key: cord-0944567-52adrmiy authors: Raval, Nirav P.; Kumar, Manish title: Geogenic Arsenic Removal through Core–shell based Functionalized Nanoparticles: Groundwater in-situ treatment perspective in the Post–COVID Anthropocene date: 2020-07-15 journal: J Hazard Mater DOI: 10.1016/j.jhazmat.2020.123466 sha: 9d00bfb935cc5f67c2d5c6b7b9e4b5e9c4585953 doc_id: 944567 cord_uid: 52adrmiy Groundwater, one of the significant potable water resources of the geological epoch is certainly contaminated with class I human carcinogenic metalloid of pnictogen family which delimiting its usability for human consumption. Hence, this study concerns with the elimination of arsenate (As(V)) from groundwater using bilayer–oleic coated iron–oxide nanoparticles (bilayer–OA@FeO NPs). The functionalized (with high–affinity carboxyl groups) adsorbent was characterized using the state–of–the–art techniques in order to understand the structural arrangement. The major emphasis was to examine the effects of pH (5.0 – 13), contact times (0 – 120 min), initial concentrations (10 – 150 µg L(–1)), adsorbent dosages (0.1 – 3 g L(–1)), and co–existing anions in order to understand the optimal experimental conditions for the effective removal process. The adsorbent had better adsorption efficiency (∼ 32.8 µg g(–1), after 2 h) for As(V) at neutral pH. Adsorption process mainly followed pseudo–second–order kinetics and Freundlich isotherm models (R(2)∼0.90) and was facilitated by coulombic, charge–dipole and surface complexation interactions. The regeneration (upto five cycles with 0.1 M NaOH) and competition studies (with binary and cocktail mixture of co–anions) supported the potential field application of the proposed adsorbent. . Amongst the listed geogenic contaminants, carcinogenic metalloid with the well-known name As has been placed in group 15 of the periodic table and it is known for its toxicity to human beings. Geogenic J o u r n a l P r e -p r o o f and antropogenic are the two primary origin/sources for As entry into groundwater. In the groundwater naturally occurring microorganisms oxidize organic matter and generate hydrogencarbonate and salt of carbonic acid which ultimately enhances the alkalinity of the solution and release As from mineral surfaces. Antropogenically As and its compounds are widely used (as fertilizers, pesticides, drugs, feed additives and wood preservatives) and finds their entry into groundwater [6] [7] [8] . In response to these widespread As contamination through various sources, it has been estimated that worldwide > 0. 15 billion people have been affected due to poisonous As polluted groundwater consumption/utilization. Any concentration above world health organization (WHO) recommended value of 10 µg L -1 for As causes harmful diseases like, hyperkeratosis, diarrhea, cerebrovascular disease, diabetes mellitus, hypothyroidism, cancer and arsenicosis [6, 9, 10] . To combat with pernicious As, variety of treatment techniques were proposed and most of them have been summerized in the form of short as well critical reviews by number of researchers [5, 7, [9] [10] [11] . However, use of conventional physicochemical methods has been confined due to excessive-energy consumption and release of hazardous chemicals into the environment. Amid these technologies, adsorption has been considered as one of the favourable techniques because of the ease of operation, easy maintenance, cost-effectiveness, absence of added reagents and applicability as a compact household module or in a community plant [12, 13] . Thus, it is a continuing need to recognize effortlessly synthesizable, economically viable, environmentally stable and sustainably regenerable adsorbent for effective and facile removal of As. Iron-oxide core based NPs i.e., magnetite (Fe3O4) [14] , hematite (α-Fe2O3) [15] , maghemite (γ-Fe2O3) [16] , Geoethite (α-FeOOH) [17] and akaganéite (β-FeOOH) [18] have been reported to be one of the best adsorbents for As removal due to their easy collecting property with external magnetic response, which enable their multiple time reuse for the adsorption process and therby reduce the secondary pollution problem. However, their high surface energy, resulted from the strong