Efficiency of surface-active natural rubber latex (NRL) films in capturing sludge wastes in seawater and river water

Efficiency of surface-active natural rubber latex (NRL) films in capturing sludge wastes in seawater and river water | 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 Efficiency of surface-active natural rubber latex (NRL) films in capturing sludge wastes in seawater and river water Nur Ayunni Ahmad Shahrul Amin, Neettha Nai Sem, Azura A. Rashid, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7361887/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract The management of sludge waste remains a critical challenge, as improper disposal can contaminate soil and water sources, impacting ecosystems and human health. In this study, polyaluminum chloride (PACl), coagulant was used to enhance the capturing process of domestic sludge waste using surface-active natural rubber latex (NRL) films. The NRL films were manufactured using the compounding and curing methods. The surface-active NRL film facilitated sludge removal by utilizing electrostatic interactions between the film and sludge particles. The incorporation of coagulant increased the sludge particle size and reduced the zeta potential of sludge water, enhancing aggregation. FTIR analysis confirmed the presence amines, proteins and peptides on the NRL film surface after the capturing process, indicating the captured sludge on the surface. The sludge deposition on the NRL films surfaces is more pronounced when the coagulant was incorporated. These findings demonstrate the effectiveness of surface-active NRL films in sludge waste removal, highlighting their potential for water treatment applications. Coagulant-assisted sludge capturing process NRL film surface-active latex wastewater treatment Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Introduction The increasing volume of sludge waste, a byproduct of wastewater treatment, is a pressing environmental concern. Sludge waste, a major by-product of biological wastewater treatment in sewage treatment plants (STPs), represents a largely untapped resource with significant potential to support the circular economy. With global production surpassing 300 million tons per year, effective management of sludge waste has become increasingly critical, especially as China accounts for nearly 20% of the total output. [ 1 ]. This rise is predominantly owing to two key factors: population growth in urban areas and advancements in wastewater treatment technologies. As highlighted by Malik et al. [ 2 ], Malaysia faces a growing challenge in managing sludge waste, with production reaching three million metric tons annually. These figures are projected to rise significantly, with estimates suggesting a potential increase to seven million metric tons by 2020. Improper sludge management poses severe environmental threats, including the contamination of surface and groundwater resources, land degradation, and the introduction of pollutants into the food chain [ 3 ]. In this context, the present study investigates a novel rubber-based film system as an alternative to conventional membrane technologies for sludge particle removal in wastewater streams. Unlike porous membranes, which rely on size exclusion and pressure-driven separation, the rubber films developed herein are nonporous and exploit surface interaction phenomena, predominantly electrostatic attraction and adhesive forces, to sequester sludge particles from aqueous media. Traditional membrane filtration, though capable of capturing particles in the micrometer to sub-micrometer range, suffers from inherent limitations such as high operational energy demands, membrane fouling, and the necessity for frequent chemical cleaning and replacement [ 4 ]. The rubber films in this study are produced using natural rubber latex (NRL), with surface charge modification achieved via incorporation of zinc diethyldithiocarbamate (ZDEC), which functions dually as a vulcanization activator and a surface functionalizing agent. Periodic mechanical stretching was employed to induce anisotropic surface structuring, further enhancing the electropositive surface character of the films. This positive charge facilitates selective adsorption of anionic sludge colloids and flocs through Coulombic attraction and interfacial adhesion mechanisms. The objective of this work is to fabricate and optimize surface-engineered NRL films for effective sludge entrapment in wastewater treatment processes. Additionally, the role of commercial coagulant agents commonly employed in industrial effluents is examined in terms of their influence on sludge particle agglomeration dynamics and subsequent capture efficiency. The performance of the rubber films is further correlated with varying ZDEC concentrations to elucidate the electrostatic binding efficacy with water quality post-capturing. Through targeted surface modification and systematic material-properties optimization, this study provides a fundamental basis for developing cost-effective, energy-efficient, and scalable sludge capture technologies using rubber-based materials. The findings contribute to advancing alternative materials in environmental remediation, particularly in sludge management within the wastewater treatment sector. Experimental Materials The sludge waste used in this study was acquired from Indah Water Konsortium (IWK). River water was collected from Sungai Kerian, Perak, while seawater was obtained from Ban Pecah Beach, Perak. The natural rubber latex (NRL) of 60% total solid content (TSC) was sourced from Zarm Scientific and Supplies (Malaysia) Sdn. Bhd. All other chemicals used in the latex compounding were procured from Merck before being made into a dispersion and solution form, including 50.9% zinc oxide (ZnO), 57.5% zinc diethyldithiocarbamate (ZDEC), 56.0% tetramethyl thiuram disulfide (TMTD), 56.5% sulfur and 10% potassium hydroxide (KOH). The coagulant, polyaluminum chloride (PACl) was produced by ACME Chemical Sdn. Bhd. Method Natural Rubber Latex (NRL) Quality test The TSC of each compounding ingredient and latex was measured. The empty petri dish was weighed first, then, approximately 1 mg of each ingredient for compounding, and the latex was weighed separately and added to the dish. The latex was gently swirled to ensure it coated the bottom of the dish. Next, 20 cm 3 of distilled water was added, and the mixture swirled thoroughly. The uncovered petri dish was then placed in an oven set to 105 ± 5°C. The sample was heated for 2 hours or until it lost its whitish appearance. After cooling to room temperature in a desiccator, the dish was weighed again. The sample was then reheated in the oven for 15 minutes, cooled in a desiccator, and weighed a final time. Eq. ( 1 ) was used to calculate the TSC, where m₁ represents the mass of the initial sample and m₀ represents the mass of the dried sample [ 5 ]. $$\:TSC=\:\frac{{m}_{1}}{{m}_{0}}\times\:100$$ 1 Natural Rubber Latex (NRL) film preparation The NRL film was produced using the formulation in Table 1 . The compounding process was initiated with NRL being stirred in an overhead mixer at 200 rpm Next, the remaining ingredients were added in a specified order: KOH, ZnO ZDEC, TMTD and sulphur. The mixture was continuously stirred at a high speed of 250 rpm for an additional 30 minutes. To eliminate any trapped air bubbles, it was then degassed in a vacuum desiccator at 25°C for 15 minutes. After degassing, the latex was casted onto a glass plate using a casting knife, which was set with a 0.5 mm gap in front of a moving beam to ensure uniform film thickness. The cast film was then dried in an oven at 100°C for 2 hours, followed by cooling at room temperature for 15 minutes. Table 1 NRL film compounding formulation Ingredients Formulation 1 (phr) Formulation 2 (phr) Formulation 3 (phr) Formulation 4 (phr) 60% NR Latex 100 100 100 100 10% KOH 1.0 1.0 1.0 1.0 50.9% ZnO 1.0 1.0 1.0 1.0 57.5% ZDEC 0.6 5.0 10.0 15.0 56.0% TMTD 2.0 2.0 2.0 2.0 56.5% Sulphur 1.0 1.0 1.0 1.0 Total 105.6 110.0 115.0 120.0 NRL film properties A multimeter was used to measure the electrical potential that developed on the surface of the NRL film. The film underwent a repeated cycle of stretching and relaxing, with each phase lasting 8 hours and 12 hours of relaxing. This cycle continued for a total of 40 hours. The stretching was performed to 30% of the initial film length. During this experiment, the NRL film was connected to the multimeter according to the specific setup shown in Fig. 1 . Performance of NRL Film in sludge removal process The performance evaluation of the NRL films for sludge waste capture was conducted using a controlled laboratory-scale water treatment setup as shown in Fig. 2 . NRL films with dimensions of 20 cm x 20 cm were arranged in a relaxed (unstressed) state within a static tank system containing a homogenous sludge suspension. The adsorption experiment was carried out over a 12-hour period to allow for sufficient interaction between the film surface and suspended sludge particulates. Pre and post-treatment analyses were performed to evaluate the sludge removal efficiency through UV–Vis spectrophotometry (PerkinElmer Lambda 35) using a wavelength range of 200–400 nm, zeta potential, and dynamic light scattering (DLS) measurements using the Malvern Zetasizer Nano ZS at an operating voltage of 40V. The percentage of sludge removal was calculated using the DLS data. To investigate the physicochemical changes at the film–sludge interface, scanning electron microscopy (SEM) imaging was performed using a ZEISS SUPRA 35 VP instrument. SEM imaging enabled high-resolution observation of the film surface morphology before and after the sludge capture process, revealing particulate adherence and topographical alterations. Fourier Transform Infrared (FTIR) spectroscopy was employed using a PerkinElmer instrument to identify the functional groups present in bare natural rubber latex (NRL) films, raw sludge, and sludge-contaminated film samples. Spectral data were recorded over the range of 550–4000 cm⁻¹ and interpreted through comparison with reference spectra to assign characteristic vibrational bands corresponding to functional group identities. The concentration of zinc (Zn) ions in the water samples pre- and post-treatment was quantified using Atomic Absorption Spectroscopy (AAS), employing a PerkinElmer Model 3300 instrument. Water samples were appropriately filtered and acidified with nitric acid to stabilize metal ions prior to analysis. Calibration was performed using standard Zn solutions, and absorbance was measured at the characteristic wavelength of 213.9 nm to determine Zn concentrations, enabling evaluation of any potential leaching from the NRL films or changes due to sludge interaction. The adsorption capacity Q of the sludge on the NRL films was calculated using Eq. 2 [ 6 ]. The data were analyzed based on the hydrodynamic size value