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 CaCl_2-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 p-hydroxybenzoic acid, coumaric acid, benzoic acid, and cinnamic acid. The study demonstrated that in the MSA-CFA 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 high-energy 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.