Preparation, characterization and aging of δ-MnO 2, for use in trace metal speciation studies

Abstract

Well characterized synthetic δ-MnO 2 can be used successfully for trace metal speciation studies in natural waters. Characteristics and adsorption behavior of δ-MnO 2 depend on the preparation method and subsequent treatment and aging. Three different recipes were used to prepare δ-MnO 2. Methods involving a redox reaction between Mn 2+ and Mn 7+ at neutral pH (δ-MnO 2-redox), and reduction of Mn 7+ at low pH (δ-MnO 2-red), yielded compounds with characteristics in agreement with literature data; they also have a high adsorption capacity for Cu. A method involving oxidation of Mn 2+ at high pH yielded a δ-MnO 2(ox), with some variant characteristics and a lower adsorption capacity for Cu. An artificial aging process, using heat treatment at neutral pH, produced cryptomelane from δ-MnO 2(ox), but not from δ-MnO 2(redox) or δ-MnO 2(red). Conversion of δ-MnO 2(redox) to cryptomelane and loss of approximately one-half of the adsorption capacity, as a result of a natural aging process, can be avoided for at least four years if the K content is 10% in the solid, and the solid is stored at low temperature. Potassium content, and not a low pH, appears to control the conversion to cryptomelane. Adsorption capacity of δ-MnO 2 for Cu appears to be related to the K and H 2O content of the solid, which is structure dependent. Because of their higher adsorption capacity for Cu, δ-MnO 2(redox) or δ-MnO 2(red) are more suitable for trace metal studies in natural waters, than δ-MnO 2(ox).