Unveiling the roles of humic acid on As(III) remediation by nanoscale zero-valent iron in anoxic water

Humic acid (HA), ubiquitous in natural groundwater, affects the efficiency and long-term performance of nanoscale zero-valent iron (nZVI) for in-situ contaminated groundwater remediation. This work aims to clarify how HA affects As(III) removal by nZVI under anoxic conditions. Batch and column experiments were conducted to investigate the removal behaviors of As(III). Multiple characterization techniques, including X-ray absorption fine structure, transmission electron microscopy integrated with energy-dispersive X-ray spectroscopy mapping, and X-ray photoelectron spectroscopy, were employed to analyze interfacial reactivity and electron transfer characteristics. The results showed that HA exhibited concentration-dependent inhibition on both As(III) removal rate and surface-mediated oxidation of As(III). The reaction rate constant decreased to approximately 80% and As(V) proportion declined to roughly 60%. HA exerted no obvious impact on Fe° corrosion and nZVI solid phase evolution. The main inhibitory mechanisms were competitive adsorption on active sites and complexation with aqueous iron ions, which restricted As(III) immobilization and hindered As(III) oxidation induced by surface iron oxides and Fe(II). Furthermore, column experiments verified that HA improved the mobility of nZVI in simulated groundwater environments, but did not offset its inhibitory effect on As(III) sequestration. These findings provide scientific insights into the influence of HA on As(III) sequestration and offer a fundamental basis for enhancing the application performance of nZVI.