Synthesis and Characterization of Magnetic Xerogel Monolith as an Adsorbent for As(V) Removal from Groundwater

: Arsenic contamination of groundwater is still a global problem due to the toxicity at low dose on human health conﬁrmed by epidemiological studies. Magnetic xerogel monoliths (MXs) were synthesized by the sol-gel polymerization using resorcinol, formaldehyde, alkaline catalyst and magnetite. The varying molar ratios of magnetite and resorcinol (M/R) in the gel were evaluated for As(V) removal from groundwater. The surface chemistry, structure and morphology of MXs related to arsenic adsorption were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and point of zero charge. Batch adsorption experiments were carried out to investigate the effects of Fe contents, initial pH and adsorbent dose on As(V) removal performance. The MXs with molar ratio of M/R at 0.15 gave the maximum As(V) adsorption capacity and removal with values of 62.8 µ g/g and 86.7%, respectively. The adsorption data were well described by the Elovich equation of the kinetic model and the Freundlich isotherm. The thermodynamic studies demonstrated that the adsorption process was endothermic and spontaneous in nature. MXs showed to be a good alternative for As(V) removal from groundwater and achieving the efﬁcient desorption, and thus fulﬁlled the Mexican standard for drinking water. the equation and the PFO The experimental to the Freundlich isotherm. Thermodynamic study conﬁrmed that the adsorption is endothermic and in to of free and value of enthalpy change. Desorption results showed that the desorption efﬁciency


Introduction
Water is an essential requirement for human beings and living things. The quality of water resources is altered by the source of natural and anthropogenic pollution. The contamination of aqueous effluents is a serious problem that directly and indirectly alters the balance of different ecosystems and affects human health. Arsenic is one of substances with potential health risk due to its toxicology and carcinogenicity. Mostly arsenic is found in groundwater caused from its distribution to the earth's crust in the naturally occurring source by geology, hydrogeology, and geochemistry especially in arid and semi-arid regions [1,2]. The main anthropogenic sources of arsenic are caused from mining, smelting and ore beneficiation processes, petroleum refining, coal burning and industries of agricultural chemicals (pesticides and insecticides) [2,3]. Arsenic in natural water is mostly presented in inorganic forms, such as arsenate As(V) and arsenite As(III). As(V) and As(III) can be found in well oxygenated (aerobic) surface water and anaerobic groundwater, respectively. Arsenic contamination of groundwater is still a health problem in the world. Millions of people in Argentina, Bangladesh, Chile, China, Hungary, India, Mexico, Mongolia, Taiwan, Thailand, Vietnam, and the USA directly affect this issue [4,5]. The health risk assessment for human exposure is concerned due to the distribution of inorganic arsenic species through drinking water. Especially the vulnerable and sensitive groups like children, pregnant women, and newborns, can lead to significant not only All the reagents used to synthesize magnetic xerogels were analytical grade which were 1, 3-dihydroxybenzene C 6 H 4 (OH) 2 (Chemistry Meyer, Mexico City, Mexico), sodium carbonate Na 2 CO 3 (J.T. Baker, Mexico City, Mexico), formaldehyde HCHO (J.T. Baker, Mexico City, Mexico), acetone (CH 3 ) 2 CO, 99.5% (J.T. Baker, Mexico City, Mexico) and magnetite Fe 3 O 4 (Lanxess, García, Nuevo León, Mexico). The preparation of As(V) solution for batch adsorption experiment using sodium arsenate dibasic heptahydrate HAsNa 2 O 4 ·7H 2 O (Sigma-Aldrich, Toluca, Mexico State, Mexico).

Synthesis of Magnetic Xerogel Monoliths
The synthesis of MXs was attended to generate the incorporation and dispersion of magnetite in the RF gel structure with indirect sonication method before the process of gelation and curing. Due to formaldehyde is one chemical for gel preparation and its high reactivity, by the way it is possible to aid the precipitation of metal ions (Fe 3+ ) under metallic state that result in the agglomeration of metal and nonhomogeneous of gel properties [38]. The preparation of MXs was carried out following the method described by Khamkure et al. [20]. In this synthesis, the molar ratios used were: R/F = 0.5, R/C = 100 and R/W = 0.04. The MXs were prepared varying the Fe contents of magnetite (M) and their molar ratios (M/R) were 0.05, 0.1, 0.15, and 0.2. The indirect sonication method applied was the ultrasonic energy transmitted from the probe through water and tubes for homogenization and deagglomeration magnetite particles into RF aqueous solution. After sonication, the MXs were produced, and they could be checked by visual observation. The glass tubes were cured 3 days at 80 • C, followed with 2 days of solvent exchange at room temperature by using a shaking water bath (BS-11; Lab companion). After vacuum filtration, the replacement of fresh solvent was applied daily. Then the drying gels were applied for 3 days in an oven at 80 • C. Finally, materials obtained were black in color and labelled as magnetic xerogel monoliths (MXs).

