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

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 (I-2) by aqueous solutions and clay minerals in the laboratory to inform estimates of the ground surface resistance for I-2 in dry deposition which photolysis limited to nighttime. We injected rectangular pulses of humidified air including I-2 into a contactor holding samples of aqueous solutions and clay minerals and monitored I-2 concentrations in air leaving the contactor to distinguish I-2 loss from I-2 adsorption. Uptake of I-2 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 I-2 by clay samples was confirmed under dry conditions (20-80% relative humidity), which suggested that it contributed to the dry deposition of I-2 onto soils as much as other processes such as reactions with organic matter. The surface resistance for I-2 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 I-2 in scaling methods. Reference values of surface resistance for SO2 above soils remain useful for estimating the resistance for I-2 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 I-2.