In Situ Forming MnFe2O4/D201 Magnetic Composite Adsorbents for High Selectivity Adsorption and Deep Treatment of As(V) from Wastewater

A new magnetic adsorbent, namely, MnFe2O4/D201, with deep-treatment ability and high selectivity adsorption for As(V) was prepared. According to isotherm adsorption and kinetics, As(V) adsorption is primarily used for chemical bonding throughout the single-layer adsorbing process. The maximum As(V) adsorption capacity of MnFe2O4/D201 can reach 35.8 mg/g at pH 3. MnFe2O4/D201 also exhibits higher selectivity adsorption against Cl-, NO3 -, SO4 2-, and PO4 3-. According to the thermodynamic results, the adsorption process was spontaneous and endothermic. The adsorption capacity is maintained at 81% of the initial after ten adsorption-desorption cycles. As(V) concentrations ranging from 1 mg/L to 10 μg/L can be treated in fixed-bed column experiments. The effectual cure volume of As(V) reaches 1332 BV (26.64 L). The removal mechanism primarily comprises electrostatic attraction and complexation.


Introduction
Arsenic, as a metalloid material with high toxicity, is widely available and is a significant synthetic material in the fields of industry, agriculture, semiconductor substances, and so on [1]. Because of the increasing demand for As, the issue of As-containing wastewater is also on the rise, causing severe water and soil pollution. For this reason, the World Health Organization (WHO) has set the maximum As concentration in drinking water at 10 μg/L to reduce As poisoning [2]. For a long time, approximately 150 million people in over 70 countries consumed As-contaminated water consumption, resulting in long-term damage from arsenic pollution [3]. As(V) is a class I carcinogenic element, and drinking water containing a significant amount of As poses severe health risks, including cancers and neurological diseases [4]. Therefore, it is critical and urgent to find an effective technology for reducing As(V) concentration.
Many strategies have been learnt and used in oxyanionpolluted wastewater treatment, including membrane separa-tion, adsorption, biological treatment, chemical precipitation, and ion exchange [5]. Among various strategies, adsorption technology has attracted significant attention because of its unique treatment effect, broad application scope, important reusability, and other advantages. Therefore, many adsorbents have been proposed to treat As(V)contaminated drinking water, including cellulose, chitosan, iron oxides, clays, zeolites, alginate, and biochar [6]. In recent years, nanomaterials have piqued interest as adsorbents because of their exceptional properties [7]. Although nanomaterials exhibit good adsorption efficiency, it is not easy to recollect the used adsorbents after pollutant adsorption [8]. The fast development of magnetized nanotechnology has provided novel possibilities for promoting the water treatment process [9]. Among these adsorbents [10], MnFe 2 O 4 is one of the most essential chosen and worthy ecofriendly materials and has piqued considerable interest because of its easy synthesis, convenient recyclability, and biocompatibility. MnFe 2 O 4 is extensively used in As(V) remediation because of its excellent sorption capacity and availability. Nevertheless, MnFe 2 O 4 is inclined to selfaggregate because of its large specific surface area and minor scales, degrading the adsorption efficiencies [11].
In addition, considering the problems of aggregation and recovery difficulty of the nanomaterial MnFe 2 O 4 , supporting materials (such as activated carbon, carbon nanotubes, resins, and graphene) are employed to provide support to enable nanoadsorbent to be well recycled through a facile "filtrating-washing" process [12], thereby overcoming the disadvantages of the traditional powdered MnFe 2 O 4 nanomaterial. Moreover, their specific surface area and dispersibility can be improved, thereby improving their reactivity [13].
Among these supporting substances, D201 resins have excellent mechanical strength, Donnan membrane impact, and stable physicochemical characteristics [14]. In particular, the D201 resin surface has positively charged functional groups conducive to heavy metal adsorption [15]. The ion exchange-surface deposition technique is one of the most widespread techniques for preparing MnFe 2 O 4 /D201 nanocomposites. The mass transfer controlling stage is the diffusion process of ionic components, because ionic components can spread out on the resin surface and the inner surface and react quickly with the functional groups on the resin surface. Thus, if modified with magnetic nanomaterials such as MnFe 2 O 4 , MnFe 2 O 4 /D201 nanocomposites can be used as excellent adsorbents for removing different types of pollutants, including anions from water. Because MnFe 2 O 4 /D201 nanocomposites contain abundant MnFe 2 O 4 and quaternary ammonium groups, such modified materials are predicted to have a high attraction for As(V) in water.
