Modified oyster shell powder with iron (II) sulfate heptahydrate to improve arsenic uptake in solution and in contaminated soils

Arsenic is a metalloid whose presence can be due to natural or anthropological causes. It is considered as a toxic chemical that puts human health at high risk. In this study, we evaluated a novel modified oyster shell (MOS) that was coated with iron (II) sulfate heptahydrate using two different proportions through batch sorption experiments in an arsenic solution and in arsenic-contaminated soils. The arsenic solution was prepared using As(III)-standard solution. The arsenic contaminated soils were extracted from a contaminated site in Cheonan, South Korea, where the average arsenic concentration of the soil was reported as 136.28 mg/kg. Different doses of oyster shell and modified oyster were used to understand the effect of the addition of iron (II) sulfate heptahydrate via sorption batch experiments in solution and sorption tests in soils. The sorption tests were conducted with 50 g of contaminated soil; then, 150 g of soils was used for the pot cultivation tests, and finally, 150 g of contaminated soils was used for column percolation test. Through the experiments, the authors observed a comparable improvement of arsenic stabilization from 10 to 60% with the addition of iron (II) sulfate heptahydrate to oyster shell.


Introduction
Anthropogenic pollution of arsenic can be traced back to mining and other industrial activities, as well as pesticide and herbicide use in agricultural soils (Kahakachchi et al. 2004). Besides anthropogenic pollution, arsenic can also be found naturally, e.g., in many subsurface drinking water aquifers (Oremland and Stolz 2003). Arsenic, similar to lead, copper, zinc, and cadmium, is a potentially toxic metal, because even trace amounts of arsenic can have a carcinogenic effect on humans (Mondal et al. 2006). The mode of toxicity varies depending on the chemical form of arsenic. Arsenic exists in four oxidation states. The inorganic forms: arsenate [As (V)], the predominant form of inorganic arsenic in aerobic aqueous environments; and trivalent arsenite [As (III)] in anoxic environments (Bilici Baskan and Pala 2011; Kahakachchi et al. 2004;Oremland and Stolz 2003;Yuan and Chiang 2008). Besides, the organic forms such as monomethyl arsenic (MMA) and dimethyl arsenic (DMA) can be traced at lower amounts in soils (Chowdhury 2015), which forms intracellularly by most living organisms (animals, plants or bacteria, and in the liver cells of mammals) (Irvine et al. 2006). Arsenic can occur naturally in the soil in the range of 1-40 mg/kg (U.S. EPA 2011;WHO 1996). Regardless of the form of arsenic, its concentration in soils needs to be controlled due to its potential toxicity. Canada has set a limit of 12 mg/kg for arsenic in agricultural soils, whereas Australia, Czech Republic, Denmark, Poland, and the UK have a cap of 20 mg/kg (Chen et al. 2018;Liu et al. 2018). Environmental Protection Agency of the USA has limited the arsenic concentration to 5.0 mg/L or 100 mg/kg for hazardous wastes. In South Korea, for an industrial land use area, arsenic concentrations in the soils should be less than 50 mg/kg. Although the regulations are established, the arsenic contamination around the abandoned mines in South Korea (Ko et al. 2012) is still an issue that needs immediate response (should be addressed). As(III) is 10 to 60 times more toxic than As(V) (Mukhopadhyay et al. 2018;Oremland and Stolz 2003). Arsenate impedes the oxidative phosphorylation, whereas arsenite impairs the function of many proteins by binding to sulfhydryl groups.
