Binding of Cd(II) to birnessite and fulvic acid organo-mineral composites and controls on Cd(II) availability

Manganese oxide minerals (MnOs) are major controls on cadmium (Cd) mobility and fate in the environment. However, MnOs are commonly coated with natural organic matter (OM), and the role of this coating in the retention and availability of harmful metals remains unclear. Herein, organo-mineral composites were synthe-sized using birnessite (BS) and fulvic acid (FA), during coprecipitation with BS and adsorption to preformed BS with two organic carbon (OC) loadings. The performance and underlying mechanism of Cd(II) adsorption by resulting BS-FA composites were explored. Consequently, FA interactions with BS at environmentally repre-sentative (5 wt% OC) increase Cd(II) adsorption capacity by 15.05-37.39% (qm = 156.5-186.9 mg g-1), attributing to the enhanced dispersion of BS particles by coexisting FA led to significant increases in specific surface area (219.1-254.8 m2 g-1). Nevertheless, Cd(II) adsorption was notably inhibited at a high OC level (15 wt%). This might have derived from the supplementation of FA decreased pore diffusion rate and generated Mn (II/III) competition for vacancy sites. The dominant Cd(II) adsorption mechanism was precipitation with min-erals (Cd(OH)2), and complexation with Mn-O groups and acid oxygen-containing functional groups of FA. In organic ligand extractions, the exchange Cd content decreased by 5.63-7.93% with low OC coating (5 wt%), but increased to 33.13-38.97% at a high OC level (15 wt%). These findings help better understand the environ-mental behavior of Cd under the interactions of OM and Mn minerals, and provide a theoretical basis for organo-mineral composite remediation of Cd-contaminated water and soil.