Unraveling immobilization mechanisms of Cd in soil by MgO-modified palygorskite/biochar composite: DFT calculation and combined-artificial aging.

In this study, a combination method of freeze-thaw cycle, dry-wet cycle, and chemical agings was used to investigate the aging effect of MgO-modified palygorskite/biochar composite (MPBC) in soil, and its immobilization capacity on Cd under aging. The immobilization mechanisms of MPBC for Cd were explored through several characterizations and DFT calculations. The results showed that MPBC effectively reduced the activate state of Cd by 56.63% at 8 mg/kg Cd concentration. Additionally, MPBC treatment improved physicochemical properties of soil, notably increasing soil pH by 0.26-0.64 units, thereby facilitating Cd immobilization. The predominant mechanism underlying Cd immobilization by MPBC involved the Cd-π complexation, ions exchange, precipitation, and complexation of surface functional groups, including C-O and C=O, with Cd. The citric acid emerged as a milder oxidizing agent combined with freeze-thaw and dry-wet aging conducive to studying the aging effect of MPBC. The dynamic calculation showed that MgO played an important role in the Cd adsorption, with a maximum probability function of 18.35 for Cd. Moreover, within the temperature range of 20 °C-30 °C, the distance between MPBC and Cd was the closest. This study provides a new idea for artificial aging of biochar and a practical method for the remediation of Cd pollution in soil.