| There is a problem of excessive discharge of total nitrogen?TN?in biochemical tail water.The TN includes ammonia?NH4+-N?,nitrate?NO3--N?and nitrite?NO2--N?.And the electrochemical technology can achieve simultaneous removal of NH4+-N,NO3--N and NO2--N through anodic oxidation and cathodic reduction.Compared with the anode?such as Ti/PbO2 electrode?which can effectively remove NH4+-N,there are many problems in the removal of NO3--N by cathode.The materials of cathode and the reaction kinetics still need to be further analyzed.The steady-state polarization curves showed that the hydrogen evolution potentials of Ti,Fe and graphite cathodes were-1.25 V,-1.30 V and-1.50 V,respectively.The linear sweep voltammetry curves showed that the working potentials of cathodic reduction of NO3--N by Ti,Fe and graphite were-1.13 V,-1.03 V and-1.14 V,respectively.And the results of potentiostatic electrolysis showed that the three cathodes of Ti,Fe and graphite could effectively remove NO3--N when the cathode potential was-1.26 V.The removal efficiency was in the order of Fe>Ti>graphite,and the selectivity of N2 was in the order of graphite>Ti>Fe.Tafel curve showed that the order of corrosion resistance was Ti>graphite>Fe.Comprehensive analysis showed that Ti cathode was suitable for application.A reactor with diaphragm equipped with Ti cathode and Ti/PbO2 anode was used for analyzing the reaction kinetics of nitrogen with different chemical valence.Apparent rate constants of each reaction were calculated by 1stOpt.The apparent rate constants of NO3--N to NO2--N and NO2--N to N2 were 2.46×10-2 min-1 and 4.03×10-2min-1,respectively.NO2--N was an unstable intermediate product.In addition to being easily oxidized to NO3--N,it was also easy to be reduced to NH4+-N(the apparent rate constants were 3.66×10-2 min-1 and 5.40×10-2 min-1,respectively).The apparent rate constants of NH4+-N to N2 and NO3--N were 5.12×10-2 min-1 and 8.74×10-4 min-1,respectively.Therefore,the electrochemical removal and conversion of NO3--N and NH4+-N to N2 required effective control and elimination of the side reactions.Based on the above results of kinetic analysis,a single-chamber reactor without diaphragm was used for investigating the effects of duty cycle,pulse frequency and current density on nitrogen removal of simulated wastewater.The initial concentration of TN in simulated wastewater is 105 mg/L?60 mg/L NH4+-N+30 mg/L NO3--N+15mg/L NO2--N?,the electrolyte is 0.1 M Na2SO4 and 1500 mg/L NaCl.The experimental results showed that the removal efficiency of TN was 84%,NH4+-N and NO2--N were completely removed,and the removal efficiency of NO3--N was 47%under the optimal parameters of pulse power supply?50%of duty cycle,0.5 kHz of pulse frequency and10 mA/cm2 of current density?.The presence of Cl-in simulated wastewater promoted the removal of NH4+-N and TN.Furthermore,the single-chamber reactor without diaphragm and dual-chamber reactor with circulation were used for treating simulated wastewater.In single-chamber reactor without diaphragm,the removal efficiency of TN was 84%with the energy consumption of 0.13 kWh/g-N,the apparent rate constant was 1.87×10-2 min-1.And NH4+-N and NO2--N were completely removed,the removal efficiency of NO3--N was47%.In dual-chamber reactor with circulation,the removal efficiency of TN was 87%with the energy consumption of 2.31 kWh/g-N,the apparent rate constant was 1.88×10-2 min-1.And NH4+-N and NO2--N were completely removed,the removal efficiency of NO3--N was 55%.Finally,the electrochemical treatment of actual biochemical tail water of pharmaceutical industry was carried out in a single-chamber reactor without diaphragm.The initial concentration of COD,TN and NH4+-N were 340 mg/L,106 mg/L and 93mg/L,respectively.With the optimized power supply parameters,the removal efficiency of COD,TN and NH4+-N were 78%,77%and 90%,respectively,after 140minutes'electrolysis,and the concentrations of pollutant indexes met the discharge standards.The energy consumption was 0.25 kWh/g-N. |
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