Characterization of Magnetic Xerogel Monoliths
The synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR) and measurement of the pH at the point of zero charge (pH PZC ) and the isoelectric point (IEP). The Fe contents in the synthesized MXs were analyzed by using an atomic absorption spectrometer Perkin Elmer Analyst 400 (Shelton, Connecticut, USA). XRD was performed on a D8 ADVANCE model of the Bruker brand (Karisruhe, Germany). The SEM and EDX techniques were analyzed by using a JSM-IT300 (Tokyo, Japan), which were coated with gold. Surface chemistry of materials was obtained by pH PZC and the isoelectric point IEP, which is determined by potentiometric titrations and electrokinetic measurements, respectively, following the method described by Amaringo and Anaguano [39]. A zeta potential analyzer Microtrac PMX 500 (Meerbuch, Germany) was used to measure the zeta potential to determine IEP and pH meter Orion Star A211 (Thermo Scientific, USA) was used to measure pH solution with a pH range (2)(3)(4)(5)(6)(7)(8)(9)(10). FTIR was carried out through a spectrophotometer Shimadzu IRAffinity-1S (Shimadzu Corp., Japan) to determine the surface functional groups that are in the synthesized organic xerogels. The FTIR analysis of the three samples was carried out before and after exposure of As. The samples in powder form were dried overnight at 60 • C before placement in a desiccator awaiting analysis. The infrared spectra results were acquired between the wave number of 400-4000 cm −1 with 45 scans per sample. Attenuated total reflection (ATR, Specac; GS10800) with the crystal puck (type IIIa monocrystalline diamond) was installed and fitted to the optical unit of the FTIR system.

Batch Adsorption Experiments for As(V)
In this experiment, the groundwater sample was obtained from a water well in Morelos, Mexico and was analyzed as stated by a standard method [40]. The batch adsorption was performed using a solution of As(V) by diluting to a required concentration with groundwater. All the experiments, MXs were applied 100 mg (dose 2 g/L) with 50 mL of As(V) solution in the centrifuge tube, except the study of adsorbent dose. All the solution tubes were shaken by using a shaking water bath at 150 rpm at room temperature (25 ± 2 • C). Afterwards, each solution was filtered through a vacuum pump with 0.45 µm of the cellulose nitrate membrane filters (Whatman) to separate the solid from the supernatant. The residual concentration of As(V) was performed on an Inductively coupled plasma atomic emission spectroscopy (ICP-AES), OPTIMA 3200 DV model Perkin Elmer brand, using the hydride generation method with KI and Ascorbic Acid as pre-reducing agents.
The batch adsorption experiments were conducted and evaluated the effect of Fe contents of eight samples of MXs. For studying the effect of pH in aqueous solution, the initial pH of the solutions was adjusted from 2 to 10 by using 0.1 M HCl and 0.1 M NaOH solutions. Effect of adsorbent dosage was studied to determine the optimum dosage of adsorbent to the percentage of arsenic removal and was varied with different dosage from 0.5 to 4 g/L. The adsorption kinetics was performed for the removal As(V) with respect to different contact time intervals. The isotherm adsorption experiment was studied by varying initial As(V) concentration from 25 to 175 mg/L. Thermodynamic analysis was also investigated with 3 different temperatures which are 25, 35, and 45 • C.
All the results from the experiment were calculated and evaluated as the percentage of arsenic removal and the adsorption capacity. The removal efficiency of As(V) can be calculated as follows: where C 0 and C e are the concentration of As(V) in aqueous solution before and after adsorption (mg/L), respectively. The As(V) adsorption capacity, q e (mg/g) is defined as follows: where V is the volume of aqueous solution (L) and m is the mass of MXs (g). Previous studies have been reported that an invaluable nonlinear regression analysis is a better way to obtain the equilibrium isotherm parameters and coefficients for describing the adsorption [41][42][43]. Therefore, in this study the adsorption kinetic and isotherm were used for mathematical description and empirical models that fit to the experimental data. The models and errors of analysis of these studied data were applied to a nonlinear least squares regression method using the QtiPlot v0.9.8.3 program. The correlation coefficient (R 2 ), the residual sum of squares (RSS) also known as the sum of squared estimate of errors (SSE) and the root mean square error (RMSE) were calculated following the equations described by Hanbali et al. [44] to represent the compatibility of the model equations.

