Improved methods for selective dissolution of manganese oxides from soils and rocks

Hydroxylamine hydrochloride (NH₂OH-HCl) and hydrogen peroxide (H₂O₂) have been used in the past for selective dissolution of manganese oxide minerals. The methods reported, however, give poor recoveries. We tried to improve the methods in this respect. By stoichiometry, incomplete dissolution of Mn oxides is due to the lack of NH ₂OH-HCl added or to the lack of acid added in the case of H ₂O₂ treatment. By increasing the amount of solution for a given amount of solid in the case of NH₂OH-HCl and by increasing the acid concentration in the case of H₂O₂ treatment, we obtained rapid and complete dissolution of synthetic Mn oxides. The dissolution of natural samples, however, was slower than that of synthetic ones. Based on the results obtained, the improved methods are: (i) 0.1 M NH₂OH-HCl, non-acidified, 2 hours' stirring, and 1 g solid in 2 litres solution; (ii) 30% H₂O₂ in 0.5 M nitric acid, 0.5 hours' stirring, and 1 g solid in 1 litre solution. The improved methods achieved almost complete release of manganese from samples without decreasing the selectivity of dissolution.