magnetic J o u r n a l P r e -p r o o f field, will cause NPs to agglomerate with each other. This agglomeration will not only reduce the surface energy but also diminishes the available surface area and theby associated adsorption capacity as well as in dynamic flow system (i.e., fixed-bed columns) it could results in pressure drops. The aggregarted and structurally unstable NPs finally leach into an aqueous medium. These leached NPs require sophisticated instrumentation facilities to separate out from the drinking water source such as groundwater aquifer after their application [19, 20] Hence, in order to increase functionality and biocompatibility as well as to reduce surface energy, unstability and chemical activity (i.e., air oxidation), the surface of the NPs needs to be grafted/functionaized with organic molecules/inorganic layer [21] . Compared to inorganic ones, coating of organic molecules would increase the potential applications of NPs by preserving their magnetic property and also by aiding the ensemble of reactive functional groups (hydroxyl, carboxyl, amino and aldehyde groups) [22] . In previous studies authors tried to directly coat iron oxide NPs with alginate biopolymer, however it could not prevent aggregation of iron NPs mainly because alginate polymeric backbone is more conformationally limited [23, 24] . Hence, recently extensive organic groups containing materials, also known as covalent organic frameworks (COFs) have been applied for the functionalization of the iron based NPs [25, 26] . However, several associated challenges such as, complex fabrication/synthesis procedure, less stability and generation of anonymous toxic breakdown products restrict their applications for the groundwater purification. Oleic acid (OA), containing C18 tail with a central single cis-double bond (forming a kink), is very well known surfactant to stabilize the magnetic NPs due to its higher affinity towards the surface of Fe3O4, compared to previously reported fatty acid (i.e., stearic acid) [27, 28] . In addition to providing stability, the OA coated iron oxide nanoparticles (OA-IONPs) have been reported as the potent and moderate biofilm inhibiting agent against gram-positive and gram-negative bacteria, respectively [29] . However, the long aliphatic hydrocarbon chain of OA would render the NPs to be immiscible in J o u r n a l P r e -p r o o f an aqueous suspension and therby reduce the interaction of NPs with the pollutants (As as the model pollutant) and ultimately decrease the adsorption efficiency [30, 31] . To circumvent this limitation, further iron oxide NPs need to be coated with the double layer of OA. This practice not only increase the hydrophilicity but it also retain magnetization of the coated NPs and increase the available functional groups on the surface of the final adsorbent. Considering the significant properties such as non-toxicity and antibacterial activity of the OA, first time this surfactant fabricated iron oxide NPs were used as an adsorbent for an eco-efficient removal of geogenic As from simple and compex water matrices. The specific objectives were: (i) synthesis and characterization (before and after sorption) of bilayer-OA@FeO NPs for As(V) removal; (ii) to examine the effects of system variables for the development of critical understanding about the optimal experimental conditions required for the effective As(V) removal; (iii) to determine the sorption capacity and reaction rate, carry out the mathematical modeling of isotherms and kinetics, respectively; (iv) to comprehend the sorption mechanism (v) to identify the potential reutilization efficiency of sorbent; and (vi) to carry out the competition study with binary and cocktail mixtures of co-anions for understanding the feasibility of the adsorbent towards field applications. The main precursors for the synthesis of iron oxide NPs, FeCl2.4H2O (98% purity) and FeCl3.6H2O (97% purity) were purchased from Loba chemie and Merck specialities LTD., respectively and used as such without further purification. All other solvents (methanol and acetone) and reagents/chemicals (oleic acid and ammonia) were procured from Finar Ltd. All reagents used were of analytical reagent (AR) grade. To prepare As stock solution (100 mg L -1 ), 0.422 g of sodium arsenate dibasic heptahydrate salt (Na2HAsO4.7H2O, >98%, Hi-Media, Mumbai, India) was dissolved in 100 mL of J o u r n a l P r e -p r o o f MQ water. By suitable dilution of this stock, different working solutions of arsenic (within the concentration range of 10 to 150 µg L -1 ) were prepared and used for the subsequent adsorption study. High purity deionised water (18.2 MΩ cm -1 , ELGA PURELAB Flex 3 system) was used for all the experimental study. Bilayer-OA@FeO NPs were chemically synthesized via simple co-precipitation method as adapted by [32] . Briefly, FeCl3.6H2O and FeCl2.4H2O were mixed with appropriate amount of the water in 1:2 ratio and stirred vigorously for 1-2 hrs at 80-100 o C temperature. After the solution becomes colloidal dark yellow colored, 20 mL ammonia solution was added drop-wise until the development of black precipitates. The further drop-wise addition of OA (10 mL) was to form a layer around nanoparticles to avoid their agglomeration caused due to high surface energy. The obtained precipitates were washed meticulously with methanol/acetone to remove unreacted residues of OA and to increase the magnetization property of the finally synthesized adsorbent. The dried Bilayer-OA@FeO NPs (adsorbent) was stored in an air tight container for further characterization and adsorption study ( NPs was exained according to the method proposed by [19] . The adsorption experiments of As(V) onto synthesized adsorbent (bilayer-OA@FeO NPs) were The adsorption rate is deliberated as the preeminent regulating factor for the numerical reckoning of removal efficiency. For the kinetic study, 0.1 g of bilayer-OA@FeO NPs was added to 100 mL of Where (µg g -1 ) is the sorption capacity of bilayer-OA@FeO NPs for As(V) at definite time t, k1 (min -1 ) and k2 (g µg -1 min -1 ) are the rate constants for pseudo-first-, second-order kinetic sorptions, respectively. α (µg g −1 min −1 ) and β (µg g −1 ) are Elovich parameters and belong to the initial sorption rate and the extent of surface coverage, respectively. The values of Elovich constants (β and α) were determined from the slope However, these commonly used kinetic rate models were not able to recognize the potential diffusion mechanism hence, the kinetic data were also examined by using the following equation based intraparticle diffusion model (proposed by Weber-Morris) [35]. Where, (µg g -1 .min -1/2 ) and (µg g -1 ) are the intraparticle diffusion rate constants. Adsorption isotherms depict the nature of the solid-solution interaction and furnish inclusive information on the adsorption efficiency of the studied sorbent [35] . Bilayer-OA@FeO NPs (0.1 g L -1 ) was equilibrated with the aqueous solutions of As(V) by agitating for 120 min at 250 rpm. The commonly used two parameter isotherm models were selected to examine the equilibrium sorption data. Langmuir isotherm is routinely practiced to determine the equilibrium state connection between the functioning sites on the adsorbents' surface and concentration of sorbate molecules in the medium. It considers homogenous type of the sorption with cipher interaction between sorbed ions/molecules. The Langmuir equation is presented as: where (L g -1 ) is constant and (µg g -1 ) is the Langmuir single-layer maximum sorption capacity. Both the parameters 1/qm and KL were determined from the slope and intercept, respectively, of the The RL value showed the type of the adsorption: RL=0 means irreversible type of the adsorption; 01 depicts unfavourable adsorption. Freundlich isotherm presumes that the sorption process takes place onto the adsorbent with heterogeneous surfaces. The linearized form of Freundlich equation is: where K F (µg 1−1/n L 1/n /g) and 1/n are the isotherm constant and heterogeneity exponent relating to the extent of sorption and adsorption intensity, respectively. Both the parameters KF and n were calculated from the intercept and slope, respectively, of the linear plot of log qe vs. log Ce. Temkin isotherm model is applied to take into account of heat of sorption for all interacting ions/molecules in the layer, which proportionally decreases with increasing surface