J. Cent. South Univ. (2021) 28: 1692-1706

DOI: https://doi.org/10.1007/s11771-021-4727-5

Deep purification of As(V) in drinking water by silica gel loaded with FeOOH and MnO₂

SHI Tong-shan(石通杉)^{1, 2}, JIANG Feng(江锋)^{1, 2}, WANG Pan(王盼)^{1, 2}, YUE Tong(岳彤)^{1, 2}, SUN Wei(孙伟)^{1, 2}

1. School of Resource Processing and Bioengineering, Central South University, Changsha 410083, China;

2. Key Laboratory of Hunan Province for Clean and Efficient Utilization of Strategic Calcium-containing Mineral Resources, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: For deep purification of As(V) from drinking water by adsorption, two adsorbents S-FeOOH and S-MnO₂ were successfully synthesized by loading FeOOH and MnO₂ nanoparticles onto silica gel in situ. Characterization of the adsorbents implied that S-FeOOH and S-MnO₂ with large particle size (diameter of 150-250 μm) still had high specific surface areas (357.0 and 334.6 m²/g) due to their specific amorphous and porous structure. In batch experiments, the influences of pH, contact time, adsorbent dosage, and temperature on the adsorption were investigated. Comparing with other adsorbents reported, the synthesized adsorbents in this study, especially S-FeOOH, showed good performance for As(V) removal in a wide pH (2-12) and temperature (25-65 °C) range. The residual As(V) concentration after S-FeOOH treatment was around 0.01 mg/L, which met the drinking water standard. The adsorption process followed the pseudo-second-order kinetic model, and the adsorption equilibrium was reached within 5 min. The equilibrium adsorption data of S-FeOOH can be well fitted by the Langmuir isotherm, while that of S-MnO₂ followed Freundlich model, which indicated their different adsorption mechanisms. The results show that S-FeOOH is superior to S-MnO₂ in eliminating As(V), and S-FeOOH could be used as a promising adsorbent for the deep purification of As(V) in drinking water.

Key words: As(V) removal; adsorption; drinking water; composite material

Cite this article as: SHI Tong-shan, JIANG Feng, WANG Pan, YUE Tong, SUN Wei. Deep purification of As(V) in drinking water by silica gel loaded with FeOOH and MnO₂ [J]. Journal of Central South University, 2021, 28(6): 1692-1706. DOI: https://doi.org/10.1007/s11771-021-4727-5.

1 Introduction

Medically, arsenic is considered to be one of the most toxic and harmful pollution elements to human health. Long-term intake of low-concentration arsenic contaminated water may lead to kidney, liver and skin cancer [1]. So, we must strictly control the content of arsenic in water to protect the water environment and ensure the safety of drinking water. A much more stringent limit (10 μg/L) provided by the World Health Organization (WHO) in 1993 has been adopted as the drinking water standard to minimize the harmful effects of arsenic on health [2]. However, with the development of social economy and industry, arsenic pollution in water has become one of the important environmental and social problems faced by many countries in the world; the major route of arsenic getting in the water has changed from the natural geological process to the human industrial process, due to a large amount of arsenious solid waste, waste water and smoke produced in the industry process, especially the mineral processing and smelting of ore resources. Harmless treatment of arsenic-bearing waste water has become a key issue of global concern. Therefore, the study of arsenic removal technology in drinking water is of great significance for protecting the ecological environment and people’s health.

In a typical aquatic environment, arsenic is primarily present in inorganic forms with arsenite (As(III)) and arsenate (As(V)) [3], and As(V) is more prevalent in oxygenated surface water while As(III) is found in anaerobic groundwater. As(III) is more toxic and harder to remove from aqueous environment than As(V). Therefore, As(III) is normally oxidized to As(V) in arsenic removal process, after which arsenic can be removed more easily using various treatment methods, such as adsorption [4], precipitation [5], ion exchange [6], reverse osmosis [7], and electro dialysis [8]. Adsorption is considered to be a simple, economical, and efficient method to remove arsenic pollution in aqueous solution [9-14]. However, the need to adjust the solution pH, the relatively low adsorption capacity, and the potential dissolution of adsorbents during the adsorption process limits the removal efficiency of arsenic from the aqueous solution. It is urgent to develop novel arsenic adsorbents to overcome these limitations, especially in the deep purification of drinking water.

