| With its special physical and chemical properties,chromium and its compounds are widely used in dustries such as the production of stainless steel,galvanization,tanning of leather,pigment,corrosion prevention,dyeing and chemical industry.The wide distribution of chromium contaminants in soil in China is from the unproper disposal of wastes containing chromium from these industries.The organic matters in soil such as low-molecular weght organic acids and humic acid can reduce Cr(Ⅵ)to Cr(Ⅲ) to allay chromium contamination.Fe as a metal element with high content is about 3.8%in soil. Althought Mn content in soil is far less than that of Fe,it has important effect on the chemical activity of chromium like Fe.Study on the influence of Mn2+ and Fe3+ on reduction of Cr(Ⅵ) by low-molecular weght organic acids and humic acid has important meaning on knowing more about the chemical activity of chromium in soil.These experiments focus on studying the influence of Mn2+ and Fe3+ adsorbed by kaolinite and montmorillinite on reduction of Cr(Ⅵ) by oxalate,citrate and tartaric acids, and the influence of Mn2+ and Fe3+ on the reaction between humic acid and Cr(Ⅵ).The main results in the study are as follows:The influence of Mn2+ adsorbed by kaolinite on the reduction of Cr(Ⅵ) by oxalate, citrate and tartaric acids is studied through batch experiments.The results show that Mn2+ adsorbed by kaolinite accelerates the reduction of Cr(Ⅵ) by the three organic acids,and the catalysis is in the order:oxalate＞citrate＞＞tartaric.When tartaric concentration is lower than 2.0 mmol·L-1,the catalysis of adsorbed Mn2+ is not obviously observed,but it becomes manifest with a higher concentration.The temperature and pH exert a great impact on the reduction of Cr(Ⅵ) by oxalate and citrate acid,the reaction rates increase with temperature. When pH ranges from 3.5-4.5,the reduction of Cr(Ⅵ) by oxalate acid increases with pH decreasing.However,the reduction of Cr(Ⅵ) by citrate acid is fastest at pH=4.0.The influence of Fe3+ adsorbed by kaolinite on the reduction of Cr(Ⅵ) by oxalate, citrate and tartaric acids is studied through batch experiments.The results show that Fe3+ adsorbed by kaolinite accelerates the reduction of Cr(Ⅵ) by the three organic acids,and the catalysis is in the order:tartaric＞oxalate＞citrate.In the absence of Fe3+,the photolysis of oxalate,citrate and tartaric acids is slight,so the reduction of Cr(Ⅵ) by photoreaction products generated from oxalate,citrate and tartaric acids are not the main reason for degradation of Cr(Ⅵ).Fe3+ photocatalytic does not exhibit a good linear correlation with the number of carboxylic groups in a molecule,pH values exert great impact on the reduction of Cr(Ⅵ) by oxalate acid,when pH ranges from 3.5-4.5,the reaction rates increase with pH decreasing.The concentration of oxalate acid has no manifest impact on the reaction.The influence of Mn2+ and Fe3+ on the reduction of Cr(Ⅵ) by humic acid is studied at 25℃and pH=2.0-6.0.The results show that pH value exerts great impact on the reduction of Cr(Ⅵ) by humic acid from black soils,the reaction rates increase with pH decreasing, when pH ranges from 2.0-6.0.Mn2+ has no effect on the reaction between humic acid from black soils or yellow-brown soils and Cr(Ⅵ).The rate of reaction between humic acid from black soils or yellow-brown soils and Cr(Ⅵ)with light is higher than that without light,and Fe3+ does not increase the rate of reaction between humic acid from black soils or yellow-brown soils and Cr(Ⅵ)with light.While,the rate of reaction between humic acid from zijinshan soils and Cr(Ⅵ)is much higher than that of reaction between humic acid from black soils and Cr(Ⅵ).The reason may be the difference on the structure of humic acid from different soils.
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