Zinc Oxide-Catalysed Photo-Oxidative Degradation of Chlorophenols
Chlorophenols are priority pollutants that must be eradicated from the environment owing to the severity of their toxicity and resistance to traditional treatment. Photocatalytic oxidation is an advanced oxidation method which has proven reliability to eliminate persistent pollutants from air and...
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| Format: | Thesis |
| Language: | English |
| Published: |
2009
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| Online Access: | http://psasir.upm.edu.my/id/eprint/5734/1/A_FS_2009_22.pdf http://psasir.upm.edu.my/id/eprint/5734/ |
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| Summary: | Chlorophenols are priority pollutants that must be eradicated from the environment
owing to the severity of their toxicity and resistance to traditional treatment.
Photocatalytic oxidation is an advanced oxidation method which has proven
reliability to eliminate persistent pollutants from air and water. The activity of zinc
oxide for pollutant removal by photocatalytic oxidation has been well established. In
this work the photocatalytic transformation of 4-chlorophenol, 2,4-dichlorophenol
and 2,4,6-trichlorophenol in irradiated ZnO suspensions at 299 K was studied. The
effect of operating parameters such as catalyst and concentration doses on the
decomposition rate of these para-chlorinated compounds has been investigated and
optimised. It was discovered that the optimum feed concentration for the phenolic
compounds is 50 mg L-1. The optimum amount of ZnO was determined for the
degradation of 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol which
decreased as with increasing of chlorine substituent. For 4-chlorophenol degradation the first clearer description of the effect of doses using response surface was
reported.
Kinetic profiles on the decomposition of chlorophenols over ZnO were consistent
with pseudo-zeroeth order rate scheme. For 2,4-dichlorophenol and 2,4,6-
trichlorophenol the decomposition was slow at the short irradiation time. It was
found that the degradability of chlorophenols increased as the number of ringchlorine
increased. The effect of pH on the destruction rate was found to be
influenced by chlorophenol adsorption and dissociation equilibrium.
The effect of different anions on the rate of chlorophenol degradation was evaluated
by utilising sodium salts as additives. Except for 4-chlorophenol it was found that,
inorganic anion additives such as SO4
2-, S2O8
2- and Cl- demonstrated inhibition to the
decomposition rate of chlorophenol. HPO4
2- was found to show strongest inhibition
and could even hamper the degradation of 4-chlorophenol.
The progression of intermediates during the mineralisation of chlorophenols was
chromatographed on high performance liquid chromatograph (HPLC). The structure
elucidation of pathway products en route to mineralisation of chlorophenols was
performed by the combined gas chromatography-mass spectrometry (GC-MS) and
HPLC methods. The study disclosed some hitherto unreported intermediates of
photocatalytic decomposition of 4-chlorophenol and 2,4-dichlorophenol. Catechol
was detected as new intermediate of 4-chlorophenol degradation. Similarly, 4-
hydroxybenzaldehyde, benzoquinone and 4-chlorophenol are for the first time
reported for 2,4-dichlorophenol degradation. The work also revealed the
intermediates of 2,4,6-trichlorophenol which have not been in literature. It is
highlighted herein the mechanism of formation of all pathway intermediates. |
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