Effect of Zn coprecipitation on the structure of layered Mn oxides

Mn oxides (MnO_x) are a group of ubiquitous metal oxides in the environment and can significantly affect the biogeochemical cycles of metals, nutrients, and contaminants. Due to their negative surface charge across a wide range of environmental conditions, metal cations have strong affinities for MnO_x, and the presence of metal cations during or after the formation of MnO_x might significantly affect their structure and reactivity. This study systematically investigates the effects of Zn²⁺ presence during mineral formation (i.e. coprecipitation) on the structure of acid birnessite and δ-MnO₂, two synthetic analogs that are structurally similar to fresh biogenic MnO_x but with different crystallinity. For both acid birnessite and δ-MnO₂, Zn²⁺ existed as surface adsorbed species at vacancy sites, interrupted layer stacking along c axis, and caused reductions of the lateral particle size. Zn²⁺ also reduced Mn(III) contents in δ-MnO₂ layers, leaving more vacancy sites (capped by adsorbed Zn²⁺). The reduction of layer stacking was more obvious for acid birnessite, while the modification of layer structure was more significant for δ-MnO₂. These structural changes will likely lead to modified reactivity of MnO_x in natural systems.