Effect of Zn coprecipitation on the structure of layered Mn oxides

Abstract 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 Zn 2+ presence during mineral formation (i.e. coprecipitation) on the structure of acid birnessite and δ-MnO 2 , two synthetic analogs that are structurally similar to fresh biogenic MnO x but with different crystallinity. For both acid birnessite and δ-MnO 2 , Zn 2+ existed as surface adsorbed species at vacancy sites, interrupted layer stacking along c axis, and caused reductions of the lateral particle size. Zn 2+ also reduced Mn(III) contents in δ-MnO 2 layers, leaving more vacancy sites (capped by adsorbed Zn 2+ ). The reduction of layer stacking was more obvious for acid birnessite, while the modification of layer structure was more significant for δ-MnO 2 . These structural changes will likely lead to modified reactivity of MnO x in natural systems.