Organic matter–induced formation and removal of floating iron films as a mechanism for arsenic remediation at the soil–water interface

Arsenic (As) contamination in flooded soils is strongly governed by redox-driven iron (Fe) cycling at the soil–water interface (SWI), yet few remediation strategies explicitly exploit this dynamic process. Here, we propose and test a novel remediation mechanism based on organic matter (OM)–induced formation and manual removal of floating iron films. Using As-contaminated soils from Foshan, China (185.66 μg As g⁻¹ soil; 22.09 mg Fe g⁻¹ soil), a 10-week flooding experiment was conducted with different OM amendments, including cassava starch (0.05% and 0.1%) and milled straw (0.1% and 0.2%), alongside an unamended control. Continuous monitoring of dissolved oxygen and redox potential revealed that OM addition markedly enhanced reducing conditions at the SWI, promoting the reductive dissolution of Fe(III) minerals and the release of dissolved As. As redox gradients migrated upward, Fe(II) was re-oxidized in the overlying water, leading to the formation of floating iron films that effectively scavenged As from the water column. Periodic manual removal of the iron films resulted in substantial As export from the system. Depth-resolved soil extractions further demonstrated a redistribution and net depletion of labile As and Fe fractions near the SWI. These results demonstrate that coupling OM-stimulated iron reduction with targeted removal of secondary iron phases offers a promising, process-based strategy for arsenic remediation in flooded soils.