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Photolytic dechlorination of 4-chlorophenol in O2-saturated aqueous solutions in the absence of photocatalysts or additives using ArF∗ (193 nm) and KrF∗ (248 nm) excimer lasers

1995; Elsevier BV; Volume: 90; Issue: 2-3 Linguagem: Inglês

10.1016/1010-6030(95)04088-w

1873-2666

Vernard R. Jr. Thomas, Anton F. Schreiner, Tianyan Xie, Chen-Loung Chen, Josef S. Gratzl,

Environmental remediation with nanomaterials

One of the toxic products from chlorine bleaching of wood pulp is 4-chlorophenol (4-CP). Detoxification of such compounds usually requires their dechlorination. The present study involves a fairly detailed comparison of the attempt and success of the photolytic dechlorination of 4-CP in O2-saturated aqueous solutions using ArF∗ (193 nm) and KrF∗ (248 nm) excimer laser radiation at higher (1.1 × 10−2 M) and lower (4.5 × 10−4 M) starting substrate concentrations and comparisons of quantum yield (φ), product distributions, etc. At the higher starting substrate concentration (1.1 × 10−2 M) the average initial quantum yields, i.e. early in the reaction process, for the disappearance of 4-CP (φ ≈2 0.30) and for the generation of chloride ions (φ ≈2 0.25) were about the same for both 193 and 248 nm radiation. However, when the number of photons absorbed (n) became greater than about 3 × 1020, more chloride ions (higher φ) were generated with the 193 nm radiation than with the 248 nm radiation. Oligomers were the major products for both wavelengths of radiation, but the quantity of oligomers generated was greater using the 193 nm radiation. At 248 nm a significant amount of hydroquinone was generated, whereas hydroquinone was not detected with the 193 nm radiation. In addition, a significant amount of 4-chlorocatechol was generated during the direct photolysis of 4-CP using either wavelength of radiation. This is a new result for the photolysis of 4-CP at a wavelength longer than 193 nm in the absence of added H2O2. At the lower starting substrate concentration (4.5 × 10−4 M) the average initial quantum yields for the disappearance of 4-CP (φ ≈2 0.55) and for the generation of chloride ions (φ ≈2 0.45) were both much greater using the 193 nm radiation than with the 248 nm radiation and 0.10 respectively). At 193 nm oligomers were still the major products generated, but the fraction of oligomeric products generated was less than the fraction at the higher substrate concentration. This is consistent with the fact that at the lower substrate concentration a significant amount of hydroquinone was generated at 193 nm, whereas it was not detected at this wavelength for the higher substrate concentration. During the 248 nm photolysis of 4-CP at this lower starting substrate concentration, hydroquinone was the only major product generated in these experiments. Oligomers were not observed at this lower substrate concentration using 248 nm radiation, whereas oligomers were generated at the higher substrate concentration. The results demonstrate the utility of using an excimer laser for the photolytic dechlorination of 4-CP without added photocatalysts or additives. It is also possible to suggest a number of explanations, given in the text, which are consistent with our findings.