Although the remediation of arsenic-contaminated soil and arsenic-contaminated water is extensively studied, solidification and stabilization technique (Hunce et al. 2012;Kargar et al. 2015;Xia et al. 2019) has captured the attention of researchers due to the benefits associated with the technology. Different stabilizer compounds, such Zeolite (Babel and Kurniawan 2003;Bilici Baskan and Pala 2011;Pan et al. 2022) and Oyster shell (OS) powder (Moon et al. 2013;Ok et al. 2010;Torres-Quiroz et al. 2021) were found to be effective in stabilizing potentially toxic metals, such as Pb, Cu, Cd, and Zn. However, these research were not conducted with arsenic contaminated soils. Researchers such as (Ko et al. 2012) investigated materials such as limestone, steel mill slag, and granular ferric hydroxide, although the results were not encouraging. Subsequently, researchers have been investigating different chemical agents that can improve the performance of low-cost binders of Arsenic with chemical modifications. For instance, Mondal et al. (2006) investigated the performance of Zeolite with chlorine and potassium permanganate (Mondal et al. 2006), while, Bilici Baskan andPala (2011) observed that NaCl and FeCl 3 solutions improved the performance of Zeolite (Clinoptilolite) in stabilizing the soils contaminated with arsenic in drinking water. Moreover, Smedley and Kinniburgh (2002) demonstrated that Ca increased the As (V) adsorption at high pH, and Jang et al. (2016) showed that phosphate (PO 4 −3 ) improved arsenic adsorption. Many researchers have investigated the use of chemicals with iron in removing arsenic from aqueous solutions. Hydrous ferric oxide was observed to be an effective compound for removing As (III) and As (V) from aqueous solutions because of the high specific area and iso-electric point (Huo et al. 2017). They also confirmed through FTIR, XPS, and XRD that main removal mechanism were precipitation and surface complexation. Granular ferric hydroxide (GFH) (An et al. (2005)), granular ferric oxide (GFO) (An et al. (2005)), and iron-coated sand (Benjamin et al. 1996) were also effective in removing arsenic from aqueous solutions. The oxidation of As(III) during sorption due to a partial redox reaction with ferric hydroxide was observed by (Ajith et al. 2021). They also observed that As (V) got reduced by Fe (II) ions in the medium later. Liu et al. (2011), used α-Fe2O3 impregnated chitosan for arsenic removal. Jeon et al. (2009) also made similar observations with iron coated zeolite. Besides, arsenic in natural waters and soils can be attributed to the presence of iron (hydro)oxides because Feoxides strongly sorb arsenate and arsenite onto their surfaces, forming inner-sphere complexes (Suzuki et al. 2015;Violante et al. 2010).
However, the binders seek to reduce the mobility of arsenic and iron forms are a good candidate to enhance the performance of low-cost binders such as OS. It is well known that OS are being discarded as waste from various industries, that emit toxic gases such as: NH 3 and H 2 S during its decomposition process (Silva et al. 2019). The calcination of OS provides good characteristics for its use as a good low-cost binder. However, calcium carbonate tends to increase the pH and has not shown good performance in removing the arsenic from aqueous mediums and stabilize soils contaminated with arsenic (Bonnard et al. 2020;Lim et al. 2009;Lu et al. 2018;Ramakrishna et al. 2018;Silva et al. 2019;Xu et al. 2019). The removal of arsenic is easily affected by the pH, in fact in soils, it was found that arsenic sorption was less in Alkaline soils when the pH range is higher than 11. Indeed, most oxyanions including arsenate become less sorbed when the pH increases (Cui et al. 2010;Dzombak and Morel 1990), which increases the arsenic mobility in alkaline soils. Therefore, the synergies of an OS and iron (II) heptahydrate combination need investigation under various pH conditions as there is no study done in this regard. Considering these factors, this study aimed to enhance the performance of the OS powder by adding low doses of Fe (II) to convert it into a new valueadded product.

As-contaminated soil
Arsenic-contaminated soil was collected from an industrial area in Seobuk-gu, Cheonan, in South Korea. The chemical characterization of the soil indicated the presence of other toxic metals, such as Pb, Cu, and Zn (Table S1) but at lower concentrations. The soil was classified as well-graded sand (SW) with a fine content lower than 5% through a particle size analysis. All the soil samples were air-dried and passed through a 2-mm mesh prior to the preliminary analysis and experiments. The initial arsenic content was 136.28 mg/kg (extraction method: aqua regia).

