Optimizing the performance of As(III) and As(V) adsorption process on magnetic carbon xerogel nanocomposites from aqueous solution and natural groundwater wells

doi:10.21203/rs.3.rs-2762742/v1

This research focuses on generating magnetic adsorbents with less expensive precursors, a simple and cheap method of subcritical drying for xerogel, and easily recovered from the aqueous medium with magnetic properties for reducing impact of pollutants in the environment. The application of the response surface methodology (RSM) in optimization of As(III) and As(V) adsorption process on carbon xerogel nanocomposites (XMCs) from aqueous solution was proposed in this study under the scheme of a central composite design 2³ with a central face. XMCs were synthesized from sol-gel polymerization of a resorcinol-formaldehyde composited with magnetite nanoparticles (MNPs) and carbonized at 600°C for 6 h. MNPs were incorporated into the structure of gels corresponding to the XRD, FTIR and SEM/EDX analysis. The varying stoichiometric of resorcinol/water ratios had a significant effect on the resulting texture and surface chemistry properties. The model obtained by RSM was able to acquire the optimal values of the variables (solution pH, dose, and initial concentration) to maximize the removal of As(V) and As(III) of 95±5.98% and 65±10.32%, respectively. The kinetic and equilibrium studies were well described by the pseudo second order and Freundlich isotherm, respectively. Thermodynamic analysis was endothermic and spontaneous in nature. The removal efficiency in groundwater found arsenic at levels lower than the WHO standards.

Adsorption

Arsenic

Groundwater

Magnetite nanoparticles

Carbon xerogels

The knowledge and understanding of the design and optimization of the adsorption process is very important, there are several factors that must be considered (Bakkal Gula et al. 2015; Mtaallah et al. 2018). One of the most important is the quantity and type of pollutants (adsorbates) in the water to be treated. These pollutants generate antagonistic effects between them, or simply decrease the functionality of the adsorbent, because of its early saturation, or the synergy between the components of the aquatic system. A problem in the availability of drinking water is the presence of contaminants in water bodies such as inorganic arsenic compounds: the trivalent arsenic (As(III), arsenites) and the pentavalent arsenic (As(V), arsenate), which is due to their toxicities in the environment and public health. Inorganic arsenic is known to be a carcinogenic element and is also the most significant chemical contaminant globally. Arsenic can be naturally present at high levels in groundwater, this condition is present in several countries of the world. In arid northern and central Mexico, the high concentration of arsenic migrates to the groundwater by weathering processes of silica volcanic rock. Arsenic can cause cardiovascular, liver, neurological, skin, immune and endocrine system diseases, diabetes, and cancer (skin, liver, lung, and bladder), through gastrointestinal, skin and respiratory tract absorption (Huang et al. 2015; Vega-Millán et al. 2021). Therefore, WHO recommends a guideline value for drinking water of 10 µg/L. Additionally, the Environmental Protection Agency (EPA) considers allowed arsenic concentrations in drinking water of 10 µg/L. While the official Mexican standard NOM-127-SSA1-2021 water for human use and consumption indicates the permissible limits of arsenic in the water quality of 0.025 mg/L.

The magnetic nanoparticle applications were significantly increased for the removal of contaminants due to high surface area, small particle size and performance behavior of magnetic materials, the chemical and physical properties, and easy separation through an external magnetic field (Tang and Lo 2013; Mehta et al. 2015). The present work proposes the synthesis of magnetite nanoparticles (MNPs) prepared by conventional co-precipitation using the Fe³⁺/Fe²⁺ molar ratio established by the reaction stoichiometry regarding the results obtained from (Khamkure et al. 2022). This synthesis method is that considered simple, cheap, high yield and potential to prepare in high production of nanoparticles, control in nanoparticle’s distribution, low temperature requirement, but the limitation of this technique is non-environmentally friendly chemical due to utilizing strong base chemicals (Nisticò 2021). The adsorption process by using magnetic nanoparticles to take into consideration water and wastewater treatment for sustainable environmental application due to their specific properties.

Gels are mesoporous materials and can be obtained from a large volume of pores by removing the liquid from the wet gel. Afterwards the solvent that saturates the pores can be removed by three following drying methods: subcritical, supercritical, and cryogenic. Moreover, the subcritical drying that applied in this study is considered as simple, fast, and low cost with high surface area and pore volumes of the obtained materials (Calvo et al. 2011). Due to the properties of mesoporous gels such as texture and structure, mechanical performance, chemical stability, and possibility of control by design according to the conditions of the synthesis, they are attractive to investigate and use for specific application (Al-Muhtaseb and Ritter 2003). A specific application of resorcinol formaldehyde (RF) gel is found in the removal of contaminants from drinking water and wastewater (Haro et al. 2011; Vivo-Vilches et al. 2018; Huang et al. 2018; Bono et al. 2021; Hristea et al. 2021). The adsorption of aqueous solution by the use of RF gels has been investigated in the removal of heavy metal ions (Ni²⁺, Cu²⁺ and Zn²⁺) using carbon aerogels and xerogels (Luzny et al. 2014), Fe³⁺ and Cu²⁺ ions using magnetic organic xerogels (Strachowski et al. 2018) and As, Fe and Mn ions using nanocarbon hybrid of carbon xerogel (Embaby et al. 2021). Moreover, iron oxide xerogels were prepared for improving water quality monitoring of arsenic(III) from groundwater samples (Bono et al. 2021).

Due to surface chemistry strongly affecting adsorption capacity of carbon-based adsorbents, the common functionalization techniques to modify surface functional groups on carbon are oxidation, nitrogenation, and sulfuration. In this study the surface oxidation route was selected to introduce oxygen containing functional groups such as –OH, –COOH, –C = O and –C–O on the surface of carbon xerogel adsorbents, which the increase in functional groups present in transmittance values or wavelengths according to the study of Yang et al. 2019. This technique is considered as common and a simple method, moreover, it is able to enhance arsenic adsorption from aqueous solution (Faizal and Zaini 2019).

