08173cas|a2200421 i 4500 20260818171816.0 191197s1994 th uzm rtt 00| a1eng d .b1049988x AIT Diss. no. EV-94-1 Gupta, Hani Photocatalytic degradation of chlorinated hydrocarbons and its combination with anaerobic digestion Bangkok : Asian Institute of Technology, 1994 209 leaves Dissertation ; no. EV-94-1 A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Technical Science, School of Environment, Resources & Development Thesis (Ph.D.) - Asian Institute of Technology, 1994 The photocatalytic degradation of two chlorinated priority pollutants, aliphatic perchloroethylene (PCE) and aromatic 246--trichlorophenol (246-TCP) was studied. The mineralization of PCE was demonstrated mainly in terms of C02 formation. On the other hand, the breakdown of 246-TCP was studied by following the decrease in 246-TCP, while . its mineralization was monitored in terms of chloride ion (Ci-) production. A 400 ml synthetically prepared solution containing 120 mg/I PCE and 1 g/1 Ti02, or a 60 mg/1246-TCP and 2 g/1 Ti02 was irradiated by UV light of intensity 98 mW/cm2 and 12 mW/ cm2 respectively. A substantial portion amounting to 35% of PCE was volatilized into the headspace of the reactor and remained unconverted by photocatalysis. From the remaining 65% of the PCE that was still in a dissolved state, almost 87% recovery of carbon (in C02) was obtained during the 5 hour illumination period. On the other hand, while almost 100% 246-TCP degradation was observed within 6 hours of illumination, the complete recovery of c1- took about 10 hours of illumination, indicating the formation of intermediates which also underwent photocatalysis. The effects of various factors such as DO, pH, organic concentration, Ti02 dosage, UV intensity were investigated on the photocatalysis of these two compounds. An increase in initial DO levels showed a remarkable increase in the C02 production rates, validating the participation of oxygen in photocatalytic transformations. In case of both organics, the initial pH of the slurry did not have a very pronounced effect on the rate of photocatalysis. The optimum Ti02 dosage was found to be 2 g/1 and a decrease in the specific catalytic activity was observed with increasing Ti02 dosages. Interestingly, while a greater surface coverage of PCE on Ti02 at higher concentrations increased the rates of C02 formation, the decrease in 246-TCP degradation was a result of the possible competition between 246-TCP and its degradation intermediates for hydroxyl radicals. No intermediates were detected during the photocatalysis of PCE. However, a lag time in the production of C02 was observed in the initial stages of the reaction. This phase was found to correspond to the time taken for adsorption of PCE onto Ti02{u2022} In case of 246-TCP, three intermediates were detected of which a phenolic compound was identified as the major 246-TCP degradation product and was found to inhibit the degradation of the parent 246-TCP. Inspite of the complete disappearance of 246-TCP within 6 hours of illumination, complete mineralization took a much longer irradiation time. The presence of chloride did not inhibit the degradation of 246-TCP at the pH ranges at which the experiments were -iv- carried out. Conversely, the degradation of 246-TCP was enhanced by the presence of chloride in amounts greater than 60 rrig/1. The kinetics of photocatalytic reactions was studied to explain the basic mechanism involved in photocatalytic reactions. A good fit of the C02 production from PCB to the Langmuir Hinshelwood model indicated that the photocatalysis of PCB was most probably a surface phenomenon and the reaction mechanism of PCB photocatalysis was via adsorption of the organic on the semiconductor surface followed by radical attack. Adsorption did not seem to play a major role in 246-TCP photocatalysis, and the attack of hydroxyl radicals on the 246-TCP molecule was realized as the major reaction pathway. A simple rate determining empirical equation was developed to determine the rate constant of 246-TCP photocatalysis within certain stipulated conditions. The photocatalytic process was applied to anaerobic degradation of 246-TCP as a pretreatment step in order to detoxify high concentrations of 246-TCP which may be inhibitory to the anaerobic populations. An increase in the photocatalytic pretreatment time for 246-TCP was found to enhance detoxification. While 246-TCP was not found to mineralize by anaerobic treatment, probably due to the absence of bacterial species capable of degrading its intermediate product 4-monochlorophenol ( 4-MCP), increased pretreatment decreased the concentration of the effluent 4-MCP. Dechlorination of the ortho- substituted chlorine was preferred to that at the meta- position. Pretreatment also enhanced acetogenesis, methanogenesis, TOC removal and sulfate reduction. Unpretreated 246-TCP was found to severely inhibit acetogenesis and methanogenesis when propionic acid was used as a co-substrate. However, this was a transitory phase which recovered once all 246-TCP was dechlorinated to 4-MCP, indicating that the inhibition was primarily due to 246-TCP and not due to the presence of the intermediates of 246-TCP dechlorination, i.e., 2,4-dichlorophenol (24-DCP) or 4-MCP. With pretreatment, the inhibitory effect was greatly reduced. A photocatalytic pretreatment time of 4 hours mitigated the toxicity of 246-TCP to acetogens and methanogens by 52% and 45%, respectively. Enrichments with propionic acid were found to be more sensitive to 246-TCP than those incubated with acetic acid, probably due to the existing thermodynamically unfavorable conditions for propionate degradation. Sulfate reducing bacteria and methane producing bacteria were found to be collectively responsible for the dechlorination of 246-TCP and 24-DCP. It was observed that the presence of any inhibitory substance capable of suppressing the activity of the sulfate reducers or methane producers could easily affect the overall degradation process. -v- Post treatment of anaerobically digested 246-TCP was unsuccessful in degrading 246-TCP or its anaerobically dechlorinated intermediates. Apparently, it was not feasible owing to the high colloidal turbidity of the effluent and/or the formation of innocuous organic/inorganic substances during anaerobic digestion that may have been capable of inhibiting photocatalysis. An increase in the photocatalytic pretreatment time upto 4 hours reduced the effluent 4-MCP concentration considerably low enough to be released into waste receiving streams. Sewage Purification Oxidation Umita, Teruyuki, Chairperson Chongrak Polprasert, Co-Chairperson Tanaka, Shuzo, Examination Committee Bhattacharya, Sribas C., Examination Committee Ohgaki, Shinnichiro, Examination Committee Asian Institute of Technology. Dissertation ; no. 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