Adsorption of CaAl-LDH for Cd2+ and its passivation on Cd-contaminated farmland around electroplating and metal processing industry cluster area

Abstract

The enrichment of heavy metal Cd in farmland soil results in significant harm to agricultural products, yet its removal poses considerable challenges. CaAl-layered double hydroxide (CaAl-LDH), a category of layered bimetallic composite hydroxides, is characterized by high efficiency, cleanliness, and stability in soil remediation. In this study, a CaAl-LDH adsorption experiment was conducted, and Cd-contaminated farmland surrounding a typical hardware processing enterprise was selected for a field trial using CaAl-LDH to passivate Cd in the soil. Alterations in Cd forms in soil and Cd content in rice were continuously monitored over three consecutive years to investigate the passivation effect of CaAl-LDH on Cd-contaminated farmland. The results indicated that the absorption of Cd²⁺ by CaAl-LDH adhered to a pseudo-second-order kinetic and the Langmuir model. At a pH of 6, CaAl-LDH's maximum adsorption capacity for Cd²⁺ reached 469.48 mg kg^–1. Compared to fields treated with CaAl-LDH, a greater proportion of ion-exchange state Cd in rhizosphere soil transitioned to residual state and Fe-Mn bound state Cd, significantly reducing Cd content in rice grains. Continuous in-situ monitoring revealed that Cd content in rice grains treated with CaAl-LDH decreased by over 37% in the second year.