In the adsorption process, various factors affect the efficiency of arsenic removal such as pH, synthesis method, initial concentration, particle size, competing ions and contact medium (Lata and Samadder 2016). The experimental design became the important tools for finding the optimization and prediction of the interactive effects in the adsorption process (Özbay et al. 2013; Mtaallah et al. 2018). The response surface methodology (RSM) is one of mathematical and statistical methods and widely used to evaluate multiple factors and identify their interactions on various response variables (Aydar 2018). The application of RSM was used to examine of the interactive effects and to predict a model building and optimization of arsenic adsorption by activated carbon derived from a food industry waste (Bakkal Gula et al. 2015) and natural zeolite with magnesium oxide (Mejía-Zamudio et al. 2009). In this study, it was proposed to establish and carry out RSM to evaluate arsenic removal. This consists of the application of a series of mathematical and statistical techniques that allow modeling and analyzing problems whose goal is to obtain the mathematical model that best fits the experimental data, relating a response variable (or dependent variable "Y") based on various variables. independent "x_i". Unlike an ordinary experimental design like 2^k factorial design, RSM allows obtaining the optimal operating conditions of a process, that is, that combination of "xi" variables that maximize or minimize the value of the response variable. "Y". This implies the use of more advanced mathematical techniques.

In present work, magnetic carbon xerogel nanocomposites (XMCs) were developed to have morphological, magnetic, texture and physicochemical properties that maximize to remove arsenates and arsenites present in groundwater. Synthesized absorbents were modified relationships of water contents, carbonized and induced surface modification, subsequently characterized with respect to their physico-chemical properties. The effect of dosage, solution pH and initial concentration of arsenic were evaluated in the removal process through an experimental design by using RSM with a three-factor 2³ full factorial and central composite. The adsorption kinetic and isotherm were carried out in a batch reactor and were calculated with non-linear regression analysis using the statistical software R v4.0.3. The adsorption enthalpy and the effect of temperature were determined using the optimal values resulting from the implemented experimental design and application of groundwater. Most of the investigations of xerogel carbons for water treatment were applied for aqueous solution however few relevant studies used groundwater for the removal of arsenic contamination by using magnetite and magnetite nanoparticle composite of xerogels (Khamkure et al. 2021, 2022). To our knowledge, this is the first study to apply magnetite nanoparticles incorporating xerogel carbons with direct sonication for As(III) and As(V) removal from natural groundwater. Therefore, these materials are capable of removing arsenates and arsenites; the latter is an anion present in groundwater wells in Mexico and its presence is associated with overexploited aquifers. The results of these adsorbents for water remediation are proposed as one of the viable alternatives with the technological developments in adsorption filters.

Synthesis of adsorbent materials

The original procedure of synthesis resorcinol-formaldehyde (RF) gels by Pekala (1989) was modified and prepared from the sol-gel polymerization of a resorcinol (1, 3-dihydroxybenzene C₆H₄(OH)₂, 99.21% Meyer brand), formaldehyde (HCHO, 37% stabilized methanol solution, JT Baker), water (W) and MNPs (M) using sodium carbonate (Na₂CO₃, anhydrous granules, JT Baker) as a catalyst following the procedure described by Khamkure et al. (2022). Deionized water was used for the synthesis and was used to prepare all study solutions. Magnetite nanoparticles were synthesized via conventional co-precipitation by using Ferric Chloride Hexahydrate (FeCl₃.6H₂O, 98.9%, Fermont), Ferrous sulfate heptahydrate (FeSO₄.7H₂O, 99%, Meyer) and Sodium Hydroxide (NaOH, 97%, Meyer).

Refer to the achievement of As(III) and As(V) adsorption obtained from the previous study of khamkure et al., 2022, therefore, this study continues to prepare magnetic carbon xerogel nanocomposites with molar ratios of M/R = 0.07, R/C = 100, and R/F = 0.5 by varying R/W = 0.04 for XMC10-600 and R/W = 0.05 for XMC5-600 in the design of materials for the application of As(V) and As(III) adsorption, respectively. Each of the mixtures regarding the relation of R/C, R/W, and M/R was prepared and stirred on a magnetic stirrer at room temperature and homogenized for 15 minutes. Afterwards MNPs were added in an RF aqueous solution, and ultrasonication was performed using an ultrasonic processor (VCX130; Sonics&Materials Inc, USA) to disperse the magnetite particles for 10 minutes. Afterwards, magnetic xerogels were produced and were cured in a hot air oven for 5 days at 80°C. After the curing of samples, they were removed from the mold and let them dry in ambient temperature. In the process of solvent exchange, Acetone ((CH₃)₂CO, 99.5%, JT Baker) was used due to lower surface tension value than water, subsequently exchanged water that contained inside the pore of gel into acetone, moreover, it aids to decrease the shrinkage of the pores during the drying process (Martin et al. 2021). Materials were dissolved in acetone for 2 days at room temperature using a shaking water bath (BS-1; Lab Companion). The solvent was replaced daily with fresh solution. Finally, the gels were dried in an oven at 80°C for 3 days. After that a porcelain mortar was used to mill the materials then sieved in stainless steel ASTM test sieve with openings No. 120 (125 µm). Finally, the obtained dry materials in powder form were black colored polymers. Magnetic xerogel samples were carbonized by using a tube furnace (STF55346C-1; Lindberg/Blue M, USA) with nitrogen flow in a nitrogen atmosphere at pyrolysis temperatures 600°C controlling the heating ramp of 2°C/min, 6 hours with nitrogen flow of 100 mL/min.

Surface modification of carbon xerogels

In the present study, functionalization of carbon xerogels were modified from the method of Embaby et al. (2021) by using hydrogen peroxide (H₂O₂, 30%, Jalmek) as an oxidizing agent. Materials were prepared with a 1:15 ratio (g of carbon: mL of H₂O₂), 1 g of carbon xerogels was weighed and taken to a three-neck round-bottom flask vertical with a capacity 250 mL, and 15 mL of H₂O₂ was added. The mixture was performed under a refluxing condition at 60°C for 2 hours while stirring. The modified carbon xerogels were filtered and washed with DI water at 75°C until the pH of the solution was constant and finally it was subjected to dry in an oven at 100°C for 15 h. This material was labelled as XMC5-600M and XMC10-600M.

