – -10.7 CF3(CF 2 ) 6 COO-…Fe 24 O 36 -29.7 -12.6 -15.2 – -27.5 -4.3 – -7.5 CF3(CF 2 ) 7 SO 3 -…Fe 24 O 36 -129.5 -107.9 -18.4 – -26.8 -0.4 – -9.7 Although FTIR studies are among the most common in environmental chemistry, we did not find a large number of publications devoted to studying the interaction between iron oxides and PFOA and PFAS. Perhaps one of the detailed studies is [5], in which the IR bands have been assigned. Besides, the fragment of the spectrum characterizing the region of vibrations of the atoms Fe and O is a wide, insufficiently resolved region, it is proposed to consider that the peaks between 550 cm-1 to 700 cm belong to the Fe–O functional groups, the peaks at 636 cm-1 and 559 cm-1 represent the vibration of Fe atoms located in tetrahedral and octahedral positions. The peaks at 1616 cm-1 correspond to the bending vibration of moisture content on the bare iron oxide nanoparticles. The peaks at 3413 cm -1 correspond to the hydroxyl functional groups on the surface of the iron nanoparticles (OH - ). The peak band at 1000–1400 cm-1 corresponds to the vibrations of the -CF 3 and -CF 2 - groups that originate from organic fluorine, indicating that the peaks at 1384 cm -1 and 1245 cm -1 represent -CF 2 - and -CF 3 bending due to the adsorption of PFOS. Although the resolutions of our computationally generated spectra appear more detailed (see Fig. 3), we are unable to provide such a detailed interpretation. The data presented in Table 2 show that all the observed bands include several vibrations. We have chosen the most intensive one to assign the band. Analyzing the vibrations of the Fe 24 O 36 globule, we are mostly able to observe only the collective motion of oxygen atoms inside the Fe 24 O 26 species. An example of the vibration representing such collective motion is displayed in Fig. 1S, where the displacement vectors of the most contributing vibration to the band with a peak at 749 cm -1 . Also, we have not identified where well-resolved vibrations of just five coordinated Fe atoms are involved. This is probably because of the size of the iron-oxygen species considered to be too small compared to the size of the experimentally fixed species. However, the assignment of the bands related to stretching C-F, C-O, and O-H motion is clearly seen from the data presented in Table 2. Table 2 Assignment of vibrational bends. Band (cm -1 ) Assignment Band (cm -1 ) Assignment Fe 24 O 36 Fe 24 O 36 .. .HOOCC 7 F 15 386.0 asymmetric stretching of low coordinated O atoms 451.9 collective motion of selected Fe and O atoms 484.2 collective motion of selected low coordinated O atoms 484.9 collective motion of selected low coordinatedlow coordinated O atoms 532.8 collective motion of selected low coordinated O atoms 564.0 stretching motion of selected low coordinatedO atoms 570.8 collective motion of selected low coordinated O atoms 751.9 mix collective motion of selected O atoms and stretching vibration of C-F bonds 624.1 stretching motion of low coordinated selected O atoms 814.5 collective motion of selected low coordinated O atoms 721.0 stretching motion of low coordinated selected O atoms 1210.1 stretching motion of selected C-F bonds 749.3 stretching motion of low coordinated elected O atoms 1282.6 stretching motion of selected C-F bonds 768.7 mix of bending and stretching motion of selected O atoms 1780.3 asymmetric stretching vibration of C-O bond 812.0 mix of bending and stretching motion of selected O atoms 3557.0 stretching vibration of O-H bond 835.9 mix of bending motion of selected O atoms 849.1 mix of bending and stretching motion of selected Fe and low coordinated O atoms 915.4 stretching motion of selected Fe and low coordinated O atoms 945.5 stretching motion of selected low coordinated O atoms 973.3 stretching motion of selected low coordinated O atoms 1211.1 stretching motion of selected C-F bonds 1780.3 stretching motion of C-O bonds 3557.0 stretching motion of O-H bonds Fe 24 O 36 . . .CO 2 C 7 F \(\:\frac{-}{15}\) Fe 24 O 36 . .. .SO 3 C 8 F \(\:\frac{-}{17}\) 470.6 collective motion of selected low coordinated O atoms 404.1 collective motion of selected low coordinated O atoms 538.6 collective motion of selected low coordinated O atoms 485.0 collective motion of selected low coordinated O atoms 660.0 stretching motion of selected C-F bonds and bonding motion of C-O bonds 532.1 collective motion of selected low coordinated O atoms 716.7 collective motion of selected low coordinated O atoms 591.4 collective motion of selected Fe and O atoms 760.0 collective motion of selected low coordinated O atoms 626.4 collective motion of selected low coordinated O atoms 808.9 collective motion of selected low coordinated O atoms 722.1 collective motion of selected low coordinated O atoms 870.4 stretching motion of selected low coordinated O atoms 767.9 collective motion of selected low coordinated O atoms 972.4 stretching motion of selected low coordinated O atoms 811.0 collective motion of selected low coordinated O atoms 12120 stretching motion of selected C-F bonds 840.6 collective motion of selected low coordinated O atoms 1283.2 stretching motion of selected C-F bonds 1189.6 asymmetric stretching vibration of S-O bond 1754.2 stretching motion of C-O bonds 1219.2 stretching motion of selected C-F bonds 1291.9 stretching motion of selected C-F bonds 1360.6 stretching motion of selected C-C bonds Bending motion of C-C bonds 4. Conclusions In light of the increasing threat posed by PFAS to the environment and humans as well as insufficient knowledge about the spread and presence of these compounds, it is necessary to search for new solutions to limit their occurrence as well as methods of their capture from the environment. In our study, we investigate the magnetic biochar as a solution for the separation and regeneration of adsorbents after adsorption. To perform computational analysis of the interactions of PFOA and PFOS with Fe 2 O 3 , a component of magnetic biochar, we designed a simplified model with the iron oxide stoichiometry of Fe 24 O 36 . Analysis of the interaction of five-coordinated Fe(III) ions of this model with CF 3 (CF 2 ) 6 COOH, CF 3 (CF 2 ) 6 COO - ), CF 3 (CF 2 ) 7 SO 3 - suggests the skewed configuration which those species possess during the adsorption. Additionally, it is worth noting that the molecular form of adsorption for CF3(CF2)6COOH has not been observed. Instead, the study revealed the proton transfer with the formation of a surface ion pair. Analysis of the interaction energies has concluded that PFOA and PFOS interact more strongly with the adsorption surfaces formed by coordinated iron ions than with the pristine carbon and oxidised carbon surfaces of biochar. The computationally generated IR spectra of adsorbed species are not fully comparable with the experimental ones due to the small size of the Fe 24 O 36 species considered in the calculations. They do not exhibit well-resolved vibrations that can identify five coordinated Fe(III) ions. However, they allow one to observe the collective vibrations of oxygen atoms in the Fe24O36 particle and assign C-F, C-O, and Fe-O-H stretching vibrations. Declarations Data availability The data supporting this article have been included as part of the Supplementary Information. Conflicts of interest The authors declare no conflicts of interest Funding This work was supported by the US Army Engineer Research and Development Center (ERDC), grant number W912HZ-23-2-0006 and the European Union’s Horizon 2020 research and innovation programme via the PROMISCES project under grant agreement Nº101036449. Acknowledgment The computation time was provided by the Mississippi Center for Supercomputer Research and Centre of Informatics Tricity Academic Supercomputer and Network of the University of Gdansk. LG thanks Dr. Mykola Ilchenko for the help with the BSSE calculations. References Samora S, Lucas S. 