STUDY ON NATURAL FORMATION OF DIOXINS: DIOXINS IN KAOLIN CLAYS FROM ASIA AND SEVERAL OTHER COUNTRIES

摘要

Elevated concentrations of dioxins in ancient ball clay from the Mississippi Embayment suggest natural formation of dioxins in the environment. In this study, kaolin clay samples collected from 11 countries were analyzed for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and dioxin-like polychlorinated biphenyls (DL-PCBs). Dioxins were found in all kaolin samples analyzed, at total concentrations ranging from 1.2 pg/g dry wt (dw) (Brazil) to 516000 pg/g dw (USA). Dioxin profiles in kaolin clays were characterized by the domination of the congener octachlorodibenzo-p-dioxin (OCDD), and the concentrations of other congeners decreased with corresponding reduction of the levels of chlorination. Furthermore, PCDFs were at very low (～ 91 pg/g dw) or non detectable (LOD: 0.008 pg/g dw) concentrations. The ratio of PCDD/PCDF concentrations in kaolin samples was extremely (250 times) higher than the ratio of pentachlorophenol (agrochemical formulation). This pattern differed from the profiles found for the anthropogenic sources of contamination. The highest TEQs (WHO 2006) were found in the American ball clays at 1620 pg-TEQ/g. No kaolin clays, except the American ball clays exceeded environmental criteria for soil (1000 pg-TEQ/g) and sediment (150 pg-TEQ/g) set by Japan (Law Concerning Special Measures against Dioxins). Dioxin inventories based on average TEQs were estimated at 1798 g-TEQ for American ball clay in 2007 and 4.8 g-TEQ for Japanese ball clay in 2005, respectively.