Cooperative effect of citric acid and pyrite on the slow-release of silicon from coal gangue and the stabilization of arsenic: Implications for soil remediation

The application of coal gangue as a soil amendment shows promise in increasing silicon availability, potentially serving as a silicon fertilizer in agriculture. However, the rapid release of silicon from coal gangue compared to the slow absorption by plants hinders its effective use. This study explored the formation of iron-containing secondary minerals on coal gangue surfaces using pyrite and citric acid to stabilize arsenic in contaminated soils and slow down silicon leaching. After co-ball milling, the silicon leaching rates for coal gangue and the composite C@PC-10 were 0.44% and 0.22% at 60 min, and 1.11% and 1.38% at 120 min, respectively. Stabilization tests showed that C@PC-10 achieved removal efficiencies of 71.3% for water-extractable arsenic and 55.9% for NaHCO₃-extractable arsenic over 30 d. Acid-soluble arsenic decreased from 32.8% to 24.1%, while residual arsenic increased from 26.5% to 36.9%. Acid rain simulations demonstrated that C@PC-10 limited leachate arsenic concentration to 28.9 mg/L over 120 d, compared to untreated soil with 59.8 mg/L. Analytical techniques like XRD, XPS, and FT-IR confirmed that pyrite oxidation during ball milling led to the formation of jarosite and FeOOH, enhancing arsenic adsorption capacity. Overall, the C@PC-10 composite shows promise as a remediation material for controlled silicate release and arsenic mitigation in soil environments.