Binders
OS powder was obtained from a domestic supplier (Jisan Industrial Co., Ltd., Busan, Korea). It was being used as an agricultural amendment. OS in. the form of 2.5 to 5 mm grains, were crushed and passed through No. 200 mesh to collect particles smaller than 0.075 mm prior to use. The SEM analysis of the OS revealed that most of the particles were larger than 1 μm. The XRF analysis conducted on the powder revealed that the OS contained 89.3% CaCO 3 (Table 1). Then, the authors modified the OS (the modified OS will be referred to as MOS from here onwards) by mixing 5 g of OS (after passing through 0.15 mm sieve) with 40 ml of 0.25 M Iron (II) sulfate heptahydrate (FeSO 4 .7H 2 O) solution in 45-ml conical tubes. The mixture in the tubes were combined using a rotary machine for 24 h at 50 rpm. The preliminary analysis with 0.25 M Iron (II) sulfate heptahydrate (FeSO 4 .7H 2 O) solution demonstrated a sorption efficiency greater than 99%. Therefore, a much lower dose of Iron (II) sulfate heptahydrate (FeSO 4 .7H 2 O) solution was needed to properly assess the effect of Iron (II) sulfate heptahydrate (FeSO 4 .7H 2 O) on the performance of the OS. Consequently, another modified OS sample was prepared with a 0.05 M FeSO 4 .7H 2 O solution (this sample will be called MOS(R)). For both doses, after mixing, the mixture was centrifuged for 10 min to separate the solid phase from the liquid phase. The solid phase (modified binder) was rinsed with DI water and filtered through a 45-µm membrane filter. Finally, the residue was oven dried at 60 °C. The adsorption characteristics of MOS and MOS(R) were evaluated using prepared arsenic (III) standard solutions.

Characterization of the materials and binders
XRF analysis was performed to characterize the chemical compounds of the arsenic-contaminated soil and the binders using S8 Tiger (Bruker, Germany). XRD analysis was conducted using S4 Pioneer by using CuKα radiation (Bruker AXS, Germany) ( Fig S1). FTIR analysis was performed to determine whether new functional groups had been generated by the addition of OS and MOS. The samples were scanned from 600 to 4000 cm −1 using Vertex-80v-Bruker, Germany ( Fig S2). The cation exchange capacity was measured using Ammonium acetate method. Finally, SEM-EDS field-emission scanning electron microscopy (SIGMA) with EDS analysis was performed on the binder (Fig. 1). All the aforementioned experiments were performed by NICEM (National Instrumentation Center for Environmental Management).

Batch experiments: sorption and desorption with arsenic solution
Batch sorption experiments were performed on OS, MOS, and MOS(R) to understand the arsenic sorption by the OS, MOS, and MOS(R). A standard arsenic solution at 1000 mg/L (As 2 O 3 ・NaCl (0.05%) ・HCl), which was purchased from Kanto Chemicals Co., Ltd, Tokyo, Japan, was used to make the arsenic standard solutions of 5, 25, and 50 mg/L. Initially, OS and MOS were mixed with arsenic solutions separately at a solid to liquid ratio (S/L ratio) of 1/20 to observe their adsorption capacity. However, the authors later decided to reduce the S/L ratio, because the adsorption was more than 99% for all the arsenic concentrations (Fig. 2). Therefore, authors decided to reduce the S/L ratio to 1/100 to observe changes in the adsorption characteristics of MOS and MOS(R). The arsenic solutions were mixed with the OS, MOS, and MOS(R) using an orbital shaker for 2 h at 150 rpm. The mixed samples were then centrifuged at 3500 rpm for 10 min and filtered through mixed cellulose ester membrane of 0.45 μm pore size. The pH values were measured before and after binder addition for all the samples using a pH meter (Thermo Scientific Orion Series -Model Meter 121,900 (USA)), as shown in Table S2. The solid retained on the filter was oven dried for 24 h at 60 °C before arsenic extraction using M-3 extraction solution.

