Distribution and Speciation of Bromine and Iodine in Volcanic Ash Soil Profiles

Abstract

Core Ideas Concentration and speciation of Br and I in volcanic ash soils are determined. Br and I were enriched in soils with andic properties, mainly as organic form. Br and I accumulated in surface soils during Andosol formation process. Bromine and iodine are halogens with similar chemical properties. Distributions of Br and I in the soil profile provide insights for understanding of their biogeochemical cycles and fate of these elements derived from artificial origin. Although generally not abundant in parent rock materials, they are often found in higher concentrations in soils, particularly volcanic ash soils in Japan. However, the process of Br and I accumulation in soils during soil formation remains unclear. We investigated the vertical distribution of Br and I to characterize the processes of Br and I accumulation in volcanic ash soils. Soils were collected from three pedons in the southern Shimokita Peninsula, Japan. Bromine (1.4 to 253 mg kg–1) and iodine concentrations (1.2 to 90 mg kg–1) in the soils were significantly correlated. Nondestructive speciation analysis by X‐ray absorption near edge structure spectroscopy showed that both Br and I mainly exist in their organic forms in soils enriched in those elements. Indices of the degree of Andosol formation (oxalate‐extractable Al and Fe, pyrophosphate‐extractable Al and Fe, phosphate absorption coefficient, and soil pH [NaF]) were significantly correlated with both Br and I concentrations in the soils. These results suggest that Br and I, derived from atmospheric deposition, were retained in surface soils after transformation to organic form and accumulated during Andosol formation from their parent tephra deposits.