Effect of Zn(II) coprecipitation on Mn(II)-induced reductive transformation of birnessite

Mn oxides (MnOx) are ubiquitous metal oxide minerals in nearly all environmental settings. They play important roles in the transport and fate of many environmental components such as metals, organics, and nutrients. In the presence of dissolved Mn(II), MnOx phases can undergo ripening and transformation, resulting in the formation of phases with higher structural order, thus strongly affect the reactivity of MnOx over extended time scale. In natural environments, metal cations can strongly interact with MnOx through mechanisms such as sorption, incorporation, and/or coprecipitation, yet much still remain unknown about the effect of metal coprecipitation on the transformation of MnOx. This study investigates the effects of Zn coprecipitation on Mn(II)-induced reductive transformation of birnessite, a common MnOx mineral phase. Pure and Zn-coprecipitated acid birnessite phases were synthesized and their transformation kinetics and pathways in the presence of Mn(II) was investigated under oxic or anoxic conditions. During the transformation process, Zn-coprecipitated birnessite showed higher capability toward Mn(II) uptake, likely due to smaller particle size and the fast consumption of Mn(II) and precipitation of a new phase hetaerolite. The formation of an intermediate phase, feitknechtite, was faster for Zn-coprecipitated birnessite than pure birnessite, which is the opposite of Zn-sorbed birnessite system. Transformation from the intermediate phase feitknechtite to the final stable phase manganite was slower for Zn-coprecipitated birnessite, due to the lower Mn(II) concentration which catalyzed the transformation. This study revealed the importance of understanding the influence of metal cation impurities on the structural stability and long term reactivity of Mn oxide minerals.