Batch experiments: sorption test in arsenic contaminated soil
The arsenic-contaminated soil (50 g) was mixed with DI water with an S/L ratio of 1:3, and this was used as the control sample. To evaluate the adsorption efficiency of the binders (OS, MOS, and MOS(R)), binders were added (3 and 5% of the total weight) to the arsenic-contaminated soil. All samples were agitated in an orbital shaker at 150 rpm for 2 h in accordance with methods used previously (Torres-Quiroz et al. 2021). The supernatant fluid was extracted for arsenic analysis through ICP-AES (ICP-730ES, Australia). The solid phase, the control samples, and stabilized soil were oven dried at 60 °C for 24 h. Finally, TCLP (US EPA 1992) and Mehlich-3 (Mehlich 1984) single extraction methods were used to extract the arsenic that was absorbed by the binder. The pH values were measured at each step to understand the variation of the pH.  As removal (%)

Pot cultivation test
For the pot cultivation test, 150 g of arsenic-contaminated soil was mixed with each binder at different proportion (3% and 5% w/w of OS or 3% and 5% of w/w MOS). The soil moisture content was maintained at 30% of the total weight. The exposure to sunlight was prevented by keeping samples in a dark and covered environment (inside polypropylene amber wide neck round bottles). Each week, a small portion of the control and treated soils was extracted and oven dried for 24 h. Then, arsenic was extracted using Mehlich extraction. Finally, the concentration in mg/L was determined by ICP-AES analysis. The test was conducted in two stages: first, using the soils that were collected in 2020 (S-I); second, using the soils that were collected in 2021(S-II) from the field.

Column percolation test
Dynamic flow adsorption experiments or column percolation tests were conducted using the methods suggested by Naka et al. (2016) and Yasutaka et al. (2017). The columns made of polypropylene (5 cm diameter and 30 cm length) were packed with the dried arsenic-contaminated soil without binder (S) and the treated soil with the binder (SB). The filled soil mass was 790 ± 10 g, which was packed as five layers of 160 g. The up-flow column experiments were conducted in triplicate for each case S and SB. Deionized water (DI) was used as an eluent solution at an up-flow rate of 0.6 ml/min. After 16 h, samples were collected. The efficiency of the binder in binding arsenic in the soil was evaluated by measuring the concentration of arsenic released from the eluent samples, which were collected at liquid to solid ratios of 0.1,0.2,0.5,1,5, 7.5, and 10. The samples were taken to the ICP-AES to determine the arsenic concentration. Additionally, the samples of S and SB were dried, and 20 g of the sample was assessed for arsenic using sequential extraction Wenzel method (Wenzel et al. 2001).

Chemical changes after modification of oyster shell
The chemical compounds observed in the X-Ray Fluorescence analysis of the arsenic-contaminated soil and the binders are presented in

Batch experiments: sorption test
The OS under a solid-to-liquid rate of 1:20 sorbed less than 8% of arsenic, which corroborated the findings of Lim et al. (2009). However, after the modification, the efficiency reached 99.9% (Fig. 2). As it was impossible to determine the maximum sorption capacity for arsenic (III) under a solidto-liquid ratio of 1:20, the authors resorted a lower solidto-liquid ratio (1:100). The MOS sorbed 97.18% of arsenic from 5 mg/L (4.99 mg/L of standard arsenic solution) and 68.17% from a 50 mg/L (52.67 mg/L of standard arsenic solution). The MOS(R) also demonstrated better performance compared with OS, but this was less efficient than MOS; 73.62% and 29.95% of arsenic was removed from 5 and 50 mg/L of standard arsenic solutions, respectively. The Freundlich isotherms illustrated in Fig. 3 showed that the performance of MOS(R) was intermediate, i.e., between the performances of OS and MOS. The XRF analysis conducted for the binders (MOS and MOS(R)) after mixing with the 25 mg/L arsenic solution revealed 0.06% of As 2 O 3 in both the binders (Table 1). Moreover, the XRD analysis showed the presence of Cu 3 AsS 4 when MOS was used as a binder. Arsenic (III) in the solution was oxidized to form arsenic(V) (Cu 3 AsS 4 ) ( Fig S1). Additionally, the desorption test performed on the solid-phase of MOS after the sorption test using M-3 extraction method was used to measure the bioavailability of the arsenic. The authors observed that the desorption rate was lower than 1%. This suggested that the binding strength was strong. Thus, bound arsenic will not be readily bioavailable ( Table 2). Surface of the binder after sorption test with arsenic solution of 25 mg/L ( Fig  S3) shows a variation con respect the surface prior analysis (Fig. 1), which can be attributed to the arsenic binding mechanism.