In this work, MnFe 2 O 4 was loaded on D201 through a coprecipitation approach for water treatment purposes, which can improve the high selectivity adsorption capacity and deep-treatment ability of materials toward pollutants such as As(V). Various characterization methods, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM), Brunauer-Emmett-Teller (BET) surface area, zeta, and X-ray photoelectron spectroscopy (XPS) analyses were used to characterize the composite structure and magnetic and physicochemical properties of the synthesized nanocomposite. Adsorption kinetics and adsorption isotherms were used to explore the adsorption mechanism in the presence of MnFe 2 O 4 /D201. The pH level, contact time, temperature, initial As(V) concentration, coexisting ions, and regeneration were studied through batch experiments. A fixed-bed column experiment with As(V) was conducted to assess its potential for practical applications. The underlying removal mechanisms were analyzed. /D201 was prepared by using a one-pot coprecipitation approach. In a standard synthesis procedure, add a certain quantity of FeSO 4 and KMnO 4 to 100 mL of deionized water, and stir until resolved. Afterward, 20 g of D201 was accurately added to the system. The solution was adopted by mixing at 298 K for several hours to permit the precursor to disperse inside D201. Then, 3 M NaOH solution was slowly added to the above mixed solution to obtain MnFe 2 O 4 /D201. The obtained MnFe 2 O 4 /D201 were soaked in alkaline environment for 12 h. Filter MnFe 2 O 4 /D201 out of the NaOH solution and clean them with 5 wt% NaCl solution several times until the supernatant arrives at neutral (pH = 7), followed by drying at 298 K to acquire the resulting nanocomposite adsorbents [10].

Materials and Methods
Adjust

Batch Adsorption Experiments.
The primary condition of batch adsorption experiments is to use 0.5 g/L adsorbent, keep the stirring speed at 150 rpm, and control the temperature at 25°C. The influences of pH, reaction time, initial concentration of As(V), and coexisting competing anions on the adsorption process were studied. For the pH in the As(V) solution, diluted HCl and NaOH solution (0.1 mol/L) are used to adjust the solution. The residual contents of As(V) were analyzed through an inductively coupled plasma atomic emission spectroscopy (ICP-AES) instrument [16]. The quantities of As(V) adsorbed onto MnFe 2 O 4 /D201 were estimated by comparing the changes in As(V) concentration before and after. The adsorption capacity was depicted as follows in Text S1.
Varying parameters' impact on the adsorption process was examined, including pH, contact time, initial concentration, and temperature. The effect of pH on As(V) removal was calculated by changing the pH from 3 to 11. For kinetic, explore the adsorption performance under different adsorption times (0-360 min). For adsorption isotherm experiments, it was examined at 298 K, 318 K, and 338 K within the initial As(V) concentration altered from 30 to 100 mg/ L. The effects of coexisting ions were examined in 100 mL of 30 mg/L As(V) solutions containing 50 Figure 2(a). The FTIR spectra of D021 and MnFe 2 O 4 /D201 indicate that they have near typical peaks and patterns, pointing out that the surface functional groups of D201 do not change in structure after the loading of MnFe 2 O 4 . For D201 resin, the broad peak at 3413 cm -1 is ascribed to -OH bands of H 2 O. The peaks at 1482 cm -1 and 1226 cm -1 are credited to -CH and -CN bands, respectively. Further, the peak at 1627 cm -1 is ascribed to C=C stretching vibration, showing the existence of D201 resin's styrene frame. The peak at 986 cm -1 is ascribed to the feature of the -N + (CH 3 ) 3 band, implying the presence of -N(CH 3 ) 3 on D201. For MnFe 2 O 4 /D201, the raw peaks at 1160 cm -1 were the same as metal hydroxyl groups (M-OH, M represented Fe or Mn), which proved that MnFe 2 O 4 was successfully loaded on D201's surface [17,18].
The thermal stability of D021 and MnFe 2 O 4 /D201 composite was measured by TGA in Figure 2(c). As illustrated in the curve, D021 and MnFe 2 O 4 /D201 have three steps in the thermal weight loss process, including bound water loss and composite degradation, and remain breakdown. For MnFe 2 O 4 /D201, the weight loss (18%) up to 328°C was probably due to the dehydration process. What is more, the 17% weight loss between 328 and 384°C was attributed to the carbonization of MnFe 2 O 4 /D201, and the 27% weight loss in the scope of 384-465°C was attributed to the degradation of MnFe 2 O 4 /D201 [21]. Compared with resin D201, the improvement of thermal stability of adsorbent MnFe 2 O 4 /D201 with loading of particle MnFe 2 O 4 will help MnFe 2 O 4 /D201 to be more suitable for application in the actual environment with higher temperature.
