| Chlorophenols (CPs) have been extensively used as broad-spectrum biocides, wood preservatives and pesticides. Due to their high toxicity and persistency, chlorophenols have widely contaminated surface water, soils, sediments, and even ground water by detected at varied concentrations. Manganese oxides are common components of minerals ubiquitously in terrestrial, aquatic and marine environments. Manganese oxides are considered the most important abiotic redox-active minerals found in the environment and are known to have an important role in catalyzing the abiotic transformation of a number of model organic compounds such as phenols and aniline. In order to have a thorough understand on the environmental chemistry behaviors of CPs in natural environment, it is highly important to study the abiotic transformation of CPs mediated by MnO2. In this study, seven isomers of chlorophenols with the different number and position of chloro substituents on the aromatic ring were selected as target compounds. A series experiments were conducted in the laboratory to determine reaction kinetics, products and pathways, and assess the effect of environmental conditions on reaction kinetics.The batch experimental method and screw-cap glass bottle -shaker system were employed for the kinetic study. The results showed that all CPs in the study were highly susceptible to transformation reaction mediated by MnC>2 except for 2,4,5-trichlorophenol and 2,3,4,5-tetrachlorophenol. Dechlorination occurred for all CPs during the transformation reaction, and the extent of dechlorination were quite significant for 2,4,6-trichlorophenol and 2,3,4,6-tetrachlorophenol with a corresponding stoichiometric dechlorination number above 1. Parallel to the CPs loss, MnO2 was reduced into dissolved Mn2+ ions. Using pseudo-first-order model in a non-linear curve fitting way to simulate the reaction kinetics of CPs gave out the good results. The reaction rate of each chlorophenol showed a positive correlation with the initial concentrations of chlorophenol itself and MnO2, but displayed a negative correlation with pH of reaction system. Addition of Mn2+, Ca2+ and substituted phenolic acids considerably decreased the reaction rate of pentachlorophenol (PCP) due to either blocking active sites on MnO2 surfaces by adsorption or occurring redox reaction with MnC>2.Using solvent extraction and GC-MS analysis, the oxidation products of each chlorophenol were determined. The products of PCP included two isomeric nonachlorohydroxybiphenylethers as major products via coupling PCP itself and tetrachloro-l,4-hydroquinone and tetrachlorocatechol as minor products via simultaneous dehydrochlorination and hydroxylation with H2O. The oxidation products of tetrachlorophenols and trichlorophenols were more than those of PCP, which mainly included polyhydroxylated compounds, dimeric and trimeric compounds. But no polyhydroxylated compounds were detected in the products of 2,4,5-trichlorophenol and 2,3,4,5-tetrachlorophenol. The structural characteristics of polymeric products demonstrated that the coupling reactions of phenoxy radicals most likely occurred via C- C or C- O coupling at the ortho or para positions on aromatic ring. According to the reaction kinetics and the structures of oxidation products of seven chlorophenols, the influence of the number and the position of chloro substitutents on the transformation reactivity exhibited different mechanisms. The different number of chloro substituents result in the different hydrophobicity of CPsand adsorption to MnC>2 increase as the number of chloro substituents is increased. Thus, the formation of surface complex increase as the number of chloro substituents is increased. The different position of chloro substituents result in the different electron density on the aromatic ring of CPs and the trend of electron transfer between CPs and MnC>2. Thus electron-transfer rates depend quite strongly upon the position of chloro substituents on the aromatic ring.
|
PDF Full Text Request