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Phys. 51: 5362 - 5367 Cossi M, Scalmani G, Rega N, Barone V(2002) New developments in the polarizable continuum model for quantum mechanical and classical calculations on molecules in solution J. Chem. Phys . 117 : 43 - 54 Goss KU (2008) The pK a Values of PFOA and Other Highly Fluorinated Carboxylic Acids. Environ Sci & Technol 42: 456 - 458 Boys SF, Bernardi F (1970) The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors. Mol Phys 19 , 553. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Petersson GA, Nakatsuji H, Li X, Caricato M, Marenich AV, Bloino J, Janesko BG, Gomperts R, Mennucci B, Hratchian HP Gaussian 16, Revision C.01 Chemcraft - Graphical software for visualization of quantum chemistry computations. Version 1.8, build 682. https://www. chemcraftprog. co. Chemcraft Encyclopedia of Inorganic Chemistry. (Wiley, 2005). doi:10.1002/0470862106 Erlebach A, Hühn C, Jana R, Sierka M, (2014) Structure and magnetic properties of (Fe 2 O 3 )n clusters (n = 1-5) Phys Chem Chem Phys 16 : 26421 Additional Declarations No competing interests reported. Supplementary Files LGorbSuplementarymaterials.docx Cite Share Download PDF Status: Published Journal Publication published 13 Sep, 2025 Read the published version in Journal of Molecular Modeling → Version 1 posted Editorial decision: Revision requested 28 Jul, 2025 Reviews received at journal 28 Jul, 2025 Reviews received at journal 17 Jul, 2025 Reviewers agreed at journal 17 Jul, 2025 Reviewers agreed at journal 15 Jul, 2025 Reviewers invited by journal 15 Jul, 2025 Editor assigned by journal 15 Jul, 2025 Submission checks completed at journal 15 Jul, 2025 First submitted to journal 13 Jul, 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. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7111972","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":485937053,"identity":"dc2bbc0c-1430-4423-be72-5d0e685fde3d","order_by":0,"name":"Leonid Gorb","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0ElEQVRIiWNgGAWjYBACPiBmZqiQIEELG1jLGaiWA0RrYWxjIEUL/+JjnwvnWcibs589+PhDxWEGfunjF/BrkXiWPHvmNgnDnT15yQYHzqQxSPblFBDQcsaYmXebRILBgRwziYNtNgwGZ3gSiNAyB6jl/BuQFgkitPD3ALU0ALXcgNvCfoCALWzJzDOOSRhuuPHG2ODMmTQeyR4evDoY+PkPH2YuqKmTNzifY/igouKwHD8P+wP8eiTQHA60gseAgDWYDidkyygYBaNgFIw0AAAvUT8zB5XxwwAAAABJRU5ErkJggg==","orcid":"","institution":"NAS of Ukraine","correspondingAuthor":true,"prefix":"","firstName":"Leonid","middleName":"","lastName":"Gorb","suffix":""},{"id":485937054,"identity":"6b30176a-e6b6-4b9e-900b-1c8cea210cfc","order_by":1,"name":"Anita Sosnowska","email":"","orcid":"","institution":"University of Gdansk","correspondingAuthor":false,"prefix":"","firstName":"Anita","middleName":"","lastName":"Sosnowska","suffix":""},{"id":485937055,"identity":"7e689480-4e67-4e0d-a046-d0b5fb6e6dfe","order_by":2,"name":"Natalia Bulawska","email":"","orcid":"","institution":"University of Gdansk","correspondingAuthor":false,"prefix":"","firstName":"Natalia","middleName":"","lastName":"Bulawska","suffix":""},{"id":485937056,"identity":"9d56d275-645e-429a-9596-ecc31801f090","order_by":3,"name":"Danuta Leszczynska","email":"","orcid":"","institution":"Jackson State University","correspondingAuthor":false,"prefix":"","firstName":"Danuta","middleName":"","lastName":"Leszczynska","suffix":""},{"id":485937057,"identity":"321c89a9-41e0-483e-92a3-af97d61f9f6d","order_by":4,"name":"Tomasz Puzyn","email":"","orcid":"","institution":"University of Gdansk","correspondingAuthor":false,"prefix":"","firstName":"Tomasz","middleName":"","lastName":"Puzyn","suffix":""},{"id":485937058,"identity":"1a5e6be3-b2f9-4fb6-8ddd-83dc338008a0","order_by":5,"name":"Jerzy Leszczynski","email":"","orcid":"","institution":"Jackson State University","correspondingAuthor":false,"prefix":"","firstName":"Jerzy","middleName":"","lastName":"Leszczynski","suffix":""}],"badges":[],"createdAt":"2025-07-13 08:08:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7111972/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7111972/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00894-025-06491-9","type":"published","date":"2025-09-13T15:57:22+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":86968754,"identity":"136fe34a-5235-4052-aef3-19e5f51e599a","added_by":"auto","created_at":"2025-07-17 18:19:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":311856,"visible":true,"origin":"","legend":"\u003cp\u003eInitial (A, B) and optimized (C, D) geometrical structure of Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e species.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7111972/v1/e34e125bc95cb54f4d1636e0.png"},{"id":86968512,"identity":"df72899e-33d4-4428-aea8-d003438eb595","added_by":"auto","created_at":"2025-07-17 18:11:54","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":220868,"visible":true,"origin":"","legend":"\u003cp\u003eGeometrical structure of adsorbed complexes PFOA (\u003cstrong\u003eA\u003c/strong\u003e– molecular form, \u003cstrong\u003eB\u003c/strong\u003e – dissociated form) and \u003cstrong\u003eC\u003c/strong\u003e – PFOS.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7111972/v1/5cf34d9b122ca5fc12adad36.png"},{"id":86968510,"identity":"fee88d67-9e63-4490-8930-a5fe67f8e558","added_by":"auto","created_at":"2025-07-17 18:11:54","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":408483,"visible":true,"origin":"","legend":"\u003cp\u003eComputationally generated vibrational spectra of Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e and adsorbed complexes.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7111972/v1/41d9a303cbe43291f31faaf1.png"},{"id":91818365,"identity":"5f2a7945-8c83-44dd-9906-2b8ebdce2083","added_by":"auto","created_at":"2025-09-22 07:04:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1706514,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7111972/v1/8c641468-58e3-4771-a31f-0adc9c64f561.pdf"},{"id":86968514,"identity":"c02d7f9e-c4ee-4b41-a6ce-c5c4cf4a1ff5","added_by":"auto","created_at":"2025-07-17 18:11:54","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":469976,"visible":true,"origin":"","legend":"","description":"","filename":"LGorbSuplementarymaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-7111972/v1/ed1a39e072ec06a3dac052c4.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Modification of Biochar by Iron Containing Adsorption Centers as a Method to Enhance the Remediation of Perfluorooctanoic (PFOA) and Perfluorooctanesulphonic (PFOS) Acids from Water and Soil: A Density Functional Theory Study","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePer- and polyfluoroalkyl substances (PFAS) are a class of chemicals characterized by having at least one fully fluorinated carbon atom in the structure. The history behind their discovery dates back approximately 90 years. In 1934, two scientists from IG Farben, a German company, discovered polychlorotrifluoroethylene (PCTFE). In 1938, Dr. Roy J. Plunkett and his group synthesized polytetrafluoroethylene (PTFE) that we know as Teflon\u0026trade;[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Today, the US EPA's database, CompTox [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], lists approximately 15,000 different types of PFAS. The size of this group is a consequence of the desired characteristics of stability and resilience. Due to this, PFAS are used in hundreds of products, including stain-resistant textiles, food-handling materials, firefighting foams, construction materials, personal care products, medical devices, and more.\u003c/p\u003e\u003cp\u003eThe enormous commercial value of added/used PFAS has brought dire side effects, such as detrimental health problems, such as various cancers, obesity/increased cholesterol, decreased fertility, etc. The common exposure occurs through the direct use of commercial products containing PFAS and indirectly through environmental contamination. The typical indirect pathways include drinking water (without removing PFAS), food grown on contaminated soil or in contaminated water, and insufficient water and wastewater treatment (not adjusted for PFAS removal), among others.