| Advanced oxidation processes(AOPs) have been used widely for degradation of refractory organic pollutants in drinking water and wastewater treatments. Among the AOPs, OH?-based AOPs haved been developed for long time and the O3-based AOPs have been applied in full scale water treatments. While SO4?--based AOPs are developing quickly in recent years, and are proposed to be used in wastewater treatment and groundwater remediation as alternative AOPs. On one hand, it concerns the degradation efficiencies of refractory organic pollutants in AOPs, which are depended on the formation of free radicals(SO4?-/OH?) and the second-order rate constants of reactions between the pollutants and radicals. Thus, it is better to predict the elimination of refractory organic pollutants for quantitation of radicals formation in AOPs. On the other hand, it is important to understand and control the undired byproducts(such as bromate(Br O3-) and halogenated byproducts) formation before the application of AOPs. Therefore, this study investigated these two aspects during AOPs:(i) determination of radicals and(ii) formation of byproducts, it aims to provide theoretical and technical support for the application of AOPs.During O3 and O3/H2O2 processes, three methods such as tert-butanol(t Bu OH) assay, methanol(Me OH) assay and competition method were developed and compared for OH? yield determination. Among these methods, the conventional t Bu OH assay was convenient for determination of OH? yield in the initiation reactoins. The results showed that:(i) organic model compounds of natrual organic matter reacted with O3 yielding 0.5~38% of OH?;(ii) the OH? yields of reactions between OH- and HO2- with O3 were 41% and 50% respectively;(iii) during ozonation, the O3 prefers to react with reactive organic compounds in the water matrix, with the increase of O3 dose, these compounds are consumed gradually and the contributions of reactions of O3 with OH- and HO2- to O3 decomposition and OH? formation become important, thus the OH? yields in the intiation reactions of water matrix increases. However, excess OH? scavenger(i.e., t Bu OH) in the t Bu OH assay would inhibit all the propagation reactions which might play an important role in these processes. The OH? yield in the propagation reactions was confirmed to be 100% by the Me OH assay. Thus, the competition method which could ac count for the contribution of propagation reactions was recommended to determine the real OH? yield. The OH? yield during ozonation was determined to be 33%~58%, which was higher than that obtained in the t Bu OH assay. During O3/H2O2 of real waters, the rate constant of O3 consumption increased significantly with the increase of H2O2 concentration([H2O2]:[O3] = 0.1~0.35). However, compared to ozonation alone, the improvement of the Ф values was negligible over a wide range of [H2O2]:[O3] = 0.1~2.0. This discrepancy was mainly ascribed to the fact that substantial O3 consumption via the propagation reactions resulted in comparable Ф values in peroxone vs. ozonation processes.Similar to OH?, scavenging assay(such as Me OH assay, ethanol(Et OH) assay and t Bu OH assay) and competition method were developed for SO4?- determination. The results showed that at p H=7, apparent formaldehyde, acetaldehyde, and formaldehyde yields for per SO4?- in Me OH assay, Et OH assay, and t Bu OH assay were 1.74, 3.30 and 0.46, respectively. Meanwhile, it should be paid attenttion that these aldehydes yields were different under conditions of different p H. The competition method was confirmed by using low concentrations of Me OH and Et OH as competitiors. Furthermore, the second-order rate constants of natural organic matter isolates SRHA, SRFA, NOM and Soli HA with SO4?- were determined by this competition method to be 1.06×104、5.57×103、7.61×103' 1.33×103(mg/L)-1 s-1. In addition, it was found that Cl- could scavenge SO4?- yielding Cl?, which converted into OH? quickly.On the other hand, the major undesired byproducts in AOPs are inorgnic bromate and brominated organic byproducts, which received great concerns before application of AOPs. Firstly, this study investigated the formation o f inorganic bromate. It was found that under the condtion of pure water, Br- could be oxidised by SO4?- into bromate(Br O3-) via intermediate of HOBr/OBr- in UV photolysis of perdisulfate(PDS), thermal activation of PDS and UV photolysis of peroxymonosulfate(PMS) processes. In UV/PDS, Br O3- formation rate increased with the increase of PDS intial concentrations, and decreased with the increase of p H. The Br O3- formation was completely suppressed at higher p H(p H>9). Compared to UV/PDS, Br O3- formation was far less in the thermal activation of PDS process at the same production of SO4?-. With the help of kinetic modeling via all the proposed reactions, it was found that the fast equilibrium of Br? and OH? acted an important role in the HOBr/OBr- and subsequent Br O3- formation.In the presence of natrual organic matter isolates and model compounds, Br O3- formation was suppressed in the SO4?--based advanced oxidation processes(such as UV/PDS, thermal activation PDS, and UV/PMS). Types of model compounds such as phenolic compounds, amino acids, carboxylic acids and olefinic compounds could suppress the Br O3- formation in different extent, and the extent of suppression increased with the increase of model compounds concentrations. It indicated that the extent of suppression was greatly affect by the mineralization of organic compounds. Based on these experimental data, the mechanism of suppression for Br O 3- formation was suggested to be that, OH? was quickly consumed by organic compounds resulted in low steady-state concentration of OH?, and thus the fast equilibrium between Br? and OH? reduced the steady-state concentration of Br?, which suppressed the HOBr/OBr- and subsequent Br O3- formation. Furthermore, in the presence of all the natrual organic matter isolates and model compounds, the mass of inorganic bromine species almost kept balance during Br- oxidation in UV/PDS process indicated that few of brominated organic byproducts formed in UV/PDS process. However, it was surprising that great amounts of inorganic bromine transformed into organic compound(TOBr) in the PMS activated SO4?--based AOPs. The discrepancy in these two SO4?--based AOPs(i.e., PDS activation and PMS activation) was mainly ascribed to the great contribution of HOBr/OBr- formed via reaction of PMS with Br-. |
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