Study Of Electro-catalytic Properties Of Doped IrO₂ Anode And Its Application To Livestock And Aquaculture Wastewater Treatment

Livestock and aquaculture wastewater has become a major source of chemical oxygen demand(COD)and ammonia nitrogen(NH4+-N)emissions in China.In addition,emerging contaminants such as antibiotics in livestock and aquaculture wastewater are receiving increasing attention.Compared with the established wastewater treatment technologies,electro-oxidation has the advantages of ⅰ)the ability to treat multiple classes of contaminants,ⅱ)in situ production of chemicals,and ⅲ)easy automation.Titanium-based noble metal oxide coated electrodes(DSA)have been commercially used for decades because of their excellent electrochemical activity and stability.However,the application of DSA has been limited in recent years due to the rising price of noble metals.Doping can improve the surface morphology of the electrode while preserving the electrochemical activity of DSA,and is a promising solution to the cost problem.A series of W,Ti-doped IrO2 anodes with different doping ratios were prepared,physically and electrochemically characterized,and the anode with the best doping ratio was selected based on the characterization results and tested for its stability.When the doping ratio does not exceed 50%,the modified electrodes show a smoother and more compact surface morphology.Despite the decrease in electrochemically active surface area(ECSA),the accelerated lifetime of the electrode is improved from 182 h to 352 h after doping.The characterization results show that the iridium,tungsten,and titanium oxides in the active layer of the electrode form amorphous solid solutions during calcination.The oxygen evolution potential(OEP)of the doped anode is～1.55 V vs.RHE,and is therefore classified as active anode.Electro-oxidation experiments of organic contaminants were conducted using Ir0.6W0.20Ti0.20 anode(optimal doping ratio),and the oxidation mechanism,conversion of sulfadiazine(SD),and the effects of current density,organic contaminant concentration and chloride ion concentration on COD removal were investigated.Indirect oxidation mediated by active chlorine is the main degradation pathway for organic contaminants.Electrolysis of synthetic wastewater containing 20 mg/L SD(J=20 mA/cm2)results in 94%conversion of SD within 1 min and complete conversion within 15 min.Electrolysis of synthetic wastewater containing 700-800 mg/L initial COD was(J=40 mA/cm2)achieves complete COD removal in 3 h with a specific energy consumption(SEC)of 100 kWh/kg COD.Electro-oxidation experiments of NH4+-N were conducted using Ir0.6W0.20Ti0.20 anode(optimal doping ratio),and the kinetic model of chlorine evolution reaction(CER)was developed,and the interaction of different components in the complex reaction system was investigated by cyclic voltammetry(CV)experiments.In the absence of NH4+-N,the generation of active chlorine follows zero-order kinetics and its consumption follows first-order kinetics.Although HCO3-does not react directly with active chlorine,it inhibits the generation of active chlorine by competing for active sites on the anode surface to promote OER side reaction.Cathodic reduction is the primary scavenging pathway for active chlorine.NH4+-N in the form of NH3 can be directly oxidized on the DSA surface.In synthetic mariculture wastewater(30-40 mg/L NH4+-N)and livestock wastewater(～450 mg/L NH4+-N),NH4+-N removal rate is 112.9 g NH4+-N/(m2·d)and 186.5 g NH4+-N/(m2·d),respectively,which is much more efficient than biological treatment,while the SEC is 31.5 and 260 kWh/kg NH4+-N,respectively.