Transformation kinetics and pathways of chlorophenols and hexachlorobenzene in fresh water lake sediment under anaerobic conditions

The anaerobic transformation of chlorophenols (CPs) and hexachlorobenzene (HCB) was examined in a fresh water lake sediment collected from Lake Kasumigaura in Japan. Anaerobic experiments were carried out by spiking each compound in a number of test tubes and the change in parent compound concentration was monitored for a year. Lag periods of 1-31 days were observed for the substrates examined. The first-order rate constants determined following the lag period, ranged between 0.0021 and 0.072 day(-1) for chlorophenols and 0.011 day(-1) for hexachlorobenzene. Two pathways for pentachlorophenol (PCP) in the sediment were observed: in the first one, under sulfidogenic conditions, PCP transformed to 3-chlorophenol (3-CP) as, 2,3,5,6-tetrachlorophenol (2,3,5,6-TeCP), 2,3,5-trichlorophenol (2,3,5-TCP), 3,5-dichlorophenol (3,5-DCP), 3-CP; and in the second pathway, under methanogenic conditions, PCP transformed via 2,3,4,5-tetrachlorophenol (2,3,4,5-TeCP), 3,4,5-trichlorophenol (3,4,5-TCP), 3,4-dichlorophenol (3,4-DCP), 3-CP. Similarly, two different transformation pathways for HCB were observed. In the first pathway HCB transformed via pentachlorobenzene (PeCB), 1,2,3,5-tetrachlorobenzene (1,2,3,5-TeCB), 1,3,5-trichlorobenzene (1,3,5-TCB), 1,3-dichlorobenzene (1,3-DCB). In the second pathway, the transformation was, HCB, PeCB, 1,2,3,4-tetrachlorobenzene (1,2,3,4-TeCB), 1,2,4-trichlorobenzene (1,2,4-TCB), 1,4-dichlorobenzene (1,4-DCB). The results showed that the removal of chlorine atoms from PCP and HCB were different in sulfidogenic and methanogenic environments.