Immobilization mechanisms of lead and cadmium by pre-formed ferrihydrite-phosphate composite

How the iron (Fe) (hydr)oxides-phosphorus (P) composites preformed in environmental media impact the distribution and mobility of toxic heavy metals is poorly understood. Given this, we combined wet chemistry experiments, Visual MINTEQ simulation, chemical sequential extraction, and spectral methods to explore the interaction mechanisms of lead (Pb) and cadmium (Cd) with ferrihydrite-phosphate (Fh-P) composite. Compared to raw Fh, Fh-P exhibited faster Pb and Cd sorption kinetics and superior sorption performance. Cd did not affect Pb's sorption rate but suppressed the formation of more stable Pb chemical forms. Conversely, Pb reduced Cd's sorption ratio but enhanced the stability of Cd chemical fractions. The mixed organic and inorganic components (i.e., OC + IC) accelerated Pb sorption during 0-3 days, with no noticeable enhancement after 7 days. OC + IC promoted Cd sorption during the entire period of 0-7 days. IC and OC dominated the enhancement of Pb and Cd sorption, respectively. OC + IC did not alter Pb chemical fractions' stabilities while enhancing Cd chemical forms' mobility and environmental risk. Pb was immobilized by Fh-P through inner-sphere surface complexation and (co)precipitation. In contrast, electrostatic interactions and/or outer-sphere surface complexation were the dominant driving forces of Cd sorption. The sorption of Pb and Cd did not alter the chemical forms and stabilities of Fe and P in the Fh-P composite. Overall, this study fills the research gap regarding the immobilization of mixed heavy metals by iron (hydr)oxide-phosphate composites under the interference effects of coexisting OC and IC. The research findings are critical for predicting the environmental fate and potential risks of Pb, Cd, Fe, and P elements in acidic water and soil systems.