\u003c/p\u003e\u003cp\u003eThe research on health effects related to PFAS concurs with the results of a study by the Centres for Disease Control and Prevention (CDC, conducted between 2000 and 2014. It was found that 98% of Americans have various detectable levels of PFAS in their blood. Currently, investigations and database updates are routinely conducted through community-wide blood testing. [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eFinally, on April 10, 2024, the U.S. EPA announced the National Primary Drinking Water Regulation (NPDWR) for six PFAS, including enforceable Maximum Contaminant Levels (MCLs). The regulated six PFAS in drinking water include perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorohexane sulfonic acid (PFHxS), perfluorononanoic acid (PFNA), and 2,3,3,3-Tetrafluoro-2-(heptafluoropropoxy)propanoic acid (HFPO-DA) with individual MCLs, and PFAS mixtures containing at least two or more of PFHxS, PFNA, HFPO-DA, and also perfluorobutanesulfonic acid (PFBS) using a Hazard Index MCL for the combined and co-occurring levels of these PFAS. In addition, the EPA finalized health-based, non-enforceable Maximum Contaminant Level Goals (MCLGs) for these PFAS. The final rules include the completion of initial monitoring (by 2027) of public water systems, implementation (by 2029) of solutions to decrease excessive levels of PFAS, and, after 2029, mandatory notification to the public when violations persist. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] Within the EU Total PFAS are limited to 0.5 \u0026micro;g/l and levels of 20 individual PFAS to 0.1 \u0026micro;g/l in drinking water under the revised Drinking Water Directive set by European Environmental Agency.\u003c/p\u003e\u003cp\u003eWhile PFOA and PFOS have been the subject of extensive research, their fate, behavior, and interaction in natural ecosystems remain inadequately understood. Therefore, further research is needed to enhance our understanding of PFAS and develop effective strategies for managing its environmental impact.[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] In light of this statement, computer methods are becoming increasingly helpful [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] (both chemoinformatic (e.g., machine learning) methods - for predicting physicochemical properties important in assessing the environmental fate of PFAS as well as computer chemistry methods allowing for modeling the processes). [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eSeveral studies have investigated diverse approaches for PFOA and PFOS remediation. [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] Such adsorbents as nanomaterials, clay, biochar (BC), ion exchange resins, polymers, graphene, carbon nanotubes, and minerals have been recognized as effective agents for the removal of PFOA and PFOS from wastewater.[\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] However, the current technological demands for adsorbents are multifaceted and encompass attributes beyond those provided by mostly natural materials. In essence, modern adsorbents must demonstrate adsorption properties that surpass those of their natural counterparts.\u003c/p\u003e\u003cp\u003eBC represents a carbon-containing product obtained by biomass pyrolysis (e.g., wood waste, agricultural waste, etc.) in conditions of limited oxygen availability (see for example [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]). This material is widely used to improve soil fertility, as a carbon storage agent, or as a filtration medium for purifying water due to its high porosity and ability to adsorb various substances. It is possible to distinguish five potential parameters of modification that affect the BC adsorption capacity of PFAS, which are (i) temperature, (ii) pH, (iii) coexisting contaminants, (iv) contact time, and (v) ionic strength.\u003c/p\u003e\u003cp\u003eMagnetic biochar (MBC) has been modified to exhibit the magnetic properties of BC, thereby expanding the scope of its applications. MBC is made by adding to biochar magnetic materials, such as iron or iron oxides [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], during or after its production process. One of the key advantages of an MBC is its ability to effectively remove contaminants from water. In addition, after the process, MBC can be easily separated from the water using a magnet.\u003c/p\u003e\u003cp\u003eIn this paper, we continue our investigations into the adsorption ability of coal-like materials and their derivatives towards interactions with various environmental contaminants. In initial studies, we developed computational protocols that enable the more accurate prediction of the Gibbs free energy of adsorption than was previously feasible using routine density functional theory (DFT) approximations.[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] The following paper reports an investigation of the nanocomposites formed by graphene oxide and polyvinyl alcohol. [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] Then, we focused on addressing potential solutions to well-known environmental issues, such as the removal of PFOA and PFOS from the environmentю.[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] Several of our investigations are devoted to the interaction of iron-containing compounds with the species which are, in fact, environmental pollutants. An investigation of iron-containing compounds is known to be the most challenging task for quantum chemistry because of the number of theoretical and computational problems associated, in particular, with the open-shell electron structure of these systems.[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] The current paper extends the previous study and provides a novel approach to investigations of the interaction of PFOA and PFOS with the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e component in magnetic biochar.\u003c/p\u003e\u003cp\u003eThe use of MBC is a promising solution for the easy separation and regeneration of adsorbents after adsorption. To enhance the sorption performance of PFOA and PFOS and facilitate the separation of post-adsorbents by an external magnetic field, a feasible method was used to prepare MBC. The synthesis of MBC involves incorporating various nanoparticles, of approximately 100 nm in size, and micro-sized ferromagnetic metals, into the feedstock materials. The magnetization of biochar can be achieved either before pyrolysis, through pre-modification, or after pyrolysis, through post-modification. To facilitate the easy separation of adsorbents after post-adsorption, magnetic properties can be induced by co-precipitating iron nanoparticles in and around feedstock materials. Furthermore, the incorporation of metal nanoparticles, such as MgO, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, CaO, La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, in BC increases the adsorbent's positive charge, making them ideal for anion sorption. The synthesis and application of MBC have been used successfully to remediate various heavy metal(loid)s and organic contaminants. However, the adsorption of PFOA and PFOS by MBC and their interfacial interaction sorption mechanisms require further verification.\u003csup\u003e5\u003c/sup\u003e\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cp\u003eAs we already mentioned, due to the large size, we were not able to model the size of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e species observed experimentally at a reliable quantum-chemical level. Instead, the crystallographic unit of hematite taken from the previous study [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] was initially adopted and multiplied twice in the direction of th\u003cb\u003ee\u003c/b\u003e crystallographic axis a. Obtained in this way, a structure having chemical composition Fe24O36 (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA) has been fully optimized at the density functional level of theory (DFT). Specifically, we employed the M06-2X exchange-correlation functional, which has been demonstrated to be effective in our recent study.