Radium: Radionuclides

Abstract

This article describes (1) the occurrence, chemistry, and bioavailability of radium (Ra) in terrestrial and aquatic environments, and its analysis in environmental samples, (2) the nature of naturally occurring radioactive materials (NORM) and their role in radium‐contamination scenarios, and (3) the geochemical processes and remedial measures associated with radium‐contaminated water and soil. Radium is the heaviest of the Group II, alkaline earth metals. Radium is exclusively divalent and its chemistry resembles that of barium. Radium has a limited environmental mobility due to sorption (e.g., by hydrous oxides of Fe, Mn, and Al) and coprecipitation reactions (e.g., with barite). All isotopes of radium (atomic number 88) are radioactive. The most important radium isotopes are 226 Ra (half life 1600 years) and 228 Ra (half life 5.7 years), decay products of, respectively, the 238 U and the 232 Th series. As members of natural radioactive decay series, these radium isotopes are found in uranium‐ and thorium‐bearing minerals. Concentrations of uranium and thorium in rocks can range from those of ore‐grade deposits to the trace levels typical of common rock‐forming minerals. In uncontaminated surface waters, radium content is generally low and within a narrow range (0.5–20 mBq l −1 ). Radium in groundwater is highly variable (<0.52–55 000 mBq l −1 ) and can arise from natural sources, resulting from the interaction of groundwater with radium‐bearing materials or indirectly from man's exploitation of the radioactive minerals of uranium and thorium. The major challenge associated with many NORM‐containing residues or NORM‐contaminated sites is the larger volumes of material and the relatively low specific activities.