Chlorine/Iron-mediated Electrochemical Oxidation Of Ammonia In Wastewater

Treatment of wastewater laden with ammonia nitrogen has attracted much attention because of its release from a variety of sources,a considerable amount of emissions,and a wide range of concentrations.Random discharge of ammonia-containing wastewater into the aquatic environment will cause eutrophication of the rivers and lakes,and seriously endanger the health of aquatic animals and plants.The commonly used methods for ammonia abatement include the biological anoxic/aerobic process and the chemical breakpoint chlorination process.However,the biological method suffers the problems of long reaction time,large land area of construction,and low nitrogen removal efficiency;the breakpoint chlorination has the limitations of difficulty in storage and transport of active chlorine,and formation of by-products toxic to human.In recent years,researchers have proposed the combination of electrochemical oxidation and active chlorine oxidation to solve the problems.The principle is the indirect transformation of ammonia by the in situ generated active chlorine in the electrochemical reactor.This strategy,however,still has the shortcomings of low reaction efficiency and production of secondary pollutants(e.g.,nitrate,chlorate,and chloramines).Herein,we report that the increasing concentration of Cl-in the active chlorine-mediated electrochemical system can facilitate ammonia oxidation,and that the presence of Fe(?)can significantly enhance reaction selectivity with distinct reduction in the levels of toxic byproducts.The chlorine/iron-mediated electrochemical system also exhibits enhanced performance of simultaneous carbon and nitrogen removal,when applied for the treatment of aged landfill leachate.First,we aim to investigate the impact of the chlorine/iron coexistence on the reactivity and selectivity of electrochemical ammonia oxidation.The results show that,in the MMO(Mixed metal oxide)-single anode and the MMO/Fe-dual anode systems,the increasing concentration of Cl-in the electrolyte results in the increased amounts of active chlorine and thus accelerates ammonia oxidation at a constant applied current to the MMO anode.The presence of Fe anode(at a current density of 2 mA cm-2)insignificantly affects ammonia oxidation,but appreciably suppresses the production of toxic byproducts such as nitrate,chlorate,and monochloramine.Under the experimental conditions of 8 mA cm-2 current density at the MMO anode,2 mA cm-2 current density at the Fe anode,500 mM Cl-,pH 3.0,and reaction time of 180 min,the first-order reaction rate constant was 0.0111 ± 0.0002 min-1,the proportion of ammonia removal was 86.5 ± 2.4%,the accumulated concentration of nitrate nitrogen was 119.0 ± 7.1 ?M,and the peak concentrations of chlorate and monochloramine were 35.3 ± 9.3?M and 92.8 ± 7.8 ?M,respectively.In contrast,in the absence of Fe anode,the peak concentrations of nitrate nitrogen,chlorate,and monochloramine were 246.0±7.1 ?M,152.2±0.6 ?M and 158.9 ± 0.6 ?M,respectively.The combination of reactive species scavenging experiments,electron spin resonance(ESR)measurements,methyl phenyl sulfoxide(PMSO)-probe tests provide preliminary evidence that Fe2+ reacts with in situ generated active chlorine to generate oxidative Fe?O2+.Based on these findings,we propose that the electrochemically generated active chlorine from the MMO anode plays an important role in oxidizing ammonia,but this leads to the extensive accumulation of the byproducts such as nitrate,chlorate and chloramines.The Fe2+ generated from the Fe anode can react with active chlorine,thus alleviating the possibility of generation of chlorate and chloramines;in addition,this induces the formation of Fe?O2+,which might enhance the selectivity of ammonia conversion into nitrogen.Secondly,we study the feasibility of utilizing the chlorine/iron-mediated electrochemical system for real wastewater treatment.It is demonstrated that the MMO/Fe-dual anode system can effectivity remove the chemical oxygen demand(COD)and total nitrogen(TN)in the aged landfill leachate.The results show that the increases in the concentration of Cl-and the current density to the MMO anode are conducive to ammonia removal.Simultaneous removal of carbon and nitrogen is achieved at the current densities of 38 mA cm-2 to the MMO anode and 8 mA cm-2 to the Fe anode,and with the addition of 500 mM Cl-.In comparison to the MMO-single anode system,the MMO/Fe-dual anode system exhibits better performance for COD removal.Although there is no significant improvement of the ammonia removal,the MMO/Fe-dual anode system enables distinctly lower production of nitrate and chlorate.This finding further confirms the positive role of Fe in alleviating the formation of byproducts for the electrogenerated active chlorine-mediated ammonia oxidation process.