锰铁氧体吸附及催化柠檬酸还原六价铬的过程及机理

Iron manganese minerals catalyzed Cr( Ⅵ) reduction by citric acid and its mechanism

本文探究了磁性纳米铁锰氧体(MnFe_2O_4)及其黏土矿物负载材料对六价铬(Cr(Ⅵ))的吸附作用,同时研究了锰铁氧体-柠檬酸溶液体系中,铁锰氧体催化柠檬酸还原Cr(Ⅵ)的机理.结果表明,MnFe_2O_4材料对Cr(Ⅵ)的吸附量随着吸附时间的增加而增加,在0—5 min内快速吸附并达到平衡,符合准二级动力学模型.伴随MnFe_2O_4负载量的增加,凹凸棒土负载铁锰氧体(ATP-Fe Mn)的吸附量大幅增加,ATP-FeMn14吸附量最大达到了29.2 mg·g^(-1),且吸附等温线均可用Langmuir方程或Freundlich方程拟合,属于单分子层吸附.MnFe_2O_4和负载ATP材料吸附Cr(Ⅵ)的最佳pH值为3—4,ATP-FeMn14对六价铬的去除率最高,最佳投加量为5 g·L^(-1).在铁锰氧体-柠檬酸体系中,溶液pH值是影响Cr(Ⅵ)的去除效率的重要因素,当溶液pH值在4和5时,Cr(Ⅵ)的去除率(76.5%、66.2%)显著高于其他处理;4 mmol·L^(-1)柠檬酸的处理去除率(89.8%)显著高于其他浓度柠檬酸的处理去除率;而在相同体系中,MnFe_2O_4的处理去除率(89.8%)显著高于其他研究材料.本研究表明铁锰氧体不仅对Cr(Ⅵ)具有良好的吸附性能,在与有机酸共存时,还可以催化有机酸还原Cr(Ⅵ),降低Cr(Ⅵ)的环境风险.

Adsorption of Cr（ Ⅵ） by nano iron manganese（ MnFe_2O_4） and the surface-catalyzed hexavalent chromium（ Cr（ Ⅵ）） reduction by citric acid were tested together in this study. The adsorption capacity of MnFe_2O_4 to Cr（ Ⅵ） increased with reaction time,and reached equilibrium gradually in the first five minutes,which fitted well to pseudo-second order kinetic model. The adsorption capacity raised with loading amount of MnFe_2O_4 on Attapugite（ ATP） increased. ATP-FeMn14 had the largest adsorption capacity of 29.2 mg·g^（-1）,which could be described by Langmuir and Freundlich equations. In the MnFe_2O_4-citrate system,p H was a critical factor to influence the reduction rate of Cr（ Ⅵ）. The reaction rate at pH 4.0 was significantly higher than that of the other treatments. Moreover,the removal rate of Cr（ Ⅵ） by MnFe_2O_4 was significantly higher than that of other materials studied under the same condition. These results suggested that MnFe_2O_4 not onlyadsorb Cr（ Ⅵ）,but acted as catalyser to reduce Cr（ Ⅵ） to Cr（ Ш） by citrate. These findings were helpful and could provide a costly strategy to remediate Cr（ Ⅵ） contaminated soil and groundwater.