Characterization of adsorbents

Various characterization analyzes were performed on the obtained materials such as Fourier Transform Infrared Spectrometry (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray energy spectroscopy (EDX), determination of N₂ physisorption and point of zero charge (pH_pzc). XRD analysis of the magnetic carbon xerogels before and after As(V) adsorption were performed using an X-ray diffractometer (D8 ADVANCE; Bruker, Germany). FTIR was used to investigate the surface functional groups of the synthesized MNPs. FTIR also studied the magnetic xerogels before and after As(V) adsorption to understand the mechanism of arsenic ion adsorption on the xerogels. A Shimadzu IRAffinity-1S (Shimadzu Corp., Japan) was performed with dry powder samples and the infrared spectra were measured by connecting to the attenuated total reflection (ATR, Specac; GS10800) containing the crystal disc (single crystal diamond of type IIIa). All spectra were recorded between the wavenumber of 400–4000 cm^− 1 with the number of 45 scans per sample. The surface morphology and pore structure of the magnetic xerogels was analyzed using an SEM/FIB combining a scanning microscope (SEM) and gallium focused ion beam (FIB) (Helios NanoLab 600; FEI company, USA). Microanalysis by energy dispersive X-ray spectroscopy (EDX) was performed on high resolution scanning electron microscope (7800F Prime; JEOL, USA), equipped with microanalysis system by energy dispersive X-ray spectroscopy (EDX) and electron backscattered diffraction system (EBSD) (Bruker brand). The textural properties of the magnetic xerogels and magnetic carbon xerogels were characterized by physical adsorption of N₂ at 77 K using a physisorption equipment (ASAP 2020; Micromeriticis, USA and NOVA touch LX2; Quantachrome Instruments, USA). Before the experiments, the samples were degassed for 12 hours at 110°C and analysis of the textural properties by physical N₂ adsorption of the samples. After data collection, the determinations of surface area and pore size were analyzed by using the Brunauer–Emmett–Teller (BET) method, the Barret–Joyner–Halenda (BJH) and density functional theory (DFT) methods (Morales-Torres et al. 2010), respectively. The pH_pzc was analyzed following the method by measuring potentiometric titrations as described by Amaringo and Anaguano (2013) and Rajput et al. (2016). The Fe content in the magnetic carbon xerogel nanocomposites were analyzed by using an atomic absorption spectrometer (AAS) (Analyst 400; Perkin Elmer, USA).

Experimental design and optimization method

The magnetic carbon xerogel nanocomposites XMC5-600 was selected to study with an experimental design and was elaborated through the RSM for evaluation of arsenic removal. RSM allows the interpolation of different values of the considered independent variables and finds the values of the combinations for an optimal value of the response variable by means of the construction of 3D surface graphs and the projection of their contours onto a 2D surface. The use of the RSM is proposed through the application of a second order polynomial model under the scheme of a central composite design 2³ with a central face. The parameters were estimated in this polynomial model by using the method of least squares. The 3D graphs and projections in 2D contour graphs of the combination of each of the different independent variables "x_i" will be obtained, as well as different statistical parameters that allow evaluating their level of adjustment and reliability, such as: R-squared (R²), adjusted R², F test statistic, lack of fit test and locating the stationary point (the maximum response point). This method can be implemented using the statistical software R v4.0.3. Table 1 summarizes the method of evaluation consisting of a response variable removal efficiency of As and three independent variables at three levels (low, medium, high) of adsorbent dose, initial concentration of As and solution pH.

Table 1

The quantitative factors of the design of experiments for adsorption of As(V) and As(III)
Factors	Coding Factors	Low (-1)	Center (0)	High (1)
Solution pH	x₁	3.0	5.0	7.0
Adsorbent dose (g/L)	x₂	0.5	2.5	4.5
As concentration (mg/L)	x₃	0.05	0.15	0.25

All the results obtained from the experiment were evaluated the removal efficiency using the following equation:

$$\% removal=\frac{{C}_{0}- {C}_{e}}{{C}_{0}}\times 100$$

1

To calculate the amount of adsorption capacity (q_e), the formula was used:

$$qe=\frac{{C}_{0}-{C}_{e}}{m}\times V$$

2

Where C₀ and C_e are the initial and final concentrations of arsenic from the solution (mg/L), m is the amount of adsorbent (g) and v is the volume of the solution (L).

Arsenic batch adsorption experiment

After the experimental design, the adsorption kinetic and isotherm were carried out in a batch reactor. The statistical software R 4.0.3 was used to calculate non-linear regression analysis of kinetic and isotherm adsorption. The adsorption enthalpy and the effect of temperature were determined using the optimal values resulting from the implemented experimental design.

To study the effect of time and equilibrium time on the adsorption kinetics, arsenic solution was used with an initial concentration of 0.1 mg/L at pH 3.0 for As(V) and pH 5.0 for As(III), with contact times of 15, 30, 60, 120, 180, 240, 300, 360, and 1440 min. The experiments were performed in 500 mL of Erlenmeyer flask with a dose of 2 g/L. The samples were stirred in a shaking water bath at 150 rpm for 1440 min, with temperature 24±2°C. The samples were taken at time intervals of 10 mL supernatant to filter and determine the residual concentrations with an atomic absorption spectrometer.

The isotherm adsorption experiment was studied by varying initial arsenic concentration from 0.05 to 0.4 mg/L to investigate the ability of adsorption capacity of carbon xerogels. The solutions of As(V) and As(III) were prepared at 0.05, 0.15, 0.2, 0.3 and 0.4 mg/L, then was adjusted to pH 3.0 for As(V) and pH 5.0 for As(III), with contact times of 1440 min. The samples were carried out in centrifuge tubes of 50 mL (PD1001N) with a dose of 2 g/L. The samples were stirred in a shaking water bath at 150 rpm for a maximum of 24 h, with temperature 24±2°C. Thermodynamic analysis was also investigated to determine the thermodynamic constants with different temperatures which are 25, 35, and 45°C with the optimum condition of adsorption process from RSM analysis.

