The simultaneous passivation of arsenic and cadmium in contaminated soil through combined use of schwertmannite and calcium carbonate

Abstract

Schwertmannite was proved to be an efficient material for the remediation of arsenic-contaminated soil. Given that arsenic (As) and cadmium (Cd) coexist in soils and have opposite chemical behaviors, stabilizing them simultaneously is challenging. Here, we reported a compound passivating agent (AMSC) which simultaneously passivated As and Cd in contaminated site soil containing 1659 mg/kg As and 32.26 mg/kg Cd. It consisted of alkali-modified schwertmannite (denoted as AMS) capable of preferentially adsorbing As and fine calcium carbonate (denoted as C) for mainly stabilizing Cd. It was found that the addition of AMSC into the contaminated soil at 5 % for 60 days could drastically reduce water-extractable As and TCLP-As concentrations by over 84 %, and water-extractable Cd and TCLP-Cd concentrations by over 94 %, respectively, due to the conversion of the labile fraction of As or Cd into more stable fractions. Additionally, pH in the AMSC-amended soil was increased to 6.74 from initial 4.50. As was mainly adsorbed into schwertmannite of AMSC by ligand exchange of SO42- with As, while Cd was jointly passivated mainly by calcium carbonate and fewly by schwertmannite. To identify the mineral phase changes of AMSC, quartz sands of different grain sizes were selected for Mössbauer analysis. The percentage of schwertmannite in the sediment amended with AMSC decreased slightly from 51.1 % to 48.9 % within 30 days, while Fe(III) oxyhydroxide (48.9 %) was converted to ferrihydrite (30.3 %) and lepidocrocite (20.8 %), respectively. Therefore, it was expected that AMSC could act as a high-performance and environmentally-friendly amendment to remediate As and Cd-contaminated soils.