(706a) Trace Metal Removal Using Metal-Complexing Chemicals

The process wastewater treatment complex (PWTC) at Oak Ridge National Laboratory uses chemical precipitation (at a pH of 11.3) and ion exchange resin to remove Sr-90 and trace metals, natural chabazite zeolite to remove Cs-137, and air stripping and activated carbon adsorption to remove organics from process wastewater and groundwater. The current processes produce various secondary wastes including, sludge (mostly calcium and magnesium hydroxides), liquid low-level waste (LLLW) from regenerating the ion exchange resin with nitric acid, and spent zeolite. A modification of the PWTC is currently being designed that will eliminate the precipitation and ion exchange processes. The zeolite will be used to remove both Sr-90 and Cs-137 at neutral pH. The new process will eliminate the LLLW waste and greatly reduce the sludge, while increasing the spent zeolite; however, the total amount of secondary waste will be reduced. A full-scale test of the new process showed that Sr-90 and Cs-137 removal was comparable to the current process, but several trace metals (Cu, U and Zn) were not removed. The concentrations of the trace metals during this test were below the discharge limits for the PWTC; however, the new system would not have the capability to remove these metals if higher concentrations were encountered in the future. The new system will use a clarifier and dual-media filters to remove suspended solids prior to the zeolite columns. Laboratory-scale tests were conducted using several metal-complexing chemicals, including dimethylthiocarbamate and trithiocarbonate, which could be used in the clarifier to precipitate heavy metals. The results of these tests will be presented.