Study On Treatment Of Malachite Green Dye Wastewater By Rotating Anode-electrocoagulation Process And Its Degradation Mechanism

In recent years,a rapid development of the dye processing industry has been seen in China.Among them,malachite green dye is a kind of multiduty dye used in industry commonly.The malachite green dye wastewater is characterized with high COD concentration,high chroma and high toxicity,the problem of its treatment has become a hot concern.Electrocoagulation technology has attracted an extensive attention as a green and efficient treatment method of industrial wastewater.However,few drawbacks associated with the traditional electrocoagulation technology are high energy consumption and short age of the electrodes,which seriously restricts its application at the industrial scale.Therefore,it is very significant to design a new type of electrocoagulation device for the treatment of malachite green dye wastewater which could offer high efficiency at low energy requirement.In this study,a novel electrocoagulation device with a rotating anode is designed.High-concentration simulated wastewater with malachite green(MG)dye is selected as the treatment object.Firstly,the effect of different anode rotation speed,current density,initial pH value of solution,electrolyte type and electrolyte concentration on the degradation of MG dye is investigated,which is using electrocoagulation device of two electrode materials.The effect of these parameters on the optimal operating conditions of MG dye wastewater in anodic rotary-electrocoagulation is evaluated.Furthermore,effect of these parameters on the morphological distribution of MG dye wastewater in nitrogen is investigated.Then the reaction kinetics model of MG dye degradation is developed and influence of different operating conditions on the reaction rate constant is investigated.Finally,X-ray Diffractometer,Fourier Transform Infrared Spectrometer,Ultraviolet Spectrophotometer and High-Performance Liquid Chromatography-The Mass Spectrometer are used to characterize the product formed after the reaction.Based on which,the degradation mechanism of MG dye wastewater using electrocoagulation device with a rotating anode,is revealed.The obtained results show that:(1)The optimal operating conditions of the electrocoagulation device with a rotating anode are:the rotatial speed of anode of 150 rpm,the current density of 6.12 mA/cm2,the initial pH of 9,and the concentration of electrolyte Na2SO4 of 3 g/L,after 30 min of electrolysis,the removal rates of COD and chromaticity reached 99.11%and 99.95%,respectively.The optimal operating conditions of the electrocoagulation device with zinc-based anode are:the rotatial speed of anode of 150 rpm,current density of 4.08 mA/cm2,initial pH of 6,and the concentration of electrolyte Na2SO4 of 4 g/L,after 30 min of electrolysis,the removal rates of COD and chromaticity reached 95.65%and 99.94%,respectively.(2)After electrocoagulation degradation,the nitrogen in the MG dye significantly reduces to trace amounts like(<2 mg/L)in the form of ammonium nitrogen,nitrate,nitrite and a small amount(<5 mg/L)in the form of organic nitrogen.(3)The degradation process of MG dye follows the first-order reaction kinetics in electrocoagulation device with iron-based anode.While,it follows the second-order reaction kinetics in the electrocoagulation device with zinc-based anode.(4)The degradation mechanism of MG dye wastewater using electrocoagulation device with a rotating anode mainly includes adsorption of metal hydroxy floc,gas flotation of hydrogen bubbles,N-demethylation reaction of chromogenic group and cleavage of central carbon atom conjugated structure.Results obtained in the present study provide a new idea for the design and development of high-efficiency and low-energy electrocoagulation process,which gives a theoretical foundation for the degradation treatment of dye wastewater.