[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] Also, the 6-31G(d,p) basis set was used to optimize the geometry and 6-311\u0026thinsp;+\u0026thinsp;+\u0026thinsp;G(d,p) for single-point calculations. To keep the system to be antiferromagnetic, electronic spin has been assigned to zero (S\u003csub\u003ez\u003c/sub\u003e = 0). To verify the ability of the optimized Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e26\u003c/sub\u003e structure to possess also a ferromagnetic state, the analysis of single determinant DFT wavefunction regarding triplet instability has been performed.\u003c/p\u003e\u003cp\u003eSince the separation of the adsorbed substances mostly occurs from the bulk water, the CPCM model, which mimics the influence of the bulk water, was applied with a dielectric permittivity of 78.3.[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] The Cartesian coordinates of all investigated species are presented in the Supplemental Materials.\u003c/p\u003e\u003cp\u003eCalculations of harmonic vibrational frequencies have verified all the structures. The Gibbs free energies have been calculated as implemented in the Gaussian 16 program package using all calculated parameters at 6-31G(d,p) level, except the total energy, which was calculated at the 6-311\u0026thinsp;+\u0026thinsp;+\u0026thinsp;G(d,p) level.\u003c/p\u003e\u003cp\u003eIt is essential to acknowledge that PFOA and PFOS have significantly acidic character, with pKa values of approximately \u0026minus;\u0026thinsp;0.1 [Values of PFOA and Other Highly Fluorinated Carboxylic [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] and much less than zero [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e], respectively. Consequently, for PFOA, the amount of PFOA anions will exceed the concentration of its non-dissociated molecular form by over five times. Given the negative pKa values, the presence of the non-dissociated form is expected to be minimal. Therefore, for PFOA, both the anionic and the non-dissociated forms were considered for adsorption, whereas for PFOS, only the anionic form was accounted for in the adsorption analysis.\u003c/p\u003e\u003cp\u003eDue to the numerical inconveniences of the routine SCF procedure implemented in Gaussian 16, quadratic convergence was applied (scf\u0026thinsp;=\u0026thinsp;qc). To correctly calculate interaction energy, a basis set superposition error has been considered as a counterpoise correction.\u003csup\u003e27\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eAll the calculations were performed using the Gaussian 16 program package.[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eIR-vibrational spectrum, as well as obtained results, has been visualized using the Chemcraft program.[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e] No scaling factors have been incorporated into computational results.\u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eThere are two drawbacks that we can only be considered partially in this work. As mentioned above, the size of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composites adsorbed or chemosorbed by the surface of biochar is near 100 nm. Previously, the largest fragment of iron(II) oxide that we were able to model computationally was Fe\u003csub\u003e13\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e.[23]\u003c/p\u003e\n\u003cp\u003eThe geometry of Fe\u003csub\u003e13\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e was frozen in this study. Currently, we have significantly extended the size of the iron oxide, investigating the Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e species. The geometry of this compound has been fully optimized. The reason to consider the interaction of PFOA and PFOS with the optimized structure of Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e is based on the obvious fact of geometrical relaxation of the Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e surface from the initial geometry upon the complexation of the investigated species. Initial geometry was the simple superposition of two crystal unit cells of hematite. A relaxation of the surface during the complexation will change surface atoms\u0026apos; adsorption ability due to the change in their positions (see Fig. 1). Obviously, not exactly such adsorption centers will be formed during the interactions of experimentally studied much large, c.a. 100 nm species; however, we believe in similarities of their active structures.\u003c/p\u003e\n\u003cp\u003eThe next issue is related to choosing the realistic electronic and spin states for the considered species. Previously, to solve this problem for the Fe\u003csub\u003e13\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e nano-particle, we considered it as an associate of 13(FeO) molecules. This assumption allows to assign the initial spin state of Fe\u003csub\u003e13\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e to be equal to S\u003csub\u003ez\u003c/sub\u003e = 26. Then the dependence of the total energy of unoptimized geometry on S\u003csub\u003ez\u003c/sub\u003e in the vicinity of 26 value was studied, and the final value of S\u003csub\u003ez\u003c/sub\u003e = 28 was assigned. Unfortunately, the size of the considered here system and the necessity to optimize the geometry prevent us from applying the procedure described above. Therefore, we simply took into account the fact that hematite, the most common mineral possessing a composition of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, is an antiferromagnetic compound. This is the reason to assign S\u003csub\u003ez\u003c/sub\u003e = 0 spin state to the considered Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e species.\u003c/p\u003e\n\u003cp\u003eWe begin the discussion by considering the changes that have occurred in the initial geometric structure of the Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e species after the optimization of the geometry (see Fig. 1). To do this, the initial structures (Fig. 1A and 1B) have to be compared with the optimized ones (Fig. 1C and 1D). The tendency to transform the initial shape of the parallelepiped into a form rather resembling a globule is clearly displayed. During this transformation, the molecular cavity formed during CPCM calculations lost 15% of its volume (from 919 \u0026Aring;\u003csup\u003e3\u003c/sup\u003e to 786 \u0026Aring;\u003csup\u003e3\u003c/sup\u003e). The formed structure still exhibits certain features of the topology of the initial species, which is especially evident from the comparison of structures B and D (Fig.\u0026nbsp;1), but with the transition of iron atoms (36, 37, 38, 39 see, Fig.\u0026nbsp;1A) from the middle layer to the formed layers of the globule (see Fig.\u0026nbsp;1D).\u003c/p\u003e\n\u003cp\u003eThe values of the coordination numbers of iron and oxygen atoms in the initial and optimized structures are given in Table\u0026nbsp;1S. Analysis of the data displayed in this table showed that during the optimization (relaxation) of the Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e structure, it is transformed from the parallellipid like to the globule (see Fig. 1). This is accompanied by an intuitively clear increase in the coordination of iron atoms by oxygen atoms with the dominance of iron atoms coordinated by five oxygen atoms. One may see that five-coordinated iron atoms are located in the centre of the globule, and lower-coordinated ones are at its boundaries. This arrangement of iron atoms prompted us to investigate the interaction of PFOA and PFOS with five-coordinated iron atoms, specifically those at positions 4, 31, and 34 (see Fig. 1). We would also like to mention that in the bulk of the hematite crystal, the coordination number of iron atoms is equal to six.