Desorption Study
Due to this research focuses on generating magnetic materials that can be easily recovered from the aqueous medium. The purpose of desorption study is important to evaluate the possibility for reusability and regenerability of the used material. Moreover, it is useful for practical applications of this adsorbent in water and wastewater treatment.
The used material from kinetic study was applied to desorption process that had an initial As(V) concentration of 75 µg/L. One hundred milligrams of As-adsorbed MXs was added in the centrifuge tube with 50 ml of alkaline solution with different time interval and concentrations. The plastic tubes were centrifuged using centrifuge (Eppendorf; model

Behavior of Fe Contents in Magnetic Xerogel Monoliths on As(V) Removal
The MXs were prepared with different magnetite loadings into the xerogels and their molar ratios of M/R were 0.05, 0.1, 0.15, and 0.2. The Fe contents in the synthesized MXs were determined using an atomic absorption spectrometer and are presented in Figure 1. It can be observed that percent of Fe contents increased continuously following the increase of molar ratios of M/R (0.05 to 0.2) and were approximately five times on the increase from the molar ratio of M/R at 0.05 to 0.15.
The As removal of MXs prepared with molar ratio of M/R at 0.05 and 0.1 had similar percentages of Fe contents which were 2.73% and 3.99%, respectively ( Figure 1). The increase of the Fe contents at 14.27% was found to be significant, when it resulted in a high As removal (77.60%). MXs synthesized with M/R at 0.2 gave the lower of As removal (57%) even contained the highest of the Fe contents (16.47%). It can be explained that the higher Fe incorporation of RF polymer might affect the saturation of arsenic into active sites on the surface of MXs. Moreover, it can be explained that some magnetite particles were leached out during the process of solvent exchange with acetone due to their particles being unbound to the polymeric matrix during synthesis [29]. Therefore, the MXs with M/R at 0.15 contained 14.27% of Fe contents was selected for characterization and applied for studying the batch adsorption for As(V) removal, which is shown later.

Behavior of Fe Contents in Magnetic Xerogel Monoliths on As(V) Removal
The MXs were prepared with different magnetite loadings into the xerogels and their molar ratios of M/R were 0.05, 0.1, 0.15, and 0.2. The Fe contents in the synthesized MXs were determined using an atomic absorption spectrometer and are presented in Figure 1. It can be observed that percent of Fe contents increased continuously following the increase of molar ratios of M/R (0.05 to 0.2) and were approximately five times on the increase from the molar ratio of M/R at 0.05 to 0.15.
The As removal of MXs prepared with molar ratio of M/R at 0.05 and 0.1 had similar percentages of Fe contents which were 2.73% and 3.99%, respectively ( Figure 1). The increase of the Fe contents at 14.27% was found to be significant, when it resulted in a high As removal (77.60%). MXs synthesized with M/R at 0.2 gave the lower of As removal (57%) even contained the highest of the Fe contents (16.47%). It can be explained that the higher Fe incorporation of RF polymer might affect the saturation of arsenic into active sites on the surface of MXs. Moreover, it can be explained that some magnetite particles were leached out during the process of solvent exchange with acetone due to their particles being unbound to the polymeric matrix during synthesis [29]. Therefore, the MXs with M/R at 0.15 contained 14.27% of Fe contents was selected for characterization and applied for studying the batch adsorption for As(V) removal, which is shown later.

XRD Analysis
The mineralogical characteristics of MXs were determined by XRD, as shown in Figure 2. The XRD profiles of the MXs with different M/R ratios had similar characterization peak patterns. The patterns of XRD show that MXs contained both of the phase of amorphous and crystalline similarly reported by Oyedoh et al. [22]. The crystalline structure of MXs was composed mainly of magnetite that had similar the diffraction peaks at 2θ values of 18°, 30°, 35.5°, 43°, 57°, and 62° which were corresponding to 111, 220, 311, 400, 511,

XRD Analysis
The mineralogical characteristics of MXs were determined by XRD, as shown in Figure 2. The XRD profiles of the MXs with different M/R ratios had similar characterization peak patterns. The patterns of XRD show that MXs contained both of the phase of amorphous and crystalline similarly reported by Oyedoh et al. [22]. The crystalline structure of MXs was composed mainly of magnetite that had similar the diffraction peaks at 2θ values of 18 • , 30 • , 35.5 • , 43 • , 57 • , and 62 • which were corresponding to 111, 220, 311, 400, 511, and 440 of the crystallographic planes of magnetite (ICDD card no. 00-01900629) according to the results of Tipsawat et al. [45]. It can be observed that MXs with low M/R ratio had low-intensity and broadened diffraction peaks than MXs with high M/R ratio. Thus, it can be indicated that MXs with high M/R ratio had the crystalline structure.
Processes 2021, 9, x FOR PEER REVIEW 6 of 15 and 440 of the crystallographic planes of magnetite (ICDD card no. 00-01900629) according to the results of Tipsawat et al. [45]. It can be observed that MXs with low M/R ratio had low-intensity and broadened diffraction peaks than MXs with high M/R ratio. Thus, it can be indicated that MXs with high M/R ratio had the crystalline structure.

