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Environmental fate of DDT-related compounds in aquatic systems

  • Kucher, Sebastian
Publication Date
Jan 01, 2018
Publikationsserver der RWTH Aachen University
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The organochlorine pesticide DDT is probably the most infamous representative of the group of persistent organic pollutants. First introduced in the 1940s, DDT was produced in massive quantities and applied worldwide for decades as insecticide in agriculture, pest control in forestry, and vector control in public health. The discovery of ubiquitous contamination of nearly all environmental compartments combined with unanticipated adverse effects on wildlife and the development of DDT resistance ultimately led to the ban of DDT in most industrialized countries in the 1970s and 1980s. However, due to the persistence of DDT and its degradation products, the legacy of production and application still poses risks for local ecosystems and the global environment. During the last 60 years the environmental fate of DDT and its transformation products have been subject to extensive studies, however, many aspects have yet to be fully explored. To enhance the knowledge of DDT and its degradates in the non-extractable fraction of aquatic matter, two submarine sediment cores obtained from the highly DDX contaminated Palos Verdes Shelf, USA, were analyzed with regard to DDT and its degradation products in both the extractable fraction and the hydrolyzable portion of non-extractable forms. GC/MS screening for 26 DDT-related compounds revealed distinct differences in the quantitative and qualitative distribution of DDX in both fractions. In contrast to studies on highly DDX contaminated fluvial sediments from the Teltow Canal, Germany, the hydrolyzable proportion represented a minor portion of the total sedimentary burden in the analyzed marine sediments. DDE, DDD, DDMS, and DDMU were the most abundant compounds in the extractable fraction, whereas DBP, DDNU, DDM, and DDA were predominant in the alkaline hydrolysis products. Since reversible ester bonds can be cleaved under natural conditions, the remobilization of water-soluble DDA points to an important degradation pathway in marine environments. The results show the necessity of a comprehensive screening for all DDT isomers and degradation products in the extractable and non-extractable fraction. Focusing on selected DDT-related compounds not only neglects the total contamination abundance but also potential environmental risks. A second aim of this study was to assess the effect of environmental processes on the positional isomer ratios of DDT-related compounds. To investigate this widely neglected aspect, previously published and newly acquired quantitative data on DDX determined in samples obtained from DDT contaminated sites was evaluated with regard to o,p’-/p,p’-DDX ratios in different environmental matrices. Shifts of the o,p’-/p,p’-ratios between the respective transformation products of DDT in a subaquatic sediment core illustrated the depletion of specific isomers during DDT degradation. Shifts of the positional isomer ratios between the initial transformation products revealed no clear trend, whereas a gradual decrease of the isomer specific ratios of second order degradation products was observed. At the same time, the isomeric composition of these compounds only showed slight variations in the vertical distribution indicating that the position of the chlorine atoms at the phenyl groups are primarily not affected by degradation. Observed compositional differences of isomeric ratios determined in marine and fluvial sediment samples can presumably be linked to the respective depositional regime. In addition, the reversible incorporation of DDX did not alter o,p’-/p,p’-ratios in marine sediment samples, however changed the isomeric composition in fluvial sediments. Shifts of the isomer specific ratios of DDX in subaquatic sediment and terrestrial soil respectively of DDA in surface water, sediment, and groundwater affected by the same source of contamination pointed to a strong impact of transfer processes and the prevailing environmental conditions on the positional isomer ratios. The observed isomeric shifts clearly demonstrated the influence of environmental processes on the isomeric composition of DDX and can potentially act as indictors to track the environmental fate of DDX. For an unambiguous assignment of these shifts to specific processes, further systematic studies and complementary lab experiments are necessary. Finally, a novel structurally highly DDT related organic pollutant group was detected in sediment and soil samples near former and current DDT production sites. In contrast to their regularly chlorinated counterparts, these compounds exhibited either one missing or additional chlorine atom at the phenyl group. This congeneric group most likely originates from production residues disposed of into the environment. As confirmed by synthesis of DDT±Cl and DDD±Cl, the formation of DDX±Cl can be linked to the usage of impure chlorobenzene as intermediate for the technical production of DDT. Besides DDT±Cl and DDD±Cl, DDMS±Cl, DDNS-Cl, and DBP+Cl were tentatively identified for the first time. The detection of DDX±Cl in environmental samples allows to draw conclusions about the purity of the production process in the associated production sites and potentially can serve as molecular indicators to differentiate between industrial DDT emissions and insecticidal applications of DDT. To confirm this hypothesis, further research on the composition of commercial DDT formulations is required to rule out that DDX±Cl were originally contained as byproducts.

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