It was widely reported that various materials, such as activated carbon, waste biomass sorbents, organic polymers, zeolite, clay, and more selective metal oxides, have been used as the adsorbents for arsenic removal [15-20]. The results of many studies revealed that the cost-effective and easily available iron/manganese (hydr)oxides were promising adsorbent materials, because they had strong affinities towards arsenic and high selectivity to arsenic in the adsorption process [21-26]. Therefore, more researches recently focused on removing arsenic with iron/manganese (hydr)oxides or based materials as adsorbents. In these effective adsorbents, iron (hydr)oxides include amorphous iron hydroxide, ferrihydrite, hematite, and goethite [27-30]; manganese oxides include nanostructured manganese oxide, alone or as iron-manganese binary oxide [31-33], are well-known for their ability to removal arsenic from aqueous system.

In order to increase the adsorption capacity, the iron/manganese (hydr)oxide materials were prepared with a micrometer scale, even nanoscale to extend the adsorption surface area. However, these fine particles tended to aggregate together due to their high surface activity energy, which decreased the effective adsorption area [14]. In addition, these fine adsorbent particles were hard to separate from aqueous solution in the recycle of adsorbents. By contrast, loading the adsorbents on the surface of large particle carriers leads the adsorbents to have better dispersibility and usability. Furthermore, the carriers may alter the adsorption performance of the loaded adsorbents through the electrostatic interaction or chemical bonding. Therefore, it is significative to develop the high-efficiency carrier material for improving arsenic removal performance of the iron/manganese (hydr)oxides adsorbents.

In this study, silica gel, which is widely used, cheap, easy to obtain, and has very good porosity and high active adsorption, was chosen as the carrier of FeOOH and MnO₂ adsorbents for arsenic removal from drinking water. The adsorbents, S-FeOOH and S-MnO₂ were synthesized through self-assembly and embedding FeOOH and MnO₂ nanoparticles in silica gel in situ. These novel materials have been characterized and then studied for As(V) adsorption from the aqueous solution under different experimental conditions in detail, such as solution pH, contact time, amount of adsorbent and temperature.

2 Material and methods

2.1 Materials

Sodium arsenate (Na₃AsO₄) was purchased from Jinjinle Chemical Co., Ltd., China. The arsenic stock solution with concentration of 1 g/L was prepared with deionized water. The arsenic working solution was also prepared by diluting the arsenic solution with deionized water in subsequent experiments. Chemicals, which were analytical grade and used without further purification, such as sulfuric acid (H₂SO₄), hydrochloric acid (HCl), sodium hydroxide (NaOH), sodium silicate (Na₂SiO₃), ferric chloride (FeCl₃), potassium permanganate (KMnO₄), and manganese sulfate monohydrate (MnSO₄H₂O), were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was solvent in all solutions.

2.2 Adsorbents preparation

The adsorbents (S-FeOOH and S-MnO₂) were prepared according to Ref. [34], which could modify the mechanical properties of the adsorbents produced. In brief, when Si(OH)₄ prepared by reaction of sodium silicate solution with hydrochloric acid contacted with metal compounds, silica-metal surface complexes between Si and metal ions formed. The complexation could enhance the binding of ultrafine metal particles and hence increase the physical strength of the adsorbents produced. The Si/Fe and Si/Mn molar ratios of the adsorbents prepared in this experiment both were1:1.

2.2.1 Preparation of S-FeOOH

S-FeOOH was mainly produced by the following two reactions:

Na₂SiO₃+H₂O+2HCl=Si(OH)₄+2NaCl        (1)

FeCl₃+3NaOH=FeOOH↓+H₂O+3NaCl        (2)

Firstly, mixing 100 mL sodium silicate solution (1 mol/L) with 300 mL sodium hydroxide solution (1 mol/L) in a stirred vessel. The 100 mL ferric salt solution (1 mol/L) was then added in the vessel under vigorously stirring conditions to precipitate gelatinous Si-Fe complexes. The pH of the precipitation process was controlled to the range of 3-10, which was adjusted with 5% HCl or 5% NaOH, and the final pH of the precipitation reaction should be maintained between 6.5 and 7.5, which ensured that the adsorbents were obtained under neutral conditions. After formation, the gelatinous complexed Si-Fe slurry went through the following four processing steps: continuously stirred for 2 h, allowed by age for 4 h, washed with deionized water, and dewatered by filtration or centrifugation. Then, the dewatered solids were granulated to the required size and shape by extrusion or other methods (In this study, spherical particles with the high specific surface area were chosen) and dried at temperature between 120 and 250 °C for several hours to get final S-FeOOH.