pre- and post-sludge capturing from the DLS and the total suspended solid (TSS) measurements. The initial TSS concentration (TSS₀) was determined by filtering a 20 mL of the sludge in seawater and riverwater separately, through a pre-weighed 0.5 µm filter. Then the retained solids were dried at 105°C, and the mass difference per volume filtered was calculated [ 7 ]. $$\:Q=\:\frac{{(C}_{O\:}-\:{C}_{e})}{A}\times\:V$$ 2 Results and discussion Surface Potential of NRL films As shown in Fig. 3 , at 0 phr ZDEC, unstretched films exhibit negligible surface potential, indicating the absence of mobile ionic species or polar moieties at the interface. Upon 8-hour of uniaxial stretching, the surface potential increases with ZDEC content, reaching ~ 65 mV at 15 phr. This enhancement is attributed to the migration and surface enrichment of ionic zinc species, facilitated by deformation-induced chain orientation and free volume generation [ 8 – 9 ]. More strikingly, samples subjected to an 8-hour stretch followed by a 12-hour relaxation and a subsequent 8-hour stretch exhibit significantly higher ϕₛ values, exceeding 95 mV at 15 phr ZDEC. This increase in surface charge correlates with the ionic and chemical coordination of ZDEC, a multifunctional additive that serves both as a vulcanization accelerator and a source of zinc ions. This finding emphasizes the critical role of ZDEC in modulating the surface electrostatic properties of the NRL film. The positive surface potential is attributed to the incorporation and redistribution of Zn²⁺ ions within the rubber matrix, particularly under the influence of periodic stretching that caused surface micromechanical deformation. ZDEC facilitates crosslink formation through the creation of zinc-sulfur complexes, while simultaneously influencing the ionization at the polymer interface. This behavior also highlights the role of cyclic mechanical stress in promoting ionic redistribution and increasing interfacial polarity, potentially via network restructuring and re-alignment of polar groups. The data suggest a near-linear dependence of ϕₛ on ZDEC concentration under both stretching conditions, which can be rationalized by an increasing density of zinc-based charge carriers or polar residues [ 10 ]. The elevated potential after relaxation-stretch cycling implies enhanced surface activation and exposure of charged domains, possibly due to microstructural recovery mechanisms that allow deeper ionic migration during rest phases. These findings are consistent with prior reports on electromechanical coupling in elastomers and surface charge generation due to filler migration and surface reorganization [ 11 – 12 ]. Comparable trends have been reported in another thin film system containing zinc-based additives, where enhanced surface potentials were observed upon mechanical stretching [ 13 ]. This phenomenon is likely linked to the orientation and migration of Zn²⁺ complex ions toward the film–air interface during the deformation, as previously proposed by Guerra et al. [ 14 ]. Although latex films inherently exhibit a negative surface charge due to polar functional groups on the polymer chains, the introduction of ZDEC appears to reverse this behavior. The periodic stretching process may facilitate the mobilization of Zn²⁺ ions, leading to localized charge accumulation and increased surface potential, as also noted by Ren et al. [ 15 ] in their studies on electrostatic charge redistribution. Moreover, it is theorized that mechanical stretching may expose or activate the inaccessible or dormant functional groups on the rubber surface [ 16 ]. This activation increases the density of electroactive sites, which can persist even after the material returns to its relaxed state, contributing to the sustained surface charge boost. This mechanism aligns with the findings reported by Posner et al. [ 17 ], who confirmed the formation of new surface-active sites upon mechanical activation within polymeric systems. This also highlights the synergistic effect of ZDEC loading and mechanical stretching in enhancing the electrostatic interactions critical to the sludge capture performance of the NRL films. Phase Angle Light Scattering (PALS) Analysis The phase angle light scattering (PALS) analysis was conducted to assess the efficiency of the surface-active NRL films in capturing sludge waste in seawater and river water, as determined by zeta potential ζ and dynamic light scattering (DLS) data. The zeta potential (ζ) profiles in Fig. 4 describe the surface charge behavior of dispersed sludge particles in seawater systems, pre- and post-capturing process using the surface-active NRL films at varying ZDEC concentrations. Prior to sludge capturing, the sludge colloids exhibit moderately negative ζ values (–11.3 to − 7.2 mV), characteristic of stable suspensions due to electrostatic repulsion between particles, primarily arising from ionized carboxylic and sulfonic groups on organic and clay-like components of the sludge [ 18 – 19 ]. After the capturing process, a stable shift toward less negative zeta potentials is observed across all ZDEC concentrations, reaching up to − 5.4 mV at 10 phr. This shift indicates a net reduction in free anionic species in the bulk water, as sludge particles are adsorbed onto positively charged regions of the NRL surface, effectively removing them from the colloidal system [ 20 ]. The increase in sludge removal efficiency at 10 phr ZDEC is attributed to the accumulation of Zn²⁺ ions, which migrate to the NRL film-water interface during film formation and create electropositive surface domains. These domains enhance electrostatic attraction with negatively charged sludge, promoting efficient adsorption, in agreement with Langmuir monolayer adsorption theory [ 21 ]. However, at 15 phr ZDEC, a slight reduction in performance is observed, possibly due to ionic oversaturation or limited binding site accessibility. This behavior is more consistent with the Freundlich isotherm, which accounts for adsorption heterogeneity and energy distribution on the NRL surface [ 22 ]. The nonlinear behavior across different ZDEC dosages suggests that surface charge modification plays a critical role in controlling sludge–NRL surface interactions in complex saline matrices. In contrast to the seawater system, the zeta potential behavior of river water in Fig. 5 demonstrates a more distinct electrokinetic response to the surface-active NRL film across ZDEC concentrations. Pre-sludge capture values are consistently negative, indicative of moderately stable colloidal suspensions in low-ionic-strength freshwater. These values reflect the natural surface charge of organic sludge particulates and humic substances commonly present in river systems [ 23 ]. After sludge capturing process, ζ-potential value increases substantially, particularly at 10 phr ZDEC, where it rises to approximately − 7.0 mV. This substantial shift toward less negative ζ values indicates effective sludge adsorption onto the electropositive regions of the modified rubber surface, reducing the anionic load within the water system [ 24 ]. The results suggest that the electrostatic interaction between the sludge particles and the rubber surface is more favorable in river water than in seawater, due to lower ionic competition and reduced double-layer compression [ 25 ]. The increase in sludge uptake at intermediate ZDEC concentrations further supports monolayer adsorption behavior described by the Langmuir isotherm, wherein a limited number of homogenous binding sites drive rapid adsorption until surface saturation [ 21 ]. At 15 phr ZDEC, a slight decline in performance and ζ shift is observed, which may be attributed to the onset of electrostatic repulsion due to excessive surface cationic density or competitive adsorption effects. This behavior is agreeable with Freundlich-type multilayer adsorption where surface heterogeneity becomes significant [ 26 ]. As reported in Fig. 6 , the DLS data reveals the average hydrodynamic diameter R H of sludge particles in seawater, where a consistent trend is observed with electrokinetic behavior, further validating the adsorption interactions at the NRL interface as depicted in Fig. 6 . Prior to sludge capturing, the sludge exhibited relatively larger particle sizes (> 1100 nm) across all systems, indicative of weakly coagulated aggregates stabilized by electrostatic repulsion in the high-ionic-strength marine environment [ 27 ]. Following contact with the surface-active rubber films, a marked reduction in R H values was observed, especially at 10 phr of ZDEC loading, where the percentage removal was maximum at about 15%. This size reduction suggests successful adsorption and partial depletion of larger aggregates, likely due to surface charge neutralization and bridging interactions with the cationic domains on the ZDEC-containing NRL [ 28 ]. The particle size behavior is consistent with zeta potential trends, where an increase in surface charge (less negative ζ) corresponds to improved sludge removal and disaggregation. The adsorption follows a monolayer-type mechanism at optimal ZDEC concentrations, supported by the Langmuir adsorption model, where the reduction in average hydrodynamic radius reflects saturation of active binding sites on the film [ 29 ]. However, at 15 phr ZDEC, the R H slightly increases again, likely due to restabilization of small aggregates or secondary floc formation triggered by excess charge agent content, well described by multilayer adsorption under the Freundlich isotherm model [ 30 ]. As shown in Fig. 7 , In the river water system, the hydrodynamic diameter R H of sludge particles exhibits a trend consistent with the zeta potential behavior, affirming the interplay between electrostatic destabilization and sludge capture by the ZDEC-modified-NRL films. Initially, at low ZDEC loading, sludge particles display large sizes, reflecting the weak electrostatic repulsion and partial aggregation in the low-ionic-strength freshwater matrix [ 31 ]. Upon increasing the ZDEC content to 10 phr, a significant reduction in particle size is observed (sub 1000 nm), corresponding with increased surface potential and enhanced adsorption capacity (Q > 18 g/L) as depicted in Fig. 8 . This result indicates efficient charge-mediated capture and flattening of aggregates on the active film surface [ 27 ]. The sharp decrease in D H after sludge interaction correlates with the shift of zeta potential toward less negative values, particularly at intermediate ZDEC levels, suggesting enhanced electrostatic attraction between the negatively charged sludge and the positively charged rubber surface. This size reduction is well-justified with monolayer-type adsorption predicted by the Langmuir isotherm, as sludge particles form compact interfacial layers, minimizing the effective hydrodynamic radius [ 32 ]. Surface Adsorption Capacity The adsorption capacity (Q) of the sludge onto the NRL film surface exhibited distinctly different trends in river water and seawater systems, revealing the influence of ionic composition and surface interactions. In the river water system, Q values increased linearly with ZDEC concentration, reaching a maximum of ~ 18 g/L at 10 phr ZDEC, indicating a strong correlation between surface activity and sludge adsorption. This linearity suggests a Langmuir-like adsorption behavior, where surface-active sites on the NRL films become increasingly available and effective with higher charging agent loading, facilitating monolayer sludge capture under relatively low ionic strength conditions [ 32 – 33 ]. Conversely, in the seawater system, the Q data were erratic, with a Q value of ~ 6 g/L for NRL with minimum ZDEC loading, which unexpectedly decreased to ~ 4.5 g/L after 5 phr of ZDEC addition, then increased and finally having the lowest value at highest ZDEC concentration. This non-monotonic Q trend suggests that the high ionic strength and abundant multivalent ions in seawater, such as Mg²⁺, Ca²⁺, and Na⁺, induce charge screening effects that suppress the electrostatic attraction between the negatively charged sludge and the positively charged NRL surface [ 34 ]. The observed behavior aligns with Freundlich-type adsorption, where heterogeneity of the surface or interactions with background electrolytes leads to variable multilayer or cooperative binding dynamics [ 35 ]. Effect of PACl coagulant on the sludge capturing efficiency In this section, the surface-active NRL film containing 10 phr ZDEC was selected to conduct the sludge capturing efficiency in the presence of 3 wt.