Arsenic binding capacity in the soil
Binder efficiency in soil decreased by 55.7% with the addition of MOS (just 5% of the total weight). In contrast, the addition of OS resulted in just a 10.4% reduction in arsenic leached by the soil. These figures corresponded to the leaching tests conducted using M-3 extraction solution, thereby demonstrating better extraction efficiencies compared with TCLP (Table 3 and Fig. 4). The addition of FeSO 4 .7H 2 O may lead to increased levels of Fe and sulfur. Thus, the concentrations of Fe and S were measured in the leachate. The addition of FeSO 4 .7H 2 O had no impact on the Fe concentration. In fact, the Fe concentration in the leachate did not change significantly for soils of MOS and MOSR compared with the soil treated with just OS. By contrast, the addition of the amendment drastically increased the sulfur concentration in the leachate (Fig S4). The mobilization of sulfur can be attributed to the repulsion of sulfur by the negative charge of the soil particles. The initial pH values of the soil, soil treated with MOS, and soil treated with MOS(R) increased slightly (Fig. 4). Later each sample was subjected to the leaching test where, the DI water, TCLP, and M-3 extraction agents had pH values of 6.50, 2.88, and 2.5, respectively. The pH of the solution did not change much when arsenic-contaminated soil was mixed with MOS-treated soil, although a slight increment could be observed for OS-treated soil. When OS was used, the pH changed from 6.50 to 8.39 (Fig S5), which may have affected the sulfur mobilization. The Canadian regulations stipulated that the Fe concentration in irrigation water used for agricultural purposes should not exceed 5 mg/L (CCME 2020). In the US, secondary drinking water should have less than 0.3 mg/L of Iron (U.S. EPA 2011). The authors were not able to find regulations for sulfur. The pot cultivation test suggested that over a period of 3 weeks. The arsenic concentration decreased in the leachate compared with the initial concentration in the soil without any binders. Adding 5% of MOS resulted in a binding efficiency of 66.9%, whereas 3% of dosage had an efficiency of 54.0% (S-II) (Fig. 5). The performance of OS in the S-II increased by more than twice for a 5% dose and four times for a 3% dose of the binder. The difference between the initial concentrations in arsenic-contaminated soil from the stage I (S-I) and from the stage II (S-II) can be attributed to the prolonged exposed to the atmospheric oxygen after the collection in 2020. Arsenic oxidizes in the presence of oxygen (natural attenuation). Descriptive statistics are shown in Fig S6. Finally, the second binder MOS(R), which was prepared using a reduced amount of iron sulfate, demonstrated that the MOS(R) can be used under dynamic conditions (up-flow percolation < 0.6 ml/ min) through the column percolation test. Arsenic concentration was reduced by 61% after the addition of the binder (Fig. 6 and Fig S7). Wenzel extractions performed on the S and SB samples after the experiment, showed a total arsenic leaching of 47.46 and 57.28 mg/ kg, respectively. These values accounted for 34.83% and 42.03% of the total (136.28 kg/mg) initial arsenic content in the soil (Table 4 and Fig S8).
The stabilization mechanism of arsenic in soils with MOS As (III) as it is more mobile in low pH conditions compared to As(V). This is due to the high tendency of As(V) to be adsorbed to solid surfaces under low-pH conditions (Dixit and Hering 2003). This phenomenon explains why more Arsenic was observed with low-pH extraction solutions. The XRD analysis in the binder showed that MOS consisted of 62% Calcite (CaCO 3 ), 33% Gypsum (CaSO 4 ), 1.63% FeCO 3, and 2.76% Goethite (FeOOH) (Fig S1). Meanwhile, MOS(R) showed no Gypsum formation, and the analysis reported over 90% calcite. The results of the Wenzel extraction (Table 4 and Fig S8) in the soil treated with the binder (SB) suggested that the increase in the percentage of the adsorbed arsenic can be associated with the amorphous and poorly crystalline and hydrous oxide of Fe and Al (Wenzel extraction step 3) and crystalline Fe oxyhydroxide (Wenzel extraction step 4) as observed by researchers such as (Liu et al. 2011). These factors may explain the stabilization of arsenic in the soil.