2.2.2 Preparation of S-MnO₂

The preparation principle of S-MnO₂ was the same as S-FeOOH, which was prepared by Eq. (1) and the following chemical equation:

2KMnO₄+3MnSO₄+2H₂O=5MnO₂↓+K₂SO₄+2H₂SO₄                                             (3)

Similarly, the preparation process remained, and just the ferric chloride solution (FeCl₃) to the 100 mL potassium permanganate (0.4 mol/L) and 100 mL manganese sulfate monohydrate (0.6 mol/L) to precipitate gelatinous Si-Mn complexes changed.

2.3 Batch adsorption tests

The optimum amount of adsorbent and contact time to reach adsorption equilibrium was determined by batch experiments. Each dosage was tested in five 100 mL flasks with 50 mL of 40 mg/L As(V) solution. Under the action of magnetic stirrer, the mixtures of five flasks were stirred for 1, 3, 5, 7, and 10 min at 150 r/min, respectively. Experiments to determine the effect of solution pH on As(V) removal were performed by adding 5 g of S-FeOOH/ S-MnO₂ into a serial of 100 mL flasks containing 50 mL of 40 mg/L As(V) solution. The pH was adjusted to desired values with 5% HCl and 5% NaOH. For these experiments, the initial pH was varied within the range 2-12 and the solution was stirred at room temperature at 150 r/min for 5 min. And the temperature experiments were carried out in an magnetic stirring water bath at 25, 35, 45, 55 and 65 °C, respectively. At each temperature, 5 g of adsorbents and 50 mL As (V) solution with 40 mg/L concentration were stirred together in a 100 mL flask, stirred at pH of 6.0-7.5 for 5 min.

The kinetics of As(V) adsorption was studied by batch experiments with varying contact times. Herein, 5 g S-FeOOH/S-MnO₂ was thoroughly mixed with 50 mL of 40 mg/L As(V) solution in 100 mL flasks. The mixtures were withdrawn at a period of time between 1 and 10 min and filtered. Then, the sample solution of residual As(V) concentration was analyzed at different contact time.

Adsorption isotherms studies were conducted, 5 g S-FeOOH/S-MnO₂ and 50 mL As(V) solution with a concentration range of 10-1000 mg/L were added into 100 mL flasks at 25 °C. The mixtures were carried out in magnetic stirring water bath for 5 min. And the specific amount of arsenic adsorbed was calculated from the following equation:

                            (4)

where Q_e (mg/g) is the equilibrium adsorption capacity; C₀ and C_e (mg/L) are the initial and equilibrium concentration of the adsorbate in solution, respectively; V is the volume (mL) of the arsenic solution; m is the mass (mg) of adsorbent used in the experiments.

2.4 Characterization methods

The morphologies and structural features were analyzed using XRD (Parnacre X-ray diffractometer), EDS (EDS300 DME), and SEM (FEI Magellan 400L XHR field emission scanning electron microscope) techniques. Specific surface area and the micropore characteristics were determined from the adsorption isotherms for liquid nitrogen at 77.3 K using a BET surface analyzer (Quantachrome NovaWin instruments. version 11.02). The sample solution was filtered with 0.45 μm membrane to analyze the residual As(V) concentration with an inductively coupled plasma mass spectrometry equipment (ICP–MS: SPECTRO-Blue II). All samples were analyzed within one day of collection.

3 Results and discussion

3.1 Adsorbents characterization

The XRD patterns of S-FeOOH and S-MnO₂ in Figure 1 show that there is a cladding peak at 2θ of 15°-30° and no obvious crystalline peak at other angles. The phenomenon indicates that the FeOOH and MnO₂ formed on silica gel with amorphous structure, without fixed crystal form. This is because the presence of silicates prevents their crystal growth. As the adsorbents, the crystalline forms of FeOOH and MnO₂ were not required, because the specific surface area of the crystal is much smaller than that of amorphous or low-crystalline form at the same particle size[34]. In addition, the existence of FeOOH and MnO₂ did not change the inherent properties of silica gel, so these two composites could be applied in all fields of silica gel application.