coverage. The linear form of Temkin isotherm equation is expressed as: where R = universal gas constant (kJ mol -1 K -1 ), T is the absolute temperature (K). b T (kJ mol -1 ) and A T (L g -1 ) are the Temkin isotherm constants, associated with the binding energy and heat of sorption and calculated from the slope and intercept of the linear plot of Qe versus ln Ce, respectively. The Dubinin-Radushkevich (D-R) isotherm is a semi-empirical model. It is usually applied to represent the sorption mechanism of contaminant with an information of Gaussian energy dissemination onto a heterogeneous surface. The linear form of D-R isotherm equation is: Where Qm (µg g -1 ) is the calculated sorption capacity, Kd (mol 2 kJ -2 ) is D-R constant and it is associated with mean free-energy of adsorption. It is often considered to be the most important constant of the D-R isotherm model because its value can be used to calculate free-energy change. is Polyani potential, related to the equilibrium concentration of adsorbate (Ce) via equation-13: Optimization process is indispensable in order to identify the suitable kinetic and isotherm models to the obtained experimental results. For the present study, correlation coefficient of determination (R 2 ) was performed for data optimization process. The isotherm model with obtained R 2 value much closer to unity is estimated to offer the best-fit with the experimental data. However, sometimes because of the inherent bias arising from linearization, non-linear regression was applied to ascertain substitute isotherm parameter sets. Hence, Chi-square test (χ 2 ) was also carried out for the current study [36, 37] . The mathematical equation for the sum of squares can be represented as: Where, ( ) (µg g -1 ) is experimental data based equilibrium capacity; ( ) (µg g -1 ) is the model based calculated equilibrium capacity. To carry out the desorption/regeneration of the applied/utilized adsorbent, the bilayer-OA@FeO NPs were separated with an external magnetic field application after completion of the adsorption cycle and redispersed into alkaline solution (50 mL) and agitated for 120 min at normal room temperature (298±2 K). After regular time interval, the remaining concentrations of metalloid arsenate were determined by GF-AAS. Considering the lower removal efficiency of As(V) at alkaline condition (refer Fig. 3 (c)), NaOH was selected as a desorbing agent for regeneration of both the biosorbents. The suitable concentration of NaOH used for desorption-regeneration cycle was determined from the preliminary experiments with 0.1 and 0.5 M NaOH solutions. These regenerated nanoparticles were washed with DI water and ethanol for many times and dried in an oven (@323 K) to reutilize in a new adsorption-desorption-regeneration cycle. To determine the effectiveness, field-scale capability and selectivity of the synthesized adsorbent (bilayer-OA@FeO NPs) towards As, the competition studies in presence of common anionic cocontaminants (F -, Cl -, NO3 -, SO4 2and PO4 3-) were performed in a batch mode. In the concentration range of 0.1 to 5 mmol L -1 , solution of the mentioned coexisting anions were added to As(V) solution with initial concentration of 100 ppb, individually (in a binary mixture) and combinely (cocktail mixture). The mixture was agitated with optimum amount of sorbent for predefined time intervals and the remaining As concentration was measured using previously mentioned model of GF-AAS. Table S1 . The SEM-EDS images of the bare FeO NPs clearely revealed the existence of aggregated particles, resulting from the high surface energy of nacked NPs and occurence of Fe (72.19 wt%) as the major element together with C, O and Cl as other components ( Fig. 1(a, b) ). After coating of NPs with OA-bilayer, the separated particles can be obtained as observed in Fig. 1(c) . Through bilayer coating OA may form shell around each core of the FeO NPs (refer Fig. S2 ). This would resulted in an increase of the NPs size, but at the same time more separated particles can provide individual binding surface for the pollutants. The EDS spectra for bilayer-OA@FeO NPs ( Fig. 1 (d) ) indicated the mass variation of major element (Fe = 53.47 wt%) along