Conclusions
MOS and MOS(R) demonstrated a removal efficiency in the range of 70-100% when introduced to the As(III) aqueous solution of 5 mg/L. The desorption test using M-3 extraction solution suggested a strong binding mechanism, in which the arsenic recovered was less than 1% with MOS(R). However, the efficiency of the binder decreased in the soil environment, which can be attributed to the pH conditions and the presence of other chemical compounds in the soil such as phosphorus aluminum forms which can switch with arsenic ions. Besides, the concentration of As(V) and As(III) was not separately measured, as such the authors cannot rule out the possibility of a redox reaction which is a limitation of this study. The extraction method had a significant impact on arsenic leaching due to the pH of the reagent solution. Accordingly, the authors observed, a higher leaching of arsenic when M-3 extraction (extraction solution with the lowest pH) method was used. This suggest that the mobility of the ions may be increased during acid rains that lowers the pH of the infiltrated water.
Pot cultivation test showed that the bioavailable arsenic decreased by 66% with the addition of 5% of the MOS. This study was conducted for a period of seven weeks and the pot cultivation test results show the average values of the last 3 weeks of the experiment. Increasing the scale of the experiment and extending the experiment period needs to be done in another study to assess the long-term performance of the binders. However, the results of this study suggest that short-term stabilization of soils contaminated with arsenic is feasible with the modified OS until a long-term solution is prepared.
The column percolation test, which evaluated the performance of the binder MOS(R), suggested that MOS(R) was effective in stabilizing the arsenic. MOS seems to be efficient in reducing arsenic mobility in soils and can be used to remove arsenic from contaminated water. The doses of the iron salt to modify the OS powder should be adjusted according to the expected arsenic reduction goals. Gypsum formation was recognized as the possible root cause for the enhancement of the efficiency of MOS, which corroborated the findings of Palansooriya et al. (2020). The creation of this compound leaves chemically available iron ions to interact with the arsenic. Further studies including XPS and AFM, are needed to identify the biding mechanisms of Fe (II) and arsenic.
Author contribution All authors contributed to the study's conception and design. Material preparation and data collection and analysis were performed by Torres Quiroz, Cecilia and Junboum Park. The first draft of the manuscript was written by Torres Quiroz, Cecilia and reviewed and edited by Janith Dissanayake. Finally, it was checked by Junboum Park. All authors commented on the previous versions of the manuscript. All authors read and approved the final manuscript.
Funding This research was supported by the Korea Ministry of Environment through the grant 2019002470002 "The SEM (Subsurface Environment Management) projects." The authors would also like to thank the Institute of Engineering Research and the Institute of Construction and Environmental Engineering at Seoul National University.
Data availability All the relevant data is provided in the supplementary document. Should other data be requested, authors will gladly comply.

Declarations
Ethical approval Not applicable Consent to participate Not applicable.
Consent for publication All the authors have read and agreed to consider publication in Environmental Science and Pollution Research.

Competing interests
The authors declare no competing interests.