Distribution coefficients, Kds, for iodide in Canadian Shield Lake sediments under oxic and anoxic conditions

Abstract

In a series of laboratory studies, we determined the solid to liquid distribution coefficient, K d , for iodine ( 125I) in Canadian Shield lake sediments and the loss rate of 125I from water to sediment for more accurate computer model predictions of the fate of 129I released from an underground disposal vault. The K d values were determined under oxic and anoxic (reducing) conditions at sediment-to-water ratios of 1, 10 and 50% for organic sediment (OS), peaty sediment (PS), clayey silt sand sediment ( CS SS ) and sand sediment (SS). Under oxic conditions and shaking for 48 h, the geometric mean (GM) ÷ × geometric standard deviation (GSD) K d values for the ratios were 247 ÷ × 1.1, 109 ÷ × 1.3 and 21 ÷ × 1.31 lkg −1 for OS, 104 ÷ × 1.3, 68 ÷ × 1.1 and 14 ÷ × 1.1 l kg −1 for PS, 32 ÷ × 1.1, 15 ÷ × 1.2 and 2.9 ÷ × 1.1 lkg −1 for CS SS and 0.1 ÷ × 2.7, 0.5 ÷ × 2.2 and 0.2 ÷ × 1.3 lkg −1 for SS. Alkalinity of water ranging from < 1.0 to 250 mg CaCO 3 had only a minor effect on K d , with values changing less than a factor of two. Treating OS with a fungicide (nystatin), bactericide (streptomycin and penicillin), irradiation (1.72 Gys −1 for 4 h) or heat (80 °C for 4 h) decreased K d values by a factor of 1.1, 2.3, 7.5 and 22, respectively. Thus, bacteria appear to be important in the sorption of 125I to sediment, although chemical and/or physical changes as a result of radiation or heat treatment may also be important. Under anoxic conditions, K d values were small (generally <1 lkg −1) and little 125I was lost from the water. Over a 48-day period, K d values increased from about 690 lkg −1 on day 6 to 3840 lkg −1 on day 48 under oxic conditions and from about 0.5 to 16 lkg −1 under anoxic conditions. Most of the 125I adsorbed to sediment under oxic conditions is released back into the water under reducing conditions. On the basis of our K d values for OS, the water to sediment loss rate for iodine in Canadian Shield lakes would be about 0.6 ÷ × 7.4 year −1. Oligotrophic lakes would be better represented by a loss rate of 3.0 ÷ × 2.6 year −1. We conclude that anoxic organic lake sediment is not a particularly effective sink for longlived 129I from an underground disposal vault and that iodine in the water column is slowly lost from water to sediment.