Biodegradation of chlordane and hexachlorobenzenes in river sediment

Contamination of river sediments by persistent organic pollutants (POPs) is a worldwide concern, and microbial degradation is regarded as an important process for removal of POPs from river sediments. To date, there is still a lack of systematic study on chlordane biodegradation in river sediments, and the information on hexachlorobenzene (HCB) biodegradation in river sediments is very limited in Japan. We investigated the anaerobic biodegradation potential of trans-chlordane (TC), cis-chlordane (CC), and HCB in sediment samples collected at three sites along the Kamogawa River in Saitama Prefecture, Japan. Lag period and biodegradation rates of TC and CC in the three sediments varied greatly with their properties and contamination by TC and CC. In contrast, biodegradation of HCB in all three sediments started immediately with the start of the experiment without lag period, and major differences in biodegradation rates among the sediments were not observed. At the end of 20-week anaerobic incubation in the dark at 30 °C temperature, degradation rates ranged from 0.0% to 33.0% for TC, 0.0% to 12.0% for CC, and 47.6% to 59.4% for HCB. Results showed that the high-to-low order of biodegradation in the river sediments was HCB > TC > CC. Although the sediments were collected in the same river, their biodegradation potential varied with properties. Sediment with rich organic content and contamination by TC and CC or HCB was observed to have high biodegradation rates for these pollutants. In addition, biodegradation of TC, CC and HCB was companied by obvious methane generation and drop of oxidation–reduction potential (ORP).