PHOTOLYTIC DECHLORINATION OF 4-CHLOROPHENOL IN O-2-SATURATED AQUEOUS-SOLUTIONS IN THE ABSENCE OF PHOTOCATALYSTS OR ADDITIVES USING ARF-ASTERISK (193 NM) AND KRF-ASTERISK (248 NM) EXCIMER LASERS

One of the toxic products from chlorine bleaching of wood pulp is 4-ch lorophenol (4-CP). Detoxification of such compounds usually requires t heir dechlorination. The present study involves a fairly detailed comp arison of the attempt and success of the photolytic dechlorination of 4-CP in O-2-saturated aqueous solutions using ArF (193 nm) and KrF* ( 248 nm) excimer laser radiation at higher (1.1 X 10(-2) M) and lower ( 4.5 X 10(-4) M) starting substrate concentrations and comparisons of q uantum yields (Phi), product distributions, etc. At the higher startin g substrate concentration (1.1 X 10(-2) M) the average initial quantum yields, i.e, early in the reaction process, for the disappearance of 4-CP (Phi approximate to 0.30) and for the generation of chloride ions (Phi approximate to 0.25) were about the same for both 193 and 248 nm radiation. However, when the number of photons absorbed (n) became gr eater than about 3 X 10(20), more chloride ions (higher Phi) were gene rated with the 193 nm radiation than with the 248 nm radiation. Oligom ers were the major products for both wavelengths of radiation, but the quantity of oligomers generated was greater using the 193 nm radiatio n. At 248 nm a significant amount of hydroquinone was generated, where as hydroquinone was not detected with the 193 nm radiation. In additio n, a significant amount of 4-chlorocatechol was generated during the d irect 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 1 93 nm in the absence of added H2O2. At the lower starting substrate co ncentration (4.5 X 10(-4) M) the average initial quantum yields for th e disappearance of 4-CP (Phi approximate to 0.55) and for the generati on of chloride ions (Phi approximate to 0.45) were both much greater u sing the 193 nm radiation than with the 248 nm radiation (Phi approxim ate to 0.20 and 0.10 respectively). At 193 nm oligomers were still the major products generated, but the fraction of oligomeric products gen erated was less than the fraction at the higher substrate concentratio n. This is consistent with the fact that at the lower substrate concen tration a significant amount of hydroquinone was generated at 193 nm, whereas it was not detected at this wavelength for the higher substrat e concentration. During the 248 nm photolysis of 4-CP at this lower st arting substrate concentration, hydroquinone was the only major produc t generated in these experiments. Oligomers were not observed at this lower substrate concentration using 248 nm radiation, whereas oligomer s were generated at the higher substrate concentration. The results de monstrate the utility of using an excimer laser for the photolytic dec hlorination of 4-CP without added photocatalysts or additives. It is a lso possible to suggest a number of explanations, given in the text, w hich are consistent with our findings.