Application of the removal of As(V) and As(III) by using magnetic carbon xerogel nanocomposites from groundwater wells

For the application of the removal of arsenic in natural groundwater, groundwater samples were obtained from wells located in the state of Morelos with coordinates 18.884195, -99.159475 and the state of Durango in Mexico that were considered as site I and site II, respectively. Analytical methods of the physical-chemical parameters of water samples were determined using a spectrophotometer (DR/1900; HACH, USA) or another equivalent to the official Mexican methods or the Standard Methods. pH and temperature were measured with a benchtop pH Meter (Orion Star A211; Thermo Fisher Scientific Inc., USA). The analysis results of groundwater samples present in Table 2. Groundwater wells in Morelos complied with the Mexican regulations of NOM-127-SSA1-2021. Because there was no presence of arsenic in this water due to below the detection limit of the equipment, then arsenic was added from the stock solution prepared for adsorption tests on synthetic samples. After dilutions were made, this water was used to prepare the corresponding dilutions to obtain a concentration of As(III) or As(V) equal to 0.1mg/L. The pH of the arsenic solution was adjusted to pH = 3 and 5 with 0.1 M of NaOH and 0.1M of HCl to evaluate the adsorption capacity of arsenic. Groundwater quality analysis results provided by the state of Durango (Table 2) revealed that the parameters exceed the permissible limit established by NOM-127-SSA1-2021 were fluoride (3.23 mg/L), arsenic (0.06 mg/L) and pH (6). Both groundwater from natural wells contains nonmetallic and heavy metals (F⁻, Fe and Mn) and abundant anion ions, which were chloride (Cl⁻), nitrate (${NO}_{3}^{-}$), phosphate (${PO}_{4}^{3-}$) and sulfate (${SO}_{4}^{2-}$) that could affect the removal of arsenic in natural conditions of the groundwater. Therefore, the condition obtained from experimental design with RSM was applied in this application of removal of As(V) and As(III) by using magnetic carbon xerogel nanocomposites. Finally, effluent arsenic concentrations were considered and compared with the Mexican and WHO standards. It is intended that the adsorbent materials made in this work constitute a viable alternative within the technological developments for water remediation.

Table 2

Physicochemical properties from the groundwater wells in the state of Morelos and Durango, Mexico used in adsorption experiments. All concentrations are in mg/L.
Name	Source of Groundwater										${{S}{O}}_{4}^{2-}$
Site I	Morelos	7.17	0.34	0.321	16	0.222	LoD^b	0.03	0.029	2.45	15.5	1.245
Site II	Durango	6.00	N.A.^c	N.A.	2.9	3.23	0.06	0.02	0.009	1.55	6.5	0.5
Standard^d		6.5–8.5	5	N.A.^c	250	1.0	0.025	0.15	0.15	10	400	N.A.
Note: ^a EC = Electric conductivity; ^bLoD = The limit of detection; ^cN.A. = not applicable; ^dStandard = NOM-127-SSA1-2021.

Determination of As(V) and As(III) concentrations in water samples

At the end of each experiment, the solution samples were filtered through a magnetic filter funnel with cellulose nitrate membrane filters, 0.45 µm pore size (Whatman) by using a vacuum pump. The residual concentration of As(V) and As(III) samples were determined using a fast sequential flame atomic absorption spectrometer (SpectrAA-220; Varian, Inc., USA) at a wavelength of 193.7 nm following the method described by Quevedo and Luna (2003).

Characterization of magnetic carbon xerogel nanocomposites

In this study water was used as solvent to examine the molar ratios of resorcinol/water (R/W) between 0.4 and 0.5 on the adsorption of arsenic. The molar ratios used for synthesis of XMC5-600 and XMC10-600 were R/C = 100 and M/R = 0.07 at different R/W ratios as shown in Table 3. It can be observed that the percentage of Fe contents of materials increased with decreasing R/W ratio. The initial solution with decreasing reactant concentration has the significant effect of the mass of dissolved solids in a fixed volume of solvent that used deionized water in this synthesis and the resulting properties of final material (Prostredný et al. 2018).

Table 3

Synthesized magnetic carbon xerogel nanocomposites with varying R/W ratios.
Sample	R/C ratio	R/W ratio	M/R ratio	Fe content (%)
XMC5-600	100	0.05	0.07	3.19
XMC10-600	100	0.04	0.07	3.56

The determination of N₂ physisorption was analyzed from the data of two materials before and after carbonization, which were XMC5 and XMC5-600, respectively. Figure 1(a) shows the nitrogen adsorption isotherms and Fig. 1(b) presents the pore size distribution of XMC5 and XMC5-600. The isotherms of both materials exhibit type IV with an H2 type hysteresis loop shape that indicate the mesoporous types corresponding to the results obtained from the mesoporous distribution (Moreno et al. 2013; Quach et al. 2017). The specific surface area from BET analysis of before and after carbonization of XMC5-600 increased from 329.1 to 579.2 m²/g (Table 4). The average pore size of XMC5 (6.171 nm) was larger than that of XMC5-600 (3.715 nm), however both were in the range of mesoporosity and identified an average pore size between 2 and 50 nm. The textural properties and particle sizes of carbon xerogels have significant changes in the carbonization process (Moreno et al. 2013). The BET surface area and total pore volume of XMC5-600 increased, but the average pore diameter decreased as shown in Fig. 1. Due to the thermal treatment, not only temperature increases and releases volatile compounds from XMC5, but also it is possible to remove non-crosslinked polymer chains and labial surface groups of oxygen and hydrogen in the carbonization process at optimum temperature under the inert gas like N₂ (Calvo et al. 2011). Finally, it can be obtained thermally stable carbonaceous materials with development of microporosity in the xerogels and resulting in the nanostructured carbon xerogel with effect on adsorption capacity.

