LEACHING TOXICITY OF PHOSPHOGYPSUN-SLAG SUBGRADE MATERIALS

Abstract

As a by-product of the phosphoric acid industry, phosphogypsum (PG) presents substantial environmental and health hazards due to residual fluorides, phosphorus compounds, and heavy metals, particularly during stockpiling or improper disposal. In this study, PG and slag were employed as primary precursors to synthesize subgrade materials, with a focus on evaluating their leaching toxicity characteristics via single-factor experimental designs. Leaching tests were conducted following the standard HJ/T 299-2007 to measure the concentrations of fluoride ion, phosphate ion, As, and Pb. Single-factor experiments were carried out to evaluate the effects of the water-to-binder ratio, hydrophobic agent dosage, and quicklime content. Results demonstrated that the optimal parameters achieved high immobilization efficiencies. Specifically, at a water-to-binder ratio of 0.32, the solidification rates for fluoride ion, phosphate ion, As, and Pb reached 98.53%, 99.12%, 76.37%, and 79.12%, respectively. Furthermore, a quicklime content of 12% and a hydrophobic agent dosage of 0.75% yielded the best comprehensive solidification performance. These findings provide crucial scientific evidence for the safe valorization of PG in subgrade engineering, contributing to the reduction of solid waste pollution and the sustainable development of the phosphorus chemical industry. It is recommended that admixture formulations be tailored to specific regional environmental conditions, accompanied by rigorous long-term monitoring of leaching toxicity.