with the change of minor elements like, C, O and N which provides an evidence for successful functionalization of NPs with OA. However, due to low concentration of As(V) solution (100 µg L -1 ) was used to equilibrate with the adsorbent, the retained element could not be observed in the EDS spectrum of bilayer-OA@FeO NPs after adsorption ( Fig. 1(f) ). The obtained outcomes are in good aggreement with the earlier reported results where authors have speculated the textural changes of synthesized nanocomposite adsorbent due to heterogenous dispersion of magnetic NPs onto supporting polyaniline hollow microsphere surface [19] . FT-IR spectra of bare iron oxide nanoparticles (FeO NPs), oleic acid (OA) and bilayer-OA@FeO NPs are shown in Fig. 2(a) . The spectrum of bare FeO NPs exhibited strong bands in 1000-500 cm -1 regions. According to [24] the spectrum of magnetite (Fe3O4) nanoparticles represented band at 570 cm -1 , whereas for maghemite (γ-Fe2O3) nanoparticles the broad band has been observed within the region of 520-610 cm -1 . In present study, the characteristic Fe-O vibrations of magnetic iron oxide can be confirmed with the band observed at 545 cm -1 (for both uncoated and coated nanoparticles). In The BET and Langmuir surface area of the adsorbent was determined to be 6.53 and 124.82 m 2 g -1 , respectively. The total pore volume of the bilayer-OA@FeO NPs was found to be 0.0163 cm 3 g -1 . The average pore size is calculated from the single-point adsorption total volume for the coated nanoparticles. The pore size distribution (Fig. S3) indicates that most of the mesoporous is around 1. It is essential to study the thermal stability of the functionalized adsorbent in order to interpret how morphological changes influence the adsorption capacity of coated NPs. The quantitative evidence for the coating of OA onto iron oxide NPs was also provided by TGA [41] . The weight losses of bare FeO NPs and bilayer-coated OA@FeO NPs were analyzed within the temperature range of 50 to 750 o C at a heating rate of 10 o C per min and results are presented in Fig. 2(d) . The TGA was carried out under inert (nitrogen) atmosphere which causes reduced oxidation of bare and OA coated NPs. In case of the bare FeO NPs the mass loss was observed in two stages. Effects of adsorbent mass on As(V) removal at 120 min contact time and 100 µg L -1 of initial metalloid concentration was studied and the acquired results are illustrated in Fig. 3 (a) in terms of As(V) removal percentage (%) and in relation of adsorbed amount at equilibrium stage (qe). It was perceived from Fig. 3(a) . This declined sorption capacity of geogenic contaminant was attributed to the overlapping/aggregation of vacant adsorption sites at higher amount of sorbent. According to this obtained results, 1 g L -1 is considered as an optimal sorbent dose and all succeeding experiments were performed with that particular amount of bilayer-OA@FeO NPs. Determination of the effect of reaction time is an essential parameter in order to identify the rapid and economical adsorption process to achieve the sustainability goal of the Anthropocene epoch. Hence, the sorption of As(V) by bilayer-OA@FeO NPs was measured at varying contact times from 0 to 120 min with altered As concentrations (10 to 150 µg L -1 ). The obtained results in terms of As(V) removal percentage (%) and adsorbed amount (µg g -1 ) are plotted in Fig. 3(b) and Fig. S4 , respectively. At equilibrium, the removal capacity of As(V) at 10 and 150 µg L -1 was 47.8% and 17.6%, respectively ( Fig. 3(b) ). It is noted that As(V) removal percentage was lower at the higher initial concentration, because at enhanced sorbate concentration the inadequate number of available adsorption sites got saturated onto bilayer-OA@FeO NPs as shown in Fig. 3(b) . With increasing initial metalloid concentrations from 10 to 150 µg L -1 , the amount of adsorption of As(V) was increased from 4.5 to 23.4 µg g -1 at room temperature ( Fig S3) . This rising adsorption capacity of toxic metalloid arsenic at higher initial adsorbate concentration was because of the high driving force to transfer the mass of The relevant results are depicted in Fig 3(c, d) . The pHZPC of the bilayer-OA@FeO