Effect mechanism of low-molecular-weight organic acids during sulfidation of As(V)-bearing ferrihydrite.

Sulfide induces the reductive dissolution of iron (oxyhydr) oxides, the primary host phases for arsenic (As), thereby triggering As release. We investigates the physicochemical mechanisms of three types of low molecular weight organic acids (LMWOAs) on sulfide-mediated reductive dissolution of As(V)-ferrihydrite and As release using batch experiments combined with hydro-chemical, spectroscopic, and microscopic analyses. Arsenate dominated the aqueous (97.2-100 %) and solid phases throughout the experiment. LMWOAs accelerated S(-II) consumption and As release by inhibiting FeS formation, with rates ordered as citric acid (CA) > oxalic acid (OA) > malic acid (MA) > control (Kb). At S(-II): Fe = 0.5, maximum As release was 11.78 % (Kb) and 14.60 % (CA); at S(-II): Fe = 1, it was 27.58 % (Kb) and 30.71 % (OA). LMWOAs enhanced As release via non-reductive ligand dissolution of As(V)-ferrihydrite. Secondary mineral formation in later stages re-immobilized As, with mineral layers ≥50 nm thick. LMWOAs interacted differently with secondary minerals: CA primarily adsorbed on surfaces, while MA integrated into the matrix. LMWOAs influenced As redistribution in secondary minerals, increasing contamination risks. Thus, the complex effects of organic matter (OM) on Fe, S, and As biogeochemistry must be considered in risk assessments and remediation strategies for As-contaminated sites in sulfidic environments.