Radiation-Initiated Dehalogenation of Trichloroacetic Acid in Aqueous Solutions

Abstract

Organochlorine compounds are widespread anthropogenic water pollutants. These compounds are toxic and carcinogenic. They are released into natural water from industrial emissions and as a consequence of agricultural use of pesticides or are formed upon the chlorination of drinking water because of the presence of various organic compounds. According to data [1], more than 150 chlorine-containing organic compounds are formed in the chlorination of water. Chlorinated organic compounds in water can be destroyed with the use of ionizing radiation [2, 3]. The knowledge of the mechanisms of radiolysis of chlorinated organic compounds from various classes is necessary for the development and application of radiation technologies to the removal of organochlorine compounds from natural water. Moreover, chlorinated organic compounds are convenient test materials for studying nucleophilic substitution reactions in radicals [4, 5]. Previously [4‐6], the reactions of chlorinated organic compounds ( CH 2 Cl 2 , ClCH 2 CH 2 Cl, CHCl 3 , CH 3 CHClCH 3 , C 6 H 5 Cl, ClC 6 H 5 CH 3 , and C 6 H 6 Cl 6 ) in aqueous alkaline solutions under exposure to ionizing radiation were examined. In this work, deaerated aqueous 0.1 M trichloroacetic acid solutions with different concentrations of an alkali were irradiated. The solutions were prepared with the use of twice-distilled water. Freshly prepared solutions were irradiated in sealed ampules. The γ -radiation dose rate was 0.34 Gy/s as measured with a ferrous sulfate dosimeter. The concentrations of chloride and oxalate ions in the samples were determined as functions of radiation dose. The radiation-chemical yields of chloride ions and oxalic acid were calculated from the dose curves. Chloride ions were determined by potentiometric titration with silver nitrate [6]. The determination error in the radiation-chemical yields of chloride ions was no higher than 15%. Oxalate ions were identified by the formation of a precipitate on the addition of an aqueous calcium nitrate solution to irradiated and acidified samples. Quantitative analysis for ions was performed by back titration with potassium permanganate [7] at room temperature. The analytical procedure was developed with standard C 2 O 4 2 ‐ oxalic acid solutions containing trichloroacetic acid (0.1 mol/l) and chloride ions in concentrations that were expected to form by irradiation. Trichloroacetic acid and chloride ions did not interfere with the determination of oxalate ions. The maximum determination error for oxalate ions was 19%. The figure illustrates the experimental results. The intensity of dehalogenation of trichloroacetic acid increased with alkali concentration in solution. The radiation-chemical yields of chloride and oxalate ions were as high as 14.4 ∠ 0.8 and 4.8 ∠ 0.9 molecule/100 eV, respectively, at [NaOH] = 0.8 mol/l. The formation of Cl — and ions on irradiation of alkaline solutions can be represented by the following reaction scheme: