TUNGSTATE ADSORPTION TO FERRIHYDRITE; EXPERIMENTAL DETERMINATION OF AN ADSORPTION CONSTANT

Adsorption onto mineral surfaces is an important mechanism that controls aqueous heavy metal concentrations. Tungsten (W) concentrations in groundwater are a recent concern due to possible links between W and human health in Fallon, NV.  Recent literature suggests W adsorption on Fe/Mn oxyhydroxides controls W concentrations in groundwater. We aim to test this hypothesis by: 1) determining a robust adsorption constant for W (as tungstate, WO₄^2-) onto ferrihydrite over a range of potential groundwater chemistries, and 2) comparing W concentrations determined with our constant to known Fallon aquifer concentrations.

            Preliminary adsorption experiments were performed at 22ºC in 0.01M  NaNO3 background electrolyte, over a pH range 4-12, with WO₄^2-(μM) /ferrihydrite (mM) ratios of 50:1, 5:1, 1:1. Experiments were conducted on a shaking platform at 270 rpm for 24 hrs.   WO₄^2- adsorption to reaction vessel walls was tested by repeating the adsorption experiments without ferrihydrite. Bulk chemical analyses and sequential extractions of samples from the shallow and intermediate Fallon aquifers were perf0rmed to quantify aquifer W concentrations and provide evidence for W association with Fe/Mn oxyhydroxide fractions.

            Preliminary batch experiments indicate for WO₄^2-/ferrihydrite ratio of 50:1, 100% adsorption occurs between pH 3-7.  Adsorption drops sharply as pH rises above 7, and is negligible at pH >10. Decreasing the ratio to 5:1 shifts the pH range of the adsorption edge to the right by approximately 0.5 pH units and there is no significant difference between isotherms measured at 5:1 and 1:1. Negligible W adsorption (<5%) to the reaction vessel walls occurred except at low pH.  Bulk aquifer sediment analyses indicate 3% Fe content and W concentrations ranging between 3-6 mg/kg.

            Experiments are underway investigating the effects on adsorption of: ionic strength 0.01-0.5 M NaNO₃, pH 3-12, adsorbate/adsorbent ratios 1-100:1, and the role of competing adsorbates, such as SO₄^2-, PO₄^2-, and AsO₄^3-.  Experimental results will be modeled with FITEQL 4.o and the Diffuse Double Layer Model to determine the best-fit adsorption constant.  The adsorption constant will be used with MINTEQ to estimate W concentrations at equilibrium in chemical conditions characteristic of the Fallon aquifers.