Dechlorination of pulp bleaching plant E-1 effluent by ArF* and KrF* excimer laser photolysis

Among the chlorophenols identified in the bleaching plant E-1 effluent, 4,5-chloroguaiacol (2), 3,4,5,6-tetrachloroguaiacol (3), 2,4,6-trichlorophenol (4) and 2,3,4,5-chlorophenol (5) were selected as model compounds for dechlorination at pH 7 and 10.5 in nitrogen- and oxygen-saturated aqueous solutions at 20 °C by ArF* (193 nm) and KrF* (248 nm) excimer laser photolyses. The ArF* (193 nm) excimer laser photolysis was more effective than the KrF* (248 nm) excimer laser photolysis in dechlorinating 2-5. The dechlorinations of 2-5 were determined to be first order reactions; first order with respect to total organically bound chlorine of substrate, first order overall. The efficiency of dechlorination was found to depend on the initial pH of reaction mixture, substituent pattern of 2-5, and the wavelength of excimer laser radiation. The dechlorination rates of chlorinated guaiacols 2 and 3 were faster than the chlorinated phenols 4 and 5 under the same reaction condition while rate of 3 was faster than that of 2. Furthermore, the dechlorination rate of a substrate increased with increasing initial pH. The dechlorination rates of chlorophenols investigated were almost the same in N2- and O2-saturated solutions. However, when 2% H2O2 per substrate was added to the initial reaction mixture of 5 in the ArF* (193 nm) excimer laser photolysis, the dechlorination rate increased considerably. Quantum yields (Φ) for the generation of chloride ions were determined for the ArF* (193 nm) and KrF* (248 nm) excimer laser photolyses of 2-5 in both N2- and O2-saturated solutions. In general, the quantum yields in O2-saturated solutions were slightly higher than the corresponding values in N2-saturated solutions and was increased appreciably with addition of 2% H2O2.