Structural Evolution of Disordered Mackinawite (FeS_m) and Its Impact on Arsenic Sorption

Disordered mackinawite (FeS_m) plays a critical role in arsenic sequestration in sulfidic environments. However, its structural evolution and the resulting impact on the variability of arsenic sorption behavior remain unexplored. Iron and arsenic K-edge X-ray absorption spectroscopy (XAS) was employed to investigate FeS_m aging effects on arsenic sorption across pH 4.9-9.1 at an arsenic concentration of 1 mM. Iron K-edge XAS reveals that aging promotes defect healing and particle growth of FeS_m. Concurrently, arsenic removal efficiency declines, with the extent varying with pH and arsenic species. Over a 60-day aging period, As(III) removal remains nearly complete at pH 4.9, but the percent removal decreases from 8.3% to 3.3% at pH 7.1 and from 6.5% to 6.0% at pH 9.1. In contrast, As(V) removal efficiency decreases under all pH conditions (e.g., from 7.4% to 4.0% at pH 4.9). Arsenic K-edge XAS shows that, at acidic-to-neutral pH, aging promotes zerovalent arsenic (As(0)) formation over arsenic sulfides due to decreased FeS_m solubility and enhanced metallic conductivity. At basic pH, arsenic initially forms surface complexes, but prolonged aging favors As(0) formation. These results challenge the conventional view of FeS_m as a static sorbent and highlight its dynamic nature, providing insights for improving its long-term effectiveness.