Hexavalent Chromium Removal From Wastewater By Electrocoagulation:Parameter Optimization And Mechanism

The electrocoagulation（EC）method is one of the most promising electrochemical techniques in the field of environmental protection,it also one of the most active research and application fields.This method has been applied to the treatment of a variety of wastewater,which can be used to treat and remove the bacteria,organic pollutants,fluoride,as well as heavy metals such as arsenic,cadmium,copper and nickel in the wastewater.In this paper,the effects of different electrode plates and operating parameters of Fe and Al electrodes on the removal of hexavalent chromium（Cr（Ⅵ））of heavy metal ions in smelting wastewater were studied by using the electrocoagulation EC method,and the mechanism analysis of electrocoagulation and the characteristics and classification of the electrocoagulation sludge were also carried out.The effects of different treatment time,electrode material and combination,current intensity,initial pH value,electrolyte type and concentration,cation and anion inorganic salt concentration on the removal of Cr（Ⅵ）concentration in wastewater by EC method was investigated in the experiment.The results of the study are as follows:（1）Through the study of different combinations of Fe and Al electrodes,using four different types of electrode materials（Fe-Fe,Fe-Al,Al-Fe and Al-Al）to remove Cr（Ⅵ）in the wastewater is more effective with the iron plate as the anode than the aluminum plate,the best electrode combination is the iron plate combination（Fe-Fe）,the iron plate combination（Fe-Fe）under the high current intensity condition.The removal of close to100%Cr（Ⅵ）can be achieved.（2）The study on the effect of different initial pH values on the removal of Cr（Ⅵ）shows that the low initial pH value is better than the high initial pH value,and the removal rate of Cr（Ⅵ）is up to 100%at the lowest initial pH value.（3）Current intensity and treatment time had a great influence on the removal efficiency of Cr（Ⅵ）.When the current intensity was 400 mA,the removal of Cr（Ⅵ）could reach 99.70%after 15 minutes.（4）Electrolytes in waste water can increase the conductivity of the solution,and also affect the efficiency of electrode passivation and removal of Cr（Ⅵ）.The effects of three different concentrations of two electrolytes（NaCl and Na₂SO₄）on the removal efficiency of Cr（Ⅵ）in wastewater were studied.Different cationic and anionic inorganic salts have been different capacities for decreasing residual Cr（Ⅵ）concentration.On the experiment,the highest efficiency of cationic and anionic are Mg²⁺and Cl^-,respectively.（5）The analysis of the sludge produced by electrocoagulation by scanning electron microscope（SEM）and X-ray diffraction（XRD）shows that,under the action of electrocoagulation,hexavalent chromium has been transformed into trivalent chromium.The flocs formed by electrocoagulation is nano crystalline and Cr（Ⅵ）has been converted to Cr（III）,and the flocs formed mainly are spherical and polygonal structures.The transverse size of these flocs is about 0.2-0.3μm.