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Linearity of Iodine Sorption and Sorption Capacities for Seven Soils

Environmental Science Branch, Atomic Energy of Canada Ltd., Whiteshell Laboratories, Pinawa, MB, Canada R0E 1L0.

^* Corresponding author. (sheppardm{at}wl.aecl.ca).

ABSTRACT

Iodine, a soluble and prevalent element in spent nuclear fuel and a pivotal element in the assessment of Canada's nuclear fuel waste disposal option, sorbs to soils rich in organics and hydrous oxides. Biotic factors, such as microbes, enzymes and plant exudates, have been implicated in the retention of I to soils. Anion exchange of I^– or IO^–₃ and chemical or biological oxidation to I₂ 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 solution concentrations (10^–7 to 10⁵ 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 correlated with extractable Al oxide content, and background I and organic matter content. Freundlich isotherm fits show that sorption of I across our intentionally large concentration range is nonlinear; however, sorption 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 not related to peroxidase enzyme activity. Desorption percentages were small implying sorption was not easily reversed, even with a strong electrolyte, KNO₃. Desorption results and simple correlations of I sorption to soil properties suggest that the oxidation of I^– to I₂ and complexation to organic functional groups or oxides are the major processes for I retention in Shield soils.