Remediation of Cd-contaminated soil through different layered double hydroxides: The weakness of delamination and mechanism

The great variability of the reaction of layered double hydroxides (LDHs) with anions and cations, especially the ultra-thin thickness, large specific surface area and fully-exposed active sites of single LDHs, may be crucial factors in the remediation of Cd-contaminated soil. In this study, the environmental behavior of Cd was investigated through sequential extraction method and plant potting tests, so as to verify the immobilization efficiency of LDHs and single LDHs in Cd-polluted soil. The results revealed that S-LDHs differed significantly from the original LDHs, and Ca-LDH were the best material for immobilizing Cd. Through sequential extraction procedures, the exchangeable fraction of Cd decreased and its residual fraction increased by forming CdCO3(s) and Cd5(OH)8(NO3)2.2 H2O. After treating the Cd-contaminated soil, the LDH layers were restacked through the XRD-pattern-confirmed S-Mg-LDH, while S-Ca-LDH were more converted to CaCO3 and Cd5(OH)8(NO3)2.2 H2O. Through investigating the structural changes of stabilizers in soil remediation via a series of characterization, the difference between Mg-LDH and S-Mg-LDH was in the generation of CdCO3(s), while that between Ca-LDH and S-Ca-LDH was in the formation of CdAl-LDH-CO3. Single LDHs and LDHs showed the opposite effect when reacting with different pollutants (Cd and Cr), and single LDHs were not as effective for Cd in soil as was previously reported. These results helped with a better understanding of the use of LDHs as a solidified agent as well as the mechanisms involved.