Sediment–Water Distribution Coefficients and Speciation of Cadmium in a Canadian Shield Lake

Sediment–water distribution coefficients for Cd were strongly pH dependent at pH > 6.5 for three surficial bottom sediments and suspended particles from a Canadian Shield lake (Lake 382). A single exponential equation in terms of pH fitted the distribution coefficients of all sediments. In the range 6.5 ≤ pH ≤ 8 the distribution coefficients were 109 × 10−3 to 1579 × 10−3 L/mg for suspended particulate matter and 0.6 × 10−3 to 129 × 10−3 L/mg for the bottom sediments. Speciation calculations performed using the computer program MACS80 and measured Cd fractions associated with suspended particles agreed well down to 7 m. In deeper water the measured fraction exceeded the calculated fraction, hypothetically because of CdS formation, differing sediment compositions, or nonequilibrium conditions. In September, H2S was 100–300 μg/L in the lower hypolimnion. Even at only 5 μg dissolved sulfide/L, some of the added Cd would precipitate as CdS at pH > 6, and the dissolved Cd, under anoxic conditions, would be largely Cd(HS)+ and Cd(HS)2. Under aerobic conditions at pH ≥ 7.5, approximately 85% of the Cd was on suspended particles and complexed with dissolved organic matter. The solids Cd(OH)2, CdCO3, and Cd3(PO4)2 played no role in the control of dissolved Cd.