Field Validation of Anaerobic Degradation Pathways for Dichlorodiphenyltrichloroethane (DDT) and 13 Metabolites in Marine Sediment Cores from China

Although the production and use of dichlorodiphenyltrichloroethane (DDT), a legacy component of persistent organic pollutants, have been highly restricted worldwide, the environmental fate of DDT has remained a great concern as it is not only ubiquitous and bioaccumulative but can also be degraded to a series of metabolites that may be more hazardous ecologically. The present study, taking advantage of the abundant levels of DDT and its metabolites in a subtropical coastal region of China, investigated into the degradation pathways of DDT in natural coastal sediment. Sediment profiles indicated that degradation of 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane (p,p?-DDT) to 1,1-dichloro-2,2-bis-(p-chlorophenyl)ethane (p,p?-DDD) mainly occurred in sediment of the top 20 cm layer. 1,1-dichloro-2,2-bis-(p-chlorophenyl)ethylene (p,p?-DDE), aerobically transformed from p,p?-DDT prior to sedimentation, was likely to degrade to 1-chloro-2,2-bis-(p-chlorophenyl)ethylene (p,p?-DDMU) which was further converted to 2,2-bis(p-chlorophenyl)ethylene (p,p?-DDNU). In addition, p,p?-DDNU could be transformed to 2,2-bis(p-chlorophenyl)ethane (p,p?-DDNS) and other high-order metabolites. On the other hand, the conversions of p,p?-DDD to p,p?-DDMU and 1-chloro-2,2-bis-(p-chlorophenyl)ethane (p,p?-DDMS) to p,p?-DDNU were deemed slow in anaerobic sediment. Therefore, the present study confirmed all the degradation pathways involving reductive dechlorination and p,p?-DDE being a more important precursor for p,p?-DDMU than p,p?-DDD in anaerobic sediment, as proposed previously. On the other hand, the present study suggested that p,p?-DDMU instead of p,p?-DDMS was more likely the precursor for formation of high-order metabolites. Based on the current assessments, use of (DDD+DDE)/DDTs to indicate whether there is fresh DDT input may lead to large uncertainties if the concentrations of high-order metabolites are not negligible. Similarly, ecological risk assessment associated with DDT should be conducted with consideration of high-order DDT metabolites.