NPs was 7.3 ( Fig. 3(c) ). At low pH, protonation of the accessible functional groups (-COO -) favours the binding of the anionic arsenic species. This resulted in the higher adsorption capacity at pHpHZPC) (Fig. 3(d) ), surface of the adsorbent becomes less positively charged, which resulting in a reduction of arsenate adsorption (via electrostatic repulsion forces) as presented in Fig. 3(e) . Similar obsevations were previously reported by other researchers, where they have applied magnetic nanoparticles for the removal of As(V) [14, 44] . However, a strong permanent dipole, created by the covalent bonding of relatively electronegative oxygen atom with carbon atom, can still favourably interact with anions (H2AsO4 − ) present in the solution via charge-dipole/ion-dipole interactions and assisted the adsorption of As(V). In addition, the formation of an inner-sphere surface (As-Fe) complexes via ligand exchange mechanism also contributes in the removal process (refer Fig. 3 (e)) [45, 46] . J o u r n a l P r e -p r o o f suitability of the sorption reaction between representative adsorbate (As(V)) and applied sorbent (bilayer-OA@FeO NPs). The respective kinetic plots are presented in Fig. 4(a, b) and statistically calculated parameters/constants from intercept (qe (cal)) and the slope (k1, k2) of these plots are mentioned in Table 1 . The closer values of experimental data (qe(exp)) pertaining to the applied sorbent towards rate model-based calculated values (qe(cal)) for the pseudo-second-order kinetic model confirmed its better applicability (refer Table 1 ). In addition, the best fits of the linear regression of the pseudo-second order rate model show the R 2 values of 0.968 for As(V) adsorption on BOA@FeO NPs. At the same time, the lower value of sum of square error function (χ 2 ) for the data of secondorder rate model confirms its applicability (refer Table 1 ). These findings clearly implied that the Fig. 4(d) . With multilinearity correlation, the plot confirmed the adsorption process of model geogenic contaminant (As(V)) onto proposed adsorbent in three consecutive steps/mechanisms (film, particle and external surface adsorption). The first step, often regarded as film diffusion is the mobilization of As from the solution to the outer-most surface of the coated nanoparticles by process of diffusion. This step was very sharp as shown in figure, mainly because initially large number of As molecules were available to transport to the adsorbent surface. The internal diffusion was the second step in which As molecules transported from surface of the NPs into interior sites. Due to the inter-connected layers of OA, this stage was very rapid. This can be explicable as the connected tails might provide channels to transport metalloid, As(V), into the interior portions of the adsorbent. The final and/or equilibrium stage approaches when the intra-particle diffusion begins to slow down. This was because of lower concentration of As(V) in solution phase and sorption of As(V) molecules from the active sites into the interior pores/sites. The intraparticle diffusion constants have been calculated using the Weber-Morris linear equation and are presented in Table 1 together with the other studied rate constants. The negative value (-1.95 µg g -1 ) of intercept for the lower initial adsorbate concentration (10 µg L -1 ) confirmed the inefficient boundary layer control. Also the plots for different As(V) concentrations were not originated from the zero which suggested the concurrent type of the adsorption mechanism together with the intraparticle diffusion. The adsorption isotherms for the removal of As(V) onto bilayer-OA@FeO NPs were evaluated using the frequently used two-parameter isotherm models as shown in Fig. 5(a-d) Table. 