Influence Factor And Mechanism Of The Indirect Electrochemical Reduction Of Nitrate By Zero-valent Titanium(ZVT)

With the continuous development of social economy and industry and agriculture,people's living level is being rised.Pollution and destruction to natural environment caused by human activities has become more serious,especially the water pollution,which has been the most important factor that restricts economy development and affects human health.In the water pollution aspect,the water contamination caused by the oxidized pollutants,particularly nitrate?NO₃^-?is more and more serious.In China,the surface and ground waters are increasingly contaminated with nitrate in recent years.Nitrate poses serious concerns to human health,ecological cycles,and overall water supply.The newly revised "Drinking Water Health Standards",stipulated that the maximum contaminant level of NO₃^--N in drinking water was 10 mg/L.In this paper,the zero-valent titanium?ZVT?was used as anode for the indirect electrochemical reduction of NO₃^-,more details are as follows:In the reaction system,the nitrate could be reduced by the anode and cathode.The results showed that NO₃^--N in water was reduced from 25.9 to 7.7 mg/L after 12 h?70.2%?,while the contribution of ZVT cathode was 21.6%at an applied current density of 2.0 mA/cm².Ammonium,nitrite and nitrogen?80.2%?were the main final product.Meanwhile,the pseudo-first-order kinetic model could simulate the indirect electrochemical reduction of nitrate.The Ti electrode could be oxidized to produce the surface oxide film and then the ZVT was oxidized during electrochemically induced pitting corrosion to produce reactive soluble species(Ti²⁺ or Ti³⁺).Related experiments were also conducted to evaluate the effects of current density,the initial chloride concentration,initial pH,and initial nitrate concentration on nitrate reduction.Experiments results showed that the removal efficiency and the kinetic rate constants of nitrate reduction increased with the increasing current density.When the current density raised from 0.6 mA/cm² to 6.0 mA/cm²,the nitrate removal efficiency and the kinetic rate constants of nitrate reduction increased from 36.5%and 0.03 h^-1 to 83.4%and 0.15 h"1,respectively.It suggested that higher current density accelerated the pitting corrosion,resulting in more production of reactive soluble species(Ti^2? or Ti^3?).The chloride had both beneficial and detrimental effects on nitrate reduction.This conclusion can be explained from two aspects.On the one hand,at the pitting corrosion part,chloride can help to induce pitting corrosion of ZVT,which improves the nitrate reduction.On the other hand,the presence of chloride also hinders the adsorption and reduction of nitrate on the ZVT surface due to the competitive adsorption.The kinetic rate constants and the removal efficiency of nitrate increase with the decrease of solution pH due to the ZVT pitting corrosion rate will be promoted in the lower pH,while the Ti³⁺ will react with hydroxyl and produce black precipitate?Ti?OH?₃?at the higher pH.When the initial pH decreased from 10.0 to 4.0,the rate constants and removal efficiency raised from 0.06 h^-1 and 56.8%to 0.13 h^-1 and 79.2%,respectively.Moreover,it was beneficial to reduce nitrate at lower initial nitrate concentration.The theoretical mechanism of nitrate reduction on the ZVT electrochemical system was discussed.Firstly,the nitrate was adsorbed on the surface of ZVT electrode.With the application of a constant current density on the electrode?2.0 mA/cm²?,corrosion of the ZVT electrode?since it was anode?took place combining with the formation of reactive soluble species(Ti²⁺ or Ti³⁺).Direction reaction between Ti²⁺ or Ti³⁺ and nitrate happened,which led to the formation of various intermediate nitrogen-containing compound and finally nitrite,ammonium,and nitrogen.However,the product analysis showed that the amount of reduced nitrate was not in accordance with that of generated products,and the loss of nitrogen should contribute to doping into the TiO₂ and volatilizing to the ambient atmosphere.In addition,the decrease in chloride concentration should be attributed to the formation volatile chlorine on the anode surface.Besides,the gaseous chlorine can dissolve into the water and oxidize the chloride to pypocholoride or chlorite,which can oxidize nitrite to nitrate.Meanwhile,some chloride and sulfur atoms can also be doped into TiO₂.