SEM Analysis
The morphology of MXs is presented in Figure 3a that MXs contained connecting microclusters and resulted in a porous structure of RF gels [22,23]. Figure 3b shows that some parts of the surface morphology of MXs had the accumulation that might be caused from the unbound of Fe contents in RF gels before the curing process. It can be explained that magnetite did not cooperate into RF gels and Fe contents in RF gels are a physical dispersion. This finding agrees to the results reported by Verma et al. [29]. Mapping analysis Figure 3c shows the distribution of Fe that was uniformly distributed throughout the RF gels [30]. EDX indicates the elemental composition of an interesting area of MXs sample as shown in Figure 3d. The presence of Fe contents can be confirmed to be approximately 29.75% within the RF gel.

SEM Analysis
The morphology of MXs is presented in Figure 3a that MXs contained connecting microclusters and resulted in a porous structure of RF gels [22,23]. Figure 3b shows that some parts of the surface morphology of MXs had the accumulation that might be caused from the unbound of Fe contents in RF gels before the curing process. It can be explained that magnetite did not cooperate into RF gels and Fe contents in RF gels are a physical dispersion. This finding agrees to the results reported by Verma et al. [29]. Mapping analysis Figure 3c shows the distribution of Fe that was uniformly distributed throughout the RF gels [30]. EDX indicates the elemental composition of an interesting area of MXs sample as shown in Figure 3d. The presence of Fe contents can be confirmed to be approximately 29.75% within the RF gel. and 440 of the crystallographic planes of magnetite (ICDD card no. 00-01900629) according to the results of Tipsawat et al. [45]. It can be observed that MXs with low M/R ratio had low-intensity and broadened diffraction peaks than MXs with high M/R ratio. Thus, it can be indicated that MXs with high M/R ratio had the crystalline structure.

SEM Analysis
The morphology of MXs is presented in Figure 3a that MXs contained connecting microclusters and resulted in a porous structure of RF gels [22,23]. Figure 3b shows that some parts of the surface morphology of MXs had the accumulation that might be caused from the unbound of Fe contents in RF gels before the curing process. It can be explained that magnetite did not cooperate into RF gels and Fe contents in RF gels are a physical dispersion. This finding agrees to the results reported by Verma et al. [29]. Mapping analysis Figure 3c shows the distribution of Fe that was uniformly distributed throughout the RF gels [30]. EDX indicates the elemental composition of an interesting area of MXs sample as shown in Figure 3d. The presence of Fe contents can be confirmed to be approximately 29.75% within the RF gel.

pHPZC and IEP Analysis
As is well known, the analysis of pHPZC and IEP is a technique to determine the nature of the surface chemistry of the adsorbents. The pHPZC corresponds to the pH at which the number of positive and negative charges is equal, so that the total surface charge is zero then the surface of adsorbent is neutral. The pHPZC of the MXs was 6.28. Therefore, the positive charge develops on the adsorbent surface of MXs at pH lower 6.28 and the MXs surface becomes negatively charged at pH above 6.28. IEP is the point that the MXs exhibit no net charge. The IEP of the MXs was 2.1. The surface charge of the composite material is important and influential in the process of the adsorption [30].

FTIR Analysis
The FTIR study was used to know the functional groups in the structure of MXs before and after the adsorption of As(V). The FTIR spectra of MXs samples are plotted and represented in Figure 4. The resultant infrared spectra were acquired and showed the major of six adsorption bands at 3227 cm −1 (-OH group), 2639 cm −1 (C-H stretching), 1605 cm −1 (C=C aromatic stretching), 1464 cm −1 (C-H bending vibration), 1209 cm −1 (C-O stretching), and 548 cm −1 (Fe-O stretching vibration), respectively. The obtained spectra peaks agreed with those of similar reported research [15,29,[46][47][48].
After As(V) adsorption, a broad band of OH was shifted to 2989 and 2903 cm −1 . It can be explained that the influence of AsO sorption and thus OH vibrations around 3300 and 3000 cm −1 developed due to the adsorption and coprecipitation [49]. It can be observed that the presence of the new band appeared with small intensity at 898 cm −1 belonging to As-O stretching vibration [50].

pH PZC and IEP Analysis
As is well known, the analysis of pH PZC and IEP is a technique to determine the nature of the surface chemistry of the adsorbents. The pH PZC corresponds to the pH at which the number of positive and negative charges is equal, so that the total surface charge is zero then the surface of adsorbent is neutral. The pH PZC of the MXs was 6.28. Therefore, the positive charge develops on the adsorbent surface of MXs at pH lower 6.28 and the MXs surface becomes negatively charged at pH above 6.28. IEP is the point that the MXs exhibit no net charge. The IEP of the MXs was 2.1. The surface charge of the composite material is important and influential in the process of the adsorption [30].

