Design of Size-Controllable MgFe-Layered Double Hydroxide for Synergistic Immobilization of Composite Arsenic and Cadmium Contamination

Arsenic (As) and cadmium (Cd) are highly toxic heavy metals that commonly coexist as mixed pollutants in mining, smelting, and agricultural soils. The effective simultaneous removal of coexisting anionic arsenic(III) and cationic cadmium(II) poses a significant challenge to conventional remediation methods and the application of restoration materials. This study synthesized layered double hydroxides (LDHs) with conventional bulk structure (MgFe-L) and ultrathin small-sized (MgFe-S) morphologies via separate nucleation and aging steps. The obtained MgFe-S showed superior removal ability compared to MgFe-L, owing to its increased specific surface area and exposed active sites. In water containing high concentrations (>100 mg·L–1) of both contaminants, it removed over 95% of As(III) and Cd(II) within 2 h. Under low concentrations, its enhanced visible-light photocatalytic activity oxidized As(III) to As(V) and immobilized Cd(II) for deep purification. Soil experiments demonstrated effective metal passivation, reduced plant uptake, and improved crop growth via Mg(II) release from the LDH structure, providing a new view in both treating pollution and providing Mg fertilizer. Detailed characterizations proved that the size effect enhanced surface defects for As(III) removal and promoted Cd(II) substitution by weakening interlayer interaction. Cd(II) incorporation further increased As(III) affinity, revealing a synergistic host–guest removal mechanism. This study provides valuable insights for designing LDHs for coexistence of anionic and cationic metal pollution in water and soil.