Stabilization effect of Mg-Fe layered double hydroxide (LDH) for arsenic-contaminated soil

Abstract

The solidification/stabilization (S/S method) has been reported as the most common treatment technology for As (arsenic)-contaminated soil in the USA. In South Korea, over the last two decades, only a few stabilizers (lime, limestone, and steelmaking slag, etc.) have been used to treat heavy metal-contaminated farmland near the abandoned mines. The application of the S/S method to various contaminated sites has been very limited in South Korea. In this study, the magnesium and iron-based layered double hydroxide (Mg-Fe LDH) was used as a stabilizer to fix the As in the soil, and it has been widely studied to remove and stabilize oxyanions. Before the Mg-Fe LDH was used as the stabilizer for As-contaminated soil, two toxicity leaching tests were performed to investigate its feasibility as the stabilizer. In this study, the bead type LDH (BLDH, 5~6 mm in diameter) mixed with starch was also manufactured. In order to evaluate the feasibility of the BLDH as the As stabilizer in the soil, the As stabilization efficiency of the BLDH was compared with the powdered type LDH (PLDH, 149 μm in diameter) by various experiments (batch and continuous column experiments). The As-contaminated soil samples were collected from the farmland, where it exists nearby the storage facility of mine tailings generated by the mining activities of gold mines. The average As concentration in soil samples measured at 112.523 mg/kg over the standards of countermeasures against soil contamination in South Korea (75 mg/kg of As concentration in 1 area). From the results of the As stabilization experiment without soil, the As sorption efficiency of PLDH and BLDH showed higher than 99%, and the pH of the solution after the addition of LDH was maintained in the range of 5.8~8.5, which satisfied the Korean municipal groundwater standard limit. From the results of the stabilization experiment with soil, the As extraction reducing efficiency of the BLDH in soil ranged from 43.5% to 80.1%, suggesting that the BLDH has a capacity to stabilize As in soil. From the results of the continuous column experiments, the As concentration of solution leached from the As-contaminated soil was maintained below the Korean municipal groundwater standard limit (0.05 mg/L) over the 3 years when applied more than 7% of BLDH in the soil, indicating the BLDH could be used as a stabilizer to stabilize As under the non-equilibrium condition in the real field. To understand the As stabilization mechanisms of two LDH stabilizers, batch experiments with a high concentration of As (100 mg/L) and several analyses were additionaly performed. Results of SEM/EDS and BET analyses supported that the Mg-Fe LDH used in this study has sufficient porous structure, and the As was fixed on the outer surface of both PLDH and BLDH. From the results of FT-IR and XRD analyses, the presence of carbonate (CO32-) and sulfate (SO42-) anions at the interlayer spaces of LDH was investigated, and it was identified that some of the anions were replaced by arsenate during the stabilization process.