FTIR Analysis
The FTIR study was used to know the functional groups in the structure of MXs before and after the adsorption of As(V). The FTIR spectra of MXs samples are plotted and represented in

pHPZC and IEP Analysis
As is well known, the analysis of pHPZC and IEP is a technique to determine the nature of the surface chemistry of the adsorbents. The pHPZC corresponds to the pH at which the number of positive and negative charges is equal, so that the total surface charge is zero then the surface of adsorbent is neutral. The pHPZC of the MXs was 6.28. Therefore, the positive charge develops on the adsorbent surface of MXs at pH lower 6.28 and the MXs surface becomes negatively charged at pH above 6.28. IEP is the point that the MXs exhibit no net charge. The IEP of the MXs was 2.1. The surface charge of the composite material is important and influential in the process of the adsorption [30].

FTIR Analysis
The FTIR study was used to know the functional groups in the structure of MXs before and after the adsorption of As(V). The FTIR spectra of MXs samples are plotted and represented in Figure 4. The resultant infrared spectra were acquired and showed the major of six adsorption bands at 3227 cm −1 (-OH group), 2639 cm −1 (C-H stretching), 1605 cm −1 (C=C aromatic stretching), 1464 cm −1 (C-H bending vibration), 1209 cm −1 (C-O stretching), and 548 cm −1 (Fe-O stretching vibration), respectively. The obtained spectra peaks agreed with those of similar reported research [15,29,[46][47][48].
After As(V) adsorption, a broad band of OH was shifted to 2989 and 2903 cm −1 . It can be explained that the influence of AsO sorption and thus OH vibrations around 3300 and 3000 cm −1 developed due to the adsorption and coprecipitation [49]. It can be observed that the presence of the new band appeared with small intensity at 898 cm −1 belonging to As-O stretching vibration [50].  After As(V) adsorption, a broad band of OH was shifted to 2989 and 2903 cm −1 . It can be explained that the influence of AsO 3− 4 sorption and thus OH vibrations around 3300 and 3000 cm −1 developed due to the adsorption and coprecipitation [49]. It can be observed that the presence of the new band appeared with small intensity at 898 cm −1 belonging to As-O stretching vibration [50]. Table 1 presents the properties of physical and chemical groundwater. The values were lower than those established by the official Mexican standard (NOM-127-SSA1-1994). In this study, groundwater was applied as the aqueous medium in the experiment contained natural anions such as SO 2− 4 , Cl − , NO − 3 , and PO 3− 4 were 37, 10.1, 4.4 and 0.82 mg/L, respectively, towards the effect of coexisting ions for the removal of As onto MXs. Mandal et al. [51] described the reduction on arsenic adsorption because of competitive ions by anions SO 2− 4 > Cl − > CO 2− 3 > HCO − 3 > NO − 3 .

Effect of Initial pH on As(V) Removal
The altering of pH of the solution directly affects and changes the surface chemistry of the adsorbent. The effect of initial pH (2-10) of 100 µg/L of As(V) solution on the capacity of adsorption by using magnetic xerogel was investigated, as shown in Figure 5. The percentages of As(V) removal at different pH were variable. The maximum of As(V) removal (65%) was observed at pH = 5. Percentage of As(V) removal decreased at pH 3, 4, 7 and 9, which is similar to the results of Khamkure et al. [52]. The percentage of As(V) removal efficiency of MXs were high values under acidic (pH = 5) and basic (pH = 8) conditions. These results agreed with those reported of Min X. et al. [15] and Wei Z. et al. [53] indicated that the predominant arsenate species are H 2 AsO − 4 should be the major As(V) species at pH 4 due to the adsorption abilities of arsenic species, water and OH − follow the order of AsO 3− 4 > OH − > HAsO 2− 4 > H 2 AsO − 4 > H 3 AsO 4 > H 2 O for As(V). The observed in the As(V) adsorption that was low at high pH solution. It could be explained regarding the increasing repulsion between the increasing of negatively charged As(V) and negatively charged surface site of iron oxide, and also to an increase in competing of hydroxide ions for adsorption sites [43].  Table 1 presents the properties of physical and chemical groundwater. The values were lower than those established by the official Mexican standard (NOM-127-SSA1-1994). In this study, groundwater was applied as the aqueous medium in the experiment contained natural anions such as SO , Cl , NO , and PO were 37, 10.1, 4.4 and 0.82 mg/L, respectively, towards the effect of coexisting ions for the removal of As onto MXs. Mandal et al. [51] described the reduction on arsenic adsorption because of competitive ions by anions SO > Cl > CO > HCO > NO .

