Combined remediation of As(Ⅲ)-contaminated soils by pre-oxidation, stabilization and solidification

The solidification/stabilization (S/S) is the most popular method for treatment of heavy metal-contaminated soils, however, the S/S treatment of As(Ⅲ)-contaminated soils is not effective due to the high mobility of As(Ⅲ). A combined remediation technique is proposed, in which As(Ⅲ) is first oxidized to As(Ⅴ) by the Fenton reagent, then stabilized by FeCl₃ and finally stabilized by cement. The unconfined compressive strength tests, toxicity characteristic leaching procedure (TCLP), synthetic precipitation leaching procedure (SPLP), pH measurements, sequential extraction procedure and spectroscopic investigations are carried out to investigate the effects and mechanism of the proposed technique. The results show that the Fenton pre-oxidation process significantly improves the remediation efficiency. Under an Fe-to-As molar ration of 1:1 and a cement dosage of 10%, the leaching toxicity of As in TCLP and SPLP is as low as 2.51 and 1.33 mg/L, and the immobilization efficiency reaches 97.46% and 98.53%, respectively. The hydration degree of the cement and the pore structure of the curing body are improved by FeCl₃ and therefore the strength increases. The combined remediation can transform As to more stable phases and effectively reduce the potential environmental risk. The majority of As is bound to hydrous oxides of Fe, but an increase in pH due to the increasing cement dosage will affect the Fe-As binding and cause potential release of As. The spectroscopic investigations show that the proposed remediation can transform 92.5% of As(Ⅲ)to As(Ⅴ) and immobilize As by the encapsulation of calcium silicate hydrate and the ion exchange of ettringite. This study provides a new insight into the effective remediation of As(Ⅲ)-contaminated soils.