Figure 1 XRD patterns of S-FeOOH and S-MnO₂

The morphologies of S-FeOOH and S-MnO₂ were confirmed by the SEM and the results in Figure 2. It could be seen that the S-FeOOH and S-MnO₂ were a spherical particle colloidal material with the diameter of 150-250 μm, and FeOOH and MnO₂ were evenly distributed on the surface of silica gel. From the EDS analysis in Figures 2(c)-(f) and Table 1, the results showed that there were iron and manganese on the surface and inner of silica gel. In other words, the effective combination of FeOOH and MnO₂ with silica gel was further confirmed. Sometimes, EDS analysis needed to spray gold to enhance the conductivity of the sample, so the Pt peak appeared in the figure. Additionally, from Table 1, the content of manganese on silica gel was higher than that of iron, which indicated that the binding ability of MnO₂ on silica gel could be better than that of FeOOH.

In order to compare the changes of specific surface area and pore size of silica gel before and after loading metal oxide materials, BET/BJH tests were carried out on pure silica gel, S-FeOOH, and S-MnO₂. The results are shown in Figure 3 and Table 2. Figures 3(a)-(c) show that the adsorption isotherms at the relative pressure (p/p₀) 0.03-0.10 of the three materials were straight lines with the great coefficient of correlation, indicating that they were multi-molecular layer adsorption, which meets the BET equation. The specific surface areas (Table 2) of pure silica gel, S-FeOOH and S-MnO₂ were 339.6, 357.0 and 334.6 m²/g, respectively. In addition, the specific surface area of the prepared absorbents S-FeOOH and S-MnO₂ were significantly greater than that of the pure FeOOH (261 m²/g) or MnO₂ (117 m²/g) [35], which increased the contact probability of functional adsorption sites on the surface of FeOOH and MnO₂ nanoparticles with As(V). Figure 3(d) shows the pore size distribution curves of the adsorbents based on the nitrogen equilibrium adsorption isotherm at 77.3 K. It shows that the pure silica gel has relatively uniform mesoporous with pore diameter at around 10 nm, while the both adsorbents have some macropores (40-80 nm). Because FeOOH/MnO₂ particles were loaded on the pore and surface of silica gel, the surface of silica gel became rougher, the specific surface area of silica gel was enlarged and the pore size decreased.

Figure 2 SEM images of S-FeOOH (a) and S-MnO₂ (b) and inner and surface EDS analysis of S-FeOOH and S-MnO₂ (c-f)

Table 1 Inner and surface metal content of S-FeOOH and S-MnO₂ by EDS

3.2 Effects of dosage and contact time

The effects of dosage and contact time were important parameters for arsenic removed by adsorption. In Figure 4(a), the equilibrated removal efficiency of As(V) for S-FeOOH rose from 70% to 95% with increasing dosage of S-FeOOH from 1 to 3 g, ascribing to the increase of adsorption sites and surface area. Further increasing the dosage, the promotion of As(V) removal efficiency slowed down due to the shortage of adsorbate in the solution [36]. Hence, the equilibrium removal efficiency of As(V)

was over 99.9% at the S-FeOOH dosage more than 5 g. Comparing with S-FeOOH, the S-MnO₂ had a worse performance for As(V) removal at the same dosage. The maximum removal efficiency of As(V) was 93%-97% when the adsorbent dosage increased to 5 and 7 g. The residual concentrations of As(V) with the optimal absorbents dosage of 5 g at different adsorption time are shown in Figure 4(c). It could be seen that the adsorption equilibrium of As(V) on S-FeOOH and S-MnO₂ occurred at around the contact time of 5 min. The equilibrium residual concentration of As(V) with S-FeOOH was 0.01 mg/L, meeting the drinking water standard (0.01 mg/L) of As(V), while it was still 1.04 mg/L for S-MnO₂. Comparing with other adsorbents which has been studied, the time required for these two adsorbents to reach adsorption equilibrium was shorter [37-39], indicating the remarkable affinity of the prepared adsorbents with As(V) in aqueous solution.

