Coordination geometry of Zn²⁺ on hexagonal turbostratic birnessites with different Mn average oxidation states and its stability under acid dissolution

Hexagonal turbostratic birnessite, with the characteristics of high contents of vacancies, varying amounts of structural and adsorbed Mn³⁺, and small particle size, undergoes strong adsorption reactions with trace metal (TM) contaminants. While the interactions of TM, i.e., Zn²⁺, with birnessite are well understood, the effect of birnessite structural characteristics on the coordination and stability of Zn²⁺ on the mineral surfaces under proton attack is as yet unclear. In the present study, the effects of a series of synthesized hexagonal turbostratic birnessites with different Mn average oxide states (AOSs) on the coordination geometry of adsorbed Zn²⁺ and its stability under acidic conditions were investigated. With decreasing Mn AOS, birnessite exhibits smaller particle sizes and thus larger specific surface area, higher amounts of layer Mn³⁺ and thus longer distances for the first MnO and MnMn shells, but a low quantity of available vacancies and thus low adsorption capacity for Zn²⁺. Zn K-edge EXAFS spectroscopy demonstrates that birnessite with low Mn AOS has smaller adsorption capacity but more tetrahedral Zn (^IVZn) complexes on vacancies than octahedral (^VIZn) complexes, and Zn²⁺ is more unstable under acidic conditions than that adsorbed on birnessite with high Mn AOS. High Zn²⁺ loading favors the formation of ^VIZn complexes over ^IVZn complexes, and the release of Zn²⁺ is faster than at low loading. These results will deepen our understanding of the interaction mechanisms of various TMs with natural birnessites, and the stability and thus the potential toxicity of heavy metal pollutants sequestered by engineered nano-sized metal oxide materials.