[24]\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eFigure 1 is here\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe last issue that we would like to discuss before analyzing the adsorption of PFOA and PFOS is the similarity in magnetic properties of hematite and the investigated structure of Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e. As follows from numerous studies (see for example [30], hematite is antiferromagnetic below 260 K and exhibits weak ferromagnetic properties between 260 K and 950 K. This finding is confirmed by very accurate computations of (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003en\u003c/sub\u003e (n\u0026thinsp;=\u0026thinsp;1\u0026ndash;5) clusters\u003csup\u003e31\u003c/sup\u003e. This investigation basically highlights two facts:\u003c/p\u003e\n\u003col\u003e\n \u003cli\u003e\n \u003cp\u003eElectronic configurations of the clusters (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003en\u003c/sub\u003e (n\u0026thinsp;=\u0026thinsp;1\u0026ndash;5) reveal the appearance of antiferromagnetic and ferromagnetic states.\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eThe electronic configurations of the clusters has only small influence on their geometric structure.\u003c/p\u003e\n \u003c/li\u003e\n\u003c/ol\u003e\n\u003cp\u003eTo study the possible emergence of ferromagnetic states, we have analyzed the obtained M06-2x/6-31G(d,p) DFT wavefunction on the stability with respect to the UHF solution (ability to form the electronic states containing unpaired electrons). Indeed, we found such instability concerning two double-occupied molecular orbitals of Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e (see Fig. 1C and D). The consequence of such RHF wave function instability could be the appearance of so-called broken symmetry ferromagnetic electronic states.\u003c/p\u003e\n\u003cp\u003eThe geometric structure of the PFOA and PFOS adsorption complexes is shown in Fig.\u0026nbsp;2. We note that both PFOA and PFOS are adsorbed in the so-called skewed form. Such an orientation indicates a particular stabilizing contribution of the electrostatic and dispersion interaction between the surface and the adsorbed molecules. This is in contrast to the interaction of considered species with the surfaces of graphene, graphene oxide, and fluorinated graphene which exhibit the parallel orientation of PFOA and PFOS regarding the adsorbing surface\u003csup\u003e21\u003c/sup\u003e Therefore, we guess that the stabilization contribution is probably smaller than the one which characterizes the interaction of those species with the graphene surface of graphene oxide and fluorinated graphene surface. By making such comparisons, we imply that the structure of the surfaces under discussion can be a simplified model of biochar, which is considered as an effective adsorbent of PFOA and PFOS (see the Introduction). Table 1 presents the interaction energy and Gibbs free energy values for all three complexes. Although both values do not contain the correction associated with the basis set superposition error, the range of the analyzed values confidently indicates virtually 100% adsorption of PFOA and PFOS by five-coordinated Fe atoms. Biochar is one of the adsorbents to which hematite (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) is added. Therefore, it is interesting to compare the adsorption capacity of biochar adsorption centers with the adsorption capacity of five-coordinated Fe atoms generated by us computationally. To facilitate this comparison, we utilized results from our previous study [21] and transferred the calculated values of interaction energies and Gibbs free energies to Table\u0026nbsp;1. It can be seen that the interaction with the adsorption centres of \u0026quot;pristine\u0026quot; biochar provides almost complete absorption of PFOA and PFAS, but the additional interaction with the adsorption centres of hematite further enhances this adsorption property. This conclusion is in line with the experimental observation that the presence of iron oxides not only makes the easy extraction of biochar from the aqueous bulk more efficient, but also enhances its adsorption properties of modified in this way a biochar surface. [5]\u003c/p\u003e\n\u003cp\u003eConcluding the section related to the discussion of the geometry and energy of adsorption, we would like to note a novel, interesting feature that is not typical for the adsorption of PFOA and PFOS by the surfaces of graphene and graphene oxide. By interacting with the five-coordinated Fe atom, the PFOA is not adsorbed in molecular form. Instead, a proton is transferred to the surface of the hematite to form an ion pair. (see Fig. 1A and B).\u0026nbsp;\u003c/p\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 1\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eAdsorption energies.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAdsorption complex\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026Delta;E\u003csub\u003eint\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026Delta;G\\(\\:\\frac{298}{\\:}\\)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026Delta;E\u003csub\u003eint\u003c/sub\u003e(BC)\u003csup\u003e21\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026Delta;G\\(\\:\\frac{298}{\\:}\\)(BC)\u003csup\u003e21\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCF3(CF\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003eCOOH\u0026hellip;Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-53.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-36.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-10.0 \u0026ndash; -14.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-6.3 \u0026ndash; -10.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCF3(CF\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003eCOO-\u0026hellip;Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-29.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-12.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-15.2 \u0026ndash; -27.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-4.3 \u0026ndash; -7.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCF3(CF\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e7\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e-\u0026hellip;Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-129.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-107.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-18.4 \u0026ndash; -26.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-0.4 \u0026ndash; -9.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003c/p\u003e\n\u003cp\u003eAlthough FTIR studies are among the most common in environmental chemistry, we did not find a large number of publications devoted to studying the interaction between iron oxides and PFOA and PFAS. Perhaps one of the detailed studies is [5], in which the IR bands have been assigned. Besides, the fragment of the spectrum characterizing the region of vibrations of the atoms Fe and O is a wide, insufficiently resolved region, it is proposed to consider that the peaks between 550 cm-1 to 700 cm belong to the Fe\u0026ndash;O functional groups, the peaks at 636 cm-1 and 559 cm-1 represent the vibration of Fe atoms located in tetrahedral and octahedral positions. The peaks at 1616 cm-1 correspond to the bending vibration of moisture content on the bare iron oxide nanoparticles. The peaks at 3413 cm\u003csup\u003e-1\u003c/sup\u003e correspond to the hydroxyl functional groups on the surface of the iron nanoparticles (OH\u003csup\u003e-\u003c/sup\u003e). The peak band at 1000\u0026ndash;1400 cm-1 corresponds to the vibrations of the -CF\u003csub\u003e3\u003c/sub\u003e and -CF\u003csub\u003e2\u003c/sub\u003e- groups that originate from organic fluorine, indicating that the peaks at 1384 cm\u003csup\u003e-1\u003c/sup\u003e and 1245 cm\u003csup\u003e-1\u003c/sup\u003e represent -CF\u003csub\u003e2\u003c/sub\u003e- and -CF\u003csub\u003e3\u003c/sub\u003e bending due to the adsorption of PFOS.