IODINE DESORPTION FROM RICE PADDY SOIL

Laboratory experiments on the desorption phenomena of iodine from rice paddy soil under waterlogged conditions, with a special reference to soil redox potential (Eh) and pH, have been conducted. Radioiodine tra cer (I-125), added to the soil, was readily sorbed on it. At the begin ning of the waterlogging, the iodine desorption was low. However, iodi ne was desorbed into soil solution with time. The iodine desorption wa s enhanced markedly by the addition of organic substances such as stra w pieces and glucose to the soil. Cultivation of rice plants in soil a lso affected the iodine desorption, suggesting root exudates and/or ro ot autolysis might be participating in the desorption process. Eh drop ped considerably after soil was waterlogged due to microbial metabolis ms. Particularly low Eh values were observed in soils with plants and also with added organic substances. A negative correlation was seen be tween the desorption and soil Eh. High desorption was frequently obser ved when the Eh dropped to about -100 mV or below. Due to the reducing conditions (low Eh) by waterlogging, iodine in soil was leached into the soil solution; consequently total iodine concentration in paddy so il was considerably lower than forest and upland field soils. These io dine desorption phenomena under anaerobic conditions should be conside red in assessing transfer of the long-lived radioiodine (I-129) in the environment, especially in rice fields and marshland.