Regulation Mechanism Of Cr(Ⅵ)-containing Wastewater Treatment By Pulse Current Electrocoagulation

In recent years,chromium has been widely used in electroplating,chemical industry,leather and other industries,which leads to the increasing amount of hexavalent chromium（Cr（Ⅵ））discharged into water,and then causes the occurrence of chromium pollution incidents.Compared with other water treatment technologies,electrocoagulation（EC）has become a practical technology for the treatment of Cr（Ⅵ）-containing wastewater due to its ease of operation,high treatment efficiency,small footprint,and freedom from secondary chemicals,but there are still many limitations and challenges in its practical application.This paper proposes to solve the problems encountered in conventional direct current EC（DC-EC）processes with the aid of pulsed currents（PC）to optimize the solutions.The effects of p H,initial Cr（Ⅵ）concentration,current density,plate spacing,conductivity,peak voltage,current frequency,duty cycle,and the ratio of positive to negative pulses on the treatment efficiency of DC-EC and PC-EC were investigated and optimized.The optimum conditions for DC-EC were found to be p H 6,conductivity 10m S cm^-1,plate spacing 15 mm,initial concentration 50 mg L^-1,and current density 20A m^-2.On the basis of the optimal conditions of DC-EC,the peak voltage of 5 V,current frequency of 5000 Hz and duty cycle of 30%are the optimal conditions of positive single pulse current EC（PSPC-EC）.When the ratio of the number of positive and negative pulses is 10,alternating pulse current EC（APC-EC）is the most effective treatment for Cr（Ⅵ）wastewater.By comparing the effects of the three kinds of EC treatment,it is found that under the premise of achieving a similar removal rate,compared with DC-EC,PSPC-EC can save energy consumption by 89%and reduce the amount of sludge by 30%.In terms of energy consumption,PSPC-EC is the best choice for Cr（Ⅵ）removal.APC-EC is the best choice for Cr（Ⅵ）removal in terms of sludge reduction.Based on single-factor experiments,the three factors of peak voltage,current frequency,and duty cycle were selected,and the conditions of PC-EC to remove Cr（Ⅵ）were optimized through a central composite design experiment in the response surface method.When Cr（Ⅵ）was used as the response value,it can be seen from the analysis of variance results that the fitted quadratic polynomial model could well predict the reaction process and results of PC-EC for removal of Cr（Ⅵ）.Analysis of the flocs produced by the three types of EC showed a difference in Cr（Ⅵ）removal mechanism between PC-EC and DC-EC:the large amount of oxygen generated during the DC-EC process causes the flocs to be oxidized toα-Fe₂O₃ andγ-Fe₂O₃ resulting in the Cr（Ⅵ）originally adsorbed by the flocs being released back into the solution,resulting in a lower removal rate.The mechanism of Cr（Ⅵ）removal by PC-EC is based on the adsorption of GRCl with effective adsorption sites,so PC-EC has better stability for Cr（Ⅵ）removal in wastewater compared with DC-EC.It has been demonstrated that the use of PC-EC for the treatment of Cr（Ⅵ）wastewater can save energy compared with conventional EC and is in line with the concept of green treatment,which lays the foundation for the practical application of PC-EC on a large scale.