REACTIVITY OF IODIDE IN VOLCANIC SOILS AND NONCRYSTALLINE SOIL CONSTITUENTS

Reaction of iodide [I-(aq)] with a series of volcanic-ash soils was co mpared with reaction onto noncrystalline materials that constitute muc h of the inorganic fraction of these soils. Our hypothesis is that the se high-surface-area materials account for iodide retention by providi ng sites for anion exchange. Iodide sorption onto imogolite and ferrih ydrite is rapid (<30 min) but not particularly extensive; imogolite ha s a threefold to fourfold greater affinity for iodide compared to ferr ihydrite on a mass basis. In contrast, rates of iodide retention by vo lcanic-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 inve rsely correlated with pH, but showed no relationship with organic matt er concentration, surface area, or imogolite and ferrihydrite concentr ations. The reaction of iodide with the volcanic-ash soils is consiste nt 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 b y dissolved oxygen to molecular iodine. Solutions of molecular iodine [I-2(aq)] react relatively quickly with laboratory-grade humic acid so lutions and the rate increases with increasing pH. The slow rate of io dination is consistent with the continual formation and reaction of I- 2(aq) or HOI(aq) by titration with soil organic matter.