The morphologies of D021 and MnFe 2 O 4 /D201 are shown in Figure 3. MnFe 2 O 4 /D201 exhibits a ball-shaped formation and has 0.6 mm in average diameter. Here, loaded MnFe 2 O 4 on D201 possesses several advantages. First, the porous morphology can significantly raise the surface accessibility between MnFe 2 O 4 /D201 and As(V). Second, D201 can effectively stop MnFe 2 O 4 's agglomeration with excellent stability. When MnFe 2 O 4 is loaded on the resin D201, there are some particles on its surface, and the surface of MnFe 2 O 4 /D201 becomes rough [22,23]. These findings support the successful synthesis of the adsorbent material MnFe 2 O 4 /D201. In addition, the mapping of D201 and MnFe 2 O 4 /D201 is displayed in Figure 3. For D201, N and O atomic contents are worked out, showing that the N and O are essential elements in D201. For MnFe 2 O 4 /D201, N, O, Mn, and Fe contents are calculated to be 5, 29, 14, and 52%, respectively, suggesting that there are MnFe 2 O 4 onto D201, which are contributing to adsorb As(V). From the distribution of different elements in the mapping of MnFe 2 O 4 / D201, it can be seen that compared with the existing nanoparticle loading technology, the nanoparticle MnFe 2 O 4 distribution in the adsorbent MnFe 2 O 4 /D201 is more uniform, which is conducive to the removal of As(V). The structure of MnFe 2 O 4 /D201 likely contributed to more adsorptive sites for As(V) removal.
To confirm that the adsorbent formation existed, N 2 adsorption-desorption isotherm characterization was  Adsorption Science & Technology pore channels of the resin D201 through D201's higher S BET . It may be expected that S BET and pore volume will advance the adsorption capacity of As(V) by providing more active adsorption sites and reducing the mass transfer resistance of As(V). The magnetic characteristics of MnFe 2 O 4 /D201 were evaluated by using a vibrating sample magnetometer (VSM). Importantly, MnFe 2 O 4 /D201 possessed a saturation magnetization (1.6 emu/g), as demonstrated in Figure S1b. Due to the nonmagnetic D201 mediums in MnFe 2 O 4 / D201, thus MnFe 2 O 4 /D201 has a lower saturation magnetization [26]. Further, the magnetic separation and dispersion of the adsorbents were drawn by a magnet, further pointing out that MnFe 2 O 4 /D201 is well separable. At the same time, the structural properties of resin D201, including the excellent mechanical strength and spherical shape, would also bring convenience to the practical application of composite materials MnFe 2 O 4 /D201. Composite materials MnFe 2 O 4 /D201 would combine their advantages to enhance the separation and recovery of nanocomposite after adsorption.
The survey XPS spectra of D201 and MnFe 2 O 4 /D201 are demonstrated in Figure S2.  Figure S3. Although the resin itself has a large specific surface area and a certain amount of effective functional groups, its adsorption capacity for As(V) still has certain limitations. Compared with many reported adsorption materials, the adsorption performance of D201 is not outstanding. With the addition of nanoparticles MnFe 2 O 4 , its adsorption performance has significantly improved. MnFe 2 O 4 /D201-15% has the most effective removal on As(V), and the adsorption capacity arrives at 35.8 mg/g. On the one hand, MnFe 2 O 4 was developed by MnFe 2 O 4 / D201 which has the potential to adsorb As(V). On the other hand, the styrene frame of D201 can distribute MnFe 2 O 4 , and the resin surface accommodates a massive quantity of -N + (CH 3 ) 3 groups. The As(V) adsorption capacities declined in the presence of MnFe 2 O 4 /D201-20% and MnFe 2 O 4 /D201-25%, which was attributed to MnFe 2 O 4 nanoparticle aggregation on the D201. Therefore, MnFe 2 O 4 /D201-15% was selected for further experiments.
