Effects Of Typical Anions On Hydrogen Peroxide Formation By Vacuum Ultraviolet Photolysis

Vacuum ultraviolet photolysis is a green and efficient method in water treatment for degradation of pollutants.It is a robust method and it can generate a series of highly-reactive radicals,so the pollutants can be effectively degraded.Therefore,this technology is widely used in advanced oxidation processes.Hydrogen peroxide is an important product during the vacuum ultraviolet photolysis process.By systematically studying the pattern of H₂O₂ formation,it is helpful to understand the vacuum ultraviolet photolysis ability and provide a valuable reference for practical engineering applications.Vacuum ultraviolet low-pressure mercury lamps can excite two different ultraviolet wavelengths（185 nm and 254 nm）.To distinguish the role of two ultraviolet wavelengths on the H₂O₂ formation,the vacuum ultraviolet lamp（185 nm and 254 nm）and ultraviolet lamp（254 nm）were compared photolysis of ultrapure water to study H₂O₂ formation,respectively.Results show the H₂O₂ comes from radicals generated by 185 nm wavelength,while the 254 nm wavelength plays a role on depredating H₂O₂.Furthermore,by studying the H₂O₂ formation under different dissolved oxygen contents（8.5 mg/L and 2.0 mg/L）,it is found that the maximum concentration of H₂O₂ is not related to dissolved oxygen,indicating that H₂O₂ was mainly derived from hydroxyl radical（·OH）recombination.In addition,the H₂O₂ formation rate is obtained by establishing the H₂O₂ kinetic model,which is well correlated with the square of the ·OH steady-state concentration（R²=0.97）.It further clarifies that the ·OH is the precursor of H₂O₂ during the vacuum ultraviolet photolysis process.In the practical engineering applications,the effect of inorganic ions in water on the photodegradation process cannot not be ignored.Therefore,in this paper,the effects of ten kinds of common inorganic anions,including hydroxide,carbonate,bicarbonate,hydrogen phosphate,dihydrogen phosphate,nitrate,nitrite,chlorine,bromine and iodide,on the formation trend of H₂O₂ are investigated.Results show that that the hydroxide ion,carbonate/bicarbonate buffer system,and hydrogen phosphate/dihydrogen phosphate buffer system mainly react with ·OH to inhibit the formation of H₂O₂.Nitrate ion and nitrite ion act as light absorbing compounds,which can inhibit the formation of ·OH by ultraviolet light cracking water.At the same time,nitrite ion generated by photolytic nitrate ions reacts with ·OH to inhibit H₂O₂ formation.Chloride ion inhibits H₂O₂ formation because it acts as a quencher for ·OH.Bromide ion not only inhibits ·OH formation due to its own light absorption properties,but also quenches H₂O₂ by the intermediate product.Therefore,the intermediate product formed by the reaction of bromide ion with ·OH accelerates H₂O₂ degradation process.Iodine ion has double effects on the H₂O₂ formation.On the one hand,due to its light absorption characteristics,the formation process of ·OH is inhibited.On the other hand,6 mol electrons are generated when 1 mol iodide ion is oxidized by ·OH to form iodate ions.6 mol electrons react with dissolved oxygen to promote H₂O₂ formation during vacuum ultraviolet process.The above phenomena and conclusions were also verified by constructing a reaction kinetic model.