% polyaluminum chloride (PACl) coagulant. This specific formulation was chosen based on its previously demonstrated optimal performance, exhibiting the highest sludge removal efficiency and surface adsorption capacity in both river and seawater systems, as illustrated in Figs. 6 – 8 . The pronounced performance at this concentration is attributed to a favorable balance between positive surface charge induction and interfacial availability of active adsorption sites, which collectively enhance electrostatic attraction and binding affinity toward negatively charged sludge particles. The integration of PACl was intended to further investigate synergistic effects between chemical coagulation and surface-mediated physical adsorption under environmentally relevant conditions. FTIR analysis Figure 9 presents the FTIR spectra of both pristine and sludge-contaminated surface-active NRL films. In the uncontaminated sample, a broad absorption band around 3000–3500 cm⁻¹ is attributed to O–H stretching vibrations, commonly associated with residual methanol in latex films [ 36 ]. After the capturing process, the contaminated films show N–H stretching vibrations, indicative of amines, proteins, and peptides commonly present in organic sludge. Notably, the distinct peak at 3389 cm⁻¹, clearly observed in the pristine film, disappears after the sludge capturing process. This change may be attributed to overlapping vibrational modes and destructive spectral interference, as similarly reported by Saini et al. [ 37 ]. for complex bio-organic interactions in the same region. Additionally, three prominent peaks observed between 2500–3000 cm⁻¹ across all samples correspond to C–H stretching vibrations arising from both the latex matrix and organic components of the sludge [ 38 – 40 ]. The presence of peaks in the 1600–1700 cm⁻¹ range in all spectra can be linked to C = C stretching vibrations and aromatic ring structures, providing strong evidence of sludge-derived compounds adsorbed onto the film surface. Furthermore, the medium-intensity peaks between 1500–1600 cm⁻¹ support the presence of sludge residues, reinforcing the effectiveness of the ZDEC-NRL films in capturing and retaining sludge materials via surface adsorption. Zeta Potential analysis The zeta potential analysis was performed before and after sludge capturing using the NRL film in both river water and seawater, with and without PACl coagulant. High absolute zeta potential values, whether positive or negative, indicate greater interparticle stability, whereas lower values suggest a tendency for particles to aggregate or flocculate. The data in Fig. 10 show all samples bearing negative zeta potential values, indicating negatively charged surfaces of sludge due to their inherent surface properties. Wastewater sludge is well known to possess a net negative surface charge, primarily due to the ionization of carboxyl, hydroxyl, and phosphate functional groups present on microbial cell walls and extracellular polymeric substances (EPS) [ 41 – 42 ]. This is evidenced by the negative zeta potential values observed in all samples during the pre-sludge capturing stage, ranging from approximately − 9.5 to − 5.0 mV. However, after the sludge capturing process, the zeta potential of the residual water shifted significantly toward less negative values or the point of zero charge (pzc). This phenomenon is not due to charge neutralization of sludge particles in-situ, but rather, a result of the physical removal of negatively charged sludge particles from suspension, facilitated by the positively charged surface of the NRL films. The positive surface charge of the NRL films arises largely from the presence of excess ZDEC. These zinc-based compounds can ionize under aqueous conditions, releasing Zn²⁺ ions that contribute to the cationic character of the rubber surface. This enables strong electrostatic attraction between the NRL films and the negatively charged sludge flocs, promoting their removal from water. The effectiveness of the capturing mechanism is supported by the observed zeta potential trend, where post-sludge capturing samples show consistently less negative ζ-values, indicating a reduction in suspended anionic species. Additionally, samples treated with polyaluminum chloride (PACl) prior to the capturing process exhibited an even greater shift in zeta potential, suggesting a synergistic effect where PACl reduced electrostatic repulsion through charge screening and aggregation, thereby enhancing the ability of the positively charged NRL films to capture the destabilized sludge particles. These findings align with previous studies demonstrating that the incorporation of metal oxides or cationic agents onto polymeric surfaces enhances their affinity for anionic pollutants via electrostatic interactions [ 43 – 46 ]. This removal mechanism is further corroborated by hydrodynamic diameter R H data in Fig. 11 , which shows a consistent decrease in particle size after the capturing process. Prior to sludge removal, the average R H ranged from ~ 1250 nm to ~ 2600 nm, indicating the presence of large, stable sludge flocs in the untreated suspensions. After exposure to the surface-active NRL films, a significant reduction in R H was observed across all conditions, particularly in the presence of PACl, where the average R H decreased from ~ 2600 nm to below 800 nm. This size reduction indicates that the larger flocs had been efficiently captured or destabilized and removed from the aqueous phase, leaving behind smaller residual particles or soluble species. The combined zeta potential and hydrodynamic size results highlight a two-stages mechanism: first, electrostatic attraction between the NRL’s positively charged surface and the anionic sludge components, and second, enhanced floc destabilization in PACl-treated samples, which allowed more effective physical capture by the NRL. From the percentage sludge removal data, the calculated data also suggested that the PACl-laden river water system has the highest value, while for seawater system, the PACl coagulant has no significant effect. These findings emphasize the effectiveness of surface-active NRL films as a sludge-capturing agent and support its integration into low-cost sludge remediation systems. The adsorption capacity (Q) of sludge onto the surface-active NRL film varied significantly depending on water matrix and the presence of PACl, indicating differences in surface interaction mechanisms. In river water, Q increased significantly from 18.32 g/L without PACl to 52.26 g/L with PACl, suggesting that PACl enhanced floc formation and neutralized the negative surface charges of sludge particles, thereby promoting their adhesion to the positively charged NRL surface. This behavior aligns with the Langmuir isotherm model, which assumes monolayer adsorption on a homogenous surface with finite binding sites, as the dramatic increase in Q implies enhanced surface saturation facilitated by PACl [ 47 ]. Conversely, in seawater, the Q value slightly decreased from 11.32 g/L without PACl to 10.16 g/L with PACl. This may be attributed to high ionic strength and salinity, which compress the electric double layer and reduce electrostatic interactions, limiting PACl’s effectiveness in improving sludge-NRL interactions [ 48 ]. The relatively lower and less responsive Q values in seawater better reflect the Freundlich isotherm, which accounts for multilayer adsorption on heterogeneous surfaces, as adsorption may be governed more by weak van der Waals forces or steric effects than by charge-based interactions. These findings highlight the dependency of water chemistry and coagulant behavior in modifying sludge surface properties and adsorption affinity, where Langmuir-type adsorption dominates under enhanced charge neutralization, and Freundlich-type behavior prevails under ionic suppression or surface heterogeneity. UV-Vis analysis Figure 12 presents the UV–Vis absorbance spectra of water samples pre- and post-sludge capture process, providing insight into the effectiveness of the NRL film in river water (12a) and seawater (12b) matrices. The absorbance measurements, taken in the 200–400 nm range, serve as a proxy for water turbidity and the presence of suspended particulates and dissolved organic matter, both of which impede UV light transmission through the aqueous medium. In untreated samples, both river and seawater exhibited high absorbance values, indicating substantial turbidity. This turbidity arises from a concentrated colloidal and organic constituents that strongly scatter and absorb UV light, thereby limiting optical penetration [ 49 ]. Such elevated absorbance is characteristic of waters with significant levels of anthropogenic or natural contamination, and is frequently used as an indirect metric of suspended solids concentration and water quality [ 50 ]. Following the capturing process using the NRL films, a notable reduction in absorbance was observed, particularly in river water samples, signifying effective sludge capture and removal of optically active particulates. Comparatively, the absorbance of seawater samples remained higher even after treatment, which may be attributed to the inherently greater ionic strength, salinity, and baseline turbidity of marine environments [ 51 ]. Seawater contains not only organic