\u003c/p\u003e\n\u003cp\u003eAlthough the resolutions of our computationally generated spectra appear more detailed (see Fig.\u0026nbsp;3), we are unable to provide such a detailed interpretation. The data presented in Table\u0026nbsp;2 show that all the observed bands include several vibrations. We have chosen the most intensive one to assign the band.\u003c/p\u003e\n\u003cp\u003eAnalyzing the vibrations of the Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e globule, we are mostly able to observe only the collective motion of oxygen atoms inside the Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e26\u003c/sub\u003e species. An example of the vibration representing such collective motion is displayed in Fig. 1S, where the displacement vectors of the most contributing vibration to the band with a peak at 749 cm\u003csup\u003e-1\u003c/sup\u003e. Also, we have not identified where well-resolved vibrations of just five coordinated Fe atoms are involved. This is probably because of the size of the iron-oxygen species considered to be too small compared to the size of the experimentally fixed species. However, the assignment of the bands related to stretching C-F, C-O, and O-H motion is clearly seen from the data presented in Table 2.\u003c/p\u003e\n\u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 2\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eAssignment of vibrational bends.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eBand\u003c/p\u003e\n \u003cp\u003e(cm\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAssignment\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eBand\u003c/p\u003e\n \u003cp\u003e(cm\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAssignment\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eFe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eFe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e.. .HOOCC\u003csub\u003e7\u003c/sub\u003eF\u003csub\u003e15\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e386.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003easymmetric stretching of low coordinated O\u003c/p\u003e\n \u003cp\u003eatoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e451.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected Fe\u003c/p\u003e\n \u003cp\u003eand O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e484.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e484.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low\u003c/p\u003e\n \u003cp\u003ecoordinatedlow coordinated O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e532.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e564.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected low coordinatedO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e570.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e751.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emix collective motion of selected O\u003c/p\u003e\n \u003cp\u003eatoms and stretching vibration of C-F bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e624.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of low coordinated selected O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e814.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e721.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of low coordinated selected O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1210.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected C-F bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e749.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of low coordinated elected O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1282.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected C-F bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e768.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emix of bending and stretching motion of\u003c/p\u003e\n \u003cp\u003eselected O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1780.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003easymmetric stretching vibration of C-O bond\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e812.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emix of bending and stretching motion of\u003c/p\u003e\n \u003cp\u003eselected O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3557.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching vibration of O-H bond\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e835.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emix of bending motion of selected O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e849.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emix of bending and stretching motion of\u003c/p\u003e\n \u003cp\u003eselected Fe and low coordinated O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e915.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected Fe and low\u003c/p\u003e\n \u003cp\u003ecoordinated O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e945.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e973.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1211.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected C-F bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1780.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of C-O bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3557.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of O-H bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003e\u003cstrong\u003eFe\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e24\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e36\u003c/strong\u003e\u003c/sub\u003e. \u003cstrong\u003e. .CO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eC\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e7\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eF\u003c/strong\u003e\\(\\:\\frac{-}{15}\\)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003e\u003cstrong\u003eFe\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e24\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e36\u003c/strong\u003e\u003c/sub\u003e. \u003cstrong\u003e.. .SO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eC\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e8\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eF\u003c/strong\u003e\\(\\:\\frac{-}{17}\\)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e470.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e404.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e538.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e485.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e660.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected C-F\u003c/p\u003e\n \u003cp\u003ebonds and bonding motion of C-O bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e532.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e716.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e591.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected Fe and O atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e760.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e626.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e808.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e722.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e870.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e767.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e972.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e811.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e12120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected C-F bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e840.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ecollective motion of selected low coordinated\u003c/p\u003e\n \u003cp\u003eO atoms\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1283.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected C-F bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1189.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003easymmetric stretching vibration of S-O bond\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1754.