Effect of Initial pH on As(V) Removal
The altering of pH of the solution directly affects and changes the surface chemistry of the adsorbent. The effect of initial pH (2-10) of 100 µg/L of As(V) solution on the capacity of adsorption by using magnetic xerogel was investigated, as shown in Figure 5. The percentages of As(V) removal at different pH were variable. The maximum of As(V) removal (65%) was observed at pH = 5. Percentage of As(V) removal decreased at pH 3, 4, 7 and 9, which is similar to the results of Khamkure et al. [52]. The percentage of As(V) removal efficiency of MXs were high values under acidic (pH = 5) and basic (pH =8 ) conditions. These results agreed with those reported of Min X. et al. [15] and Wei Z. et al. [53] indicated that the predominant arsenate species are H AsO should be the major As(V) species at pH 4 due to the adsorption abilities of arsenic species, water and OH follow the order of AsO > OH > HAsO > H AsO > H AsO > H O for As(V). The observed in the As(V) adsorption that was low at high pH solution. It could be explained regarding the increasing repulsion between the increasing of negatively charged As(V) and negatively charged surface site of iron oxide, and also to an increase in competing of hydroxide ions for adsorption sites [43]. In this study, the effect of adsorbent dose on the removal of As(V) was conducted to determine the optimum mass of adsorbent regarding the adsorptive capacity. The varying

Effect of Adsorbent Dose on the Removal of As(V)
In this study, the effect of adsorbent dose on the removal of As(V) was conducted to determine the optimum mass of adsorbent regarding the adsorptive capacity. The varying of adsorbent doses was 0.5-10 g/L, in 50 mL of 100 µg/L of As(V) solution. The obtained results between adsorbent dose vs As(V) removal and q e are presented in Figure 6. It can be seen that the percentage of As(V) removal increased from 35% to 76% with the increased adsorbent dose from 1 to 2 g/L and after that it seems that percentages of As removal (65%) were relatively constant although the adsorbent dose increased. It can be explained that the particle size of magnetite added into the gels influence the adsorption process due to effective particle size change with increasing adsorbent dose [54]. According to the particle size analysis of magnetite by using Laser Scattering Particle Size Distribution Analyzer (LA-950, Horiba), the mean size was 9.27 µm which was larger than average pore diameter of the RF gel which is in the range of the mesopore (approximately 50 nm). As in the case of magnetite particle size on the adsorption and desorption, affect the behavior of arsenite and arsenate, the adsorption capacity of arsenite and arsenate increases with decreasing magnetite particle size described by Yean et al. [55].
Processes 2021, 9, x FOR PEER REVIEW 9 of 15 of adsorbent doses was 0.5-10 g/L, in 50 mL of 100 µg/L of As(V) solution. The obtained results between adsorbent dose vs As(V) removal and qe are presented in Figure 6. It can be seen that the percentage of As(V) removal increased from 35% to 76% with the increased adsorbent dose from 1 to 2 g/L and after that it seems that percentages of As removal (65%) were relatively constant although the adsorbent dose increased. It can be explained that the particle size of magnetite added into the gels influence the adsorption process due to effective particle size change with increasing adsorbent dose [54]. According to the particle size analysis of magnetite by using Laser Scattering Particle Size Distribution Analyzer (LA-950, Horiba), the mean size was 9.27 µm which was larger than average pore diameter of the RF gel which is in the range of the mesopore (approximately 50 nm). As in the case of magnetite particle size on the adsorption and desorption, affect the behavior of arsenite and arsenate, the adsorption capacity of arsenite and arsenate increases with decreasing magnetite particle size described by Yean et al. [55].

Kinetic Adsorption Models
In order to understand the reaction pathway and the mechanism of adsorption process, the change of the amount of As(V) on solution that was removed following the adsorption time, were observed until reach to the equilibrium. Several kinetic models were used to determine the constant of kinetic adsorption. É.C. Lima et al. [56] summarized kinetic models based on the order of reaction and the most used for adsorption analysis are a pseudo first order (PFO) and pseudo second order (PSO) kinetic adsorption models. Therefore, four kinetic models PFO, PSO, Elovich and Power function were applied to fit the kinetic data by non-linear regression to describe the As(V) adsorption with MXs adsorbent, as follows [57]: Pseudo first-order equation: Pseudo second-order model: = Elovich: = ln( ) + ln ( ) Power function: = where qt and qe are the amount of As(V) adsorbed at time t (mg/g) and the equilibrium adsorption capacity (mg/g), respectively. k1 and k2 are the pseudo first-order rate constant