particulates but also inorganic salts and fine mineral debris, which may reduce the efficiency of electrostatic and adhesion-based particle removal mechanisms. The main difference in treatment efficacy between river and seawater could be linked to the electrostatic double layer behavior at the film–water interface. The high ionic content of seawater likely compresses the Debye length, thereby weakening the electrostatic interactions between the positively charged NRL film surface and negatively charged sludge particulates. In contrast, river water having lower ionic strength enabling more effective long-range electrostatic attractions, thereby enhancing sludge capture efficiency [ 52 – 53 ]. Atomic Absorption Spectroscopy (AAS) AAS measurements revealed that the zinc (Zn) concentrations in treated river and seawater samples were 3.00 mg/L and 4.15 mg/L, respectively. Notably, these values were higher than those recorded in their respective untreated water systems, indicating the leaching of zinc species during the sludge capturing process using the NRL films. This observation suggests that the NRL films, particularly those with high ZDEC may serve as a secondary source of zinc ions in the aqueous phase upon exposure to aquatic environments. The elevated Zn content is likely due to the partial leaching of Zn²⁺ ions from the film surface into the surrounding water, a behavior previously observed in elastomeric systems containing zinc-based vulcanization accelerators and activators [ 54 – 55 ]. ZDEC, being a coordination compound of zinc, can dissociate or degrade under aqueous conditions, especially in the presence of ions and dissolved organics that facilitate complexation or chelation. Furthermore, mechanical stretching and environmental exposure may disrupt the zinc coordination environment, enhancing Zn²⁺ mobility from the film surface into the bulk solution [ 56 ]. The more pronounced zinc release in seawater compared to river water could be attributed to seawater's higher ionic strength, pH buffering capacity, and complexing agents such as chloride ions and organic matter, all of which can enhance the solubilization or displacement of weakly bound Zn²⁺ species from the rubber matrix [ 57 – 58 ]. Although the released Zn concentrations remain below the acute toxicity thresholds for short-term exposure (commonly reported at 5–10 mg/L for aquatic organisms), the elevated Zn concentrations detected post-treatment indicate that while the NRL films are effective in sludge capture, additional post-treatment steps are necessary to reduce residual zinc levels and ensure compliance with environmental water quality standards. Conclusion This study demonstrated the effectiveness of zinc diethyldithiocarbamate (ZDEC)-modified natural rubber latex (NRL) films as surface-active adsorbents for capturing sludge particles from both river and seawater systems. The introduction of ZDEC significantly enhanced the positive surface charge of the NRL films, facilitating electrostatic interactions with negatively charged colloidal sludge, as confirmed by zeta potential, FTIR, and hydrodynamic size analyses. The adsorption capacity (Q) exhibited a linear increase with ZDEC concentration in river water, aligning with Langmuir adsorption behavior, while seawater data showed erratic trends due to high ionic strength and charge screening, suggesting a more complex, Freundlich-like mechanism. The addition of PACl coagulant further improved adsorption efficiency in freshwater but had limited effects in saline conditions. Overall, the combination of DLS, surface potential measurements, and FTIR spectroscopy provided a comprehensive understanding of sludge-film interactions. These findings highlight the potential of modified NRL films as a sustainable, scalable, and low-cost solution for advanced water purification applications, especially in low-ionic strength environments. Declarations Acknowledgement Authors would like to thank the Universiti Sains Malaysia (USM) for funding this work through UNIVERSITI SAINS MALAYSIA APEX ERA RESEARCH GRANT (1001.PBAHAN.881008). The authors declared that they have no conflict of interest. Funding: Universiti Sains Malaysia (USM)- APEX ERA RESEARCH GRANT (1001.PBAHAN.881008) Ethical approval: N/A Informed consent: N/A Author Contributions: Nur Ayunni Ahmad Shahrul Amin and Neetha Nai Sem : Experimental and Data Analysis Azura A. Rashid: Natural rubber analysis and properties Ahmad Shaiful Abdul Razak: Coagulant production and analysis Mohamad Danial Shafiq: Overall project analysis Data Availability Statement: The authors confirmed that the data required for this work are available within this article. Conflict of Interest: The authors declare no conflict of interests. Clinical Trial Number in the manuscript: N/A References Verma, A., & Arora, S. (2024). Enhancement in antimicrobial efficacy and biodegradation of natural rubber latex through graphene oxide/nickel oxide nanoparticles. International Journal of Biological Macromolecules , 265, 131046. https://doi.org/10.1016/j.ijbiomac.2024.131046 Malik, N. N. 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Marine Pollution Bulletin , 60(2), 159–171. Additional Declarations No competing interests reported. Supplementary Files GraphicalAbstract2.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 16 Sep, 2025 Reviews received at journal 14 Sep, 2025 Reviewers agreed at journal 06 Sep, 2025 Reviewers invited by journal 06 Sep, 2025 Editor assigned by journal 21 Aug, 2025 Submission checks completed at journal 21 Aug, 2025 First submitted to journal 13 Aug, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-7361887","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":514774296,"identity":"1e2234b3-e92b-4fda-b1f9-f1ebf98ecb18","order_by":0,"name":"Nur Ayunni Ahmad Shahrul Amin","email":"","orcid":"","institution":"Universiti Sains Malaysia","correspondingAuthor":false,"prefix":"","firstName":"Nur","middleName":"Ayunni Ahmad Shahrul","lastName":"Amin","suffix":""},{"id":514774297,"identity":"a86fc5a5-040a-46d7-9db2-7f39c90cc02d","order_by":1,"name":"Neettha Nai Sem","email":"","orcid":"","institution":"Universiti Sains Malaysia","correspondingAuthor":false,"prefix":"","firstName":"Neettha","middleName":"Nai","lastName":"Sem","suffix":""},{"id":514774298,"identity":"fc21b0ff-1546-4991-8ddd-bd5540cda52f","order_by":2,"name":"Azura A. 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9","display":"","copyAsset":false,"role":"figure","size":107409,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra for the raw sludge samples, pure NRL films and captured sludge samples on NRL films in both seawater and river water, without and with PACl coagulant\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7361887/v1/2a80eb595b6283d1cad1d812.png"},{"id":91338005,"identity":"57110cb4-8e08-4547-9e32-2c71dbae1071","added_by":"auto","created_at":"2025-09-15 12:24:31","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":50724,"visible":true,"origin":"","legend":"\u003cp\u003eZeta potential values of the seawater and river water samples pre- and post-sludge capturing using surface-active NRL films without and with PACl coagulant\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7361887/v1/12eacda0c60db75c3bbc4f67.png"},{"id":91337101,"identity":"5cf06690-06c6-417e-9677-a6ec8dbbedcc","added_by":"auto","created_at":"2025-09-15 12:16:31","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":87757,"visible":true,"origin":"","legend":"\u003cp\u003eAverage hydrodynamic size R\u003csub\u003eH\u003c/sub\u003e of sludge particles in river water, pre- and post-capturing process and its calculated removal percentage, without and with PACl coagulant\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-7361887/v1/f450c1389d4b2c3b635596ff.png"},{"id":91335915,"identity":"6027321d-d879-4d76-884c-b1a582e6a97c","added_by":"auto","created_at":"2025-09-15 12:00:31","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":73955,"visible":true,"origin":"","legend":"\u003cp\u003eUV-Vis absorbance of the (a) river water sample and (b) seawater sample, pre- and post-sludge capturing, without and with PACl coagulant\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-7361887/v1/472a9bd3f55922a17534f97e.png"},{"id":91338185,"identity":"43d733ae-9f8c-4fcc-9826-ed79bf43f66d","added_by":"auto","created_at":"2025-09-15 12:32:32","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1428876,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7361887/v1/86b3e858-6f36-427b-ae59-3450b3b02001.pdf"},{"id":91335910,"identity":"fab61d43-7c8a-47c7-85db-1a88838828ff","added_by":"auto","created_at":"2025-09-15 12:00:30","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":253519,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract2.docx","url":"https://assets-eu.researchsquare.com/files/rs-7361887/v1/d05524ec7b68f43ad589b766.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Efficiency of surface-active natural rubber latex (NRL) films in capturing sludge wastes in seawater and river water","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe increasing volume of sludge waste, a byproduct of wastewater treatment, is a pressing environmental concern. Sludge waste, a major by-product of biological wastewater treatment in sewage treatment plants (STPs), represents a largely untapped resource with significant potential to support the circular economy. With global production surpassing 300\u0026nbsp;million tons per year, effective management of sludge waste has become increasingly critical, especially as China accounts for nearly 20% of the total output. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. This rise is predominantly owing to two key factors: population growth in urban areas and advancements in wastewater treatment technologies. As highlighted by Malik et al. