Toxicity-oriented control of advanced oxidation processes a case study on phenol derivatives

Abstract

Advanced oxidation processes are expected to be carefully operated and monitored based on their ecotoxicological impact because the partial oxidation of organic contaminants may result in the formation of intermediates more toxic than parent compounds. Toxicity tests in combination with the transformation product analyses could be an important tool for the control of advanced oxidation processes. Considering the commercial importance as well as toxicological properties of chlorophenol and nonylphenol polyethoxylate group phenolic compounds, degradation and detoxification of 2,4-dichlorophenol (2,4-DCP) and nonylphenol decaethoxylate (NP-10) in distilled water and synthetically prepared freshwater were investigated by applying the H2O2/UV-C, Fenton and photo-Fenton advanced oxidation processes that are well-known for their effectiveness in removing many organic pollutants from waste streams. Although not an advanced oxidation process, UV-C photolysis was also included in the study due to the fact that it is becoming an attractive technology in water treatment facilities due to several advantages. The marine photobacteria Vibrio fischeri were employed as the test organism to assess changes in acute inhibitory effect during the studied treatment processes, whereas the umu-test using Salmonella typhimurium TA1535/pSK1002 strain was selected as the genotoxicity assay. Toxicity results were complemented by transformation product analyses performed by means of high performance liquid chromatography, gas chromatography-mass spectrometry, liquid chromatography-mass spectrometry, colorimetry and ion chromatography in order to gain a deeper insight into acute toxicity and genotoxicity patterns during application of the studied treatment processes. The H2O2/UV-C and photo-Fenton processes ensured complete 2,4-DCP removal and near-complete mineralization as well as effective abatement of UV280/254 absorbances. Hydroquinone, chlorohydroquinone, maleic and formic acids and aldehydes were identified as the common transformation products of the studied treatment processes. The most rapid decrease in the acute inhibitory effect was achieved by the H2O2/UV-C and photo-Fenton processes, however a re-increase in toxic effect was evidenced in the former advanced oxidation process when extending the treatment time beyond 30 min. The acute inhibitory effect ultimately measured in synthetic freshwater was higher than that recorded in distilled water for UV-C photolysis and lower for the H2O2/UV-C and Fenton processes, whereas the photo-Fenton process was not significantly affected by the inorganic constituents of synthetic freshwater based on acute toxicity test results. During application of the studied treatment processes in distilled water no significant genotoxic effect was observed with and without metabolic activation. Complete NP-10 degradation that is very fast accompanied with high total organic carbon removal efficiencies were achieved by the H2O2/UV-C and photo-Fenton processes, whereas the Fenton’s reagent provided only poor NP-10 abatement and insignificant mineralization. Aliphatic carboxylic acids including formic, acetic and oxalic acids, aldehydes and polyethylene glycols containing 2-8 ethoxy units were all identified as the degradation products of NP-10 by the studied treatment processes. The photo-Fenton process appeared to be toxicologically safer both in distilled water and synthetic freshwater based on the acute toxicity tests since the inhibitory effect did not increase relative to the original NP-10 solution after treatment. The genotoxic effects obtained in distilled water both with and without metabolic activation followed the order; Fenton > H2O2/UV-C > photo-Fenton > UV-C. Formation of weakly to moderately genotoxic transformation products was evidenced in the presence and absence of metabolic activation when the H2O2/UV-C process was applied to synthetically prepared freshwater. The genotoxic effect levels were typically lower during photo-Fenton treatment of synthetic freshwater contaminated with NP-10 as compared to those achieved with the H2O2/UV-C process both in the presence and absence of metabolic activation.