The preferential retention of VI Zn over IV Zn on birnessite during dissolution/desorption

Abstract Zn is a common heavy metal in soils and sediments. In this study, the release behaviors of octahedral ( VI Zn) and tetrahedral ( IV Zn) Zn complexes on synthesized hexagonal birnessite were explored by solution chemistry method in combination with spectroscopic analysis. In acidic dissolution processes, the release of adsorbed Zn 2+ from birnessite occurred into two stages: in the first stage, ~60% of Zn 2+ was desorbed rapidly, with only 8% of Mn being released, and the ratio of VI Zn/ IV Zn increased with time; in the second stage, the residual Zn 2+ was mostly VI Zn and released slowly at a nearly constant rate until complete dissolution of the matrix mineral was observed. During desorption of Zn 2+ by Pb 2+ , the ratio of VI Zn/ IV Zn on birnessite also increased, while the residual percentage of VI Zn remained nearly constant. However, it is known that IV Zn-triple corner-sharing (TCS) is more stable than VI Zn-TCS, suggesting that part of the remaining IV Zn-TCS on birnessite might transform to VI Zn-TCS immediately when VI Zn-TCS is replaced by H + or Pb 2+ . Additionally, the possible distribution of Mn 3+ and IV Zn or the partial charge compensation by protons can lead to the preferential retention of VI Zn on birnessite or the preferential re-adsorption of VI Zn at the new edge sites. These results can provide new insights into the geochemical behavior of Zn 2+ contaminant in soil and aquatic environments.