Electrochemical Method For Simultaneous Removal Of COD And TN From Wastewater

The reduction of chemical oxygen demand?COD?and total nitrogen?TN?is the important content of wastewater treatment in the Chinese 13th five-year plan.Targeted pollutants in wastewater can be removed effectively by combining cathodic reduction with anodic oxidation in electrochemical method.The factors and its control are the key to the efficient transformation of pollutants in electrochemical process.Due to the phenomenons of low energy efficiency,over-oxidation and over-reduction in electrochemical wastewater treatment,electrochemical workstation and in-situ electrochemical infrared spectroscopy were used to study the working potentials,process parameters,and regulatory mechanism of potential for simultaneous removal of COD and TN.On this basis,differential control of paired electrolysis was proposed,which realized the efficient removal of COD and TN,and achieved high current efficiency.Finally,the feasibility and treatment capacity of this method were also verified by actual wastewater treatment.Firstly,the working potentials of COD,NH₄⁺,NO₃^-and NO₂^-on anode/cathode were investigated respectively.The Ti/PbO₂ was selected as anode,and the oxygen/chlorine evolution potential was determined by the polarization curve.The oxygen evolution potential and hydrogen evolution potential were measured as 1.89 V and 1.42 V,respectively.Three commonly used electrodes,i.e.,Ti,Cu and stainless steel,were selected to conduct the electrode characterization and NO₃^-/NO₂^-reduction.It showed that Ti electrode had negative hydrogen evolution potential,and optimal reduction performance of NO₃^-/NO₂^-by the experiments of polarization curve,cyclic voltammetry,constant potential electrolysis,and Tafel curve.Based on the selected electrode materials,the working potential ranges of COD and NH₄⁺on Ti/PbO₂ electrode were determined as 0.65 V¹.00 V and 0.75 V¹.05 V,respectively;the working potential ranges of NO₃^-and NO₂^-on Ti electrode were measured as 0.80 V¹.26 V and 0.48 V⁰.80 V,respectively.According to the working potentials,constant potential electrolysis was used for COD/NH₄⁺oxidation on Ti/PbO₂ and NO₃^-/NO₂^-reduction on Ti respectively.The results showed that 1.60V was suitable for COD/NH₄⁺oxidation,and-1.26 V was optimal for NO₃^-/NO₂^-reduction.Secondly,the influence rules of parameters of electrochemical wastewater treatment in static divided electrolysis cell were studied,and the influence of power supply parameters?pulse frequency and duty cycle?,process parameters?current density and initial pH value?and electrolyte environment?Na₂SO₄ concentration and Cl^-concentration?on pollutant removal were investigated.By analyzing and inferring the experimental results,the optimal parameters were obtained:pulse frequency of500 Hz,duty cycle of 50%,current density of 10 mA/cm²,initial pH value of 6,Na₂SO₄ concentration of 0.1 mol/L,and Cl^-concentration of 1000 mg/L.Furthermore,the mechanisms of COD/NH₄⁺direct/indirect oxidation and NO₃^-/NO₂^-direct/indirect reduction were also explored.The reaction paths of COD,NH₄⁺,NO₃^-and NO₂^-were measured by in-situ electrochemical infrared spectroscopy,and the obtained results were consistent with the previous experiments.According to the above investigation,a paired electrolysis system was constructed to realize the anodic oxidation of COD/NH₄⁺and the cathodic reduction of NO₃^-/NO₂^-simultaneously.The comparison of pollutants removal in static and flowing divided electrolysis cell were investigated,and the results proved that the paired electrolysis method could be used for the efficient removal of COD and TN.In this system,COD/NH₄⁺could be oxidized on anode and NO₃^-/NO₂^-could be reduced on cathode;at the same time,the generated NO₃^-/NO₂^-in over-oxidation process,and the generated NH₄⁺in over-reduction process could also be effectively removed.Furthermore,the relationships between current density/electrode area ratio and the anode/cathode potential were investigated;thus,the differential control of paired electrolysis was proposed,i.e.,controlling the cathodic current density and the area of the electrode.Under the conditions of cathodic current density of 5.0 mA/cm² and cathode/anode surface area ratio of 2:1,the obtained cathode/anode potential was close to the optimal working potential of each pollutant.The removal rates of COD and TN were 91.9%and 86.2%respectively,and current efficiency of 82.3%was achieved.In addition,the pseudo-first-order model could precisely describe the kinetics of COD removal;and TN removal followed zero-order kinetics.Finially,the differential control of paired electrolysis was applied for the treatment of actual wastewater to verify the feasibility and treatment capacity of COD/TN removal.These results clearly demonstrated that not only this process could improve the selectivity of desired product and achieve excellent removal efficiencies of targeted pollutants,but also obtain high current efficiency.The removal efficiencies of 68.0%,100%,50.9%,100%and 88.5%were achieved for COD,NH₄⁺,NO₃^-,NO₂^-and TN,respectively.High current efficiency of 84.8%was also obtained.Thus,the electrochemical method adopted in this paper can simultaneous removal of COD and TN from wastewater,and the proposed differential control of paired electrolysis can not only efficiently remove COD and TN,but also obtain high current efficiency.The results will provide theoretical support on the precision optimization control of pollutants removal in electrochemical process.