Speciation and migration of 129I in soil profiles

A method has been developed for speciation analysis of ultra low level 129I in soil using sequential extraction combined with coprecipitation for separation of carrier free iodine and AMS measurement of 129I. Two loess profiles collected from northwest China were analyzed for species of 129I and 127I. Similar partitioning of 129I and 127I was observed in the loess profiles, the distribution of iodine isotopes followed an order of organic > leachable > reducible > residue. The 129I concentrations and 129I/127I 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 129I in soil. The mobility of 129I 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.

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► 129I concentration decreases exponentially with the depth of soil profile.
► The mobility of 129I in fractions decreases: leachable > organic > oxides > residue.
► Iodine shows less mobility in oxides and residue fractions.
► High organic soil content can effectively reduce the migration of 129I in the environment.
► Carrier free iodine AMS is an effective method for natural 129I speciation analysis in the environment.