Acidic stability and mechanisms of soil cadmium immobilization by layered double hydroxides intercalated with mercaptosuccinic acid

Layered double hydroxide intercalated with mercaptosuccinic acid (MSA-CFA) holds considerable promise for remediating cadmium (Cd)-contaminated soils through selective immobilization; however, its stability under acidic conditions has yet to be investigated. The acidic stability of MSA-CFA was investigated by acid stability investigation and simulated soil acidification. In the immersion test, the cadmium dissolution rate (DR) for the Cd immobilized products of MSA-CFA (MSA-CFA-Cd) was significantly lower (2.06% in nitric acid and 5.91% in malic acid) compared to 53.74% and 62.76%, respectively, for the Cd immobilized products of lime (lime-Cd). Furthermore, the CaCl2-Cd in soils immobilized with the MSA-CFA increased by 8.49% and 17.4%, respectively, after the soils were treated with inorganic acid solution of H2SO4 and HNO3 and organic acid solution of phydroxybenzoic acid, coumaric acid, benzoic acid, and cinnamic acid. The study demonstrated that in the MSACFA treatment, bioavailable Cd was transformed into a more stable Fe/Mn oxide-bound state. This was attributed to the formation of Cd-containing hydrotalcite through isomorphous substitution and the formation of highenergy S-Cd complexes via sulfhydryl binding. In comparison, lime treatment resulted primarily in precipitation, which was less effective at stabilizing the Cd. The replacement of interlamellar S-Cd by hydrolyzed organic acid ions increased the remobilization risk (/\CaCl2-Cd//\pH) of the soil Cd compared to inorganic acid treatment. This study preliminarily elucidated the resistance mechanisms of MSA-CFA in a complex acidic environment, providing insights into its long-term stability for soil Cd remediation.