Evaluation of iodide and iodate for adsorption-desorption characteristics and bioavailability in three types of soil.

Adsorption-desorption of iodine in two forms, viz., iodide (I(-)) and iodate (IO (3) (-) ), in three types of soil were investigated. The soils were: red soil developed on Quaternary red earths (REQ)- clayey, kaolintic thermic plinthite Aquult, Inceptisol soil (IS) and alluvial soil (AS)-Fluvio-marine yellow loamy soil. The isothermal curves of iodine adsorption on soils were described by Langmuir and Freundlich equation, and the maximum adsorption values (y (m)) were obtained from the simple Langmuir model. As compared with the iodide, the iodate was adsorbed in higher amounts by the soils tested. Among three soils, the REQ soil adsorbed more iodine (I(-) and IO (3) (-) ) than the IS and AS. The distribution coefficient (K (d)) of iodine in the soils decreased exponentially with increasing iodine loading concentration. Desorption of iodine in soil was increased correspondingly with increasing adsorption values. The REQ soil had a greater affinity for iodine than the IS and AS at the same iodine loadings. In the pot experiment cultivated with pakchoi (Brassica chinensis L.) and added with two exogenous iodine sources, the iodide form was quickly taken up by pakchoi and caused more toxicity to the vegetable. The rate of iodine loss from soil was higher for iodide form as compared with the iodate. The iodine bioavailability was the highest but the persistence was the weakest in AS among the three soils tested, and the REQ soil showed just the opposite trend to that of the AS soil. This study is of theoretical importance to understand the relationship between iodine adsorption-desorption characteristics and their bioavailability in different soils and it also has practical implications for seeking effective alternatives of iodine biofortification to prevent iodine deficiency disorders.