Figure 3 Nitrogen adsorption-desorption isotherms (a-c) and corresponding pore-size distribution curves (d) for pure silica gel, S-FeOOH and S-MnO₂

Table 2 Textural properties of pure silica gel, S-FeOOH, and S-MnO₂ adsorbents

3.3 Effect of pH

In Figure 5, different from S-MnO₂, the removal efficiency of As(V) for S-FeOOH was around 98% with pH range from 2 to 12. In other words, the As(V) adsorption capacity of S-FeOOH hardly depends on pH. Such excellent adsorption performance, stabilized and high-efficiency arsenic adsorption capacity in a wide pH range, had not been reported on the previous studies. Additionally, it indicated that the main As(V) removal mechanism of S-FeOOH is the ligand interchange between arsenic and iron oxyhydroxide, rather than the electrostatic attraction [40]. Since the pH_pzc of amorphous FeOOH was about 8.0 [41], S-FeOOH had positive charge when pH<8.0. So the As(V) was removed by S-FeOOH through electrostatic adsorption under the alkaline condition, and S-FeOOH still had a certain adsorption capacity, which indicated that S-FeOOH interacted with As(V) through the specific adsorption of coordination complexation that As(V) was adsorbed to S-FeOOH surface to form an inner-sphere complex with S-FeOOH surface. Therefore, it could be concluded that in the whole process of adsorbing As(V) adsorption on S-FeOOH, besides electrostatic adsorption, ligand exchange adsorption was also involved.

Figure 4 Effects of dosage and contact time for As(V) adsorption efficiency on S-FeOOH (a) and S-MnO₂ (b) at (25±0.5) °C, residual concentration of As(V) in treated water (c)

Figure 5 Effect of pH on As (V) adsorption onto S-FeOOH and S-MnO₂ at (25±0.5) °C

For S-MnO₂, the removal efficiency of As(V) was gradually decreased from 97.80% to 85.00% as the pH increased from 2 to 10. When the pH further increased to 12, the removal efficiency slumped to 25%. The variety of the above adsorption capacity of S-MnO₂ to As(V) was attributed to the adsorption of As(V) on the surface of S-MnO₂ belonging to electrostatic adsorption [42]. The high adsorption of As(V) under acidic and neutral conditions was due to the fact that the positively charged adsorbent surface sites were easily occupied by the negatively charged arsenate species as well as the lower concentration of OH^- could less compete for active sites on the adsorbent. With alkalinity increasing, the positive charge of S-MnO₂ surfaces gradually became negatively charged, the arsenate gradually dissociated and formed strong electrostatic repulsion between OH^- and arsenate. Hence a sharply decrease of adsorption was seen over pH=10. Accordingly, the As(V) removal by S-MnO₂ was more evidently dependent on pH than that by S-FeOOH and the greatest adsorption occurring under acidic conditions and decreased with higher pH.

3.4 Effect of temperature

Figure 6 depicts the adsorption efficiency of S-FeOOH and S-MnO₂ to As(V) at different ambient temperatures (25-65 °C). Whether S-FeOOH or S-MnO₂, the removal efficiency of As(V) remained approximately constant at each temperature. The adsorption efficiency of S-FeOOH to As(V) was maintained at 100%, which was obviously higher than that of S-MnO₂ (93.83%). This was consistent with the above results that S-FeOOH was better than S-MnO₂ to the adsorption of As(V) under pH>2 conditions. Apparently, the adsorption properties of the two adsorbents were not affected by the temperature range set in this experiment, implying that these two adsorbents were relatively excellent heat-resistant adsorption materials.

Figure 6 Effect of temperature on As(V) adsorption onto S-FeOOH and S-MnO₂ at 25-65 °C

3.5 Adsorption kinetics

The study of adsorption kinetics was very important for the description of adsorption behavior of adsorbents. So, the pseudo-first-order kinetic model and the pseudo-second-order kinetic model were used to investigate the adsorption rate of As(V) adsorption on S-FeOOH and S-MnO₂ and evaluate the dominant adsorption mechanism. Their linear forms were as follows:

                       (5)

                            (6)

where Q_t (mg/g) is the adsorption capacity of the adsorbent at time t (min); K₁ (min^-1) and K₂ (g/(mg·min)) are the adsorption rate constants of the pseudo-first-order adsorption and pseudo-second-order adsorption, respectively.

