EFFECTS OF ZN PRESENCE ON THE STRUCTURE AND REACTIVITY OF MN OXIDES TOWARDS PHOSPHATE SORPTION

Phosphate (P) is an essential nutrient and sometimes a limiting factor for all organisms. Thus it is critical to understand P biogeochemical cycling. Manganese oxides (MnOx) is a group of metal oxide minerals that is ubiquitous in almost all environmental settings and can be an important P sink. Although P adsorption on MnOx is not significant at circumneutral conditions, we hypothesize that during and after the formation of MnOx metal cations can incorporate into the structure or strongly complex with their surface, thus potentially change the reactivity of MnOx toward P sorption under otherwise unfavored condition.

We test our hypothesis by comparing the effect of Zn presence during (i.e. doping) and post (i.e. sorption) Mn oxide formation toward P sorption. Two types of MnOx were tested, birnessite and d-MnO₂, both structurally similar to the most common fresh MnOx phases produced by microorganisms. Batch experiments were conducted to evaluate P sorption onto pure or Zn doped MnOx. Morphological and structural analysis techniques such as X-ray diffraction (XRD), synchrotron X-ray adsorption spectroscopy (XAS), transmission electron microscopy were performed to understand the local structure of Zn doped MnOx and reaction mechanism between MnOx and P (e.g. formation of ternary surface complex, surface precipitation).

Results reveal that Zn presence during MnOx formation has different effects on the structure of both birnessties and δ-MnO₂. Decreased crystallinity was observed for both MnOx. However, P uptake by birnessite and δ-MnO₂ are effected differently. Adsorption is greatly enhanced onto Zn-doped δ-MnO₂ but more complicated in the case of birnesste. Overall, P uptake by MnOx pretreated with metal cations points to the important roles of MnOx in controlling P availability and cycling.