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of C-O bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1219.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected C-F bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1291.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected C-F bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1360.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003estretching motion of selected C-C bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eBending motion of C-C bonds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"1\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n"},{"header":"4. Conclusions","content":"\u003cp\u003eIn light of the increasing threat posed by PFAS to the environment and humans as well as insufficient knowledge about the spread and presence of these compounds, it is necessary to search for new solutions to limit their occurrence as well as methods of their capture from the environment. In our study, we investigate the magnetic biochar as a solution for the separation and regeneration of adsorbents after adsorption. To perform computational analysis of the interactions of PFOA and PFOS with Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, a component of magnetic biochar, we designed a simplified model with the iron oxide stoichiometry of Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e. Analysis of the interaction of five-coordinated Fe(III) ions of this model with CF\u003csub\u003e3\u003c/sub\u003e(CF\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003eCOOH, CF\u003csub\u003e3\u003c/sub\u003e(CF\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003eCOO\u003csup\u003e-\u003c/sup\u003e), CF\u003csub\u003e3\u003c/sub\u003e(CF\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e7\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e suggests the skewed configuration which those species possess during the adsorption. Additionally, it is worth noting that the molecular form of adsorption for CF3(CF2)6COOH has not been observed. Instead, the study revealed the proton transfer with the formation of a surface ion pair.\u003c/p\u003e\u003cp\u003eAnalysis of the interaction energies has concluded that PFOA and PFOS interact more strongly with the adsorption surfaces formed by coordinated iron ions than with the pristine carbon and oxidised carbon surfaces of biochar.\u003c/p\u003e\u003cp\u003eThe computationally generated IR spectra of adsorbed species are not fully comparable with the experimental ones due to the small size of the Fe\u003csub\u003e24\u003c/sub\u003eO\u003csub\u003e36\u003c/sub\u003e species considered in the calculations. They do not exhibit well-resolved vibrations that can identify five coordinated Fe(III) ions. However, they allow one to observe the collective vibrations of oxygen atoms in the Fe24O36 particle and assign C-F, C-O, and Fe-O-H stretching vibrations.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u0026nbsp;Data availability\u003c/p\u003e\n\u003cp\u003eThe data supporting this article have been included as part of the Supplementary Information.\u003c/p\u003e\n\u003cp\u003eConflicts of interest\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflicts of interest\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Funding\u003c/p\u003e\n\u003cp\u003eThis work was supported by the US Army Engineer Research and Development Center (ERDC), grant number W912HZ-23-2-0006 and\u0026nbsp;the European Union\u0026rsquo;s Horizon 2020 research and innovation programme via the PROMISCES project under grant agreement N\u0026ordm;101036449.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAcknowledgment\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe computation time was provided by the Mississippi Center for Supercomputer Research and Centre of Informatics Tricity Academic Supercomputer and Network of the University of Gdansk.\u003c/p\u003e\n\u003cp\u003eLG thanks Dr. Mykola Ilchenko for the help with the BSSE calculations.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eSamora S, Lucas S. The history of PFAS: From World War II to your Teflon pan. https://www.manufacturingdive.com/news/the-history-behind-forever-chemicals-pfas-3m-dupont-pfte-pfoa-pfos/698254/\u003c/li\u003e\n\u003cli\u003eUS EPA: CompTox Chemicals Dashboard. https://comptox.epa.gov/dashboard/\u003c/li\u003e\n\u003cli\u003eU.S. Department of Health and Human Services, The Agency for Toxic Substances and Disease Registry (ATSDR), PFAS in the U.S. Populatione. \u003cem\u003ehttps://www.atsdr.cdc.gov/\u003c/em\u003e\u003c/li\u003e\n\u003cli\u003eUS EPA: The final National Primary Drinking Water Regulation (NPDWR) for six PFAS. https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas\u003c/li\u003e\n\u003cli\u003eHassan M, et al (2022) Magnetic biochar for removal of perfluorooctane sulphonate (PFOS): Interfacial interaction and adsorption mechanism Environ Technol Innov 28:, 102593\u003c/li\u003e\n\u003cli\u003eSosnowska A, et al (2023) Expanding the applicability domain of QSPRs for predicting water solubility and vapor pressure of PFAS. \u003cem\u003eChemosphere\u003c/em\u003e 340: 139965\u003c/li\u003e\n\u003cli\u003eCzapla M, Skurski P (2023) Degradation of selected perfluoroalkyl substances (PFASs) using AlF3 in water. Phys Chem Chem Phys 25:18095\u003c/li\u003e\n\u003cli\u003eBanks D et al (2020) Selected advanced water treatment technologies for perfluoroalkyl and polyfluoroalkyl substances: A review. Sep Purif Technol 231: 115929\u003c/li\u003e\n\u003cli\u003eCheng J et al (2010) Sonochemical degradation of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) in groundwater: Kinetic effects of matrix inorganics. Environ. Sci. Technol. 44: 445 - 450.\u003c/li\u003e\n\u003cli\u003eMahinroosta R, Senevirathna L (2020) A review of the emerging treatment technologies for PFAS contaminated soils.(2020) J Environ Manage 255: 109896\u003c/li\u003e\n\u003cli\u003eGagliano E, et al (2020) Removal of poly- and perfluoroalkyl substances (PFAS) from water by adsorption: Role of PFAS chain length, effect of organic matter and challenges in adsorbent regeneration. Water Res 171: 115381\u003c/li\u003e\n\u003cli\u003eMejia-Avenda\u0026ntilde;o S, \u003cem\u003eet al\u003c/em\u003e (2020) Sorption of Polyfluoroalkyl Surfactants on Surface Soils: Effect of Molecular Structures, Soil Properties, and Solution Chemistry. \u003cem\u003eEnviron. Sci. Technol.\u003c/em\u003e 2020, 54: 1513 - 1521\u003c/li\u003e\n\u003cli\u003eZhang DQ, et al (2019) Adsorption of perfluoroalkyl and polyfluoroalkyl substances (PFASs) from aqueous solution - A review. Sci Total Environ \u003cstrong\u003e 694\u003c/strong\u003e:133606.\u003c/li\u003e\n\u003cli\u003eLehmann J, Joseph S (2012) Biochar for Environmental Management.Routledge, London doi:10.4324/9781849770552\u003c/li\u003e\n\u003cli\u003eTomczyk A, Sokołowska Z, Boguta P (2020) Biochar physicochemical properties: pyrolysis temperature and feedstock kind effects. Rev. Environ. Sci. Biotechnol. 19: 191 - 215\u003c/li\u003e\n\u003cli\u003eE. Weidner, et al (2022) Hybrid Metal Oxide/Biochar Materials for Wastewater Treatment Technology: A Review. ACS Omega 7: 27062\u003c/li\u003e\n\u003cli\u003eZeng H, Li J, Liu JP, Wang ZL, Sun S (2002) Exchange-coupled nanocomposite magnets by nanoparticle self-assembly. Nature 420: 395 -408 \u003c/li\u003e\n\u003cli\u003eRaj K, Moskowitz R (1990) Commercial applications of ferrofluids. \u003cem\u003eJ. Magn. Magn. Mater.