Selectivity characteristics of immobilization effects of sulfhydryl grafted palygorskite on heavy metals in soils

Abstract

Sulfhydryl-based amendments are emerging immobilization agents for heavy metal contaminated soils, though their immobilization selectivity remains incompletely understood. To elucidate the sorption preferences and immobilization selectivity of sulfhydryl grafted palygorskite (SGP), series of experiments including solution sorption, spectroscopic characterization, density functional theory (DFT) calculation, soil incubation, and plant pot cultivation were conducted. Sulfhydryl grafting significantly enhanced SGP's sorption capacities for Cu²⁺, Hg²⁺ and Cd²⁺. The selectivity coefficients determined by the competitive sorption highlighted the sorption preference of SGP towards Fe³⁺, Hg²⁺ and Cu²⁺, but Fe³⁺ was attributable to the palygorskite substrate itself. The sorption selectivity was confirmed by the sorption energies calculated by the DFT. In six model soils, SGP consistently reduced available Cd and Cu contents with the minimum reductions of 35.74 % and 20.93 % respectively, regardless of acid or alkaline soils, while showing no consistent effect on other cations. Correspondingly, SGP decreased Cd and Cu accumulation in pakchoi leaves with the minimum reductions of 40.51 % and 29.67 %, demonstrating selective immobilization. The maximal immobilization efficiency for Cd (5.66 mmol/kg) and Cu (5.37 mmol/kg) quantitatively characterized SGP's preferential selectivity of the immobilization effect in acid and alkaline soils. This study advances the theoretical framework for selective immobilization and provides a scientific foundation for the remediation of Cd contaminated soils.