Strategic combined use of oyster shell and phosphogypsum for simultaneous immobilization of As(V) and Cd in upland arable soils

This study evaluated the simultaneous immobilization of arsenate and cadmium in upland arable soils using oyster shell and phosphogypsum, which are calcium-rich industrial by-products. Soil microcosm experiments were conducted to systematically evaluate the individual and combined effects of oyster shell and phosphogypsum on immobilizing arsenate and cadmium across acidic, neutral, and alkaline soils. The results showed that oyster shell significantly immobilized cadmium, achieving up to a 76.1 % reduction in acidic soils by increasing soil pH, which enhanced the saturation index for cadmium carbonate (p < 0.05). In contrast, phosphogypsum substantially decreased extractable arsenate by up to 20.6 % in neutral soils by lowering soil pH and enhancing the adsorption of arsenate onto soil particles (p < 0.05). Using quadratic regression analysis, an optimal oyster shell to phosphogypsum ratio of 40:60 was identified, which achieved simultaneous reductions of extractable arsenate and cadmium concentrations by 12.9 % (p < 0.001) and 31.4 % (p < 0.001), respectively. A mesocosm pot experiment further validated the practical effectiveness of a mixed calcium fertilizer with the optimal oyster shell to phosphogypsum ratio of 40:60. When applied at a rate of 4 Mg ha⁻¹, the mixed calcium fertilizer effectively reduced plant-available Arsenic and cadmium, leading to reductions of 16.4 % (p < 0.001) and 43.8 % (p < 0.05) in radish, respectively without any adverse effects on crop productivity. These findings suggest that reconciling oyster shell and phosphogypsum offers a promising immobilization strategy for simultaneously mitigating multi-metal contamination in arable soils.