Degradation of pentachlorophenol by ozonation and biodegradability of intermediates

The degradation pathway of pentachlorophenol (PCP) under ozonation and the biodegradability of the resulting intermediates were investigated. The objectives were to: (1) provide mechanistic details in the ozone-mediated degradation, (2) evaluate the biodegradability of resultant intermediates at various progressive stages of ozonation, and (3) thereby, assess the feasibility for a coupled chemical–biological treatment scheme for PCP. Tests of BOD5, COD, and E. coli toxicity along with qualitative and quantitative GC analyses were performed for aliquots withdrawn before and after various stages of ozonation and biological incubation. Ozonated PCP decomposed under the direct nucleophilic attack of ozone through an addition–elimination mechanism, resulting in tetrachloro-p-benzoquinone and tetrachloro-p-hydroquinone intermediates that were further degraded by O3 and OH√ to other open-ring products including ketones and acids that eventually led to simple oxalic acid and quantitative release of chloride ion. As ozonation progressed toward but prior to complete mineralization, reaction intermediates became increasingly more biodegradable prior to complete degradation, which suggested the potential of using ozonation in conjunction with biological treatment for the effective control of chlorinated aromatics.