| Abstract: | Hexagonal turbostratic birnessite,with the characteristics of high contents of vacancies,varying amounts of structural and adsorbed Mn~(3+),and small particle size,undergoes strong adsorption reactions with trace metal(TM)contaminants.While the interactions of TM,i.e.,Zn~(2+),with birnessite are well understood,the effect of birnessite structural characteristics on the coordination and stability of Zn~(2+)on the mineral surfaces under proton attack is as yet unclear.In the present study,the effects of a series of synthesized hexagonal turbostratic birnessites with different Mn average oxide states(AOSs)on the coordination geometry of adsorbed Zn~(2+)and its stability under acidic conditions were investigated.With decreasing Mn AOS,birnessite exhibits smaller particle sizes and thus larger specific surface area,higher amounts of layer Mn~(3+)and thus longer distances for the first Mn\O and Mn\Mn shells,but a low quantity of available vacancies and thus low adsorption capacity for Zn~(2+).Zn K-edge EXAFS spectroscopy demonstrates that birnessite with low Mn AOS has smaller adsorption capacity but more tetrahedral Zn(~(IV)Zn)complexes on vacancies than octahedral(~(VI)Zn)complexes,and Zn~(2+)is more unstable under acidic conditions than that adsorbed on birnessite with high Mn AOS.High Zn~(2+)loading favors the formation of~(VI)Zn complexes over~(IV)Zn complexes,and the release of Zn~(2+)is faster than at low loading.These results will deepen our understanding of the interaction mechanisms of various TMs with natural birnessites,and the stability and thus the potential toxicity of heavy metal pollutants sequestered by engineered nano-sized metal oxide materials. |