The kinetics simulation and kinetics parameters of As(V) adsorption on S-FeOOH and S-MnO₂ are shown in Figure 7 and Table 3, respectively. As being indicated from R² values in Table 3, the pseudo-second-order kinetic model gave a higher R² value for the As(V) removal by S-FeOOH and S-MnO₂(R²>0.999), and the Q_e,_cal value of the pseudo-first-order kinetic model was close to the Q_e,_exp, which further confirmed that the pseudo-second-order kinetic model fitted the kinetics data well. The k value indicated the relative adsorption rate of metal ions, which was relative to the diffusion rate of metal ions from the bulk solution to adsorbents [43]. The K values in Table 3 showed K_S-MnO2≈K_S-FeOOH, indicating that As(V) is adsorbed by S-MnO₂ and S-FeOOH at equal rates, which is consistent with the experiment result of contact time and dosage (they both only take 5 min to reach equilibrium adsorption). It could be concluded that the rate limiting step is chemisorption involving valence forces through the chemisorption involving valence forces through the sharing or exchange of electrons between adsorbents and As(V) [44, 45], indicating the dominant mechanism of chemisorptions involved in As(V) adsorption.Table 4 compares the adsorption rate of S-FeOOH with other similar reports. The K value represented the relative adsorption rate of metal ions on adsorbent. It is obvious that the K value of S-FeOOH is close to 21.92, much larger than other adsorption materials, and it takes only 5 min for the adsorption of As(V) on S-FeOOH to reach equilibrium which could be concluded from Figure 4. It is also inferred that S-FeOOH is more attractive to As(V) than other adsorbents in Table 4.

Figure 7 Pseudo-first-order kinetics model (a) and pseudo-second-order kinetics model (b) for adsorption of As(V) on S-FeOOH and S-MnO₂, respectively

Table 3 Kinetics parameters for adsorption of As(V) by S-FeOOH and S-MnO₂

Table 4 Comparison of As(V) adsorption rate for some related adsorbent materials

3.6 Adsorption isotherms

To obtain more information about the As(V) adsorption on S-FeOOH and S-MnO₂, the isotherm models of Langmuir and Freundlich were used to describe the adsorption equilibrium data. The Langmuir isotherm model assumes that monolayer adsorption occuring on a homogeneous surface. When monolayer adsorption is completely saturated, there is no interaction between adsorbates. On the contrary, the Freundlich isotherm model describes the multilayer adsorption on a heterogeneous surface on which the adsorbates are interactive [47-50].

Langmuir and Freundlich isotherm equations are given as follows:

                             (7)

                                (8)

where Q_max (mg/g) and Q_e (mg/L) are the maximum adsorption capacity and the mass of As(V) adsorbed at equilibrium, respectively; C_e (mg/L) is the As(V) equilibrium concentration; K_l (L/mg) and K_f (mg/g) are Langmuir constant and Freundlich constant, which indicate the attraction between As(V) and the adsorbents.

A non-linear regression was applied to obtaining all Langmuir and Freundlich isotherm parameters presented in Table 5. From Figure 8, the As(V) adsorption on S-FeOOH, Langmuir model (R²>0.946) had a better regression coefficient than Freundlich model (R²>0.943), which was similar to the result obtained by ZHANG et al [33]. And the adsorption data of S-MnO₂ could be described well by the Freundlich model (R²>0.988) rather than Langmuir model (R²>0.964). In isotherm study, the saturated adsorption capacity (Q_max) was the practical maximum of S-FeOOH. However, the BET surface area analysis had determined that S-MnO₂ had a lower specific surface area (334.6m²/g) than that of S-FeOOH (357 m²/g). And the pH_pzc of SMnO₂ was 4.8, which was lower than that of S-FeOOH (8.0). The surface charge of S-MnO₂ was more negative at lower pH, and there was an electrostatic repulsion S-MnO₂ and arsenate. So, the above two reasons could be eliminated. According to the isotherm adsorption results, it was known that S-FeOOH was monomolecular layer adsorption and S-MnO₂ was multimolecular layer adsorption, which may explain why S-MnO₂ had the larger adsorption capacity. It was obvious that K_f>K_l from Table 5, indicating that S-MnO₂ might have a higher affinity for As(V) than S-FeOOH. In fact, the above result was not consistent with Figure 4(c), which may be due to the fact that the hypothesis of Langmuir model was far from the reality, and the information obtained had a certain amount of error. In addition, the adsorption capacity of S-FeOOH with other similar reports is compared in Table 6. It could be clearly obtained that S-FeOOH had a higher adsorption capacity than other adsorbents. Combined with the previous analysis, it is indicated that S-FeOOH was a promising adsorbent for the treatment of low concentration arsenic contaminated drinking water.

