Simultaneous Removal Of Cr(â…¥) And Cd(â…¡)in Groundwater By PRB With Mixture Of Fe-AC

The problem of groundwater polluted in varying degrees.by toxic and harmful substances is becoming increasingly worse in recent years.The pollution caused by heavy metals such as Cr（Ⅵ）and Cd（Ⅱ）is frequent,which becomes a serious threaten to the environment and human health.In groundwater,Cr（Ⅵ）exists in the form of complex anions(such as CrO₄^2-),while Cd（Ⅱ）exists in the form of cations.Thus,in order to remove these two ions simultaneously,the different charge properties must be taken into account and the remover need to be selected.Futhermore,the simultaneous removal process and the separate removal process of these two ions may be different,which needs further analysis.In this paper,zero valent iron（Fe）,zeolite and activated carbon（AC）were chosen as PRB media to remove Cr（Ⅵ）and Cd（Ⅱ）in groundwater.The optimal proportion of these composite materials were made by batch experiments.Column experiments were performed to simulate the whole removal process.And the differences of simultaneous removal of Cr（Ⅵ）and Cd（Ⅱ）by different media were analyzed.Futhermore,the differences between single heavy metal ion removal and simultaneous removal of two ions were analyzed by batch experiments and column experiments.At last,the reaction mechanism was analyzed and discussed.Main research results of this paper are described in detail as following:The screening test of the remover showed that:When the reaction media were Fe-AC,Fe-zeolite and Fe-AC-zeolite in different proportions,the reaction of Cr（Ⅵ）accorded with the pseudo first order reaction kinetics,and the reaction of Cd（Ⅱ）accorded with the pseudo-2nd-order adsorption kinetics.Fe-AC was the best composite material for simultaneous removal of Cr（Ⅵ）and Cd（Ⅱ）in groundwater.The reaction rate constants of the reaction between Fe-AC in different proportion and Cr（Ⅵ）were related by linear equations.The saturated adsorption capacity of the reaction between Fe-AC in different proportion and Cd（Ⅱ）were related by quadratic equations.In conclusion,the best proportion of Fe:AC was 4:1,which is used as the optimum proportional reaction medium for simultaneous removal of Cr（Ⅵ）and Cd（Ⅱ）in groundwater.Column experiments on simultaneous removal of Cr（Ⅵ）and Cd（Ⅱ）in groundwater indicated that:The efficiency of Fe-AC-quartz sand column in removing Cr（Ⅵ）and Cd（Ⅱ）in groundwater was superior to Fe-quartz sand column.And Fe-AC with a ratio of 4:1 can be used to remediate ground water contaminated by Cr（Ⅵ）and Cd（Ⅱ）.Fe and AC numerous tiny primary cells formed between Fe and AC.Cr（Ⅵ）acted as an electron acceptor,could simultaneously acquire electrons from the iron surface and the reaction point associated with the galvanic cell,which led to an increase in the redox rate.Under the unfavorable conditions（pH below 8）,because of the addition of AC（AC-pHPZC<Fe-pHPZC）,the favorable range of pH for Cd（Ⅱ）removal was broadened.At the same time,the adsorption capacity of Cd（Ⅱ）was increased.Differences between simultaneous removal of Cr（Ⅵ）and Cd（Ⅱ）and single heavy metal ion indicated that:In the composite reaction system,Cr（Ⅵ）promoted the adsorption of Cd（Ⅱ）,while Cd（Ⅱ）inhibited the removal of Cr（Ⅵ）.Cd（Ⅱ）is in the form of precipitation when pH greater than 8.In addition with the precipitation produced by the reaction between Cr（Ⅵ）and Fe,the reactive sites were covered,resulting in the inhibition of the removal of Cr（Ⅵ）.On the other hand,when the value of pH was greater than 8,the removal rate of Cd（Ⅱ）increased due to the precipitations.