3.2.2. The pH pzc of MnFe 2 O 4 /D201 and the Influence of pH on As(V) Removal. The pH action is a critical factor because of its double effect on the ionic form of the adsorbate and the surface charge of the adsorbent. To probe the effect of pH and achieve excellent As(V) adsorption, the zeta potential analysis of adsorbents and adsorption experiments at varying pH levels were conducted. According to the references, the nanoparticle MnFe 2 O 4 has a low pH pzc value, so MnFe 2 O 4 presents negative charge in a wide range of pH. MnFe 2 O 4 has a repulsive effect with As(V), which results in poor adsorption capacity. There are a large number of -N + (CH 3 ) 3 groups on the resin D201, which can improve the electrical properties of the composite within the whole pH range and increase the material's pH pzc value. For MnFe 2 O 4 /D201, as acidity increases, the charge of the adsorbent particles tends to be positive, as shown in Figure 4(a). Both D201 and the impregnated MnFe 2 O 4 are positively charged and adsorb anions by electrostatic attraction [28].
Meanwhile, for MnFe 2 O 4 /D201, the composite adsorbent's removal efficiency gradually declines as pH increases, as shown in Figure 4(b). For As(V), H 2 AsO 4 is the dominate species at pH 2-7, whereas the dominant As ion formed is HAsO 4 2as pH increases, as shown in Figure 4(c). When pH < 7, MnFe 2 O 4 /D201 with positively charged would adsorb more H 2 AsO 4 by electrostatic attraction [29]. With an increase in pH, a large amount of OHwould lead to competitive adsorption with As(V) on MnFe 2 O 4 /D201, resulting in insufficient adsorption capacity. Notably, the adsorption capacity is 26.3 mg/g at pH 8. The removal efficiency slightly declines in the pH range of 3-7, whereas it declines sharply in the range of 8-11. A highly acidic water environment would lead to the risk of posttreatment. Thus, the best pH for As(V) adsorption was 7, which is the optimum pH of adsorption. These finding confirmed that pH has a critical effect on As(V) adsorption. MnFe 2 O 4 /D201 exhibited a positive charge at pH 3-8 and promoted the adsorption of negatively charged As(V) by electrostatic attraction. As is visible, all kinetic adsorption curves could be around segmented into two consecutive adsorption steps; the As(V) adsorption capacities by the MnFe 2 O 4 /D201 were sharply raised throughout the first 60 min and the adsorption capacity would not increase after 240 min. While contact time was passed 240 min, adsorption capacity's raises were nearly insignificant, indicating that 240 min could be considered with the adsorption equilibrium time. The maximum equilibrium adsorption capacities (q e ) are 38 and 38.9 mg/g for As(V), respectively. The rapid adsorption of As(V) may be associated with a lot of directly uncovered active adsorption sites and the porous structure of the MnFe 2 O 4 /D201 [30], which was contributed to the adsorption and diffusion of As(V). After that, unsaturated adsorption sites were filled, and the growth rate of adsorption began to went down [31].
To better investigate and interpret the adsorption behavior, pseudo-first-order [32], pseudo-second-order [33], and Elovich models [34] (Text S1) were analyzed. The parameters derived from adsorption kinetic curves are listed in Table 1. The fitting data in Table 1 indicate that the pseudosecond-order model describes the sorption kinetics better than the pseudo-first-order and Elovich models. The adsorption mechanism corresponding to the pseudo-second-order kinetic is that the control factor of adsorption is chemical interaction, which occurred via electrostatic attraction or complexation between adsorbent and As(V) [35]. The k 2 values of As(V) on D201 and MnFe 2 O 4 /D201 were 0.0004 and 0.0008 g/(mg * min) ( Table 1), respectively. This indicated that MnFe 2 O 4 /D201 had the highest adsorption rate and quickly promoted As(V) removal from aqueous solution. The reason may be that MnFe 2 O 4 on the surface of the MnFe 2 O 4 /D201 is conducive to the adsorption of As(V). The fit of the data to the pseudo-first-order and Elovich models also matches well. The data overall suggest that the rate-limiting kinetic process for the removal of As(V) from solution was mainly the electrostatic attraction and complexation, with a certain degree of diffusion on highly heterogeneous surface of the adsorbent. In a word, the adsorption process may include the following steps: the As(V) migrated from the liquid phase to the outer surface of the MnFe 2 O 4 /D201, continued to migrate on the surface until enter the pores of MnFe 2 O 4 /D201, and loaded to the active site of the MnFe 2 O 4 /D201 through this process until the adsorption reaches saturation.