Kinetic Adsorption Models
In order to understand the reaction pathway and the mechanism of adsorption process, the change of the amount of As(V) on solution that was removed following the adsorption time, were observed until reach to the equilibrium. Several kinetic models were used to determine the constant of kinetic adsorption. É.C. Lima et al. [56] summarized kinetic models based on the order of reaction and the most used for adsorption analysis are a pseudo first order (PFO) and pseudo second order (PSO) kinetic adsorption models. Therefore, four kinetic models PFO, PSO, Elovich and Power function were applied to fit the kinetic data by non-linear regression to describe the As(V) adsorption with MXs adsorbent, as follows [57]: Pseudo first-order equation : q t = q e 1 − e −k 1 t Pseudo second-order model : q t = k 2 q 2 e t 1 + k 2 q e t Elovich : q t = 1 Processes 2021, 9, 386 10 of 15 Power function : q t = at b (7) where q t and q e are the amount of As(V) adsorbed at time t (mg/g) and the equilibrium adsorption capacity (mg/g), respectively. k 1 and k 2 are the pseudo first-order rate constant (min −1 ) and the pseudo second-order rate constant (min −1 ), respectively and t is the contact time (min). α (mg/g min) is a constant related to chemisorption rate and β (g/mg) is a constant of the extent of surface coverage, finally a and b are constants. A non-linear regression analysis was used to compute the experimental data and solve the adsorption kinetic models. The adsorption kinetics study with four kinetic models PFO, PSO, Elovich, and Power function are shown in Figure 7 and the corresponding parameter estimates and correlation coefficients are presented in Table 2. It can be observed that the Elovich equation was best described on As(V) adsorption data due to higher R 2 value (0.94), and lower RSS and RMSE values that are considered to represent goodness of conformity. The Elovich equation is generally applied to chemisorption kinetic for adsorbing to heterogeneous surfaces and has covered a wide range of slow adsorption rates [56]. Due to the result of kinetics of liquid phase batch adsorption experiments, it presents the slow stage of As(V) adsorption following an Elovich equation. This result agrees with a study reported by Zhang and Stanforth [54] indicating that may be caused by the heterogeneity of surface binding sites or other reactions for the removal of arsenic. Moreover, the equilibrium adsorption was fitted well to the power function equation (R 2 = 0.92). The R 2 values were relatively lower for the pseudo second-order equation (0.75) and the pseudo first-order equation (0.50).
Processes 2021, 9, x FOR PEER REVIEW 10 of 15 (min −1 ) and the pseudo second-order rate constant (min −1 ), respectively and t is the contact time (min). α (mg/g min) is a constant related to chemisorption rate and β (g/mg) is a constant of the extent of surface coverage, finally a and b are constants. A non-linear regression analysis was used to compute the experimental data and solve the adsorption kinetic models. The adsorption kinetics study with four kinetic models PFO, PSO, Elovich, and Power function are shown in Figure 7 and the corresponding parameter estimates and correlation coefficients are presented in Table 2. It can be observed that the Elovich equation was best described on As(V) adsorption data due to higher R 2 value (0.94), and lower RSS and RMSE values that are considered to represent goodness of conformity. The Elovich equation is generally applied to chemisorption kinetic for adsorbing to heterogeneous surfaces and has covered a wide range of slow adsorption rates [56]. Due to the result of kinetics of liquid phase batch adsorption experiments, it presents the slow stage of As(V) adsorption following an Elovich equation. This result agrees with a study reported by Zhang and Stanforth [54] indicating that may be caused by the heterogeneity of surface binding sites or other reactions for the removal of arsenic. Moreover, the equilibrium adsorption was fitted well to the power function equation (R 2 = 0.92). The R 2 values were relatively lower for the pseudo second-order equation (0.75) and the pseudo first-order equation (0.50).

Adsorption Isotherm
The adsorption isotherm is the relation of amount of solute adsorbed per unit of the adsorbents reaching to the equilibrium concentration at the constant temperature. Several models can be applied to describe the experimental adsorption data and the most common models like Langmuir and Freundlich equations are widely used for adsorption isotherm.