Table 4

Specific surface area, average pore diameter and total pore volume of magnetic xerogel XMC5 and magnetic carbon xerogel XMC5-600.
Samples	BET surface	Average pore diameter (nm)	Total pore volume
Samples	area (m²/g)	Average pore diameter (nm)	(cm³/g)
XMC5	329.1	6.171	0.508
XMC5-600	579.2	3.715	0.538

Figure 2 shows SEM images that were achieved by using secondary electron (SE) and backscattering electron (BSE) detection with a magnification ranging from 10,000x to 400,000x, spatial resolution of 50 to 300 nm. The actual acceleration voltage of 5 kV was applied to all images. The microstructure of magnetic carbon xerogels at R/C = 100 and M/R = 0.07 were compared with the different of R/W ratios between XMC10-600 (R/W = 0.04) and XMC5-600 (R/W = 0.05) as shown in Fig. 2(a, b, c) and 3(e, d, f), respectively. All SEM images represent carbon xerogels in the form of nanosized particles composed of spherical particles less than 100 nm as shown in Fig. 2(a) and 2(d). It can be observed that RF carbon xerogels both of XMC10-600 and XMC5-600 had uniformly interconnected large numbers of microclusters like a three-dimensional polymer and result in porous materials for adsorption application (Yoon et al. 2019). The decreasing of R/W ratios can be controlled the microclusters and porous properties on the resulting RF carbon xerogel because decreasing reactant concentration and affecting to have less compaction or less voids of RF gel structure and increase surface areas (Al-Muhtaseb and Ritter 2003). The structure of void was a resulting of solvent removal and evaporation during the process of carbonization, although the solvent exchange by using acetone was applied on the preparation to protect the collapse of the porous structure of the gel (Thubsuang et al. 2014; Canal-Rodríguez et al. 2017). Figures 2(b) and 2(e) illustrate that XMC5-600 had more compaction of nanoporous xerogels than XMC10-600. Microtexture determination by ESBD shows that MNPs were deposited and distributed homogeneously onto the surface of carbon xerogels. The brightened regions in Figs. 2(c) and 2(f) represent the area where Fe networks were formed, also the gray and dark regions illustrate nanopores. Moreover, the results of the determination of Fe contents by AAS confirmed that the percentage of Fe contents of XMC5-600 and XMC10-600 at different R/W ratio increased with decrease in R/W ratio which were 3.19% and 3.56%, respectively. The corresponding EDX image in Fig. 3 shows that magnetic carbon xerogel XMC10-600 after adsorption of As(V), contained C, O, Al, Si, Fe and As, and most of the bulk of XMCs composed of carbon approximately 84.5% wt. (Embaby et al. 2021).

The XRD patterns of XMC5 before and after thermal treatment with carbonized temperature at 600°C, XMC5-600M and XMC10-600M with surface modification are presented in Fig. 4. The XRD examination indicated the presence of magnetite nanoparticles composed in carbon xerogels that displayed crystallization peaks of the samples at 30.2°, 35.5°, 43.1°, 57.0°, and 62.6° corresponding to (220), (311), (400), (511), and (440) planes of the characteristic peaks of magnetite (JCPDS No. 88–0866) (Sundrarajan and Ramalakshmi 2012). It can be observed that the peaks at 2θ between 20–30° that confirm the structure of materials are amorphous (Huang et al. 2018). The XRD diffraction profile at 20° and 43° corresponds to the (002) and (100) planes of graphene sheets and graphite, respectively, that are reflected in a carbon structure (Kiciński et al. 2011; Arsić et al. 2016).

Figure 5(a) shows FTIR spectra of magnetic carbon xerogels with R/W molar ratios of 0.4 and 0.5 before and after carbonization at 600°C as named XMC10, XMC10-600, XMC5, and XMC5-600. The FTIR bands at 3400 cm^− 1 are attributed to –OH groups bonding to benzene rings caused by the stretching and bending vibration (Attia et al. 2017). The bands at 1620 cm^− 1 are attributed to H-O-H bonds ending vibration of hydroxyl groups of water (Min et al. 2017). It can be observed that both adsorption peaks at 3400 and 1620 cm^− 1 of XMC5-600 and XMC10-600 decreased when compared with the FTIR spectra of XMC5 and XMC10, respectively. Therefore, the influence of the carbonization process with increasing the temperature of thermal treatment to 600°C that evaporated water inside the pores and released the gases (Morales-Torres et al. 2010; Huang et al. 2018). The bands at 1584 cm^− 1 are associated with the C = C stretching in the aromatic ring (Gore et al. 2018). The band in the region of 1271 cm^− 1 is due to the C-O stretching vibrations in C–O–C structure caused by the formation of methylene and methyl ether bridges between the aromatic rings (Morales-Torres et al. 2012). The 571 cm^− 1 band is attributed to Fe-O stretching vibration that can be present in magnetite composite in RF carbon xerogels affecting the arsenic adsorption (Min et al. 2017).

Figure 5(b) illustrates the FTIR of XMC10-600 before and after As(V) adsorption. After As(V) adsorption on XMC10-600, it can be observed the decreased bands at 2950 cm^− 1 (C–H stretching) and 1584 cm^− 1 (C = C aromatic stretching) that corresponded to ring C–C bond stretch of aromatic compound of resorcinol-formaldehyde interacted with arsenic on the adsorption process (Gore et al. 2018). It can also be found in the presence of the new band peak with small intensity at 1490 cm^− 1 due to Fe = O bonds (Ruiz-Mora et al. 2022).

Models fitted and optimal parameters.

The evaluation of the removal of arsenic using XMC5-600 was carried out executing a RSM and consisted of a response variable which was removal efficiency of arsenic, three independent variables dose of adsorbent, initial concentration of arsenic and solution pH with three levels (low, medium, high). The experimental method and data analysis was implemented using the statistical software R v4.0.3.

Equations 3 and 4 describe the most significant parameters after applying the RSM method with respect to the removal efficiency of As(V) and As(III), respectively. Figure 6 shows the contour plot of the removal efficiency of As(V) with two different factors. In Fig. 7 the x and y axes show the two variable input parameters (pH, dose, or initial concentration) that maximize As(V) removal. It can be seen that the most efficient As(V) removal was achieved for the significant quantitative factors which were x₁ = pH and x₂ = adsorbent dose (Fig. 6). Moreover, as can be seen in Fig. 7a 3D surface estimation plot represents As(V) removal efficiency as a function of parameter variations pH vs. dose (Fig. 7(a)) and pH vs. concentration. initial (Fig. 7(b)). The maximum efficiency of As(V) removal was obtained when a low pH and a high dose of adsorbent were applied.

