A laboratory study on the uptake of gaseous molecular iodine by clay minerals at different relative humidities

Abstract

Dry deposition of iodine is a crucial parameter for estimating the public health risk of radioiodine released in the Fukushima Dai-ichi nuclear power plant accident. We measured the uptake of molecular iodine (I₂) by aqueous solutions and clay minerals in the laboratory to inform estimates of the ground surface resistance for I₂ in dry deposition which photolysis limited to nighttime. We injected rectangular pulses of humidified air including I₂ into a contactor holding samples of aqueous solutions and clay minerals and monitored I₂ concentrations in air leaving the contactor to distinguish I₂ loss from I₂ adsorption. Uptake of I₂ proceeded much more rapidly by aqueous ascorbic acid than by water; the former was limited by mass transfer in the gas-film layer. Uptake of I₂ by clay samples was confirmed under dry conditions (20–80% relative humidity), which suggested that it contributed to the dry deposition of I₂ onto soils as much as other processes such as reactions with organic matter. The surface resistance for I₂ increased with repeated experiments on the same clay samples, reaching 240–670 s m⁻¹, and its dependence on relative humidity differed from that for sulfur dioxide (SO₂), a commonly used proxy for I₂ in scaling methods. Reference values of surface resistance for SO₂ above soils remain useful for estimating the resistance for I₂ above vegetate surfaces at 80% RH in atmospheric transport and dispersion model calculations but may result in substantial errors at 20% RH unless organic matter in soils contributes to ground surface resistance for I₂.