Reduction of porewater arsenic and accumulation in rice grains by novel composite membranes under flooded conditions

Reductive dissolution of As-bearing Fe-oxyhyr)oxides in paddy soils under flooded conditions can trigger mass As liberation into porewater as a primary hotspot of bioavailable As and thus favor As uptake by rice. To tackle this challenge, we fabricated a ferrihydrite-polyvinyl alcohol composite membrane to target and extract porewater As in paddy soils, aiming at decreasing As accumulation in rice grains. The treatment deploying the membranes at three depths reduced porewater As concentration by 34% and average diffusive gradients in thin-films (DGT)-measured As levels by 36% at the 0–20 ​cm soil depth relative to control. The As content in rice grains decreased by 34% accordingly. Furthermore, ammonium phosphate application not only enhanced the As extraction efficiency of the composite membranes from porewater, but also increased the oxidation percentage of AsIII on the membranes based on X-ray photoelectron spectroscopy analysis. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy reveals Si enrichment on ferrihydrite surfaces within the membrane under flooded conditions. This surface-bound Si effectively inhibits ferrihydrite crystallization and retards its reductive dissolution, thereby sustaining high As removal efficiency throughout extended extraction cycles. This study provides a promising remediation strategy for in-situ removal of porewater As in paddy soils, significantly mitigating As accumulation in rice grains. • An efficient ferrihydrite-PVA membrane extraction method for porewater As was developed. • The As content in rice grains decreased after extraction using ferrihydrite-PVA membrane. • Active sorption sites toward As on the membrane increased with Fe precipitation. • Ferrihydrite-PVA membrane promoted arsenite oxidation.