Ball-Milled Humic Acid–Functionalized Layered Double Hydroxides Enable Cd–As Stabilization in Contaminated Agricultural Soils

Layered double hydroxides (LDHs) are promising amendments for simultaneous cadmium (Cd) and arsenic (As) immobilization, but pristine materials often require relatively high application rates. Here, MgFe LDHs were functionalized with two bio-derived modifiers, cysteine and humic acid, using calcination–reconstruction or mechanochemical ball milling. The resulting materials were evaluated in separate aqueous Cd(II) and As(III) adsorption experiments and in soil incubation and pot experiments using Cd–As contaminated soil. In separate aqueous adsorption experiments, ball-milled humic acid–functionalized LDHs (M-H-LDHs) exhibited the strongest performance, with fitted maximum Cd(II) and As(III) adsorption capacities of 443 and 93.3 mg g−1, respectively, representing increases of 243% and 104% relative to pristine LDHs. When applied at 5 g kg−1 soil in soil incubation experiments, M-H-LDHs reduced extractable soil Cd and As by 67.3% and 50.4%, respectively. Pot experiments further demonstrated decreases in Cd and As concentrations in Chinese cabbage shoots by 34.5% and 13.1%, respectively. This enhanced immobilization may be associated with humic acid functional groups and mechanochemical changes to the material surface, as well as changes in soil pH and microbial community structure. These findings provide a promising strategy for efficient co-stabilization of Cd and As in agricultural soils.