Impacts of aged nanoscale zero-valent iron on the weathering of arsenopyrite: Arsenic transformation and environmental implications.

Arsenopyrite, an arsenic-bearing sulfide mineral, is highly susceptible to weathering, causing contamination with toxic arsenic and hydrogen ions. Nanoscale zero-valent iron (nZVI), which is widely employed in environmental remediation, is gradually oxidized into iron hydroxides accompanied by aging phenomena after being applied to soil or groundwater, thereby affecting the oxidation process of arsenopyrite. In this study, electrochemical measurements, immersion experiments and various characterization techniques were employed to explore the effects of aged nanoscale zero-valent iron (A-nZVI) on the oxidation mechanism of arsenopyrite. Experimental results indicate that within the low concentration range (0-1.0 g·L-1), A-nZVI accelerates the oxidation rate of arsenopyrite and raises arsenic concentration. The corrosion current density reaches its peak at 1.0 g·L-1. A-nZVI can adsorb and reduce arsenic in the solution, markedly decreasing the concentration of As(V). This inhibits the formation of ferric arsenate passivation products and reduces the charge transfer resistance at the mineral-solution interface. As A-nZVI concentration increase from 1.0 to 1.5 g·L-1, excess A-nZVI covered the mineral surface, increasing the adsorption layer resistance and charge transfer resistance, which in turn inhibited the oxidation of arsenopyrite. Characterization results verified that A-nZVI surface consists of lepidocrocite (γ-FeOOH) and magnetite/maghemite (Fe3O4/γ-Fe2O3). A-nZVI increases As(III) levels and total arsenic release without aggravating proton pollution originating from arsenopyrite. This work offers data and scientific evidence for evaluating A-nZVI performance in environmental/industrial uses and its large-scale application near mining sites.