Elucidation of Zn surface complexation and precipitation mechanism in coprecipitation with birnessite

Abstract

Birnessite (δ-MnO2) is recognized as an adsorbent for heavy metal ions in soil and aqueous environments. In particular, coexisting metal ions can be incorporated into the crystalline structure of δ-MnO2 when δ-MnO2 is formed under a high Mn oxidation rate. Thus; in this research we studied the mechanism of Zn coprecipitation and the surface complexation associated with the formation of δ-MnO2 under neutral pH conditions (pH 6.0, and pH7.5), considering the results of surface complex modeling and X-ray absorption fine structure (XAFS) analysis. As a result, the coprecipitation mechanism was estimated as the three-dimensional uptake of Zn accompanying the formation of Zn-Mn minerals on the surface of δ-MnO2, in addition to the surface complexation with Zn between the layers of δ-MnO2. In addition, the produced Zn-Mn minerals differed depending on the pH and Zn/Mn molar ratio, and a large difference was confirmed in the amount adsorbed. From the XAFS analysis, it is considered that this could be caused by the transformation of the mineral composition and crystal structure by a slight difference in pH change.