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], Malaysia faces a growing challenge in managing sludge waste, with production reaching three million metric tons annually. These figures are projected to rise significantly, with estimates suggesting a potential increase to seven million metric tons by 2020. Improper sludge management poses severe environmental threats, including the contamination of surface and groundwater resources, land degradation, and the introduction of pollutants into the food chain [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn this context, the present study investigates a novel rubber-based film system as an alternative to conventional membrane technologies for sludge particle removal in wastewater streams. Unlike porous membranes, which rely on size exclusion and pressure-driven separation, the rubber films developed herein are nonporous and exploit surface interaction phenomena, predominantly electrostatic attraction and adhesive forces, to sequester sludge particles from aqueous media. Traditional membrane filtration, though capable of capturing particles in the micrometer to sub-micrometer range, suffers from inherent limitations such as high operational energy demands, membrane fouling, and the necessity for frequent chemical cleaning and replacement [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe rubber films in this study are produced using natural rubber latex (NRL), with surface charge modification achieved via incorporation of zinc diethyldithiocarbamate (ZDEC), which functions dually as a vulcanization activator and a surface functionalizing agent. Periodic mechanical stretching was employed to induce anisotropic surface structuring, further enhancing the electropositive surface character of the films. This positive charge facilitates selective adsorption of anionic sludge colloids and flocs through Coulombic attraction and interfacial adhesion mechanisms. The objective of this work is to fabricate and optimize surface-engineered NRL films for effective sludge entrapment in wastewater treatment processes. Additionally, the role of commercial coagulant agents commonly employed in industrial effluents is examined in terms of their influence on sludge particle agglomeration dynamics and subsequent capture efficiency. The performance of the rubber films is further correlated with varying ZDEC concentrations to elucidate the electrostatic binding efficacy with water quality post-capturing.\u003c/p\u003e\u003cp\u003eThrough targeted surface modification and systematic material-properties optimization, this study provides a fundamental basis for developing cost-effective, energy-efficient, and scalable sludge capture technologies using rubber-based materials. The findings contribute to advancing alternative materials in environmental remediation, particularly in sludge management within the wastewater treatment sector.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eMaterials\u003c/h2\u003e\u003cp\u003eThe sludge waste used in this study was acquired from Indah Water Konsortium (IWK). River water was collected from Sungai Kerian, Perak, while seawater was obtained from Ban Pecah Beach, Perak. The natural rubber latex (NRL) of 60% total solid content (TSC) was sourced from Zarm Scientific and Supplies (Malaysia) Sdn. Bhd. All other chemicals used in the latex compounding were procured from Merck before being made into a dispersion and solution form, including 50.9% zinc oxide (ZnO), 57.5% zinc diethyldithiocarbamate (ZDEC), 56.0% tetramethyl thiuram disulfide (TMTD), 56.5% sulfur and 10% potassium hydroxide (KOH). The coagulant, polyaluminum chloride (PACl) was produced by ACME Chemical Sdn. Bhd.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eMethod\u003c/h3\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003eNatural Rubber Latex (NRL) Quality test\u003c/h2\u003e\u003cp\u003eThe TSC of each compounding ingredient and latex was measured. The empty petri dish was weighed first, then, approximately 1 mg of each ingredient for compounding, and the latex was weighed separately and added to the dish. The latex was gently swirled to ensure it coated the bottom of the dish. Next, 20 cm\u003csup\u003e3\u003c/sup\u003e of distilled water was added, and the mixture swirled thoroughly. The uncovered petri dish was then placed in an oven set to 105\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u0026deg;C. The sample was heated for 2 hours or until it lost its whitish appearance. After cooling to room temperature in a desiccator, the dish was weighed again. The sample was then reheated in the oven for 15 minutes, cooled in a desiccator, and weighed a final time. Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) was used to calculate the TSC, where \u003cem\u003em₁\u003c/em\u003e represents the mass of the initial sample and \u003cem\u003em₀\u003c/em\u003e represents the mass of the dried sample [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:TSC=\\:\\frac{{m}_{1}}{{m}_{0}}\\times\\:100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eNatural Rubber Latex (NRL) film preparation\u003c/h3\u003e\n\u003cp\u003eThe NRL film was produced using the formulation in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The compounding process was initiated with NRL being stirred in an overhead mixer at 200 rpm Next, the remaining ingredients were added in a specified order: KOH, ZnO ZDEC, TMTD and sulphur. The mixture was continuously stirred at a high speed of 250 rpm for an additional 30 minutes. To eliminate any trapped air bubbles, it was then degassed in a vacuum desiccator at 25\u0026deg;C for 15 minutes. After degassing, the latex was casted onto a glass plate using a casting knife, which was set with a 0.5 mm gap in front of a moving beam to ensure uniform film thickness. The cast film was then dried in an oven at 100\u0026deg;C for 2 hours, followed by cooling at room temperature for 15 minutes.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eNRL film compounding formulation\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eIngredients\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFormulation 1 (phr)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eFormulation 2 (phr)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eFormulation 3 (phr)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFormulation 4 (phr)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e60% NR Latex\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e10% KOH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e50.9% ZnO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e57.5% ZDEC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e10.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e15.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e56.0% TMTD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e56.5% Sulphur\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e105.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e110.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e115.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e120.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\n\u003ch3\u003eNRL film properties\u003c/h3\u003e\n\u003cp\u003eA multimeter was used to measure the electrical potential that developed on the surface of the NRL film. The film underwent a repeated cycle of stretching and relaxing, with each phase lasting 8 hours and 12 hours of relaxing. This cycle continued for a total of 40 hours. The stretching was performed to 30% of the initial film length. During this experiment, the NRL film was connected to the multimeter according to the specific setup shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003ePerformance of NRL Film in sludge removal process\u003c/h2\u003e\u003cp\u003eThe performance evaluation of the NRL films for sludge waste capture was conducted using a controlled laboratory-scale water treatment setup as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. NRL films with dimensions of 20 cm x 20 cm were arranged in a relaxed (unstressed) state within a static tank system containing a homogenous sludge suspension. The adsorption experiment was carried out over a 12-hour period to allow for sufficient interaction between the film surface and suspended sludge particulates. Pre and post-treatment analyses were performed to evaluate the sludge removal efficiency through UV\u0026ndash;Vis spectrophotometry (PerkinElmer Lambda 35) using a wavelength range of 200\u0026ndash;400 nm, zeta potential, and dynamic light scattering (DLS) measurements using the Malvern Zetasizer Nano ZS at an operating voltage of 40V. The percentage of sludge removal was calculated using the DLS data. To investigate the physicochemical changes at the film\u0026ndash;sludge interface, scanning electron microscopy (SEM) imaging was performed using a ZEISS SUPRA 35 VP instrument. SEM imaging enabled high-resolution observation of the film surface morphology before and after the sludge capture process, revealing particulate adherence and topographical alterations. Fourier Transform Infrared (FTIR) spectroscopy was employed using a PerkinElmer instrument to identify the functional groups present in bare natural rubber latex (NRL) films, raw sludge, and sludge-contaminated film samples. Spectral data were recorded over the range of 550\u0026ndash;4000 cm⁻\u0026sup1; and interpreted through comparison with reference spectra to assign characteristic vibrational bands corresponding to functional group identities. The concentration of zinc (Zn) ions in the water samples pre- and post-treatment was quantified using Atomic Absorption Spectroscopy (AAS), employing a PerkinElmer Model 3300 instrument. Water samples were appropriately filtered and acidified with nitric acid to stabilize metal ions prior to analysis. Calibration was performed using standard Zn solutions, and absorbance was measured at the characteristic wavelength of 213.9 nm to determine Zn concentrations, enabling evaluation of any potential leaching from the NRL films or changes due to sludge interaction.\u003c/p\u003e\u003cp\u003eThe adsorption capacity Q of the sludge on the NRL films was calculated using Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. The data were analyzed based on the hydrodynamic size value pre- and post-sludge capturing from the DLS and the total suspended solid (TSS) measurements. The initial TSS concentration (TSS₀) was determined by filtering a 20 mL of the sludge in seawater and riverwater separately, through a pre-weighed 0.5 \u0026micro;m filter. Then the retained solids were dried at 105\u0026deg;C, and the mass difference per volume filtered was calculated [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:Q=\\:\\frac{{(C}_{O\\:}-\\:{C}_{e})}{A}\\times\\:V$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003eSurface Potential of NRL films\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, at 0 phr ZDEC, unstretched films exhibit negligible surface potential, indicating the absence of mobile ionic species or polar moieties at the interface. Upon 8-hour of uniaxial stretching, the surface potential increases with ZDEC content, reaching\u0026thinsp;~\u0026thinsp;65 mV at 15 phr. This enhancement is attributed to the migration and surface enrichment of ionic zinc species, facilitated by deformation-induced chain orientation and free volume generation [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. More strikingly, samples subjected to an 8-hour stretch followed by a 12-hour relaxation and a subsequent 8-hour stretch exhibit significantly higher ϕₛ values, exceeding 95 mV at 15 phr ZDEC. This increase in surface charge correlates with the ionic and chemical coordination of ZDEC, a multifunctional additive that serves both as a vulcanization accelerator and a source of zinc ions. This finding emphasizes the critical role of ZDEC in modulating the surface electrostatic properties of the NRL film. The positive surface potential is attributed to the incorporation and redistribution of Zn\u0026sup2;⁺ ions within the rubber matrix, particularly under the influence of periodic stretching that caused surface micromechanical deformation. ZDEC facilitates crosslink formation through the creation of zinc-sulfur complexes, while simultaneously influencing the ionization at the polymer interface.