Impacts of organic acids of varying molecular weights on alkaline transformation and cadmium immobilization in bauxite residue.

Ecological restoration of bauxite residue (BR) dump has proved great potential in the large-scale disposal of BR, during which organic amendments with varying molecular weight organic acids play critical roles on alkaline regulation and heavy metal (e.g., cadmium) migration in BR, yet the mechanisms remain unclear. Building on a systematic investigation on the impacts of organic acids (oxalic, citric, malic acids) on the alkalinity transformation of BR, this work further explores the geochemical interplay between the dominant alkaline minerals (sodalite and calcite) and organic acids in mediating Cd immobilization. The results revealed that low molecular weight organic acids (LMWOAs) induced alkalinity evolution of BR via pH adjustment, surface charge modification, alkaline minerals dissolution and aggregate structure improvement. Sodalite and calcite could immobilize Cd via monolayer chemisorption, with sodalite exhibiting higher capacity and faster kinetics. LMWOAs suppressed Cd retention on both minerals by competing binding sites and promoting mineral dissolution, while high molecular weight organic acids (HMWOAs, humic/fulvic acid: HA/FA) enhanced Cd adsorption on sodalite through ternary complexes (sodalite-HA/FA-Cd) but impeded immobilization on calcite by blocking active sites and generating soluble Cd-organic complexes. These insights provide a basis for tailoring organic amendments to optimize soil properties and limit Cd bioavailability in BR remediation.