Adsorption Isotherm
Analysis. The effect of initial As(V) concentration on As(V) adsorption over the surface of MnFe 2 O 4 /D201 was studied as shown in Figures 5(d)-5(f). When the As(V) initial concentration was increased from 30 to 100 mg/L at 298 K, the As(V) adsorption capacity was expanded from 35.8 mg/g to 41 mg/g. That is because the adsorption process enhances in the mass transfer driving force [36]. Then, the adsorption capacity of MnFe 2 O 4 /D201 has not significantly changed as the As(V) concentration increases again, because the active sites of MnFe 2 O 4 /D201 are saturated.
To better understand the interaction mechanisms between As(V) and MnFe 2 O 4 /D201, the isotherm models of Langmuir [37], Freundlich [38], and Temkin [39] (Text S1) were evaluated (Figures 5(c) and 5(d)). The suitability of isotherm fit of experimental data was evaluated using nonlinear chi-square test (χ 2 ) [40] and hybrid fractional error function [41], which are calculated using the following at Text S1. The relation between temperature and equilibrium concentration was examined at varying temperatures (298, 318, and 338 K). The Langmuir model supposed monolayer sorption's structure onto a concrete surface with similar and energetically equivalent active sites, whereas the Freundlich isotherm model thought multilayer sorption's network onto a heterogeneous surface. These models' linear

Adsorbents
The pseudo-first-order The pseudo-second-order Elovich k 1 (min -1 ) q e (mg/g) R 2 k 2 (g/(mg·min)) q e (mg/g)   [42]. Therefore, the adsorption of As(V) ions took place as a monolayer on MnFe 2 O 4 /D201 nanocomposites [43]. When the temperature is 298, 318, and 328 K, the expected maximal capacities of MnFe 2 O 4 /  9 Adsorption Science & Technology D201 were taken from the Langmuir model, and the value is 42.2, 43.9, and 46.4 mg/g, showing that the rise of temperature promotes the progress of adsorption process. Moreover, 1/n (298-338 K) are all under 1, pointing out that MnFe 2 O 4 / D201 can adsorb As(V) spontaneously [44]. The values of R L (0 < R L < 1) for the adsorption of As(V) ions lie between 0.467 and 0.602, indicating the suitability of the sorption process and the excellent affinity between the As(V) and the MnFe 2 O 4 /D201 composite adsorbent. As a result, the adsorption process was single-layer adsorption, suggesting that the MnFe 2 O 4 /D201 surface was homogeneously distributed adsorption active sites [45].

Adsorption
Thermodynamics. Adsorption thermodynamic characteristics were examined at 298-338 K to determine the effect of temperature on As(V) removal capacity. Three thermodynamic parameters, including Gibbs free energy, entropy, and enthalpy, are presented in Table 3 and Figure S4. The entropy and enthalpy were calculated from the incline and intercept of the adsorption data fitted adopting equation (Text S1). The Gibbs free energy as negative values for the As(V) uptake reactions by MnFe 2 O 4 /D201 indicates spontaneity of the adsorption process ( Table 3). As the temperature rose, it was also noticed that the Gibbs free energy value rose, implying a greater driving force and thus a higher adsorption affinity at higher temperatures. The high positive values of enthalpy (5.34 kJ/mol) indicate that As(V) adsorption process over the MnFe 2 O 4 /D201 is endothermic and the adsorption process is involved in chemical interactions [40,46]. The presence of a positive entropy values (0.15 kJ/ (mol * K)) indicates that As(V) variability rises during process and the MnFe 2 O 4 /D201 has a significant affinity for As(V) [41,47]. , PO 4 3-) possess a more substantial affinity for MnFe 2 O 4 /D201 than monovalent anions (Cl -, NO 3 -) in Figure 6(a). The effect of Cland NO 3 -(0.01-0.1 mol/L) on the adsorption capacity of As(V) by using MnFe 2 O 4 /D201 is nearly unchanged. Only when the concentration of Cland NO 3 increases to 1 mol/L, the adsorption capacity would decrease slightly. Compared with Cland NO 3 -, the addition of SO 4 2and PO 4 3leads to an apparent drop in adsorption capacity, but further raising the concentration of the competing anions results in an unvarying q e value at 27.5 mg/g [48]. The As(V) adsorption of MnFe 2 O 4 /D201 reduced in the following order: PO 4 3->SO 4 2->NO 3 ->Cl -. A potential reason is that both SO 4 2and PO 4 3have more charges, which easily make them inhibit the electrostatic attraction between the D201 host and As(V) [49]. At the same time, the inner sphere complex structure between As(V) and MnFe 2 O 4 was unaffected [50]. The adsorption sites on D201 would be affected by the competition between the As(V) and the coexisting anions, thereby having a slight impact on the adsorption capacity of MnFe 2 O 4 /D201.
