Anomalous dependence of leachability on surface area during sulphuric acid leaching of nickel- and copper-doped birnessites

Studies on nickel and copper-doped birnessite samples reveal, quite unexpectedly, that the leachability of the doped element in sulphuric acid decreases with an increase in specific surface area of the host phase. The observed anomalous dependence of leachability on surface area is explained in terms of the transformation of birnessite during leaching into more compact structure phase(s), such as nsutite (γ-MnO2), which are difficult to leach.