Construction Of UV/NO₃^- System And Its Degradation Of Refractory Organics

With the rapid development of industry and agriculture,the types and quantities of refractory organics are increasing rapidly.They are difficult to remove through traditional biological or physical methods and are increasingly becoming a serious problem that threatens human health.On the basis of UV and its coupling process,this project attempts to build a UV/NO₃^- system and use it to degrade the refractory organics such as methylene blue and p-chlorophenol.In this study,low-pressure mercury lamps and medium-pressure mercury lamps were used as UV light sources to investigate the effects of different UV light sources on the NO₃^-photochemical activity in the presence of different concentrations and pH values.It was found that NO₃^-has photochemical activity under both UV light sources and can be converted into ammonia nitrogen,nitrite nitrogen,nitrogen oxides and other nitrogen forms.EPR test results also directly demonstrate that NO₃^- can produce hydroxyl radicals when irradiated by medium-pressure mercury lamps.Acidic conditions are conducive to the formation of hydroxyl radicals,and the alkaline environment is conducive to the accumulation of nitrite nitrogen.Based on the study of NO₃^- photochemical activity,methylene blue was degraded using low-pressure mercury lamp/NO₃^- and medium-pressure mercury lamp/NO₃^- processes,respectively.The study found that the decomposition of methylene blue by low-pressure mercury lamp/NO₃^- process was based on the indirect photolysis induced by NO₃^-.NO₃^- accelerated the degradation of methylene blue by generating hydroxyl radicals.The degradation of methylene blue under medium-pressure mercury lamp/NO₃^- process is dominated by direct photolysis,and NO₃^- mainly inhibits the degradation of methylene blue by forming an inert filter layer.The acidic conditions favor the degradation of methylene blue,while the pH of the solution in the alkaline environment fluctuates greatly and is essentially neutral at the end of the reaction.In addition,this topic also attempts to degrade p-chlorophenol using a medium-pressure mercury lamp/NO₃^- process,and investigate the effect of NO₃^- concentration,pH,humic acid,and carbonate on it.The study found that NO₃^- has different effects in different concentration ranges:the degradation of p-chlorophenol is dominated by direct photolysis at low concentrations,and NO₃^- inhibits the photolysis of p-chlorophenol;as the concentration increases,the indirect photolysis induced by NO₃^- is gradually enhanced;when the concentration reaches a certain value,the indirect photolysis induced by NO₃^- predominates,and the addition of NO₃^- promotes the degradation of p-chlorophenol.Compared with single-pressure mercury lamp irradiation,adding a certain amount of NO₃^- can significantly increase the rate of p-chlorophenol mineralization.The characteristic absorption peak of p-chlorophenol can be red-shifted under strong alkaline condition,which is beneficial to the accelerated degradation of p-chlorophenol by absorbing more light radiation.Humic acid can inhibit the direct photolysis and indirect photolysis of p-chlorophenol under the reaction system,leading to a slower degradation of p-chlorophenol.Carbonate has less comprehensive effect on the degradation of p-chlorophenol under the reaction system.In summary,the UV/NO₃^- process is a viable and promising method for treating refractory organics.NO₃^- is widely sourced and can be obtained from the wastewater itself or other wastewater.This method can significantly accelerate the degradation of refractory organics or increase their mineralization rate without additional input of oxidants or catalysts,and is worthy of further research or application in practice.