Table 5 Isotherm parameters for As(V) adsorption on S-FeOOH and S-MnO₂

Figure 8 Adsorption isotherms for different concentrations of As(V) with S-FeOOH and S-MnO₂

Table 6 Comparison of As(V) adsorption capacity for some related adsorbent materials

3.7 Adsorption thermodynamics

Through the study of adsorption thermodynamics, it could be understood the driving process of adsorption reaction, the degree of adsorption, and the effects of various factors on adsorption. So, S-FeOOH with better adsorption performance was selected for thermodynamic study. And the thermodynamic parameters of adsorption involve enthalpy (H), entropy (S), and Gibbs free energy (G).

These parameters can be obtained from the following equations:

                                 (9)

                          (10)

                        (11)

                        (12)

where K_D is equilibrium adsorption partition coefficient, which is reliable to S-FeOOH when C_e<40 mg/L; R (8.314 J/(mol·K)) is gas constant; T (K) is temperature; △G^Θ, △H^Θ, and △S^Θ are standard Gibbs free energy △G (kJ/mol), standard enthalpy change, and standard entropy change, respectively.

The △G parameter was calculated as -2.31, -3.62, -6.53 and -10.29 kJ/mol for 298, 308, 318, and 328 K in Table 7, respectively. In the temperatures range, the negative △G indicated that the adsorption process was spontaneous. The absolute value of △G increased with the increase of temperature. It was concluded that the net driving force and the degree of spontaneity gradually increased in adsorption process. The reasonable explanation was that the pore size on adsorbent surface increased or adsorbent surface was activated with the increasing temperature [55, 56]. As shown in Figure 9, the K_D -1/T plot was used to determine △H and △S. These parameters were found to be 73.97 and 225.71 kJ/mol, respectively. △H>0 indicated that the adsorption process was endothermic, which was confirmed by the increased K_D values with increasing temperature. △S>0 indicated that the adsorption reaction makes the whole system confusing. S-FeOOH adsorbed As(V) in water, which makes the degree of freedom of the whole system decrease and leads to the decrease of entropy. In adsorption process, some molecules would be separated from the system; the △S of the system was the sum of the increase of entropy due to As(V) adsorption on S-FeOOH and the decrease of entropy of S-FeOOH surface analytical molecules.

Table 7 Thermodynamics parameters of S-FeOOH

Figure 9 lnK_D-T^-1 plot used in determination of thermodynamic parameters for adsorption of As(V) onto S-FeOOH

In this study, the value of entropy increase was greater than the value of entropy decrease in adsorption process, so the value of △S was positive value [57].

3.8 Regeneration and wastewater treatment

In order to reduce the cost and recycled utilization of adsorbent, desorption experiments of S-FeOOH were conducted by mixing the As(V)-loaded S-FeOOH with different pH. On the premise that the structure of S-FeOOH was not destroyed, the result of desorption is shown in Figure 10.

In Figure 10, the desorption efficiency increased with the increase of pH, but decreased suddenly under strong alkali due to the fact that S-FeOOH was easily corroded under strong alkali. The desorption efficiency reached its maximum when pH was 10, but the desorption efficiency was also not ideal, only about 32.13%.

Figure 10 Desorption efficiency of S-FeOOH

So, another method was adopted to maximize utilization and minimize costs. It was generally known that FeOOH was soluble in strong acid, while silica gel was acid resistant. After mixing S-FeOOH which has been saturated for As(V) adsorption and 0.2 mol/L H₂SO₄ at the mass ratio of 1:30, the mixture was stirred at a constant speed (200 r/min). After 0.5 h, FeOOH was completely dissolved from silica gel and As(V) was desorbed simultaneously. Then, the filtered silica gel was cleaned by deionized water and dried at 60 °C. FeOOH was loaded on silica gel by the same preparation method as new adsorbent which would be used in next round and their ability of As(V) treatment did not decrease. However, there was a high concentration of As(V) in the acid washing wastewater, which needed to be further treated to prevent secondary pollution. Solidification landfill was a common treatment method for arsenic contaminated wastewater. Based on many literatures, the lime, cement and ferrous sulfate were used as solidified agent and the solidification was conducted by mixing sludge, lime, cement, and ferrous sulfate with the mass ratio of 5:0.7:1:2 [57, 58].

The specific steps are shown in Figure 11. First, because the acid washing wastewater was arsenic-high iron wastewater (mainly Fe³⁺) and ferric iron was easy to hydrolyze, NaClO solution (V_NaClO:V_water=1:400) was added to promote its hydrolysis to form FeOOH colloid and adsorb arsenic to form sludge. Second, a certain amount of sludge and deionized water twice the weight of the sludge were mixed and stirred fully to the paste,adding the solidified agent prepared according to the proportion. Third, adding cement and a small amount of deionized water to the mixture of step 2 with stirring. After the sludge was fully mixed, it was poured into the mold, sealed, and placed in a cool and ventilated place for natural maintenance and air drying.

