Lead/cadmium impacts on zeolite-tobermorite: Nutrient release and sediment stability

To realize the comprehensive utilization of large amounts of high-ash coal slime and comprehensively understand the excellent performance of nutrient release and lead and cadmium adsorption of high-ash coal slime silicon composite materials, green and safe mild hydrothermal conditions (200 °C) were used to prepare the rich-rich coal slime. Zeolite/tobermorite composites (Z-TOBs) were used in this study. Batch adsorption tests and repeated extraction tests were used to determine whether silicon, potassium, and calcium nutrients of Z-TOBs have sustained release properties and are affected by pH. Through temperature gradient analyses, batch adsorption experiments at the water-soil interface, and density functional theory (DFT) calculations, the nutrient release mechanisms and the passivation of lead (Pb) and cadmium (Cd) by Z-TOBs were comprehensively investigated. The findings indicated that the release of silicon, calcium, and potassium nutrients from Z-TOBs was influenced by both ambient temperature and Pb and Cd concentrations. As the ambient temperature increased, the release patterns of different nutrients from Z-TOBs varied significantly, and the release of Pb and Cd was enhanced; however, the adsorption capacity for Pb consistently exceeded that for Cd. The passivation effect of soil amended with Z-TOBs on Pb remained stronger than that on Cd, significantly impacting the release of silicon. Characterization results revealed that silicon participated in the formation of silicon-containing compounds such as Cd₂SiO₄, CdAl₂Si₃O₁₂, and Pb₃Si₂O₇. The skew density calculated using DFT indicated that the silicic acid compounds formed with Pb and Cd exhibited greater stability than those formed with CO₃^2- and SO₄^2- groups, with Pb compounds demonstrating superior stability compared to Cd compounds. This study offers both practical and theoretical insights for the comprehensive utilization of high-ash coal slime in mild environments, presenting an alternative pathway for sustainable agricultural development.