Effect And Mechanism Of Degradation Of Nitro(Chloro)Benzene And Tannin Acid In Aqueous Solution By O₃/H₂O₂

Organic pollutions in water gradually become a serious threaten to human health and water environment. In order to keep water secure, proper treatment process should be innovated and adopted to eliminate or remove these pollutants. Aryls, such as NB and CNBs, occupies more than 50 percent of Priority Organic Compounds(POCs) listed in most countries. They are difficult to be eliminated with routine processes of water treatment. Ozonation and AOPs, which could produce·OH, perform powerfully in the removal of these aryls. The cooperation of O₃ and H₂O₂ can generated·OH which is a strong oxidatant.Selecting nitrobenzene (NB) as the model pollutant, the efficiency and mechanism of the degradation of NB in aqueous solution by O₃/H₂O₂ system were investigated. Effects of pH, H₂O₂ dose and the inhibitor or accelerant of·OH on the removal rate of NB were studied. H₂O₂ could obviously improve the ozonation of NB below pH 7. H₂O₂ dose was increased from 1.0mg/L to 4.0mg/L, within the time of 5-min reacting, the removal rate of NB was enhanced. However, as H₂O₂ dose increased from 4.0mg/L to 20mg/L, the removal efficiency of NB decreased. Different quantities of H₂O₂ were yielded in different reaction phases of single ozonation system. The addition of some organisms could make the removal of NB decreased with different degrees. Both systems of single ozonation and H₂O₂-catalysed ozonation could not reduce TOC observably. During the NB degradation process, organonitrogen was almost completely converted to nitrate and the pH value reduced significantly. Results of LC-MS and GC-MS show that main intermediate products were phenolic compounds and carbonyl compounds. Finally, a possible reaction pathway of the catalytic ozonation of NB was proposed. It had been found that the catalytic ozonation of NB can be divided into two steps. First, hydroxyl radical attacked phenyl ring, then the ring opened, forming into various aliphatic compounds or being mineralizd to inorganic compounds.pCNB is another stubborn organic pollutant in water. The kinetics and mechanism of ozonation of p-nitrochlorobenzene (pCNB) were investigated. The reaction rate constants of pCNB with O₃ was 1.6L/(mol·s) by direct measuration. With nitrobenzene and chlorobenzene as the reference compounds, the reaction rate constants of pCNB with·OH, measured by means of a competition kinetics, was 2.6×109L/(mol·s) During the ozonation of pCNB, organic nitrogen and organic chlorine were almost completely converted to nitrate and chloride, TOC could not be obviously reduced. Results of LC-MS and GC-MS show that main intermediate products were aromatic substances such as p-chlorophenol, p-nitrophenol, 2-chloro-5-nitrophenol, etc., and nonaromatic substances such as oxalic acid, malonic acid, maleic acid, muconic acid, etc. A possible reaction pathway of ozonation of pCNB , proposed according to these products, was found to be in accord with that of NB. Due to the dissimilar place of either nitryl or chlorine, nitrochlorobenzene has three isomers as p-nitrochlorobenzene(pCNB), m-nitrochlorobenzene(mCNB) and o-nitrochlorobenzene(oCNB). Basing on the quantum chemistry, electron cloud density of carbon atom on phenyl and NOB were calculated to be different of these three isomers. The secondary rate constants of these three CNBs reacting directly with O₃ were 1.6, 1.5, 1.6L/(mol·s), respectively. Taking nitrobenzene and chlorbenzene as reference compounds, measured with competition kinetics method, the secondary rate constants of three CNBs reacting with·OH were 2.6×109, 2.8×109, 3.2×109L/(mol·s), respectively. It suggested that the free radical reaction is the primary reaction in the ozonation process of CNBs. It was found that pH value of the solution decreased during the process, the color of the solution turned yellow suddenly, then came back to colorlessness rapidly, and the concentration of IC,Cl- and NO₃- in the system increased gradually. H₂O₂ was generated in each different phase of the process, when the concentration of the pCNB is 50mg/L, the concentration of H₂O₂ could reach maximally to 1.8mg/L. The degradation efficiency of CNBs was 99%, but the removal rate of TOC was only 45%. Intermediate products were analyzed by GC-MS/LC-MS. The products of these three CNBs mainly contain phenolic matters such as chlorophenol, nitrophenol, nitrochlorophenol, and carboxylic matters such as oxalic acid, malonic acid, maleic acid,muconic acid. In the ozonation process of pCNB, -Cl and -NO2 could be replaced by·OH, chlorophenol and nitrophenol were formed meanwhile. But in the ozonation process of oCNB and mCNB, neither nitrophenol nor chlorophenol was detected. The hydrogen on the phenyl of all these three CNBs could be replaced by·OH, thus nitrochlorophenol was produced, and ten different nitrochlorophenols were detected by GC-MS/LC-MS. The difference of the substitutional place of -Cl or -NO₂ on the phenyl could lead to the difference of the electron cloud density of carbon and the natural bond orbital (NBO) population. The probability of the reaction between·OH and the hydrogen on the phenyl has some relationship with electron cloud density of carbon and NOB, but little relationship with bond energy. The reaction between·OH and CNBs is probably similar to the electrophilic substitution reactions of aryl.In the degradation of NB and CNBs by O₃-O₃/H₂O₂, aryl ring was hydroxylated by·OH first, electron cloud density on the aryl increased, then the instable aryl was cracked by ozone. Tannic acid, being polyhydroxyl, is one of macromolecule nature oganic matters, the efficiency and mechanism of tannic acid degradation by O₃ and O₃/H₂O₂ were studied. Results show that the addition of H₂O₂ has no obvious promotion on the ozonation efficiency of tannic acid. But the system of O₃/H₂O₂ performs 10 percent more than solo ozonation in the removal of TOC. The relative generating intensity of·OH in the O₃/H₂O₂ system was detected by Electronic Spin Resonance(ESR). It was found that tannic acid consumingly restrained the generation of·OH, it mainly attributes to the polyhydroxyl structure of tannic acid. In addition, ozone reacts speedily with tannic acid. Ozonation of tannic acid are mainly aldehydes such as acetaldehyde, glyoxal, methyl glyoxal. Analysis of Ion Chromatogram show that final products of tannic acid are oxalic acid, ketomalonic acid, they occupies 30 percent of TOC in both systems.