Coprecipitation mechanisms of Zn by birnessite formation and its mineralogy under neutral pH conditions

Birnessite(δ-Mn(IV)O_(2))is a great manganese(Mn)adsorbent for dissolved divalent metals.In this study,we investigated the coprecipitation mechanism of δ-MnO_(2) in the presence of Zn(II)and an oxidizing agent(sodium hypochlorite)under two neutral pH values(6.0 and 7.5).Themineralogical characteristics and Zn–Mn mixed products were compared with simple surface complexation by adsorption modeling and structural analysis.Batch coprecipitation experiments at different Zn/Mn molar ratios showed a Langmuir-type isotherm at pH 6.0,which was similar to the result of adsorption experiments at pH 6.0 and 7.5.X-ray diffraction and X-ray absorption fine structure analysis revealed triple-corner-sharing innersphere complexation on the vacant sites was the dominant Zn sorption mechanism on δ-MnO_(2) under these experimental conditions.A coprecipitation experiment at pH 6.0 produced some hetaerolite(ZnMn(Ⅲ)_(2)O_(4))and manganite(γ-Mn(Ⅲ)OOH),but only at low Zn/Mn molar ratios(<1).These secondary precipitates disappeared because of crystal dissolution at higher Zn/Mn molar ratios because they were thermodynamically unstable.Woodruffite(ZnMn(IV)_(3)O_(7)•2H_(2)O)was produced in the coprecipitation experiment at pH 7.5 with a high Zn/Mn molar ratio of 5.This resulted in a Brunauer–Emmett–Teller(BET)-type sorption isotherm,in which formation was explained by transformation of the crystalline structure ofδ-MnO_(2) to a tunnel structure.Our experiments demonstrate that abiotic coprecipitation reactions can induce Zn–Mn compound formation on theδ-MnO_(2) surface,and that the pH is an important controlling factor for the crystalline structures and thermodynamic stabilities.