Research On Development And Organic Degradation Characteristics Of Ozone/Hydroxylamine System

Nowadays,organic pollution is one of the problems to be resolved in the water treatment.During the ozonation systems,the molecular ozone is effective to degrade the electron-donating compounds,but fails to degrade many refractory organic contaminants.In comparison,·OH is capable of degrading the majority of organic contaminants at high reaction rates.As a result,constructing an ozonation system that produces ·OH is beneficial for the degradation of refractory contaminants.In this work,we developed the ozone/hydroxylamine system that produces ·OH,and investigated its ·OH formation,oxidation and bromate formation.We proposed deethylatrazine?DEA?as a new ·OH probe compound to replace p-chlorobenzoic acid?pCBA?,the commonly used one during ozonation.It is difficult to qunatify ·OH accurately using pCBA as the probe compound,because pCBA-derived propagation reactions largely accelarated the rate of ozone decay by 50%-175% and led to additional ·OH exposures,which accounted 3.3%-37.5% of the total amount.DEA exhibited a negligible effect on the ozone decomposition.The degradation of DEA was exclusively contributed by ·OH formed in the waters tested using DEA as the ·OH probe compound.pCBA is applicable only for the water with high ·OH scavenging capacity,due to the pCBA-derived propagation reactions are fully inhibited.In comparison,DEA is applicable for waters with different ·OH scavenging capacities.Thus,DEA is a more approperiate ·OH probe compound during ozonation.Ozone/hydroxylamine process was developed.·OH formation has been demonstrated using DEA as the ·OH probe compound in the ozone/hydroxylamine process.The ·OH yield was determined to be 0.258 by “the t-Bu OH assay”.We investigated the effect of different factors on ·OH formation in ozone/hydroxylamine process using DEA as the ·OH probe compound.The results indicated that the ozone/hydroxylamine system is capable of producing ·OH fast with a second-order rate constant of 3.03×104M^-1s^-1.It is applicable for the waters over a wide p H range.The ozone/hydroxylamine system still produced ·OH even at acidic p H.Nitrate and nitrite were identified as the main products from the reaction of ozone with hydroxylamine.Based on the ·OH yield and the end products of the reaction of ozone with hydroxylamine,we propsed the mechanism of ·OH formation in the ozone/hydroxylamine system.The degradations of atrazine by the ozone/hydroxylamine system were investigated under different conditions to evaluate its oxidation capacity.Results indicated that the degradation of atrazine increased from 24% to 80% with increasing hydroxylamine concentrations.But when the concentrations of hydroxylamine were in excess,it would compete ·OH with DEA with a second-order rate constant of 9.5×109M^-1s^-1,and thus resulted a decrease in the degradation of atrazine at a high dosage of hydroxylamine.The degradation rate of atrazine increased with increasing p H in the ozone/hydroxylamine system,which still degraded 80% of the initial atrazine even at acidic pH but with a lower rate.In addition,bicarbonate and temperature could also affect the degradation of atrazine in the ozone/hydroxylamine system.Based on the identification of degradation products of atrazine,we proposed the mechanism of the atrazine degradation as dealkylation,dechlorination-hydroxylation and olefination.As the main product from the reaction of ozone with hydroxylamine,nitrite reacts with ozone to produce ·OH.This work investigated the effect of nitrite on the degradation of atrazine by ozonation.The results indicated that the presence of nitrite enhanced the degradation of atrazine by ozonation.Thus,the formed nitrite from the reaction of ozone with hydroxylamine would exert enhancing effect on the degradation of atrazine in the ozone/hydroxylamine system.The bromate formation exhibited a decrease with increasing p H in the ozone/hydroxylamine system in the bromide-containing water,which was different with that in other ozone systems.Thus,the study investigated the bromate formation in ozone/hydroxylamine system.Results indicated that ·OH plays an essential role in the formation of bromate in ozone/hydroxylamine system.Based on the evolutions of the formed concentrations of HOBr/BrO-and the degradations of DEA,we proposed that bromide was initially oxidized by ·OH to form ·Br,which was then further oxizide by ozone and formed bromate eventually in the ozone/hydroxylamine system at acidic p H.The different p H-dependency of bromate formation in ozone/hydroxylamine system could be ascribed to the increase in the rate of ·Br formation and the decrease in the rate of ozone decay by pH depression.