Some considerations on the sorption and desorption phenomena of iodide and iodate on soil

Abstract

Radiotracer experiments on the sorption of I⁻ (iodide) and IO ³_inf ^p− (iodate) from water by soils such as field soil, rice paddy soil and sandy soil, as well as by some soil components, have been carried out with special reference to the effects of heating and gamma-irradiating the soil. Desorption phenomena of I from soil to various solutions were also studied. The sorption of I⁻ by soil was markedly reduced through treatments of air-drying and heating the soil prior to its equilibration with water. The results indicated that I⁻ sorption was by the soil fraction which was unstable at about 150 °C, while IO ³_inf ^p− sorption was by the soil fraction which was relatively stable to heating. Gamma-irradiation at 27 kGy affected the sorption to a smaller extent than heating at 150 °C. A very high sorption (or soil-water distribution coefficient, Kd) was found in untreated field soil (andosol) with a low organic C (humus) content, while the sorption by sandy soil was considerably smaller than the other soils. Neither I⁻ or IO ³_inf ^p− were well sorbed by clay minerals, Al₂O₃ and quartz sand, while the sorption by Fe₂O₃ was IO ³_inf ^p− were desorbed by 1N NaOH solution. By acidifying this solution, only a part of the desorbed I was re-precipitated with humic acid. The desorption by solutions containing K₂SO₃ or KI was also high, while that by solutions containing HCI, CH₃COONH₄ or chemical fertilizer was considerably lower. These findings suggested the possibility that I was not directly associated with humic acid itself.