Mercury(II) Uptake at the Vermiculite Surface with Relevance to the East Fork Poplar Creek Floodplain

The uptake of mercury(II) onto vermiculite was studied to gain insight into the ultimate fate of this toxic metal in floodplain soils, in particular those of East Fork Poplar Creek, located near Oak Ridge, Tennessee. Batch experiments were conducted for 48 h at room temperature with an initial Hg concentration of 4.98 x 10-7 M (100 ppb) and 10 mg vermiculite to determine the effect of pH from 2.1 to 9.5 and the presence of dissolved organic matter at pH 5.3 to 7.3 on mercury(II) uptake. Mercury(II) desorption from vermiculite also was investigated under varying conditions to examine how the natural cycle of wetting and drying may affect the mobility of mercury. In general, mercury(II) uptake onto vermiculite increased with increasing final pH, with a relatively steep increase from 11.5% at pH 8 to 88.5% at pH 9.5. The pH-dependence of the uptake curve is correlated with the distribution of aqueous mercury(II) chloride and mercury(II) hydroxide species, which were investigated at a chloride concentration of 0.0003 to 0.02 M. Variation of background ionic strength, from 0.01 to 0.04 M, did not show an effect on Hg(II) adsorption. At final pH values from 5.3 to 7.3, the presence of dissolved organic matter at 15 mg/L DOC did not enhance mercury uptake onto vermiculite. Lastly, mercury adsorbed to the vermiculite structure was strongly bound. A maximum of only 17.5% of the adsorbed mercury(II) was removed when an Hg-sorbed sample was immediately resuspended in solution of a lesser pH for 48 h. Drying the vermiculite sample for up to 11 days prior to resuspension resulted in 1.6 to 14.4 % mercury removal, when resuspended in the presence of 15 mg/L DOC. Vermiculite, and likely other 2:1 phyllosilicates, may immobilize soluble mercury in floodplain environments.