\u003c/p\u003e\u003cp\u003eThis behavior also highlights the role of cyclic mechanical stress in promoting ionic redistribution and increasing interfacial polarity, potentially via network restructuring and re-alignment of polar groups. The data suggest a near-linear dependence of ϕₛ on ZDEC concentration under both stretching conditions, which can be rationalized by an increasing density of zinc-based charge carriers or polar residues [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The elevated potential after relaxation-stretch cycling implies enhanced surface activation and exposure of charged domains, possibly due to microstructural recovery mechanisms that allow deeper ionic migration during rest phases. These findings are consistent with prior reports on electromechanical coupling in elastomers and surface charge generation due to filler migration and surface reorganization [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eComparable trends have been reported in another thin film system containing zinc-based additives, where enhanced surface potentials were observed upon mechanical stretching [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. This phenomenon is likely linked to the orientation and migration of Zn\u0026sup2;⁺ complex ions toward the film\u0026ndash;air interface during the deformation, as previously proposed by Guerra et al. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Although latex films inherently exhibit a negative surface charge due to polar functional groups on the polymer chains, the introduction of ZDEC appears to reverse this behavior. The periodic stretching process may facilitate the mobilization of Zn\u0026sup2;⁺ ions, leading to localized charge accumulation and increased surface potential, as also noted by Ren et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] in their studies on electrostatic charge redistribution.\u003c/p\u003e\u003cp\u003eMoreover, it is theorized that mechanical stretching may expose or activate the inaccessible or dormant functional groups on the rubber surface [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. This activation increases the density of electroactive sites, which can persist even after the material returns to its relaxed state, contributing to the sustained surface charge boost. This mechanism aligns with the findings reported by Posner et al. [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], who confirmed the formation of new surface-active sites upon mechanical activation within polymeric systems. This also highlights the synergistic effect of ZDEC loading and mechanical stretching in enhancing the electrostatic interactions critical to the sludge capture performance of the NRL films.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003ePhase Angle Light Scattering (PALS) Analysis\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe phase angle light scattering (PALS) analysis was conducted to assess the efficiency of the surface-active NRL films in capturing sludge waste in seawater and river water, as determined by zeta potential ζ and dynamic light scattering (DLS) data. The zeta potential (ζ) profiles in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e describe the surface charge behavior of dispersed sludge particles in seawater systems, pre- and post-capturing process using the surface-active NRL films at varying ZDEC concentrations. Prior to sludge capturing, the sludge colloids exhibit moderately negative ζ values (\u0026ndash;11.3 to \u0026minus;\u0026thinsp;7.2 mV), characteristic of stable suspensions due to electrostatic repulsion between particles, primarily arising from ionized carboxylic and sulfonic groups on organic and clay-like components of the sludge [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. After the capturing process, a stable shift toward less negative zeta potentials is observed across all ZDEC concentrations, reaching up to \u0026minus;\u0026thinsp;5.4 mV at 10 phr. This shift indicates a net reduction in free anionic species in the bulk water, as sludge particles are adsorbed onto positively charged regions of the NRL surface, effectively removing them from the colloidal system [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe increase in sludge removal efficiency at 10 phr ZDEC is attributed to the accumulation of Zn\u0026sup2;⁺ ions, which migrate to the NRL film-water interface during film formation and create electropositive surface domains. These domains enhance electrostatic attraction with negatively charged sludge, promoting efficient adsorption, in agreement with Langmuir monolayer adsorption theory [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. However, at 15 phr ZDEC, a slight reduction in performance is observed, possibly due to ionic oversaturation or limited binding site accessibility. This behavior is more consistent with the Freundlich isotherm, which accounts for adsorption heterogeneity and energy distribution on the NRL surface [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The nonlinear behavior across different ZDEC dosages suggests that surface charge modification plays a critical role in controlling sludge\u0026ndash;NRL surface interactions in complex saline matrices.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn contrast to the seawater system, the zeta potential behavior of river water in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e demonstrates a more distinct electrokinetic response to the surface-active NRL film across ZDEC concentrations. Pre-sludge capture values are consistently negative, indicative of moderately stable colloidal suspensions in low-ionic-strength freshwater. These values reflect the natural surface charge of organic sludge particulates and humic substances commonly present in river systems [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. After sludge capturing process, ζ-potential value increases substantially, particularly at 10 phr ZDEC, where it rises to approximately \u0026minus;\u0026thinsp;7.0 mV. This substantial shift toward less negative ζ values indicates effective sludge adsorption onto the electropositive regions of the modified rubber surface, reducing the anionic load within the water system [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe results suggest that the electrostatic interaction between the sludge particles and the rubber surface is more favorable in river water than in seawater, due to lower ionic competition and reduced double-layer compression [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The increase in sludge uptake at intermediate ZDEC concentrations further supports monolayer adsorption behavior described by the Langmuir isotherm, wherein a limited number of homogenous binding sites drive rapid adsorption until surface saturation [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. At 15 phr ZDEC, a slight decline in performance and ζ shift is observed, which may be attributed to the onset of electrostatic repulsion due to excessive surface cationic density or competitive adsorption effects. This behavior is agreeable with Freundlich-type multilayer adsorption where surface heterogeneity becomes significant [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs reported in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the DLS data reveals the average hydrodynamic diameter R\u003csub\u003eH\u003c/sub\u003e of sludge particles in seawater, where a consistent trend is observed with electrokinetic behavior, further validating the adsorption interactions at the NRL interface as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Prior to sludge capturing, the sludge exhibited relatively larger particle sizes (\u0026gt;\u0026thinsp;1100 nm) across all systems, indicative of weakly coagulated aggregates stabilized by electrostatic repulsion in the high-ionic-strength marine environment [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Following contact with the surface-active rubber films, a marked reduction in R\u003csub\u003eH\u003c/sub\u003e values was observed, especially at 10 phr of ZDEC loading, where the percentage removal was maximum at about 15%. This size reduction suggests successful adsorption and partial depletion of larger aggregates, likely due to surface charge neutralization and bridging interactions with the cationic domains on the ZDEC-containing NRL [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe particle size behavior is consistent with zeta potential trends, where an increase in surface charge (less negative ζ) corresponds to improved sludge removal and disaggregation. The adsorption follows a monolayer-type mechanism at optimal ZDEC concentrations, supported by the Langmuir adsorption model, where the reduction in average hydrodynamic radius reflects saturation of active binding sites on the film [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. However, at 15 phr ZDEC, the R\u003csub\u003eH\u003c/sub\u003e slightly increases again, likely due to restabilization of small aggregates or secondary floc formation triggered by excess charge agent content, well described by multilayer adsorption under the Freundlich isotherm model [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, In the river water system, the hydrodynamic diameter R\u003csub\u003eH\u003c/sub\u003e of sludge particles exhibits a trend consistent with the zeta potential behavior, affirming the interplay between electrostatic destabilization and sludge capture by the ZDEC-modified-NRL films. Initially, at low ZDEC loading, sludge particles display large sizes, reflecting the weak electrostatic repulsion and partial aggregation in the low-ionic-strength freshwater matrix [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Upon increasing the ZDEC content to 10 phr, a significant reduction in particle size is observed (sub 1000 nm), corresponding with increased surface potential and enhanced adsorption capacity (Q\u0026thinsp;\u0026gt;\u0026thinsp;18 g/L) as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. This result indicates efficient charge-mediated capture and flattening of aggregates on the active film surface [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe sharp decrease in D\u003csub\u003eH\u003c/sub\u003e after sludge interaction correlates with the shift of zeta potential toward less negative values, particularly at intermediate ZDEC levels, suggesting enhanced electrostatic attraction between the negatively charged sludge and the positively charged rubber surface. This size reduction is well-justified with monolayer-type adsorption predicted by the Langmuir