Dioxin pollution of aquatic sediments has been one of the important environmental problems in Japan. Grain size distribution and organic constituents may play important roles in dioxins contents in sediments, which have not been well investigated. This paper aims to quantify dioxins (PCDDs/Fs and co-PCBs) in sieved surface sediment fractions obtained from Kizu and Sumiyoshi Rivers in Osaka, Japan. The samples were prepared and analyzed following the ‘Manual of the Survey and Measurement of Dioxins in Sediment’ (Environment Agency of Japan, 2000). The results showed that TEQ concentrations of dioxins increased with decreasing grain size in sediment fractions. Sediment fractions with less than 20μm grain size showed the highest dioxin contents. Dioxins with five and six chlorine atoms dominated other congeners in less than 20μm fractions. The dioxin TEQ concentrations increased with total organic carbon (TOC), total carbon (TC), ignition loss (IL), total sulphur and nitrogen contents in the sediment fractions. Thus, reporting dioxin contents in sediments without considering grain size distribution may be erroneous, and conventional sedimentation and separation techniques might not be useful for treating contaminated sediments. This information can be useful for effective remediation of dioxin-contaminated sediments.

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