In-situ remediation for arsenic-contaminated soils using iron and aluminum amendments under varying redox conditions.

Arsenic (As) contamination can threaten groundwater quality and human health, particularly where redox conditions fluctuate. Although iron (Fe) and aluminum (Al) (hydr)oxides strongly adsorb arsenate under oxic conditions, flooding can promote As(V) reduction to more toxic and mobile As(III) and destabilize the Fe(III)-As(V) complexes. This study evaluated ferrous sulfate and aluminum sulfate as in-situ soil amendments to immobilize As and assess their stability under submergence. A batch, column leaching, and anaerobic incubation study was conducted to monitor redox potential (Eh), As speciation, and Fe/Al behavior. Results showed that both amendments reduced water-soluble and leachable As at pH 7-9. In batch tests, Fe and Al amendments lowered water-soluble As by 78.3% and 83.3%, respectively. Column tests showed that most As eluted within the first 10 pore volumes, with Fe and Al amendments reducing cumulative leachable As by 60.3% and 89.7%, respectively. During incubation, soil Eh declined to strongly reducing conditions (-259 mV). Around days 20-30 of incubation, the As(III)/total As ratio was >50%. Fe amendment showed substantial Fe(II) accumulation consistent with reductive dissolution and As remobilization. Although the Al amendment remained redox-stable and maintained lower dissolved As, strongly reducing conditions could still increase risk by shifting As(V) toward As(III). Overall, aluminum sulfate is preferred for flood-prone or redox-fluctuating soils because it remains effective under reducing conditions, whereas ferrous sulfate can be applied for persistently oxic sites to minimize redox-driven Fe(III)-As(V) destabilization.