isotherm, as sludge particles form compact interfacial layers, minimizing the effective hydrodynamic radius [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003eSurface Adsorption Capacity\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe adsorption capacity (Q) of the sludge onto the NRL film surface exhibited distinctly different trends in river water and seawater systems, revealing the influence of ionic composition and surface interactions. In the river water system, Q values increased linearly with ZDEC concentration, reaching a maximum of ~\u0026thinsp;18 g/L at 10 phr ZDEC, indicating a strong correlation between surface activity and sludge adsorption. This linearity suggests a Langmuir-like adsorption behavior, where surface-active sites on the NRL films become increasingly available and effective with higher charging agent loading, facilitating monolayer sludge capture under relatively low ionic strength conditions [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eConversely, in the seawater system, the Q data were erratic, with a Q value of ~\u0026thinsp;6 g/L for NRL with minimum ZDEC loading, which unexpectedly decreased to ~\u0026thinsp;4.5 g/L after 5 phr of ZDEC addition, then increased and finally having the lowest value at highest ZDEC concentration. This non-monotonic Q trend suggests that the high ionic strength and abundant multivalent ions in seawater, such as Mg\u0026sup2;⁺, Ca\u0026sup2;⁺, and Na⁺, induce charge screening effects that suppress the electrostatic attraction between the negatively charged sludge and the positively charged NRL surface [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The observed behavior aligns with Freundlich-type adsorption, where heterogeneity of the surface or interactions with background electrolytes leads to variable multilayer or cooperative binding dynamics [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003eEffect of PACl coagulant on the sludge capturing efficiency\u003c/h2\u003e\u003cp\u003eIn this section, the surface-active NRL film containing 10 phr ZDEC was selected to conduct the sludge capturing efficiency in the presence of 3 wt.% polyaluminum chloride (PACl) coagulant. This specific formulation was chosen based on its previously demonstrated optimal performance, exhibiting the highest sludge removal efficiency and surface adsorption capacity in both river and seawater systems, as illustrated in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. The pronounced performance at this concentration is attributed to a favorable balance between positive surface charge induction and interfacial availability of active adsorption sites, which collectively enhance electrostatic attraction and binding affinity toward negatively charged sludge particles. The integration of PACl was intended to further investigate synergistic effects between chemical coagulation and surface-mediated physical adsorption under environmentally relevant conditions.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003eFTIR analysis\u003c/h2\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e presents the FTIR spectra of both pristine and sludge-contaminated surface-active NRL films. In the uncontaminated sample, a broad absorption band around 3000\u0026ndash;3500 cm⁻\u0026sup1; is attributed to O\u0026ndash;H stretching vibrations, commonly associated with residual methanol in latex films [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. After the capturing process, the contaminated films show N\u0026ndash;H stretching vibrations, indicative of amines, proteins, and peptides commonly present in organic sludge. Notably, the distinct peak at 3389 cm⁻\u0026sup1;, clearly observed in the pristine film, disappears after the sludge capturing process. This change may be attributed to overlapping vibrational modes and destructive spectral interference, as similarly reported by Saini et al. [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. for complex bio-organic interactions in the same region. Additionally, three prominent peaks observed between 2500\u0026ndash;3000 cm⁻\u0026sup1; across all samples correspond to C\u0026ndash;H stretching vibrations arising from both the latex matrix and organic components of the sludge [\u003cspan additionalcitationids=\"CR39\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. The presence of peaks in the 1600\u0026ndash;1700 cm⁻\u0026sup1; range in all spectra can be linked to C\u0026thinsp;=\u0026thinsp;C stretching vibrations and aromatic ring structures, providing strong evidence of sludge-derived compounds adsorbed onto the film surface. Furthermore, the medium-intensity peaks between 1500\u0026ndash;1600 cm⁻\u0026sup1; support the presence of sludge residues, reinforcing the effectiveness of the ZDEC-NRL films in capturing and retaining sludge materials via surface adsorption.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003eZeta Potential analysis\u003c/h2\u003e\u003cp\u003eThe zeta potential analysis was performed before and after sludge capturing using the NRL film in both river water and seawater, with and without PACl coagulant. High absolute zeta potential values, whether positive or negative, indicate greater interparticle stability, whereas lower values suggest a tendency for particles to aggregate or flocculate. The data in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e show all samples bearing negative zeta potential values, indicating negatively charged surfaces of sludge due to their inherent surface properties. Wastewater sludge is well known to possess a net negative surface charge, primarily due to the ionization of carboxyl, hydroxyl, and phosphate functional groups present on microbial cell walls and extracellular polymeric substances (EPS) [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. This is evidenced by the negative zeta potential values observed in all samples during the pre-sludge capturing stage, ranging from approximately \u0026minus;\u0026thinsp;9.5 to \u0026minus;\u0026thinsp;5.0 mV. However, after the sludge capturing process, the zeta potential of the residual water shifted significantly toward less negative values or the point of zero charge (pzc). This phenomenon is not due to charge neutralization of sludge particles in-situ, but rather, a result of the physical removal of negatively charged sludge particles from suspension, facilitated by the positively charged surface of the NRL films. The positive surface charge of the NRL films arises largely from the presence of excess ZDEC. These zinc-based compounds can ionize under aqueous conditions, releasing Zn\u0026sup2;⁺ ions that contribute to the cationic character of the rubber surface. This enables strong electrostatic attraction between the NRL films and the negatively charged sludge flocs, promoting their removal from water.\u003c/p\u003e\u003cp\u003eThe effectiveness of the capturing mechanism is supported by the observed zeta potential trend, where post-sludge capturing samples show consistently less negative ζ-values, indicating a reduction in suspended anionic species. Additionally, samples treated with polyaluminum chloride (PACl) prior to the capturing process exhibited an even greater shift in zeta potential, suggesting a synergistic effect where PACl reduced electrostatic repulsion through charge screening and aggregation, thereby enhancing the ability of the positively charged NRL films to capture the destabilized sludge particles. These findings align with previous studies demonstrating that the incorporation of metal oxides or cationic agents onto polymeric surfaces enhances their affinity for anionic pollutants via electrostatic interactions [\u003cspan additionalcitationids=\"CR44 CR45\" citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThis removal mechanism is further corroborated by hydrodynamic diameter R\u003csub\u003eH\u003c/sub\u003e data in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e, which shows a consistent decrease in particle size after the capturing process. Prior to sludge removal, the average R\u003csub\u003eH\u003c/sub\u003e ranged from ~\u0026thinsp;1250 nm to ~\u0026thinsp;2600 nm, indicating the presence of large, stable sludge flocs in the untreated suspensions. After exposure to the surface-active NRL films, a significant reduction in R\u003csub\u003eH\u003c/sub\u003e was observed across all conditions, particularly in the presence of PACl, where the average R\u003csub\u003eH\u003c/sub\u003e decreased from ~\u0026thinsp;2600 nm to below 800 nm. This size reduction indicates that the larger flocs had been efficiently captured or destabilized and removed from the aqueous phase, leaving behind smaller residual particles or soluble species. The combined zeta potential and hydrodynamic size results highlight a two-stages mechanism: first, electrostatic attraction between the NRL\u0026rsquo;s positively charged surface and the anionic sludge components, and second, enhanced floc destabilization in PACl-treated samples, which allowed more effective physical capture by the NRL. From the percentage sludge removal data, the calculated data also suggested that the PACl-laden river water system has the highest value, while for seawater system, the PACl coagulant has no significant effect. These findings emphasize the effectiveness of surface-active NRL films as a sludge-capturing agent and support its integration into low-cost sludge remediation systems.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe adsorption capacity (Q) of sludge onto the surface-active NRL film varied significantly depending on water matrix and the presence of PACl, indicating differences in surface interaction mechanisms. In river water, Q increased significantly from 18.32 g/L without PACl to 52.26 g/L with PACl, suggesting that PACl enhanced floc formation and neutralized the negative surface charges of sludge particles, thereby promoting their adhesion to the positively charged NRL surface. This behavior aligns with the Langmuir isotherm model, which assumes monolayer adsorption on a homogenous surface with finite binding sites, as the dramatic increase in Q implies enhanced surface saturation facilitated by PACl [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. Conversely, in seawater, the Q value slightly decreased from 11.32 g/L without PACl to 10.16 g/L with PACl. This may be attributed to high ionic strength and salinity, which compress the electric double layer and reduce electrostatic interactions, limiting PACl\u0026rsquo;s effectiveness in improving sludge-NRL interactions [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. The relatively lower and less responsive Q values in seawater better reflect the Freundlich isotherm, which accounts for multilayer adsorption on heterogeneous surfaces, as adsorption may be governed more by weak van der Waals forces or steric effects than by charge-based interactions. These findings highlight the dependency of water chemistry and coagulant behavior in modifying sludge surface properties and adsorption affinity, where Langmuir-type adsorption dominates under enhanced charge neutralization, and Freundlich-type behavior prevails under ionic suppression or surface heterogeneity.