Speciation and migration of (129)I in soil profiles

A method has been developed for speciation analysis of ultra low level (129)I in soil using sequential extraction combined with coprecipitation for separation of carrier free iodine and AMS measurement of (129)I. Two loess profiles collected from northwest China were analyzed for species of (129)I and (127)I. Similar partitioning of (129)I and (127)I was observed in the loess profiles, the distribution of iodine isotopes followed an order of organic > leachable > reducible > residue. The (129)I concentrations and (129)I/(127)I ratios decreased exponentially with the depth, and 2 orders of magnitude lower in the deepest layer (60 and 90 cm) compared with the top layer, indicating a significant contribution of anthropogenic input in the upper layer, and high retention of (129)I in soil. The mobility of (129)I in different fractions decreased in an order of leachable > organic > oxides > residue. The results suggest that migration of iodine downwards in the soil profile is a slow process; the oxides and residue are the less mobile fractions of iodine.