| Popis: |
The interactions between 10−4 M Cr3+aq and seven different Mn-oxides (six natural samples, including hausmannite, manganite, romanechite, cryptomelane, lithiophorite, and pyrolusite; and one synthetic sample, birnessite) have been observed in aqueous solution at pH 4.4 (pH 5.2 in the case of hausmannite) and room temperature. For each mineral-solution interaction, the aqueous chemical concentrations of [Mnaq], [Cr]Taq and [Cr6+aq] were measured with time. Reacted samples were examined by XPS to determine if and to what extent the surface chemical states of Cr, Mn, and O had changed. Microscopic observations of the reacted surfaces were obtained using AFM and high-resolution, low-voltage FESEM. Cr-uptake onto/into the Mn-oxides is observed to occur in Cr-hydroxide/hydrate undersaturated solutions as a result of adsorption, absorption, and surface precipitation. In addition, the rate and extent of reductive dissolution and Cr3+ aq oxidation varied considerably between the seven Mn-oxides. The measured ratios of [Mn]aq:[Cr6+]aq were in agreement with the values expected from the proposed stoichiometric reactions where they could be easily written. The chromium as detected by XPS on the surface of the reacted Mn-oxides was predominately Cr3+, however pyrolusite contained both Cr3+ and Cr6+. Several previous studies have implicated a chromium surface precipitate to be responsible for the cessation of the Cr3+ aq oxidation reaction. Our surface sensitive FESEM and AFM observations tend to suggest that Cr uptake is by isolated site binding, and very small ( |
