The Reduction Of Cr(Ⅵ) By Pyrite Assisted By Organic Acids

Cr(Ⅵ) is harmful to environment and human, which has become one of the most serious environmental contaminants. Pyrite is the most common sulfide mineral of the Earth’s crust. Cr(Ⅵ) can be effectively transformed to Cr(Ⅲ) by both Fe(Ⅱ) and S2(-Ⅱ), the constituents of pyrite. And Fe(Ⅱ) can be transformed to Fe(Ⅲ) which can be chelated with many kinds of organic acids. Fe(Ⅲ)-organic acids complexes are high photo-activity formations. However, in the previous research, the reduction of Cr(Ⅵ) by pyrite is handicapped in pyrite-Cr(Ⅵ) system, which can results in precipitates including Fe(OH)3and FexCr1-x(OH)3. In this paper, the mechanism and the impacting factors of reduction of Cr(Ⅵ) by pyrite assisted by organic acids were investigated. The main results in the study are as follows:Under mimicking solar light irradiation, the mechanism of reduction of Cr(Ⅵ) by pyrite assisted by organic acids in different systems were studied. And the mechanism and the impacting factors including pH and the concentration of pyrite were also investigated. The results indicated that the reduction of Cr(Ⅵ) was markedly accelerated in the system that both pyrite and organic acids exist because of (1) the formation of Fe(Ⅲ)-organic acids complex and (2) the removal of precipitates including Fe(OH)3and FexCr1-x(OH)3. pH is an important factor because of its influence on the distributions of Fe(Ⅲ) forms. Lower pH and higher loading of pyrite in suspension benefited the conversion of Cr(Ⅵ) to Cr(Ⅲ). Cr(Ⅵ) was effectively transformed to Cr(Ⅲ) by both Fe(Ⅱ) and S2(-Ⅱ), the constituents of pyrite.The effect of these three organic acids on the reaction rate was in the order:tartaric acid>oxalic acid>citrate acid.The mechanism of reduction of Cr(Ⅵ) by pyrite assisted by organic acids without light was studied. The results demonstrated that the reduction of Cr(Ⅵ) by pyriyte without light was slow due to the formation of Fe(OH)3and FexCr1-x(OH)3.