The measurement of the organic-carbon normalized partition coefficient, K_oc, for dioxin from contaminated sediment

The K_oc of dioxin, 2,3,7,8-TCDD, was measured by desorption from Lake Ontario sediment (470 pg dioxin/g dry wt., 2.37% organic carbon); it is concluded that log K_oc > 7.1. Suspensions of various solids concentration were prepared in 2 or 4-l cylinders. After 4-14 days, the solids and water were separated by sedimentation. The measured aqueous quantities were solids concentration, C_s,w, 0.06-60 g/l; organic carbon, C_'o_c,w, 0.23-65 mg/l; and dioxin, 2-1000 pg/l. Measured coefficients follow log K^m_oc = constant - log C, where C = C_s,w or C_sorb,w. The quantity of sorbent in the aqueous phase is C_sorb,w = C_oc,w - 0.25 mg/l. The level of dioxin in water depends on organic carbon when C_oc,w > 0.25 mg/l. The apparatus was spiked with ¹³C-labeled dioxin and ratios of native-to-all dioxin in water, sediment and dioxin associated with the apparatus indicate the approach to equilibrium. Steady state was reached in water for four experiments. Analysis of recovered solids shows significant desorption of material. It appears that equilibrium is reached when levels of dioxin in the organic carbon associated with the sediment, water, and apparatus are the same.