Adsorption Characteristics And Oxidation Of Cr(Ⅲ) Versus PH Of Several Manganese Oxide Minerals

As the essential source of Mn element for the nutrition of animals and plants, one of the important adsorbents, catalysts and redox agents, manganese oxide minerals are important components in soil. For it's low PZC, large surface area, great deal of surface negative charge and active surface character, manganese oxide minerals not only can adsorb many heavy metals but also can oxidize many elements with variable value so that influence these elements' validity and toxicity. Investigation on the adsorption and oxidization characteristics of manganese oxide minerals could shed light on mechanism of soil pollution and the relation between the soil quality and entironments, so it is very significant to both academic and practice.Isotherm adsorption method, chemical equilibrium method and chemical form analysis method were used to study the adsorption and oxidization characteristics of manganese oxide minerals, such as birnessit(H-birnessit, Na-birnessit), todorokite, cryptomelane and hausmannite. The main results as follows:1 Among the tested synthetic manganese oxide minerals, H-birnessite possessed the greatest adsorption capacity on the heavy metals, such as Pb2+,Cu2+,Co2+,Cd2+ and Zn2+,hausmannite possessed the weakest one. Except hausmannite, other three minerals (birnessit, todorokite and cryptomelane) had the greatest adsorption capacity on Pb2+. All these four minerals adsorbed the greatest amount of Pb2+ in mixed solution, and then is Cu2+. On the surface of H-birnessite and cryptomelane, Zn2+ has no specific or preferred binding sites of it's own. Hausmannite has the greatest adsorption capacity for Cu2+ in single element solutuion, but in the mixed solution it has greatest adsorption capacity on Pb2+.this is because reaction rate of Pb2+ is faster than other ions.2 The maximum amounts of Cr(III) oxidized by the tested manganese oxide minerals in order were(mmol/kg):H-birnessite(1330.0) > cryptomelane(422.6) > Na-birnessite(217.0)>todorokite(59.7). The amounts of Cr(III) oxidized by the tested manganese oxide minerals has no significant correlation with ion strength. The amounts of Cr(III) oxidized by different manganese oxide minerals have different change with the increasing of temperature. Cr(III) oxidation by the three Mn oxide minerals peaked in pH3~3.5. When pH<5, redox reaction between Cr(III) and the Mn oxide minerals reached thermodynamic equilibrium. When pH>5~5.5, Cr(OH)3 precipitate was produced in the system, pH has little effect on the amounts of Cr(III) oxidized.