2. For the determination of felicitousness and aptness of the model parameters, statistical hypothesis tests (correlation coefficient (R 2 ), chi-square test (χ 2 )) were performed and their obtained values are mentioned in the same table together with isotherm parameters. The extent of the goodness of fit function (R 2 ) were high and relatively comparable for all the studied isotherm models ( Table 2 ). The maximum monolayer sorption capacity (qm) of 32.8 ± 0.5 µg g -1 was for As(V) onto the surface of bilayer-OA@FeO NPs. The sorption capacity of present bilayer-OA@FeO NPs adsorbent, prepared by chemical co-precipitation, was compared with the other magnetic nanoparticles reported in contemporary literature (Table 3) . Another important constant (KL) of monolayer isotherm which is related to the sorption energy was found to be 0.72 ± 0.04 L g -1 . RL (separation factor) values (0.26 to 0.83) were between the range of 0 to 1 for different initial As concentrarions (10 to 150 µg g -1 ) which further confirm the favorable adsorption process of modeled pollutant onto applied adsorbent. The Freundlich isotherm, which assumes multilayer coverage of adsorbate over a well distributed heterogenous adsorbent surface, simulated the experimental data very well with the R 2 value of 0.99 and χ 2 value of 0.27. It has been reported that favorable adsorption was indicated when the values of '1/n' obtained from Freundlich isotherm were between 0.1 to 1 [48] . In the present work, the value of '1/n' was found to be in the suggested range (0.55 ± 0.001) indicated that arsenate can favorably adsorbed onto bilayer-OA@FeO NPs. By comparing the values of goodness of fit (R 2 ) and standard error (χ 2 ) of the isotherm models on the experimental data, the Freundlich > Langmuir > Temkin models gave higher R 2 and the lower error χ 2 values. Therefore, it is conceivable to consider that the adsorption of As(V) occurs by the formation of consecutive multilayer/monolayer coverages and difference in heat of adsorption onto proposed adsorbent (bilayer-OA@FeO NPs) [49] . The thermodynamic transfer of the free energy of solute to the external surface of the adsorbent can be calculated from the D-R constants (numerical derivation of ε and E). The E values (0.18 kJ mol -1 ) of arsenate metalloid indicated that the adsorption process is controlled by physisorption (see Table 2 ). Similar findings were reported by other researchers, where the mean energy of adsorption was in the range of 0.18-0.96 kJ mol -1 and physisorption dominates chemisorption over the entire equilibrium concentration range [50] . The possible reutilization efficiency, essential to determine the robustness, of the applied adsorbent towards As(V) removal was estimated by performing desorption experiments under alkaline environment. It was found that the synthesized adsorbent can be successfully regenerated up to consecutive five desorption/regeneration cycle with application of 0.1 M NaOH as a desorbing agent ( Fig. 6(a) ). From Fig. 6(a) , it has been observed that the As(V) removal efficiency was 14.8% for bilayer-OA@FeO NPs after fifth cycle which was only 5.6% less than that obtained with the fresh adsorbent (20.4%). At higher pH, stong competition between hydroxyl anions and complexed ligands causes desorption of adsorbed As(V). The obtained results are in line with the previously published work where alkaline solution has been reported as the favourable regenerant for desorption of anionic (negatively charged) geogenic contaminants from bio-nano polymer based composite adsorbents because of the high affinity of the complexed metal oxides for hydroxyl anions [19] . The results clearly demonstrates that the engineered adsorbent (bilayer-OA@FeO NPs) could be sustainably regenerated with NaOH treatment and reutilized multiple times for the management of geogenic contaminants to provide potable water for the human development which is considered to be one of the prime challenges of the Anthropocene era. J o u r n a l P r e -p r o o f It has been very well-known that As is naturally present in the groudwater, which is considered to be one of the significant drinking water sources in the contemporary time, along with the other common geogenic cationic (i.e., Ca 2+ , Mg 2+ and Fe 2+ ) and anionic (i.e., F -, Cl -, SO4 2-, NO3 -, PO4 3-, CO3 2and SiO4 4-) species [26] . These differently charged species (anions/cations) may have synergistic and/or antagonistic effects on the adsorption of As. Hence, As(V) adsorption by bilayer-OA@FeO NPs (1 g L -1 ) was examined in the presence of other counter-anions, i.e., F -, Cl -, NO3 -, SO4 2-, PO4 3and their