REACTIVITY OF IODIDE IN VOLCANIC SOILS AND NONCRYSTALLINE SOIL CONSTITUENTS

Abstract

Reaction of iodide [I-(aq)] with a series of volcanic-ash soils was compared with reaction onto noncrystalline materials that constitute much of the inorganic fraction of these soils. Our hypothesis is that these high-surface-area materials account for iodide retention by providing sites for anion exchange. Iodide sorption onto imogolite and ferrihydrite is rapid (<30 min) but not particularly extensive; imogolite has a threefold to fourfold greater affinity for iodide compared to ferrihydrite on a mass basis. In contrast, rates of iodide retention by volcanic-ash soils were slow and did not attain a steady-state after 300 h. The extent of this largely irreversible reaction can be attenuated by sterilization, but it cannot be suppressed. The iodide retained by the soils can only be completely recovered by treatment with boiling 2 M sodium hydroxide. The amount of iodide retention by soils was inversely correlated with pH, but showed no relationship with organic matter concentration, surface area, or imogolite and ferrihydrite concentrations. The reaction of iodide with the volcanic-ash soils is consistent with a rapid initial uptake by soil mineral surfaces, followed by a slower reaction of soil organic matter with oxidized forms of iodide. Under our experimental conditions, iodide is likely slowly oxidized by dissolved oxygen to molecular iodine. Solutions of molecular iodine [I-2(aq)] react relatively quickly with laboratory-grade humic acid solutions and the rate increases with increasing pH. The slow rate of iodination is consistent with the continual formation and reaction of I-2(aq) or HOI(aq) by titration with soil organic matter.