It shows that MnFe 2 O 4 modification enhanced the selective adsorption on MnFe 2 O 4 /D201. D201 can just capture As(V) through the structure of an outer sphere complex or nonspecific electrostatic attraction. In contrast, the immobilized MnFe 2 O 4 nanoparticles could separate As(V) following ligand exchange or a specific Lewis acid-base interaction. This study's consequences demonstrated that MnFe 2 O 4 / D201 is efficient and selective in removing possibly hazardous As(V) from complex polluted industrial effluents and groundwaters.
3.2.7. Desorption and Reusability Study. In general, the recyclability of adsorbents can enhance the economics of the remediation process. To investigate the recyclability and reusability of adsorbents, the adsorption capacity of MnFe 2 O 4 /D201 was explored for ten cycles. The adsorption capacity is 33.3 mg/g for two reusing runs, higher than 30.4 mg/g for five reusing runs, and higher than 29.1 mg/g for the ten reusing runs (Figure 6(b)). For the adsorbent MnFe 2 O 4 /D201, in addition to keeping the adsorption capacity at a high level, its other advantage over MnFe 2 O 4 is that the material quality of MnFe 2 O 4 /D201 is basically unchanged. In the field of adsorption, most powdered adsorbents have limitations, including the quality loss of adsorbents and the difficult operation on recovery in practical application. Desorption efficiency of adsorbent MnFe 2 O 4 / D201 is shown in Figure 6(c). MnFe 2 O 4 /D201 presented a good desorption efficiency over 81.3% after ten times of adsorption-desorption cycle. The small drop in the removal efficiencies of As(V) ions over MnFe 2 O 4 /D201 nanocomposite was attributed to the damaged adsorption sites after each cycle [51]. This conclusion is consistent with SEM results of MnFe 2 O 4 /D201 before and after adsorption-desorption

10
Adsorption Science & Technology cycle. As shown in Figure 3, the surface of the adsorbent MnFe 2 O 4 /D201 is relatively rough before adsorption. According to the mapping results, the relative percentage contents of N, Mn, and Fe elements at this time are 5%, 14%, and 52%, respectively. After adsorption-desorption cycle, the surface of the adsorbent MnFe 2 O 4 /D201 becomes smoother, and the relative percentage content of N, Mn, and Fe elements has slightly decreased in Figure S5. The smooth surface may be caused by the shedding of loaded nanoparticles MnFe 2 O 4 during the desorption process. The relative percentage contents of N, Mn, and Fe elements are 3%, 11%, and 42%, respectively. The active site of the adsorbent mainly contains N, Mn, and Fe elements. It is obvious that the active site of the adsorbent MnFe 2 O 4 / D201 is destroyed by the desorption agent during the desorption process, and the content of the active groups has slightly decreased. The controlled drops in sorptiondesorption performances indicate the excellent stability and high recyclability of MnFe 2 O 4 /D201.

Fixed-Bed Adsorption.
To further explore MnFe 2 O 4 / D201 applied in wastewater, fixed-bed adsorption toward As(V)-contaminated water was studied according to MnFe 2 O 4 /D201 into the separated fixed-bed columns. As described in Figure 6(d), the bed volume of MnFe 2 O 4 / D201 was about 774 BV (298 K), 1134 BV (318 K), and 1332 BV (338 K) as reported by the breakthrough curves (breakthrough level was 10 μg/L) [52], implying that MnFe 2 O 4 /D201 has good As(V) adsorption capacity in wastewater. Also, it was shown that MnFe 2 O 4 /D201 possessed the potential for As(V) species removal in water. 11 Adsorption Science & Technology -OH functional groups was decreased after adsorption of As(V), suggesting that the surface complexation between -OH functional groups and As(V) substantially contributed a lot to the adsorption process.

