The sorption of iodine by an inorganic zinc primer

The purpose of this work was to identify and evaluate significant parameters in the sorption of I2(g) onto Carbo Zinc 11 inorganic primer, a paint used in the containment structure of some CANDU reactors. Air containing known amounts of 131I2(g) was passed through 0.64 cm diameter glass tubing coated on the inner surface with paint. The accumulation of iodine on the surface was continuously monitored using two scintillation detectors. The test parameters covered were relative humidity, flow rate, I2 concentration and paint temperature. Adsorption was rapid at 23oC and predominantly gas phase mass transfer limited: the deposition velocity of 0.7±0.4 cm/s was similar to the gas phase mass transfer coefficient of 1.2 cm/s estimated for the system. The deposition velocity observed at a higher paint surface temperature was an order of magnitude smaller. A similar deposition velocity was observed at 23oC for adsorption of I2(g) from essentially dry air suggesting that the low deposition velocity observed for high surface temperature was limited by the amount of water on the paint surface. The rate of adsorption on the paint was directly proportional to the I2(g) concentration over the range in concentration studied, justifying the use of deposition velocities. The majority of the iodine retained by the paint could not be removed by washing with methanol or chloroform, but it was removed by water indicating that it was in an ionic form. Analysis of the speciation of the iodine in the wash water indicated that only a third of it was in the form of I-; the form of the remaining iodine could not be resolved. Desorption from the paint was negligible at room temperature but was detectable at higher temperatures. These low desorption rates and the ionic nature of the surface iodine indicated that adsorption occurred predominantly through a chemisorption process. A number of possible mechanisms were proposed to explain the role of water in the adsorption process. (author) 5 figs., 2 tabs., 6 refs