Structure of Zn Surfaces Complexes on Biogenic Hexagonal Birnessite

Birnessite minerals produced by bacteria and fungi are ubiquitous environmental nanoparticles participating in important geochemical processes, particularly trace metal scavenging. These layer-type Mn oxide minerals are poorly-crystalline, with significant structural disorder created by Mn(IV) vacancies and random stacking arrangements. Negatively-charged cation vacancies largely account for the metal-scavenging capacity of these minerals. Among the trace metals of major interest, Zn appears to be influenced strongly by adsorption on birnessite minerals. 3O7. 3H2O), in which Zn is in octahedral coordination with three surface O around a Mn(IV) vacancy and three O from H2O. However, the local coordination environment of adsorbed Zn does not replicate that in chalcophanite. In addition to ZnVI-TCS, tetrahedral surface complexes (ZnIV-TCS) have been shown to exist at low surface coverage.