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003eUV-Vis analysis\u003c/h2\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e presents the UV\u0026ndash;Vis absorbance spectra of water samples pre- and post-sludge capture process, providing insight into the effectiveness of the NRL film in river water (12a) and seawater (12b) matrices. The absorbance measurements, taken in the 200\u0026ndash;400 nm range, serve as a proxy for water turbidity and the presence of suspended particulates and dissolved organic matter, both of which impede UV light transmission through the aqueous medium. In untreated samples, both river and seawater exhibited high absorbance values, indicating substantial turbidity. This turbidity arises from a concentrated colloidal and organic constituents that strongly scatter and absorb UV light, thereby limiting optical penetration [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Such elevated absorbance is characteristic of waters with significant levels of anthropogenic or natural contamination, and is frequently used as an indirect metric of suspended solids concentration and water quality [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFollowing the capturing process using the NRL films, a notable reduction in absorbance was observed, particularly in river water samples, signifying effective sludge capture and removal of optically active particulates. Comparatively, the absorbance of seawater samples remained higher even after treatment, which may be attributed to the inherently greater ionic strength, salinity, and baseline turbidity of marine environments [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. Seawater contains not only organic particulates but also inorganic salts and fine mineral debris, which may reduce the efficiency of electrostatic and adhesion-based particle removal mechanisms. The main difference in treatment efficacy between river and seawater could be linked to the electrostatic double layer behavior at the film\u0026ndash;water interface. The high ionic content of seawater likely compresses the Debye length, thereby weakening the electrostatic interactions between the positively charged NRL film surface and negatively charged sludge particulates. In contrast, river water having lower ionic strength enabling more effective long-range electrostatic attractions, thereby enhancing sludge capture efficiency [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003eAtomic Absorption Spectroscopy (AAS)\u003c/h2\u003e\u003cp\u003eAAS measurements revealed that the zinc (Zn) concentrations in treated river and seawater samples were 3.00 mg/L and 4.15 mg/L, respectively. Notably, these values were higher than those recorded in their respective untreated water systems, indicating the leaching of zinc species during the sludge capturing process using the NRL films. This observation suggests that the NRL films, particularly those with high ZDEC may serve as a secondary source of zinc ions in the aqueous phase upon exposure to aquatic environments.\u003c/p\u003e\u003cp\u003eThe elevated Zn content is likely due to the partial leaching of Zn\u0026sup2;⁺ ions from the film surface into the surrounding water, a behavior previously observed in elastomeric systems containing zinc-based vulcanization accelerators and activators [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. ZDEC, being a coordination compound of zinc, can dissociate or degrade under aqueous conditions, especially in the presence of ions and dissolved organics that facilitate complexation or chelation. Furthermore, mechanical stretching and environmental exposure may disrupt the zinc coordination environment, enhancing Zn\u0026sup2;⁺ mobility from the film surface into the bulk solution [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe more pronounced zinc release in seawater compared to river water could be attributed to seawater's higher ionic strength, pH buffering capacity, and complexing agents such as chloride ions and organic matter, all of which can enhance the solubilization or displacement of weakly bound Zn\u0026sup2;⁺ species from the rubber matrix [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. Although the released Zn concentrations remain below the acute toxicity thresholds for short-term exposure (commonly reported at 5\u0026ndash;10 mg/L for aquatic organisms), the elevated Zn concentrations detected post-treatment indicate that while the NRL films are effective in sludge capture, additional post-treatment steps are necessary to reduce residual zinc levels and ensure compliance with environmental water quality standards.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study demonstrated the effectiveness of zinc diethyldithiocarbamate (ZDEC)-modified natural rubber latex (NRL) films as surface-active adsorbents for capturing sludge particles from both river and seawater systems. The introduction of ZDEC significantly enhanced the positive surface charge of the NRL films, facilitating electrostatic interactions with negatively charged colloidal sludge, as confirmed by zeta potential, FTIR, and hydrodynamic size analyses. The adsorption capacity (Q) exhibited a linear increase with ZDEC concentration in river water, aligning with Langmuir adsorption behavior, while seawater data showed erratic trends due to high ionic strength and charge screening, suggesting a more complex, Freundlich-like mechanism. The addition of PACl coagulant further improved adsorption efficiency in freshwater but had limited effects in saline conditions. Overall, the combination of DLS, surface potential measurements, and FTIR spectroscopy provided a comprehensive understanding of sludge-film interactions. These findings highlight the potential of modified NRL films as a sustainable, scalable, and low-cost solution for advanced water purification applications, especially in low-ionic strength environments.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAuthors would like to thank the Universiti Sains Malaysia (USM) for funding this work through UNIVERSITI SAINS MALAYSIA APEX ERA RESEARCH GRANT (1001.PBAHAN.881008).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe authors declared that they have no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u0026nbsp;\u003c/strong\u003eUniversiti Sains Malaysia (USM)- APEX ERA RESEARCH GRANT (1001.PBAHAN.881008)\u003cstrong\u003e\u003cbr\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval:\u0026nbsp;\u003c/strong\u003eN/A\u003cstrong\u003e\u003cbr\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInformed consent:\u0026nbsp;\u003c/strong\u003eN/A\u003cstrong\u003e\u003cbr\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u0026nbsp;\u003c/strong\u003eNur Ayunni Ahmad Shahrul Amin and Neetha Nai Sem\u003cstrong\u003e:\u0026nbsp;\u003c/strong\u003eExperimental and Data Analysis\u003c/p\u003e\n\u003cp\u003eAzura A. Rashid: Natural rubber analysis and properties\u003c/p\u003e\n\u003cp\u003eAhmad Shaiful Abdul Razak: Coagulant production and analysis\u003c/p\u003e\n\u003cp\u003eMohamad Danial Shafiq: Overall project analysis\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement:\u0026nbsp;\u003c/strong\u003eThe authors confirmed that the data required for this work are available within this article. \u003cstrong\u003e\u003cbr\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest:\u0026nbsp;\u003c/strong\u003eThe authors declare no conflict of interests.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e\u003cbr\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eClinical Trial Number in the manuscript:\u0026nbsp;\u003c/strong\u003eN/A\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eVerma, A., \u0026amp; Arora, S. (2024). 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Marine pollution from antifouling paint particles. \u003cem\u003eMarine Pollution Bulletin\u003c/em\u003e, 60(2), 159\u0026ndash;171.\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":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Coagulant-assisted, sludge capturing process, NRL film, surface-active latex, wastewater treatment","lastPublishedDoi":"10.21203/rs.3.rs-7361887/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7361887/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe management of sludge waste remains a critical challenge, as improper disposal can contaminate soil and water sources, impacting ecosystems and human health. In this study, polyaluminum chloride (PACl), coagulant was used to enhance the capturing process of domestic sludge waste using surface-active natural rubber latex (NRL) films. The NRL films were manufactured using the compounding and curing methods. The surface-active NRL film facilitated sludge removal by utilizing electrostatic interactions between the film and sludge particles. The incorporation of coagulant increased the sludge particle size and reduced the zeta potential of sludge water, enhancing aggregation. FTIR analysis confirmed the presence amines, proteins and peptides on the NRL film surface after the capturing process, indicating the captured sludge on the surface. The sludge deposition on the NRL films surfaces is more pronounced when the coagulant was incorporated. These findings demonstrate the effectiveness of surface-active NRL films in sludge waste removal, highlighting their potential for water treatment applications.\u003c/p\u003e","manuscriptTitle":"Efficiency of surface-active natural rubber latex (NRL) films in capturing sludge wastes in seawater and river water","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-15 12:00:26","doi":"10.21203/rs.3.rs-7361887/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-16T05:55:05+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-14T09:05:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"324258010326121771858491934858626585771","date":"2025-09-06T14:58:44+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-06T04:38:37+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-22T03:00:13+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-22T02:59:08+00:00","index":"","fulltext":""},{"type":"submitted","content":"Colloid and Polymer Science","date":"2025-08-13T07:09:58+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"dac7df97-2ee4-4902-ade1-090c4ada362b","owner":[],"postedDate":"September 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-10-31T01:39:02+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-15 12:00:26","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7361887","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7361887","identity":"rs-7361887","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}