Mechanistic Insights into Fe(II)-Mediated Phase Transformation of Birnessite and Its Impacts on Cd Immobilization

Manganese minerals are a vital container for cadmium (Cd) scavenging in anoxic soils and sediments, where the redox reaction between ferrous ions (Fe(II)) and manganese minerals critically controls its fixation and release. However, how the Fe(II)-mediated reductive transformation of manganese minerals affects cadmium binding speciation remains unknown. Here, the kinetics of birnessite transformation mediated by Fe(II) and its impacts on cadmium mobility across a range of Fe(II)/Mn ratios and pH were systematically investigated. During the redox reaction, Fe(II) oxidation into ferrihydrite and the reductive dissolution of birnessite into divalent and trivalent manganese were facilitated by higher Fe(II) loadings and pH, resulting in the formation of more Fe-Mn complex minerals. After the redox reaction, no other crystalline iron and manganese minerals were detected. Despite efficient immobilization, a portion of cadmium, initially bound to birnessite, was redistributed to newly formed ferrihydrite. Interestingly, greater cadmium immobilization by Fe-Mn complex minerals was observed at higher pH values and lower Fe(II) loadings. The cadmium immobilization was primarily attributed to its binding to adsorption sites and vacancies of minerals, with minor contributions from possible physical encapsulation and the formation of Cd-Mn coprecipitates. Shell-by-shell fitting revealed that cadmium formed double corner-sharing, edge-sharing, and triple-corner-sharing complexes with the Fe-Mn complex minerals. These findings provide novel insights into the redox reaction between Fe(II) and birnessite and the associated cadmium dynamics, advancing our understanding of iron, manganese, and cadmium geochemical cycling in anoxic environments.