cocktail mixture, at different concentrations (0.1, 1 and 5 mmol L -1 ) and the obtained results are depicted in Fig. 6(b) . From the plotted results, it was observed that the As(V) removal was least affected in the presence of Cl -, SO4 2and NO3ions whereas, more significantly reduced with the common co-contaminants i.e., F -, PO4 3-. The substantial reduction in the removal efficiency of arsenate in presence of Fand PO4 3was mainly because of the smaller size/highest electronegativity of the Fions and structural similarity between arsenate and phosphate which create strong competition amongst them for the available binding sites (-COOH group) present on the bilayer-OA@FeO NPs. Wang et al. (2014) also reported that presence of phosphate significantly deteriorate the adsorption of arsenate onto magnetic nanoparticles impregnated chitosan beads (MICB) adsorbent [51] . The potential of adsorbent to remove the As(V) in presence of other competitive ions was considerably good and the antagonism of the common anions follows the order of: Cl -< SO4 2-< NO3 -< F -< PO4 3 . To simulate the actual environmental condition, further experiments were performed using cocktail mixture of the major geogenic anions (H2AsO4 -, F -, Cl -, NO3 -, SO4 2-, PO4 3-) under the same experimental conditions and the acquired results are plotted in Fig. 6(b) . Results confirmed that present In order to comprehend the plausible sorption mechanism of As(V), which is usually present in the form of oxoacids/inorganic acids (arsenic or arsenous acid), as well as the leverage of stabilizing agent (OA) on the magnetic NPs' adsorption properties, comparative characterizations were performed. The surface of the adsorbent became protonated and positively charged when the solution pH was ≤ 7.3, the isoelectric point of the sorbent, which was beneficial to adsorption because As(V) was adsorbed as anionic species via coulombic forces/electrostatic attraction. At higher pH (pH≥pHZPC), the negative charge prevails on the surface of the adsorbents which resulting in a reduction of As(V) sorption (via electrostatic repulsive forces). However, a strong permanent dipole can still favourably interact with anions via charge-dipole/ion-dipole interaction. In addition, the surface complexation, film diffusion/absorption and intraparticle diffusion of adsorbates into the sorbent play vital role in the adsorption process. Comparing the SEM-EDS spectra of synthesized adsorbent in the presence and absence of As(V), it was noticed that after loading of the geogenic contaminant onto adsorbent surface, the clearly separated particles became invisible and represented changes in their surface morphology (refer Fig. 1(e) ). In the present stuy, bilayer-OA@FeO NPs showed the presence of -COOH and -C=O at ~1423 and ~ 1705 cm -1 regions (refer Fig. 2(a) ). These groups were mainly responsible for the adsorption of However, two identical peaks at 2Ɵ values of ~ 43.5 • and 54 • , have changes in the shape/intensity which could be attributed to the formation of precipitates of ferric arsenate (FeAsO4) (refer Fig. 2(b) ). Based on the above analyses, the probable sequestration of As(V) onto proposed adsorbent (bilayer-OA@FeO NPs) can be elucidated by the electrostatic attraction/repulsion, coulombic interaction, iondipole/charge-dipole interaction, intra-particle diffusion and inner-sphere complex formation (Monodentate-mononuclear and/or bidentate-binuclear) mechanisms (refer Fig. 3(e) ). The proposed sorption mechanism is in agreement with previously reported results where it has been very-well documented that the inner-sphere complexes are mainly responsible for the sorption of As(V) onto various iron-based adsorbents [6, 19] . The bilayer OA@FeO NPs (core-shell) adsorbent was successfully synthesized and first time The authors declare no conflict of interest ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Tables Table 1: Parameters for pseudo-first-order, pseudo-second-order, Elovich and intraparticle diffusion kinetic models for the sorption of As(V) onto bilayer-OA@FeO NPs (Adsorbent dose: 1 g L -1 ; solution volume: 100 mL; pH: 7.0 ± 0.2; temperature: 298 ± 2 K). 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