Degradation and detoxification of industrially important phenol derivatives in water by direct UV-C photolysis and H₂O₂/UV-C process: A comparative study

H₂O₂/UV-C oxidation and UV-C photolysis of 2,4-dichlorophenol (2,4-DCP) and the nonionic industrial surfactant nonylphenol decaethoxylate (NP-10) were comparatively studied in terms of changes in parent pollutant, total organic carbon (TOC), oxidation products and acute toxicity to the marine photobacterium Vibrio fischeri. The H2O2/UV-C process was superior over UV-C photolysis in terms of parent compound and TOC removals. After 90 min H₂O₂/UV-C treatment, complete degradation of both parent compounds accompanied with 95% and 78% TOC removals for 2,4-DCP and NP-10, respectively, could be achieved. The highest concentrations of oxidation products were obtained for carboxylic acids and aldehydes during H₂O₂/UV-C treatment and UV-C photolysis of 2,4-DCP, with 21 and 19 mg L^-1 carboxylic acids and 0.96 and 0.77 mg L^-1 aldehyde formation, respectively. In the case of NP-10, substantially higher concentrations of carboxylic acids (32 mg L^-1) were formed throughout the course of H₂O₂/UV-C treatment as compared to UV-C photolysis alone (3.4 mg L^-1). Although the original inhibitory effect of 75 mg L^-1 2,4-DCP (99% relative inhibition) was reduced to 17% after 30 min H₂O₂/UV-C treatment, the toxic effect re-appeared after complete degradation of 2,4-DCP being measured as 75% after 90 min treatment. During UV-C photolysis of 2,4-DCP the relative inhibition decreased steadily to 34% after 90 min treatment. The inhibitory effect of NP-10 fluctuated during H₂O₂/UV-C ultimately resulting in a slightly higher toxicity value (16% relative inhibition) than the original pollutant (9% relative inhibition). The acute toxicity of NP-10 gradually increased to a relative inhibition of 36% throughout UV-C treatment.