Figure 8 illustrates the results of the As(III) removal efficiency contour profile. The highest As(III) removal efficiency was obtained for the significant quantitative factors that were x₂ = adsorbent dose and x₃ = initial concentration. Equations 3 and 4 describe the most significant parameters after applying the RSM method with respect to the percentage of As(V) and As(III) adsorption, respectively. Code categorical variables were described according to Table 1.

As(V) removal [%] = 28.6177–23.4317x₁ + 10.8115x₂ − 11.9858x₁x₂ + 26.0798${x}_{1}^{2}$ − 9.5217${x}_{3}^{2}$ (3)

As(III) removal [%] = 51.96523–7.42122x₁ + 13.36756x₂x₃ − 18.22614${x}_{1}^{2}$ + 15.53519${x}_{3}^{2}$ (4)

In the statistical study, the optimal values were found using the compute estimated marginal means for factor combinations in a linear model by means of the package emmeans (Lenth 2023) into the R software. The optimum values of pH, dose of adsorbent and initial concentration for the removal of As(V) 95.03±5.98% were 3.0, 4.5 g/L, and 0.15 mg/L, and the removal of As(III) 65.04±10.32% were 4.0, 4.5 g/L, and 0.18 mg/L, respectively, that calculated regarding the range of quantitative factors of the design of experiments as displayed in Table 1. However, the use of adsorbent dose at 4.5 g/L is quite high and not economical, therefore the continuing experiments were considered to apply only 2 g/L by computing the optimum values with the Eqs. 3 and 4.

Preliminary test for the removal of arsenic

The preliminary test of arsenic removal using magnetic carbon xerogels was studied in a batch experiment. Figure 9. shows the removal efficiency and adsorption capacity of As(V) and As(III) using XMC10-600 were higher than XMC5-600. Both of them demonstrate that their adsorption capacities for As(V) were higher than As(III). Regarding the obtained result, this research was to evaluate XMC10-600 and XMC5-600 in detail the removal of As(V) and As(III), respectively.

Adsorption of As(V) on magnetic carbon xerogel nanocomposites XMC10-600 from aqueous solution

Adsorption kinetics models

In adsorption, as a time-dependent process, it is necessary to know its speed for the design and evaluation of adsorbents. Various kinetic models were used to determine the kinetic adsorption constant. The most used for adsorption kinetics analysis are pseudo first order (PFO) and pseudo second order (PSO) kinetic adsorption models (Lima et al. 2015). Therefore, two kinetic models PFO and PSO were applied to fit the kinetic data by non-linear regression to describe As(V) and As(III) adsorption with XMC10-600 and XMC5-600M adsorbents, respectively, as follows:

Pseudo first-order equation: ${q}_{t}={q}_{e}\left(1-{e}^{{-k}_{1}t}\right)$ (5)

Pseudo second-order model: ${q}_{t}=\frac{{k}_{2}{q}_{e}^{2}t}{1+{k}_{2}{q}_{e}t}$ (6)

Where qt is the amount of arsenic adsorbed at time t (mg/g) and qe is the equilibrium adsorption capacity (mg/g). k₁ is the pseudo first order rate constant PFO (min^− 1) and k₂ is the pseudo second order rate constant PSO (min^− 1). t is the contact time (min). α and β are constants of the rate of chemisorption (mg/g min) and the degree of coverage of the surface (g/mg), respectively.

The experimental data was calculated by nonlinear regression analysis to obtain the adsorption kinetic models PFO and PSO as shown in Fig. 10. The corresponding parameter estimates, and correlation coefficients are presented in Table 5. It can be seen that the PSO equation was best described in the As(V) adsorption data due to higher R² value (0.8427) and lower RMSE value which are considered to represent goodness of conformity.

Table 5
Estimation of the kinetic model parameters and correlation coefficients for the adsorption of As(V) by magnetic carbon xerogel nanocomposites XMC10-600.
Pseudo first-order equation	k₁	q_t (µg/g)	R²	RMSE
Pseudo first-order equation	0.027	43.367	0.770	4.127
Pseudo second-order equation	k₂	q_t (µg/g)	R²	RMSE
Pseudo second-order equation	0.001	45.986	0.843	3.413

Adsorption isotherm

An adsorption isotherm describes the phenomenon that governs the retention (or release) of a substance from solution to a solid surface at a temperature and pH constants. Adsorption equilibrium is established when one adsorbate phase is in contact with the adsorbent for a period. Normally, the isotherm is represented graphically by expressing the concentration of adsorbent against its residual concentration. It provides insight into the adsorption mechanism, the degree of harmony of adsorbents and surface properties. In this experiment was analyzed isotherm data by applying Langmuir and Freundlich isotherm models with non-linear regression to describe the relative concentrations of As(V) and As(III) adsorbed to the XMC10-600 and XMC5-600 adsorbents, respectively, as follows:

Langmuir isotherm: q_e$=\frac{\text{b} \text{q}\text{m} \text{C}\text{e}}{(1+\text{b} \text{C}\text{e})}$ (7)

Freundlich isotherm: q_e =$K{Ce}^{\frac{1}{n}}$ (8)

Where qe is the amount of arsenic adsorbed by the XMCs (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 adsorption energy (L/mg) of the Langmuir. K indicates the measure of adsorption capacity and n are Freundlich constants of intensity.

The experiment of the adsorption isotherm for As(V) removal using XMC10-600, was applied with different As initial concentrations between 53–325 µg/L with fixed dose of adsorbent 2 g/L at 25°C. Figure 11 shows the Langmuir and Freundlich isotherms for As(V) adsorption. It can be seen that the As(V) adsorption capacity increased with increasing As(V) concentration. Table 6 summarizes the isotherm parameters and correlation coefficients. It can be seen that a similar value of R² was found for the Langmuir and Freundlich isotherm. Furthermore, the Freundlich model indicated a better fit to describe the experimental equilibrium data of As(V) adsorption using XMC10-600 due to the higher value of R². It is evident that As(V) adsorption is similar to having adsorption on a heterogeneous surface instead of monolayer adsorption.

Table 6

Parameters of the Langmuir and Freundlich isotherm models for the adsorption of As(V) on magnetic carbon xerogel nanocomposites XMC10-600.
Langmuir				Freundlich
q_m (µg/g)	b (L/mg)	R²		K	n	R²
153.4	2.797	0.827		158.2	1.496	0.857

Thermodynamics of adsorption

The application of thermodynamics to adsorption is important to realize the behavior of adsorbent react to variable temperatures. Temperature is one of the key parameters in the adsorption process. In the experiment of adsorption thermodynamics, the effect of arsenic adsorption with three different temperatures at 298, 308 and 318 °K was studied using the magnetic carbon xerogel XMC10-600. According to the van't Hoff equation, the thermodynamic parameters are the Gibb free changes (ΔG), the entropy of adsorption (ΔS) and the enthalpy of adsorption (ΔH) of the experimental data were calculated as follows:

ΔG= $-RTIn\left({K}_{D}\right)$ (9)

ΔG = ΔH-TΔS (10)

ln(K_D)= $-\frac{H}{RT}$+$\frac{S}{R}$ (11)

where R is the universal gas constant, T is the absolute temperature, and K_D is the equilibrium constant.

According to the results obtained from the experiment of thermodynamics of adsorption, their parameters for the adsorption of As(V) with three different temperatures on magnetic carbon xerogel XMC10-600 are presented in Table 7. It can be seen that negative values of ΔG with all studied temperatures demonstrates that the adsorption of As(V) by using XMC10-600 is a spontaneous process. In the case of ΔH, obtaining the positive value of change in enthalpy of the adsorption indicates the endothermic nature of the adsorption process. Finally, ΔS was positive and indicates that the process is irreversible. Therefore, thermodynamic study of As(V) adsorption on the magnetic carbon xerogel nanocomposites XMC10-600 confirmed that the adsorption process is endothermic and spontaneous in nature due to the negative value of free energy change and positive value of enthalpy change.

Table 7

Thermodynamic parameters for As(V) adsorption on XMC10-600
Temperature (K)	ΔG (KJ/mol)	ΔH (KJ/mol)	ΔS (J/mol)
298	-1.6191	170.8270	0.5808
308	-10.0988
318	-9.5648

Adsorption of As(III) on magnetic carbon xerogel nanocomposites XMC5-600M from aqueous solution

In the case of the adsorption of As(III) with XMC5-600M, the conditions for the time effect experiment were: a solution of As(III) with a concentration of 0.1mg/L at pH = 4.6 and kept stirring at 150 rpm. The adsorption kinetics experiment of XMC5-600M fits a pseudo-second order model according to the data shown in Fig. 12 and the values of parameters presented in Table 8. Therefore, XMC5-600M and XMC10-600 can be used for the removal of As(III) and As(V) due to the fact that the materials have a high BET surface area (579.2m²/g), as well as contain of iron with a value of 3.19 and 3.56, respectively. The pH_pzc defines the state in which the surface electric charge density is zero, that is, it is neutralized under certain pH conditions, therefore, the point of zero charge after carbonization increases from pH_pzc of 4.77 (XMC10) to pH_pzc of 9.6 (XMC10-600) (data not shown). The surface of the materials is charged positively at a higher pH and pH dependence of arsenic species in various dominant inorganic forms: H₃AsO₃, H₃AsO₄, ${HAsO}_{4}^{2-}$, H₂AsO₃ (Min et al. 2017).

Table 8

Estimation of the kinetic model parameters and correlation coefficients for As(III) adsorption by magnetic carbon xerogel nanocomposites XMC5-600M.
Pseudo first-order equation	k₁	q_t (µg/g)	R²	RMSE
Pseudo first-order equation	0.054	14.242	0.708	1.055
Pseudo second-order equation	k₂	q_t (µg/g)	R²	RMSE
Pseudo second-order equation	0.007	14.834	0.808	0.855

The experiment of the adsorption isotherm for As(III) removal using XMC5-600M, was applied with different As initial concentrations between 23–210 µg/L with fixed dose of adsorbent 2 g/L at 25°C. The adsorption constants obtained from the experiment of As(III) adsorption isotherm are listed in Table 9. The values of R² reveal both models of the Langmuir and Freundlich isotherm were similar. After the surface modification of XMC5-600M, the maximum adsorption capacity of As(III) is 859.5 µg/g. Therefore, an oxidizing agent H₂O₂ used in the surface oxidation by generating oxygen-containing functional groups of carbon xerogels, consequently enhances the adsorption of arsenic.

Table 9

Parameters of the Langmuir and Freundlich isotherm models for As(III) adsorption on magnetic carbon xerogel nanocomposites XMC5-600M.
Langmuir				Freundlich
q_m (µg/g)	b (L/mg)	R²		K	n	R²
859.5	0.253	0.976		196.1	1.021	0.975

Application of the removal of As(V) and As(III) by using magnetic carbon xerogels in groundwater

Evaluation of the effect of solution pH on As(V) and As(III) adsorption from groundwater in the field experimental site I.

This study area was carried out in Jiutepec, Morelos, Mexico. The effects of pH on As(V) and As(III) removal were studied using XMC10-600 (Fig. 13(a)) and XMC5-600M (Fig. 13(b)), respectively, from groundwater with solution pH = 3 and pH = 5, dose of 2 g/L, initial concentration of arsenic solution of 100 µg/L, contact time of 5 h, and temperature 25°C. The results showed that the As(V) removal efficiency in XMC10-600 for pH 3 and 5 was 74.37% and 47.46%, respectively. The As(III) removal efficiency in XMC5-600 for pH 3 and 5 was 76.62% and 88.41%, respectively. It can be indicated that the effect of pH for As(V) adsorption on XMC10-600 was a significant change in arsenic removal efficiency and adsorption capacity which is different from As(III) adsorption on XMC5-600M which no significant changes are observed in the pH 3 and pH 5.

The reduction of the R/W ratio during the synthesis of xerogel, it can be explained that resorcinol is an important part of formation of the nodules or clusters, and formaldehyde generates the interconnected structure gaining strength of gels, therefore the volume of water added in this synthesis affects to the distance between nodules (Canal-Rodríguez et al. 2020). The higher volume of water added into preparing the RF solution, results in the high segregation of the nodules that are confirmed with less the compactness of XMC10-600 (R/W = 0.4) than XMC5-600 (R/W = 0.5) appearance in the analysis of SEM.

