DETERMINING THE MINERALOGICAL ASSOCIATIONS OF MERCURY IN FGD PRODUCTS BY SEQUENTIAL EXTRACTION

The natural mode of retention of mercury (Hg) in flue gas desulfurization (FGD) products (primarily FGD-produced gypsum) is a key issue in the utilization of coal byproducts as environmentally acceptable resources. This is being investigated with a sequential extraction scheme that subjects FGD material to a series of phase-targeted reagents. Mineral phases with the greatest affinity for Hg and the form in which Hg is naturally immobilized can be discovered by observing the amount of Hg mobilized by each successive extracting solution. The procedure involves a series of batch extractions, including a water rinse to remove water soluble and loosely sorbed ions, 0.11M acetic acid to target carbonate minerals and exchangeable ions, 0.1 M hydroxylamine hydrochloride to dissolve manganese oxides and hydroxides, 0.25 M hydroxylamine hydrochloride in 0.25 M HCl to dissolve iron oxides and hydroxides, and hydrogen peroxide and 0.1 M ammonium acetate to oxidize organic matter and dissolve sulfide minerals. All extractions were conducted at room temperature except the second hydroxylamine hydrochloride step and the hydrogen peroxide steps, which were performed in a bath heated to at least 80°C. Some samples were tested after a prolonged water rinse to dissolve the gypsum phase, others were tested as-received. Results indicate that Hg is associated with two distinct fractions of FGD materials. Most of the solubilized Hg is extracted by the iron oxide and hydroxide dissolution reagent. Repeated applications of this reagent suggest that Hg is retained with more resistant iron phases. Hg release occurred under extremely acidic (pH<1) and reducing conditions. Analysis of the extracts indicates the presence of a minor phase (possibly clay minerals) that may also be associated with retained Hg. The organic matter and sulfide minerals fraction typically yields lower but still significant amounts of Hg.