Sulfhydryl grafted palygorskite amendment with varying loading rates: Characteristic differences and dose-effect relationship for immobilizing soil Cd

Heavy metal contamination in agricultural soil and immobilization remediation have generated widespread concern in all areas of society. Sulfhydryl-functionalized materials as emerging amendments exhibit application potential, but the dose-effect relationship and immobilization mechanism are poorly understood. To understand the relationship between the immobilization effect and total sulfhydryl content, sulfhydryl-grafted palygorskite (SGP) with three sulfhydryl loading rates (0.88 mmol/g, 1.83 mmol/g, and 2.77 mmol/g) was prepared and characterized in the current study. The Cd immobilization efficiency and the dose-effect relationship were investigated via sorption in solutions, soil incubation, and field-scale wheat cultivation. ²⁹Si nuclear magnetic resonance, X-ray diffraction, differential scanning calorimetry, X-ray photoelectron spectroscopy, zeta potential, and potentiometric titration analyses confirmed that the sulfhydryl loading rates had little impact on the mineral structure but had a significant effect on the SGP surface properties. The sorption process of Cd²⁺ on SGP can be described by the 2nd order kinetic model and the Langmuir isotherm. The maximal sorption capacities had a linear relationship with the experimental sulfhydryl content in SGP. Meanwhile, SGP with three loading rates under different doses reduced available Cd concentrations in alkaline soil by 28.40 %–87.78 % in soil incubation and wheat grain Cd by 21.95 %–80.19 % in field-scale demonstration The relationship between immobilization efficiency and the total sulfhydryl group could be adequately described using the Michaelis-Menten equation. The sulfhydryl group was the key site for SGP amendments, which when added to the soil, reduced the bioavailability of soil Cd. Clarification of the dose-effect relationship will provide theoretical support for the accurate regulation of safe utilization of Cd-contaminated farmland.