Adsorption
The SEM and mapping of MnFe 2 O 4 /D201 after the adsorption process are exhibited in Figure S6. Compared with MnFe 2 O 4 /D201, the surface of MnFe 2 O 4 /D201 has no apparent changes. And As elements appear on the surface of MnFe 2 O 4 /D201 about 3% after adsorption, with decreased Mn and Fe element contents. This result indirectly proves that metal atoms Mn and Fe play a significant part in the adsorption of As(V).
To further realize the adsorption mechanism, XPS spectra were used to probe the chemical state on the surface of the MnFe 2 O 4 /D201 composites before and after adsorption, as shown in Figure 8. The high-resolution spectra of N1s of MnFe 2 O 4 /D201 are illustrated in Figures 8(a) and 8(b). The characteristic peaks at 402.34 and 399.54 eV belong to -N + (CH 3 ) 3 and -NH, respectively. After contact with As(V), -N + (CH 3 ) 3 content is reduced from 90.47% to 76.19%, as shown in Table 4, which is ascribed to the electrostatic attraction between As(V) anions and D201 resin [53,54]. , respectively. After absorbing As(V), the peak area proportion of O 2increased significantly from 42.19% to 50.00%, whereas that of OHdeclined from 50.00% to 41.18%. The results further confirmed that more -OH on the MnFe 2 O 4 /D201 surface reacted with As(V). The number of -OH decreased after the reaction, and the inner sphere complexes were formed by complexation between -OH and As(V), which was more steady than the outer sphere complexes formed by electrostatic. To further analyze the role of MnFe 2 O 4 , the high-resolution XPS spectra of Mn2p and Fe2p were observed (Figures 8(e)-8(h)). As shown in Figures 8(e) and 8(f), the Mn2p peaks exhibited major peaks before adsorption at 656.42, 652.81, 644.47, and 641.11 eV. The Fe2p peaks exhibited major peaks before adsorption at 711.13, 715.07, 724.66, and 731.55 eV. The binding energies of Mn2p and Fe2p shifted after As(V) adsorption, indicating that Mn and Fe species reacted with As(V) [55,56]. After adsorption, a raw peak at 45.25 eV is observed in Figure 8(i), attributed to the As V -O. The As3d's peak indicates that the MnFe 2 O 4 /D201 surface adsorbs a significant amount of As(V) during the adsorption process.
Concentrating on the experimental data under varying pH, the adsorption capacity of MnFe 2 O 4 /D201 decreases with the increase in pH. The physical electrostatic repulsion enhanced due to the deprotonation of sorbents accompanied  These above consequences confirmed that the adsorption mechanism was electrostatic attraction and complexation. The results were also consistent with the isotherm and kinetic study. Besides, the schema of As(V) removal mechanisms by MnFe 2 O 4 /D201 is illustrated in Figure 9. First, D201 would increase the zeta potential of MnFe 2 O 4 /D201, which is more favorable for electrostatic attraction with As(V). Second, MnFe 2 O 4 modified the surface structure of MnFe 2 O 4 /D201 and provided more functional groups such as -OH, contributing to complexation with As(V).

Conclusion
In this study, the successful combination of the two components (MnFe 2 O 4 and D201) of the nanocomposite MnFe 2 O 4 /D201 was confirmed by various characterization techniques. Different parameters such as pH, contact time, and initial concentration were examined to define the optimum adsorption conditions. The adsorption of As(V) is affected by the pH value, with As(V) adsorption being the most effective at pH 3. The kinetics and adsorption isotherms of MnFe 2 O 4 /D201 for As(V) followed the Langmuir isotherm model and pseudo-second-order kinetic model, respectively. The adsorption behavior is controlled by chemical adsorption. The maximal adsorption capacity of MnFe 2 O 4 /D201 was 35.8 mg/g, and it attained equilibrium within 240 min. Thermodynamic results showed that the adsorption process was endothermic and spontaneous. The magnetic separation of MnFe 2 O 4 /D201 enhanced its reusability for As(V) removal up to ten successive cycles. Finally, the superior performance in the fixed-bed column experiments confirmed the potential use of MnFe 2 O 4 / D201 as an excellent adsorbent. The efficient As(V) removal by MnFe 2 O 4 /D201 is based on the combined electrostatic attraction and complexation. In summary, because of its facile fabrication process, excellent selectivity adsorption capacity, and deep treatment for As(V) removal, MnFe 2 O 4 /D201 has promising application capability for As(V) sequestration from polluted water.

Data Availability
The data used to support the findings of this study are included within the supplementary information file(s).