The Reduction Of Cr(Ⅵ) BY Sodium Sulfide Catalyzed By Schwertmannite

Chromium (Cr) is widely applied in industries, such as electrofacing, tanning, pigment, painting, dyeing, offset printing etc. On the other hand, it poses serious harm to all beings since it is improperly discharged into soil and water environments. In natural environment, chromium presents mainly in two oxidation states:Cr(Ⅵ) and Cr(Ⅲ). Cr(Ⅵ) is highly soluble, mobile and toxic to humans, animals and plants. Conversely, Cr(Ⅲ) has relatively low toxicity and mobility. Thus, the approaches that convert Cr(Ⅵ) into Cr(Ⅲ) in contaminated soils and waters have received considerable attention. Schwertmannite exits in acid waste water of mine. Fe(Ⅲ), dissovled from Schwertmannite, is effectively reductive to Fe(Ⅱ) by S(-Ⅱ). The produced Fe(Ⅱ) easily reduces Cr(Ⅵ) to Cr(Ⅲ), and Fe(Ⅱ) itself turns to Fe(Ⅲ) again. Therefore, a circle catalytic system of Fe(Ⅲ)/Fe(Ⅱ) in the reaction system forms. In this thesis, the schwertmannite-catalytic mechanism of Cr(VI) reduction by sulfide and related affecting factors were investigated. The main results were listed as follows:1. Under the conditions of no light and oxygen, it is found that Cr(Ⅵ) was greatly adsorbed by Schwertmannite, which could be markedly depressed by NaH2PO4with a concentration of0.1mol/L. Temperatures exert a weak effect on the adsorption of Cr(Ⅵ) on Schwertmannite.2. Under the conditions of no light and oxygen, a series of experiments were performed to study the reduction mechanism of Cr(Ⅵ) and its affecting factors concluding temperature, pH, the initial concentration of the reactants and the mass concentration of Schwertmannite. The result demonstrated that the reduction of Cr(Ⅵ) by sulfide alone was slow, but the addition of Schwertmannite could significantly accelerate the reaction. The Schwertmannite-catalytic reduction of Cr(Ⅵ) was obviously influenced by temperature, pH, the initial concentration of the reactants and the mass concentration of Schwertrnannite. Lower pH, higher temperature and higher concentration of the reactants and the mass concentration of Schwertmannite were benefit to Cr(Ⅵ) reduction by sulfide catalyzed by Schwertmannite. However, the added o-phenathroline and NaF, a complex agent for Fe(Ⅱ) and Fe(Ⅲ), respectively, markedly reduced the reaction rate. A possible catalytic mechanism was proposed in this study. Fe(Ⅲ) on the surface of Schwertmannite was reduced by sulfide, the produced Fe(Ⅱ) reduced Cr(VI) to Cr(Ⅲ), meanwhile Fe(Ⅱ) itself was converted into Fe(Ⅲ), which was transformed into Fe(Ⅱ) by sulfide again. Then, a catalytic circle system of Fe(Ⅲ)/Fe(Ⅱ) for Cr(Ⅵ) reduction was established. It is well known that the reduction of Cr(Ⅵ) by Fe(Ⅱ) was much greater than that by sodium sulfide at the same pH. As a result, the reduction of Cr(Ⅵ) by sulfide in the presence of Schwertmannite was enhanced greatly.