Study On Efficient Removal Of Hexavalent Chromium From Wastewater By Asymmetric AC Electrochemistry

Industrial production process will discharge a large number of heavy metal pollution wastewater,including hexavalent chromium wastewater is highly toxic,long-term exposure will cause damage to human organs and tissues.Electrochemical method is one of the effective methods to treat chrome-containing wastewater because of its advantages of no additional chemical reagent and simple operation.However,the lack of effective active sites on the surface of traditional electrode(iron plate,graphite plate,etc.)is not conducive to the convective diffusion of Cr(Ⅵ)to the electrode.At the same time,due to the action of coulomb force,the cathode mediated by direct current(DC)system will reject the negatively charged hexavalent chromium containing oxygen,so that it can not be effectively reduced on the electrode surface.Therefore,it is urgent to develop green and efficient new electrochemical repair technology.In this paper,functional electrodes with high adsorption affinity for Cr(Ⅵ)were prepared to solve the common problems in traditional direct-current electrochemical treatment of Cr(Ⅵ).A penetrating electrochemical treatment device superior to the traditional parallel plate electrode electrochemical reactor was designed,and a hexavalent chromium wastewater remediation method mediated by asymmetric AC electrochemical system was constructed.The efficient removal mechanism of Cr(Ⅵ)containing wastewater by asymmetric AC electrochemical coupled functional electrode and its influencing factors were studied.The main research contents of this paper are as follows:(1)Polyacrylonitrile(PAN),Super P carbon black and N,n-dimethylformamide(DMF)were used as precursor paste to dip the original commercial carbon felt(O-CF)to obtain Pan-CF electrode.PAN-CF electrode was modified by sodium carbonate and hydroxylamine hydrochloride solution to obtain amidoxime-based carbon felt electrode(Ami-CF).The morphology,structure and composition of O-CF,PAN-CF and Ami-CF electrodes were characterized by scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FTIR).The adsorption kinetics and isotherm of Cr(Ⅵ)on amidoxime carbon felt electrode were studied by batch experiments.It was found that O-CF and PAN-CF had little adsorption capacity for Cr(Ⅵ).In contrast,Ami-CF has excellent adsorption capacity for Cr(Ⅵ),and the saturated adsorption capacity can reach 101.73 mg/g,which is 100 times higher than O-CF.(2)Whereas direct current can only adjust voltage(current)to control the electrochemical reaction,asymmetric pulsed square wave alternating current(AC)can achieve accurate electrode interface reaction control because it has four parameters:frequency,duty ratio and positive and negative bias.In this chapter,based on the prepared amidoxime carbon felt electrode(Ami-CF),a penetrating asymmetric AC electrochemical system is established to study its removal effect on wastewater containing Cr(Ⅵ).The effects of applied positive and negative bias,duty ratio,initial Cr(Ⅵ)concentration,initial solution p H,flow rate and other coexisting ions on Cr(Ⅵ)removal were discussed through batch experiments.Meanwhile,the removal mechanism of Cr(Ⅵ)in AC electrochemical system was discussed combined with material characterization.The results show that the Coulomb repulsion effect and the side reaction of electrolytic water are inhibited by the high frequency anode and cathode conversion(asymmetric AC),the diffusion mass transfer rate of Cr(Ⅵ)in the solution is improved,the reduction efficiency of Cr(Ⅵ)to Cr(Ⅲ)is significantly promoted,and the efficient removal of Cr(Ⅵ)is realized.Under optimal operating conditions(positive bias 1 V,negative bias 2.5V,duty ratio 20%,frequency 400 Hz,solution p H=2),The asymmetric AC electrochemistry based on the carbamide group carbon felt electrode can achieve a fast(30 s)and efficient removal effect(>99.97%)of Cr(Ⅵ)with a high flux of300 L/h/m~2and 0.5～100 mg/L.At the same time,durability test verified the sustainability of the method.For Cr(Ⅵ)containing wastewater with initial concentration of 50 mg/L,the effluent quality could still reach drinking water grade(<0.05 mg/L)after 10 experiments.This study provides a new idea for the rapid green and efficient removal of Cr(Ⅵ)containing wastewater at low and medium concentration.