LINEARITY OF IODINE SORPTION AND SORPTION CAPACITIES FOR 7 SOILS

Iodine, a soluble and prevalent element in spent nuclear fuel and a pi votal element in the assessment of Canada's nuclear fuel waste disposa l option, sorbs to soils rich in organics and hydrous oxides. Biotic f actors, such as microbes, enzymes and plant exudates, have been implic ated in the retention of I to soils. Anion exchange of I- or IO3- and chemical or biological oxidation to It followed by reactions with the soil organic matter are possible retention mechanisms. We have carried out sorption and desorption studies across a wide range of soil solut ion concentrations (10(-7) to 10(5) mg I/L, 10(-12) to 1 M) for seven soils typical of upland and lowland soils of the Canadian Precambrian Shield. Soil solid-liquid partition values (K-d), required for impact assessments, varied from 6 to 1800 L/kg and were significantly correla ted with extractable Al oxide content, and background I and organic ma tter content. Freundlich isotherm fits show that sorption of I across our intentionally large concentration range is nonlinear; however, sor ption of I at environmental concentrations (< 0.1 mg I/L soil solution ) is linear and can be described by the K-d model. Sorption of I was n ot related to peroxidase enzyme activity. Desorption percentages were small implying sorption was not easily reversed, even with a strong el ectrolyte, KNO3. Desorption results and simple correlations of I sorpt ion to soil properties suggest that the oxidation of I- to I-2 and com plexation to organic functional groups or oxides are the major process es for I retention in Shield soils.