Photocatalytic ozonation of 4-chloro-2-methylphenoxyacetic acid and its reaction intermediate 4-chloro-2-methyl phenol

Abstract

Aqueous 4-chloro-2-methylphenoxyacetic acid (MCPA) has been treated by the systems UVA/TiO2/N2, O3, TiO2/O3, UVA/O3, UVA/TiO2/O2, and UVA/TiO2/O3. Under the conditions investigated (T = 20 °C, pH = 4.5, Qgas = 30 L/h, V = 1 L, CO3= 5 ppm, CMCPA = 5 ppm, CTiO2= 0.5 g/L), MCPA is removed in less than 30 min. Photocatalytic ozonation is the most efficient process both in terms of MCPA removal rate (100% conversion in less than 15 min) and mineralization extent (60% after 3 h and 25 °C). 4-Chloro-2-methyl phenol (CMP) is detected in those systems combining TiO2 and UVA radiation. The presence of ozone involves the complete depletion of CMP following its generation. The direct rate constant between CMP and ozone corroborates the high reactivity observed (7.2 ± 0.3 × 104 (M s)−1, 4.4 ± 0.2 × 105 (M s)−1, and 2.9 ± 0.7 × 106 (M s)−1 at pHs 4, 7 and 10, respectively). Identified intermediates detected in the UVA/TiO2/O3 applied to MCPA correspond to oxygenated species derived from the parent compound after loss of some substitution groups. No significant toxicity of intermediates is observed in BOD5, Daphnia parvula, and Culex pipiens larvae tests.