Tungsten in groundwaters, surface waters, and associated sediments: Closing some of the knowledge gaps
Wednesday, 17 December 2014: 11:50 AM
Interest in the geochemistry of tungsten (W) in the environment is on the rise in part due to the United States Environmental Protection Agency’s (USEPA) recent recognition that it represents an emerging contaminant of environmental concern. As such, we undertook a multi-year investigation of W in groundwaters and surface waters to help augment the relatively sparse understanding of the biogeochemistry of W in environmental waters. We quantified W concentrations in natural waters and sediments from a variety of settings, including shallow and deep groundwater flow systems, river waters, estuaries, and marine sediments and associated porewaters. In addition, we measured the equilibrium constants for the formation of thiotungstate ions in laboratory experiments to investigate the possible formation of these thioanions in sulfidic waters. Earlier investigations showed that the chemically similar element molybdenum (Mo) forms thiomolybdate ions in sulfidic waters. Our investigations indicate that pH-related adsorption/desorption reactions and the available quantity of Fe(III) oxide/oxyhydroxide minerals in substrate sediments are important controls on the amount of dissolved W in the environment with higher pH and lower Fe(III) oxide/oxyhydroxide content both favoring greater W mobility. Moreover, W is mobile in oxic, suboxic, and anoxic, including sulfidic waters occurring as the tungstate oxyanion and/or various thiotungstate anions depending of the dissolved sulfide concentrations. Specifically, thiotungstate anions predominate over the tungstate oxyanion for natural waters where dissolved sulfide concentrations attain millimolal levels. Investigation of sediments from a Louisiana salt marsh indicates that W is sequestered into pyrite similar to Mo.