The Importance of Understanding the Chemical Form of a Metal in the Environment: The Case of Barium Sulfate (Barite)

ABSTRACT

Environmental assessments of metals are often simplified by referring to metals in terms of their elemental names without further consideration of specific metal compounds. Such simplifications can obscure important information on the fate and effects of the particular metal compounds. Barium sulfate (barite) provides a good example of the importance of knowing the specific compound when evaluating exposure, effects, and risks. We illustrate this by comparing the solubility and toxicity of barite to the soluble barium salts used to derive the U.S. Environmental Protection Agency barium ecological soil screening levels (Eco-SSLs). For ecologically relevant exposure pathways, the toxicity of barium compounds is largely controlled by their solubility. In contrast to soluble barium salts (e.g., barium acetate, barium chloride, and barium nitrate), barite is sparingly soluble and practically nontoxic to invertebrates, plants, or wildlife. No-effect levels for soil invertebrates (17,000 to 1,000,000 mg/kg) are several orders of magnitude higher than the barium Eco-SSL for soil invertebrates (330 mg/kg). And, low solubility likely limits the bioaccumulation of barium associated with barite and potential for effects on wildlife species. The importance of knowing the form of a metal compound is underscored by the fact that barite is one of the most commonly occurring barium compounds in soils and is widely used in commerce. The large differences between barite and the soluble barium compounds used to derive the Eco-SSL indicate that this screening tool cannot be usefully applied to soils containing the most commonly found barium compound, barite. The value of considering the specific metal compounds present in exposure media applies to other metals and indicates a need to go beyond simplified characterizations of metals expressed solely in terms of their elemental names.

Key Words:

• Barite
• barium sulfate
• solubility
• bioavailability
• toxicity
• Eco-SSL

ACKNOWLEDGMENTS

Funding for the development of this article was provided by the American Petroleum Institute (API) in Washington, DC, USA. Technical reviews from Yvette Lowney, Rick Bodishbaugh, Michael Harrass, Mark Johnson, Randall Wentsel, and Lawrence Tannenbaum are appreciated. The contributions to this article made by Patti Warden and Betty Dowd are acknowledged and appreciated.