Radionuclides in the environment : uranium

This article describes the occurrence, chemistry, and bioavailability of uranium (U) in terrestrial and aquatic environments, its analysis in environmental samples, and remedial measures applicable to uranium-contaminated water and soil. Uranium is widely distributed throughout the world. There are three main isotopes present in natural uranium, which comprises 234U (0.0055%), 235U (0.72%), and 238U (99.27%). Uranium can occur either in its reduced state (U(IV)), which is generally highly immobile, or in its more soluble and mobile higher oxidation state (U(VI)). Concentrations of uranium in rocks can range from those of ore-grade deposits to the trace levels typical of common rock-forming minerals. Uranium mobility and bioavailability are governed by oxidation state, complexation by organic and inorganic ligands, pH, sorption by minerals including clays and hydroxides, and interactions with organic matter. In uncontaminated surface waters, uranium content is generally low (