EFFECT OF HUMIC ACIDS ON ADSORPTION AND RELEASE OF TUNGSTEN ON GOETHITE-WATER INTERFACE

Although originally considered to be environmentally benign, recent studies have demonstrated that tungsten (W) is not stable in soils. Now, USEPA considers W an emerging contaminant, which is subject to dissolution as a function of soil chemical properties. However, geochemical fate of W and its species in aquatic environments is still poorly understood, particularly in the presence of humic substances that are ubiquitous in soils. Here we report the effect of two standard humic acids, Elliot Soil Humic Acid (ESHA) and Pahokee Peat Humic Acid (PPHA) on the sorption of tungstate anion (WO₄^2-) on a common iron oxide in soils, goethite. Our primary objective was to understand the relation between carboxylic (-COOH) and phenolic (-OH) functional groups of ESHA and PPHA and the amount of WO₄^2- release from the goethite surface. Various concentrations of WO₄^2- (5 - 817 µM) was added to a 5 g L^-1 goethite suspension to construct a sorption isotherm at pH 5.5. After 24 hours equilibration, suspensions were centrifuged and filtered, and W concentration measured by ICP-MS. To study the effects of ESHA and PPHA on W sorption, solutions of these humic acids containing 10, 25, and 50 µM –COOH were added to goethite onto which 272 µM WO₄^2- had been pre-adsorbed. The suspension was shaken for 24h, centrifuged and filtered to measure W concentration, with the amount of W release calculated from the difference. A linear correlation between the amount of –COOH in ESHA/PPHA and the amount of W released was obtained, with a statistically significant R² value. The ratio between the added –COOH moiety, and the combination of –COOH and phenolic-OH moieties of ESHA/PPHA to W release was 1.36 to 1, and 1.77 to 1, respectively. Further studies on the tungsten-goethite system are ongoing, which will help us better understand the geochemical fate of W in the soil-water environment.