Origin of PCDDs in Ball Clay assessed with compound-specific chlorine isotope analysis and radiocarbon dating

TitleOrigin of PCDDs in Ball Clay assessed with compound-specific chlorine isotope analysis and radiocarbon dating
Publication TypeJournal Article
Year of Publication2006
AuthorsHolmstrand, H, Gadomski D, Mandalakis M, Tysklind M, Irvine R, Andersson P, Gustafsson Ö
JournalEnvironmental Science and Technology
Volume40
Issue12
Pages3730 - 3735
Abstract

Polychlorinated dibenzo-p-dioxins (PCDDs) of high concentrations in a ball clay deposit from the Mississippi Embayment were found to be consistent with a natural abiotic and non-pyrogenic origin by investigation with bulk radiocarbon analysis, compound-specific chlorine isotope analysis (CSIA- δ 37Cl) of octachlorodibenzo-p-dioxin (OCDD), and black carbon (BC) analysis. The conventional radiocarbon date of total organic carbon from a depth of approximately 10 m in three parallel cores ranged from 14 700 years to >48 000 years, indicating that the strata with elevated levels of PCDDs have remained isolated from recent anthropogenic input in these >40 Ma old clay sediments. The CSIA-δ 37Cl of OCDD yielded a δ 37Cl of -0.2%, which is significantly higher than the postulated range for biotic chlorination by chloroperoxidase enzymes, -11 to -10‰, and falls within the known range for abiotic organochlorines, -6 to +3‰. The absence of correlations between concentrations of PCDDs and corresponding pyrogenic black carbon (BC), together with estimations of BC sorptive loadings and the absence of polychlorinated dibenzofurans (PCDFs), suggest that vegetation fires did not form these ball-clay PCDDs. Results from this study indicate that the high levels of the toxic and carcinogenic PCDDs found in kaolinite-bearing clays may result from natural abiotic formation via in situ surface-promoted reactions on the clay mineral, including a so-far unknown organic precursor, rather than being the result of anthropogenic contamination. © 2006 American Chemical Society.

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