Penta- and 2,4,6-tri-chlorophenol biodegradation during municipal solid waste anaerobic digestion

• Partial biodegradation of PCP and 2,4,6-TCP in mesophilic methanogenic condition.
• Accumulation of 4-CP during PCP and 2,4,6-TCP biodegradation in mesophilic methanogenic condition.
• Phenol appears not to be a metabolite of PCP and 2,4,6-TCP during MSW anaerobic digestion.
• Persistence of PCP and 2,4,6-TCP in methanogenic thermophilic conditions.

In this study isotopic tracing using 13C labelled pentachlorophenol (PCP) and 2,4,6-trichlorophenol (2,4,6-TCP) is proposed as a tool to distinguish the loss of PCP and 2,4,6-TCP due to biodegradation from other physical processes. This isotopic approach was applied to accurately assess in situ PCP and 2,4,6-TCP degradation under methanogenic conditions in several microcosms made up of household waste. These microcosms were incubated in anaerobic conditions at 35 °C (mesophilic) and 55 °C (thermophilic) without agitation. The volume of biogas produced (CH4 and CO2), was followed for a period of 130 days. At this stage of stable methanogenesis, 13C6-PCP and 13C6-2,4,6-TCP were introduced anaerobically in microcosms and its monitoring at mesophilic and thermophilic conditions was performed in parallel by gas chromatography mass spectrometry (GC–MS) and gas chromatography isotope-ratio mass spectrometry (GC–IRMS). This study proved the almost total dechlorination of bioavailable PCP and 2,4,6-TCP into 4-CP at 35 °C. Nevertheless, high rate adsorption in particular materials of the two compounds was observed. Furthermore, Carbon-13 Nuclear Magnetic Resonance (13C-NMR) Spectroscopy analysis of 13C labelled 2,4,6-TCP mesophilic incubations showed the partial mineralization of 4-CP at 35 °C to acetate and then to HCO3−. Consequently, NMR results confirm the biogas isotopic results indicating the mineralization of 13C labelled 2,4,6-TCP into 13C (CH4 and CO2). Concerning 13C labelled PCP mesophilic incubations, the isotopic composition of the biogas still natural until the day 262. In contrast, no dechlorination was observed at 55 °C. Thus PCP and 2,4,6-TCP were persistent in thermophilic conditions.