| Abstract | 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• 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
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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 LangmuirHinshelwood 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.
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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. |