Adsorption Isotherm
The adsorption isotherm is the relation of amount of solute adsorbed per unit of the adsorbents reaching to the equilibrium concentration at the constant temperature. Several models can be applied to describe the experimental adsorption data and the most common models like Langmuir and Freundlich equations are widely used for adsorption isotherm.
The Langmuir isotherm considers sorption as a chemical phenomenon and describes the monolayer adsorption of a solute onto a homogeneous surface with a finite number of similar active sites. The Freundlich isotherm describes multilayer adsorption on the heterogeneous surface having a non-uniform energy distribution of the active sites [29,44]. Therefore, Langmuir and Freundlich isotherm models were applied to fit the isotherm data by non-linear regression to describe the relative As(V) concentrations adsorbed to MXs adsorbent as follows [57]: Freundlich isotherm : q e = KCe 1 n (9) where q e is the amount of As(V) ions adsorbed by the adsorbent (mg/g), Ce is the equilibrium concentration of adsorbate in solution (mg/L), q m is the maximum adsorption capacity (mg/g), and b is the Langmuir constant that is related to the energy of adsorption (L/mg). K and n are Freundlich constants indicating the measurement of the adsorption capacity and intensity, respectively. The adsorption isotherm for As(V) removal using MXs, was studied by varying As(V) solution with different concentrations (25-175 µg/L) and the fixed of MXs dose (2 g/L) at 25 • C. The plot of q e versus Ce of As(V) adsorption can be determined parameters of the isotherm models [41]. The Langmuir and Freundlich isotherms for As(V) adsorption is presented in Figure 8. It can be observed that the adsorption capacity of As(V) increased with the increase of the As(V) concentration. The isotherm parameters and the correlation coefficients are shown in Table 3. It can be observed that a similar value of R 2 was found for Langmuir and Freundlich isotherm, corresponding to the results reported by Perdigoto et al. [35]. Furthermore, the Freundlich model indicated as a better fit to describe the experimental equilibrium data of As(V) adsorption using MXs due to the higher value of R 2 . It is apparent that the As(V) adsorption is similar to having the adsorption onto a heterogeneous surface rather than monolayer adsorption. This result was in good agreement with the study of adsorption kinetic as discussed above that the slow stage of arsenate adsorption follows an Elovich equation due to the heterogeneity of the surface. The Langmuir isotherm considers sorption as a chemical phenomenon and describes the monolayer adsorption of a solute onto a homogeneous surface with a finite number of similar active sites. The Freundlich isotherm describes multilayer adsorption on the heterogeneous surface having a non-uniform energy distribution of the active sites [29,44]. Therefore, Langmuir and Freundlich isotherm models were applied to fit the isotherm data by non-linear regression to describe the relative As(V) concentrations adsorbed to MXs adsorbent as follows [57]: Freundlich isotherm: qe = KCe (9) where qe is the amount of As(V) ions adsorbed by the adsorbent (mg/g), Ce is the equilibrium concentration of adsorbate in solution (mg/L), qm is the maximum adsorption capacity (mg/g), and b is the Langmuir constant that is related to the energy of adsorption (L/mg). K and n are Freundlich constants indicating the measurement of the adsorption capacity and intensity, respectively. The adsorption isotherm for As(V) removal using MXs, was studied by varying As(V) solution with different concentrations (25-175 µg/L) and the fixed of MXs dose (2 g/L) at 25 °C. The plot of qe versus Ce of As(V) adsorption can be determined parameters of the isotherm models [41]. The Langmuir and Freundlich isotherms for As(V) adsorption is presented in Figure 8. It can be observed that the adsorption capacity of As(V) increased with the increase of the As(V) concentration. The isotherm parameters and the correlation coefficients are shown in Table 3. It can be observed that a similar value of R 2 was found for Langmuir and Freundlich isotherm, corresponding to the results reported by Perdigoto et al. [35]. Furthermore, the Freundlich model indicated as a better fit to describe the experimental equilibrium data of As(V) adsorption using MXs due to the higher value of R 2 . It is apparent that the As(V) adsorption is similar to having the adsorption onto a heterogeneous surface rather than monolayer adsorption. This result was in good agreement with the study of adsorption kinetic as discussed above that the slow stage of arsenate adsorption follows an Elovich equation due to the heterogeneity of the surface.

Adsorption Thermodynamics
In this experiment, adsorption thermodynamics were studied to understand the effect on As(V) adsorption process with different temperatures at 298, 308, and 318 K. The thermodynamic parameters are the changes of Gibb's free (∆G), entropy of the adsorption (∆S), and enthalpy of adsorption (∆H). According to the van't Hoff equation, ∆G, ∆S and ∆H of the experimental data were calculated as follow: where R is the universal gas constant, T is the absolute temperature and K D is the equilibrium constant.
The results of the calculation of the thermodynamic parameters for As(V) adsorption on MXs are shown in Table 4. A negative value of ∆G indicates that the adsorption of As(V) by MXs is a spontaneous process. The positive value of ∆H demonstrates the endothermic nature of the adsorption process. The value of ∆S was positive. Not only it reflects the affinity adsorption of the MXs toward As(V) and may affect to the change of the structure in the arsenic species and the adsorbent, but also it indicates the increasing of randomness at the solid and liquid interface during As(V) adsorption on the MXs [43].

Desorption Study
In this study, 1 M KOH solution was employed to promote the desorption of used adsorbent over 90 and 180 min with centrifuge at 200 rpm. It can be observed that the desorption efficiency of used MXs with centrifugation time 180 min (70%) was higher than centrifugation time 90 min (45%) [37]. This result indicates that this adsorbent had ability and potential for As removal in the practical application. Therefore, additional studies should be conducted to investigate the method and condition of regenerability and lifetime of the adsorbent to have good performance on arsenic removal and least environmental impact.

Conclusions
The synthesized magnetic xerogel monoliths (14.27% of Fe contents) with molar ratio of magnetite and resorcinol at 0.15 was applied for As(V) removal from groundwater. Magnetite was incorporated in the structure of RF gels corresponding to XRD patterns and SEM figures that was physical without participation in the polycondensation reaction of RF. pH in groundwater (pH = 5) with MXs dose (2 g/L) and particle size of magnetite in RF gels play an important role in As(V) adsorption. Adsorption kinetics of As(V) on MXs was better described by the Elovich equation, followed by the power function equation, the PSO equation and then the PFO equation. The experimental data was correlated to the Freundlich isotherm. Thermodynamic study confirmed that the adsorption process is endothermic and spontaneous in nature due to negative value of free energy change and positive value of enthalpy change. Desorption results showed that the desorption efficiency was 70%.