\u003c/em\u003e 85: 233 -245\u003c/li\u003e\n\u003cli\u003eMichalkova A, et al (2011) Can the Gibbs Free Energy of Adsorption Be Predicted Efficiently and Accurately: An M05-2X DFT Study. J Phy. Chem A, 115: 2423 -2430\u003c/li\u003e\n\u003cli\u003eCheng J, et al. (2010) Sonochemical degradation of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) in groundwater: Kinetic effects of matrix inorganics. Environ Sci Technol 44:\u003cstrong\u003e,\u003c/strong\u003e 445 - 450\u003c/li\u003e\n\u003cli\u003eGorb L, Ilchenko M, Leszczynski J (2020) A density functional theory study of simplest nanocomposites formed by graphene oxide and polyvinyl alcohol: geometry, interaction energy and vibrational spectrum J. Mol. Model26:183\u003c/li\u003e\n\u003cli\u003eIsayev O, Gorb L, Zilberberg I, Leszczynski J (2007) Electronic Structure and Bonding of {Fe(PhNO2)}6 Complexes: A Density Functional Theory Study J Phys Chem A 111: 3571 - 3576\u003c/li\u003e\n\u003cli\u003eGorb L, Ilchenko M, Leszczynski J (2022) Decomposition of 2,4,6-trinitrotoluene (TNT) and 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) by Fe\u003csub\u003e13\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e nanoparticle: density functional theory study. Environ. Sci. Pollut. Res. \u003cstrong\u003e\u003cem\u003e29\u003c/em\u003e\u003c/strong\u003e: 68522\u003c/li\u003e\n\u003cli\u003eFinger LW, Hazen RM (1980) Crystal structure and isothermal compression of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e to 50 kbars. J. Appl. Phys. 51: 5362 - 5367\u003c/li\u003e\n\u003cli\u003eCossi M, Scalmani G, Rega N, Barone V(2002) New developments in the polarizable continuum model for quantum mechanical and classical calculations on molecules in solution J. Chem. Phys\u003cem\u003e.\u003c/em\u003e \u003cstrong\u003e117\u003c/strong\u003e: 43 - 54\u003c/li\u003e\n\u003cli\u003eGoss KU (2008) The pK\u003csub\u003ea\u003c/sub\u003e Values of PFOA and Other Highly Fluorinated Carboxylic Acids. Environ Sci \u0026amp; Technol 42: 456 - 458\u003c/li\u003e\n\u003cli\u003eBoys SF, Bernardi F (1970) The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors. Mol Phys\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e19\u003c/strong\u003e, 553.\u003c/li\u003e\n\u003cli\u003eFrisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Petersson GA, Nakatsuji H, Li X, Caricato M, Marenich AV, Bloino J, Janesko BG, Gomperts R, Mennucci B, Hratchian HP Gaussian 16, Revision C.01\u003c/li\u003e\n\u003cli\u003eChemcraft - Graphical software for visualization of quantum chemistry computations. Version 1.8, build 682. https://www. chemcraftprog. co. Chemcraft\u003c/li\u003e\n\u003cli\u003eEncyclopedia of Inorganic Chemistry. (Wiley, 2005). doi:10.1002/0470862106\u003c/li\u003e\n\u003cli\u003eErlebach A, H\u0026uuml;hn C, Jana R, Sierka M, (2014) Structure and magnetic properties of (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e)n clusters (n = 1-5) Phys Chem Chem Phys \u003cstrong\u003e16\u003c/strong\u003e: 26421\u003c/li\u003e\n\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":"journal-of-molecular-modeling","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jmmo","sideBox":"Learn more about [Journal of Molecular Modeling](https://www.springer.com/journal/894)","snPcode":"894","submissionUrl":"https://submission.nature.com/new-submission/894/3","title":"Journal of Molecular Modeling","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Magnetic biochar, poly-fluoroalkyl substances, perfluorooctanoic acid, perfluorooctanesulphonic acid, PFAS remediation, density functional theory (DFT)","lastPublishedDoi":"10.21203/rs.3.rs-7111972/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7111972/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eContext\u003c/p\u003e\u003cp\u003ePer- and polyfluoroalkyl substances (PFAS), with over 15,000 types listed in the US EPA\u0026rsquo;s CompTox database, are found in everyday items like textiles, packaging, firefighting foams, and medical devices. Their widespread use has led to concerning health effects\u0026mdash;including cancers, elevated cholesterol, and fertility issues\u0026mdash;with detectable levels present in 98% of Americans.\u003c/p\u003e\u003cp\u003eWhile PFOA and PFOS are among the most studied, their environmental behavior and ecological interactions remain poorly understood. Advances in computer-based methods, including chemoinformatics and quantum modeling, now aid in predicting properties and simulating PFAS dynamics.\u003c/p\u003e\u003cp\u003eBiochar (BC), produced via biomass pyrolysis under limited oxygen, is known for its porosity and adsorption capabilities. Magnetic biochar (MBC), enhanced with iron-based compounds, adds the benefit of magnetic separation, making it ideal for water decontamination. This paper explores the use of MBC to remove PFOA and PFOS from the environment, leveraging computational tools to investigate molecular interactions and adsorption properties.\u003c/p\u003e\u003cp\u003eMethods\u003c/p\u003e\u003cp\u003eA doubled crystallographic unit of hematite (Fe₂₄O₃₆) was constructed and fully optimized using density functional theory (DFT) with the M06-2X functional. Geometry optimization used the 6-31G(d,p) basis set, while single-point energies were calculated with 6-311\u0026thinsp;+\u0026thinsp;+\u0026thinsp;G(d,p). Antiferromagnetic conditions were ensured by setting the total spin to zero (Sz\u0026thinsp;=\u0026thinsp;0), and triplet instability analysis was performed to evaluate ferromagnetic potential.\u003c/p\u003e\u003cp\u003eTo simulate bulk water effects on adsorption, the CPCM solvation model (ε\u0026thinsp;=\u0026thinsp;78.3) was applied. Harmonic frequency analysis confirmed structural minima, and Gibbs free energies were calculated using Gaussian 16. PFOA and PFOS, with highly negative pKa values (~\u0026ndash;0.1 and \u0026lt;\u003c/p\u003e\u003cp\u003eQuadratic SCF convergence (scf\u0026thinsp;=\u0026thinsp;qc) addressed numerical challenges, and interaction energies were corrected for basis set superposition error using the counterpoise method. Calculated IR spectra and molecular visualizations were generated with Chemcraft, without applying scaling factors.\u003c/p\u003e","manuscriptTitle":"Modification of Biochar by Iron Containing Adsorption Centers as a Method to Enhance the Remediation of Perfluorooctanoic (PFOA) and Perfluorooctanesulphonic (PFOS) Acids from Water and Soil: A Density Functional Theory Study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-17 18:11:50","doi":"10.21203/rs.3.rs-7111972/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-07-28T13:57:13+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-28T13:36:40+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-17T20:31:16+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"327680047125838393593056398806552030634","date":"2025-07-17T18:27:49+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"239709490752675762727580733033432020660","date":"2025-07-15T18:51:47+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-15T10:15:58+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-15T04:57:40+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-07-15T04:55:44+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Molecular Modeling","date":"2025-07-13T07:59:22+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-molecular-modeling","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jmmo","sideBox":"Learn more about [Journal of Molecular Modeling](https://www.springer.com/journal/894)","snPcode":"894","submissionUrl":"https://submission.nature.com/new-submission/894/3","title":"Journal of Molecular Modeling","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"4d58bce1-1f07-4df7-a62b-e57a4ccdfac4","owner":[],"postedDate":"July 17th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-09-22T06:59:59+00:00","versionOfRecord":{"articleIdentity":"rs-7111972","link":"https://doi.org/10.1007/s00894-025-06491-9","journal":{"identity":"journal-of-molecular-modeling","isVorOnly":false,"title":"Journal of Molecular Modeling"},"publishedOn":"2025-09-13 15:57:22","publishedOnDateReadable":"September 13th, 2025"},"versionCreatedAt":"2025-07-17 18:11:50","video":"","vorDoi":"10.1007/s00894-025-06491-9","vorDoiUrl":"https://doi.org/10.1007/s00894-025-06491-9","workflowStages":[]},"version":"v1","identity":"rs-7111972","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7111972","identity":"rs-7111972","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}