Evaluation of the effect of contact time on arsenic adsorption from groundwater in the field experimental site II.

According to the result of XMC10-600 using for As(V) and As(III) adsorption in groundwater provided from well in the state of Durango as illustrated in Fig. 14, shows the removal efficiency and q_e for pH 3 and pH 5 approximately 48–74% and 18.70–29.30 µg/g, respectively, that was lower than results obtained from XMC5-600M. Therefore XMC10-600M was modified to surface with H₂O₂ as described above to evaluate the arsenic adsorption in groundwater provided from wells in the state of Durango that heavy metals As and F⁻ were over than the permissible limit established by NOM-127-SSA1-2021. The condition obtained from experimental design with RSM was applied in this application of groundwater and performed in the condition: an initial arsenic concentration of 0.06 mg/L, an adsorbent dose of 2 g/L, solution pH of 3 a stirring speed of 150 rpm at a temperature of 25–28°C with different contact time of 4 h and 24 h. The performance of XMC10-600M in the removal efficiency of arsenic were 96.79% and 97.5% at contact time 4 h and 24 h, respectively (Fig. 14). The adsorption capacity with different contact time of 4 h and 24 h were similar of 104.5-105.3 µg/g, therefore, the optimum contact time was 240 minutes, and the appropriate pH was found to be 3.0 can lower the arsenic from groundwater to comply with the requirements of NOM-127-SSA1- 2021. Regard to the physical-chemical characterization of the real water, this natural groundwater contains nonmetallic and heavy metals (F⁻, Fe and Mn) and abundant anion ions such as Cl⁻, ${NO}_{3}^{-}, {SO}_{4}^{2-}{ and PO}_{4}^{3-}$ but the presence of these ions did not significantly affect the performance of arsenic removal. It can be explained that the presence of interfering ions of groundwater replace in the adsorption process of arsenic, but in the case of lower solution pH, showing a higher selectivity for adsorbing arsenic and a higher number of sites (Bono et al. 2021).

The surface modification of XMC5-600M and XMC10-600M exhibits surface chemistry heterogeneity strongly due to containing of various functional groups and influence to increase the adsorption capacity, also the main mechanisms controlling the surface chemistry such as repulsive and dispersive interactions, and hydrogen bonds (Pego et al. 2019). The results of porous structural morphology with high surface area and low pore volume, were caused by the increment of the ratio of surface area to overall volume of magnetic carbon xerogels that were affected to have higher the adsorption capacity of As(III) and As(V) (Bono et al. 2021). The mesoporous matrix of resorcinol-formaldehyde carbon xerogel provide the hierarchical pathways for the adsorbate molecules like arsenic in the adsorption process and the surface modification of these adsorbents improved and enhanced the adsorption capacity (Faizal and Zaini 2019). Moreover, the carbonization under an inert atmosphere removes oxygen and hydrogen, can control the microclusters and porous properties on the resulting magnetic carbon xerogel with high specific surface areas, thermally stable nanostructure carbon and affect the adsorption capacity of arsenic (Canal-Rodríguez et al. 2017).

In this study, magnetite nanoparticles were prepared by coprecipitation method and incorporated on a xerogel, subsequently carbonized at 600°C for 6 h and H₂O₂-induced surface modification. The developed magnetic carbon xerogel nanocomposites contained 3.19–3.56% Fe content. A molar ratio of magnetite nanoparticles and resorcinol was applied at 0.07 but the varying of water contents were modified by 0.04 and 0.05 for the removal of As(V) and As(III) from the aqueous solution, respectively. In agreement with the reduction of the R/W ratio, the compactness and voidness of the RF gel structure decreased. Moreover, the surface modification to generate functional groups of carbon xerogel enhanced the adsorption capacity of arsenic. From the model obtained by the experimental design using RSM, the pH of solution and dose of XMCs play an important role in arsenic adsorption. As(III) and As(V) adsorption data were well described by the pseudo second order kinetics. The equilibrium adsorption of As(III) and As(V) fitted well the Freundlich isotherm model. Thermodynamic studies showed that the adsorption process was endothermic and spontaneous in nature. XMC5-600M gave the best performance within the groundwater application range of pH 3 and 5. The effect of pH for As(V) adsorption on XMC10-600 was a significant change in arsenic removal efficiency and adsorption capacity which is different from As(III) adsorption on XMC5-600M. The removal efficiency of As(V) was 91% at 2 h and the equilibrium time was achieved in 3 h for aqueous solution. However, the removal efficiency in groundwater using XMC5-600M and XMC10-600M at were 88.41% of As(III) within 5 h and 96.79% of As(V) within 4 h, respectively, with arsenic at levels lower than the Mexican and WHO standards. According to the study results, magnetic carbon xerogel nanocomposites were developed as a potential adsorbent for the effective removal of arsenic from groundwater.

Acknowledgement

Khamkure S. thanks for the financial support by the Catedras-CONACyT Program of the National Council of Science and Technology (Project No. 159). The authors would like to thank José Martín Baas-López (Yucatan Scientific Research Center), Manuel Sanchez-Zarza and Gloribel Vázquez-Cornejo (Mexican Institute of Water Technology) for the characterization and technical analysis.

Authors Contributions

All authors participated in the study of conception, design, and methodology. Material preparation and characterization were achieved by SK, VBT, DEPC, and PGM. Data collection and investigation were performed by SK and MFCL. Data analysis, modelling and data interpretation were accomplished by SK and ARR. Fund acquisition and project administration were managed by SK and SEGH. The first draft of the manuscript was written by SK and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding This work was supported by Mexican Institute of Water Technology (Grant No. DP2101.1).

Availability of data and materials The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Ethical Approval Not applicable.

Consent to Participate Not applicable.

Consent to Publish Not applicable.

Competing Interests The authors have no relevant financial or non-financial interests to disclose.

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Optimizing the performance of As(III) and As(V) adsorption process on magnetic carbon xerogel nanocomposites from aqueous solution and natural groundwater wells

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Abstract

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Introduction

Material And Method