Investigation of physicochemical form of uranium in sediment of brackish Lake Obuchi using sequential extraction procedure

The physicochemical form of uranium (²³⁸U) in the sediment of brackish Lake Obuchi was investigated using the standard procedures of sequential extraction. Approximately 60% to 70% of ²³⁸U was composed of the form bound to carbonates and iron (Fe)-manganese (Mn) oxides in silty sediments, whereas more than 50% of ²³⁸U existed in the minerals of sandy sediments. The proportions of ²³⁸U bound to carbonates in the sediments of brackish Lake Obuchi were larger than those in fresh-water lake sediments. Most of the uranium in the brackish lake sediment was considered to be mainly influenced by UO₂(CO₃)₃ ⁴⁻ in seawater, judging from the fact that the ²³⁴U/ ²³⁸U activity ratios (1.13 to 1.16) of silty sediments were very close to that of seawater (1.14). The high correlation found between the ²³⁸U concentration in the sediment and the amount of organic matter demonstrated that organic matter acts as a scavenger for ²³⁸U accumulation on the lake bottom. Received: July 2, 1999 / Accepted: May 25, 2000