Model of radioiodine speciation and partitioning in organic-rich and organic-poor soils from the Savannah River Site

•Developed model describes iodine speciation and partitioning on soil.•The modeling demonstrates important role of iodine–SOM interactions.•Novel colloidal organic-iodine formation was suggested by the model simulation.•The modeling captures the dynamics of iodine transformation with SOM in soil.

Radioiodine biogeochemistry was investigated by developing an integrated comprehensive model describing multiple physiochemical and enzyme catalyzed reactions based on detailed iodine speciation data obtained previously from laboratory experiments using organic-rich and organic-poor soils from the Savannah River Site, South Carolina. The model accounted for iodine speciation, inter-conversion kinetics (I−, IO3−, organo-I, and colloidal-I), reversible partitioning to soil organic matter (SOM) and mineral surfaces, irreversible covalent bonding to SOM, and abiotic and biotic (enzymatic/catalyst-type) reactions. Modeling results strongly supported the assertion that iodine–SOM interactions dominate iodine geochemistry; the iodine uptake coefficient for SOM was an order-of-magnitude greater than that for mineral surface. The proposed model simulated well the iodine partitioning among the soil, colloid, and solution phases. The previously proposed process of soil reduction of IO3− to I− was strongly supported through model simulations. The model revealed that during the first 14 days of contact most iodine in soil was comprised of I− or IO3− associated with mineral surfaces and reversibly bound to SOM. After 14 days, the continued uptake of iodine by soil was attributed primarily to the irreversible bonding of organo-I to SOM. Finally, the model was successfully validated using an independent experimental data set. This biogeochemical modeling study underscores the importance of capturing the dynamic nature of iodine speciation transformations and the importance of treating SOM as a sink (irreversible covalent bonding) and a source (colloidal- and organo-iodine mobile species) for subsurface iodine.