Figure 11 Flowchart of S-FeOOH regeneration and wastewater treatment

By comparing the arsenic content of the sludge before solidification and the leaching solution after solidification, it could be found that the As(V) in the solidified sludge leaching solution greatly decreased (6 ug/L), and the solidification rate was 99.6%, which ensured that the solidified sludge met the landfill requirements.

4 Conclusions

1) In this study, S-FeOOH and S-MnO₂ are synthesized by metal (hydr)oxides formed in situ on silica gel as the adsorbents to adsorb As(V).

2) S-FeOOH and S-MnO₂ both show excellent adsorption properties towards As(V). The adsorption is efficient, because the adsorption equilibrium could be achieved within 5 min. The adsorption capacities of S-FeOOH and S-MnO₂ for As(V) remain stable in a wide pH range (2–12), especially S-FeOOH, which is a limitation in the study of arsenic adsorption.And the adsorption performance is not easily affected by the solution temperature (25-65 °C). In addition, there was no secondary pollution in adsorption process and the solid-liquid separation was easy to achieve.

3) From the practical point of view, S-FeOOH is more likely to be the promising adsorbent for the treatment of arsenic-contaminated natural water than S-MnO₂, especially in the field of lower concentration, because S-FeOOH is more stable and efficient than S-MnO₂ in As(V) adsorption.

Contributors

The overarching research goals were developed by YUE Tong and SUN Wei. SHI Tong-shan and JIANG Feng provided the test plan and test data, and the measured data were analyzed by SHI Tong-shan and WANG Pan. The initial draft of the manuscript was written by SHI Tong-shan and YUE Tong. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

SHI Tong-shan, JIANG Feng, WANG Pan, YUE Tong, and SUN Wei declare that they have no conflict of interest.

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(Edited by FANG Jing-hua)

中文导读

硅胶负载FeOOH和MnO₂对饮用水中As(V)的深度纯化

摘要：为了研究吸附法对饮用水中As(V)的深度净化，采用原位生成法，将FeOOH和MnO₂纳米粒子负载到硅胶上，成功地合成了两种吸附剂(S-FeOOH和S-MnO₂)。吸附剂的表征表明，S-FeOOH和S-MnO₂具有较大的颗粒尺寸(粒径150~250 μm)，由于其特殊的非晶和多孔结构，使其具有较高的比表面积(357.0和334.6 m²/g)。在间歇实验中，研究了pH、接触时间、吸附剂用量、温度对吸附剂吸附性能的影响。将实验结果与其他吸附剂相比，本研究合成的吸附剂，特别是S-FeOOH，在较宽的pH(2~12)和温度(25~65 °C)范围内，均表现出良好的As(V)去除性能。在被S-FeOOH处理的砷污水中，As(V)的残留浓度约为0.01 mg/L，达到了饮用水标准。吸附过程遵循伪二级动力学模型，在5 min内吸附达到平衡。S-FeOOH的平衡吸附数据可通过Langmuir等温线拟合，而S-MnO₂的吸附过程遵循Freundlich模型，说明它们具有不同的吸附机理。结果表明，S-FeOOH在去除As(V)方面优于S-MnO₂，即S-FeOOH可作为一种有发展前景的吸附剂用于饮用水中As(V)的深度净化。

关键词：As(V)的去除；吸附；饮用水；复合材料

Foundation item: Projects(2019YFC0408305, 2018YFC1901601) supported by the National Key Scientific Research of China; Project(2018CX036) supported by the Innovation-Driven Plan of Central South University, China; Project(2018TP1002) supported by the Key Laboratory of Hunan Province for Clean and Efficient Utilization of Strategic Calcium-Containing Mineral Resources, China

Received date: 2020-06-28; Accepted date: 2020-10-16

Corresponding author: YUE Tong, PhD, Associate Professor; Tel: +86-13607430125; E-mail: yuetong@csu.edu.cn; ORCID: https://orcid.org/0000-0002-3330-3468; SUN Wei, PhD, Professor; Tel: +86-13507310692; E-mail: sunmenghu@csu.edu.cn; ORCID: 0000-0002-0204-4520