WP3 Metals in the Environment: Chemistry and Fate Issues
254 Portland Ballroom
1:20 PM - 4:40 PM, Wednesday
() Fate and behaviour of EDTA in a metal-mining polishing pond.
Taylor, L.1, Martin, A.2, Novak, L.1, Wren, C.3, 1 Stantec, Guelph, Ontario, Canada2 Lorax Environmental Services, Vancouver, British Columbia, Canada3 C. Wren & Associates Inc., Guelph, Ontario, Canada
ABSTRACT- A gold mine (Northern Ontario, Canada), now in a state of closure, was given approval for the addition of Ethylenediamine Tetraacetic Acid (EDTA) to wastewater to reduce copper bioavailability and toxicity in the receiving waters. As part of the permitting process, the Ontario Ministry of the Environment requested that the fate and behaviour of EDTA in the receiver be determined prior to active treatment. Rapid metal-EDTA complexation kinetics will result in the immediate decrease in Cu bioavailability to levels defined by the metal-EDTA equilibrium, whereas slow reaction rates may result in the persistence of labile Cu species for some time after EDTA addition. In a flowing system, this reaction time translates to the distance downstream of the treatment point and has obvious implications with respect to changes in metal bioavailability in the receiving environment. The EDTA feasibility assessment involved the following components: 1) a desktop modeling exercise to determine the EDTA dose needed to bind available Cu; 2) assessment of the site-specific acute toxicity of Cu using the Biotic Ligand Model (BLM); 3) bioassays with Daphnia magna to confirm the reduction of Cu toxicity by EDTA; 4) laboratory assessment of the metal-EDTA complexation kinetics via measurements of labile Cu using DGT (Diffusive Gradients in Thin films) at two temperatures; and 5) laboratory assessment of the effect of EDTA on sediment metal remobilization. Toxicity results were consistent with BLM predictions, and indicated the EDTA effectively mitigates Cu toxicity in mine effluent. Measured rates of Cu-EDTA complex formation demonstrate that such complexes are formed rapidly, with faster kinetics being observed at higher temperatures. The potential for sediment Cu remobilization could not be discounted at cold temperatures; however, at higher temperatures EDTA did not remobilize Cu from the sediments in two weeks. Collectively, the results demonstrate that EDTA addition will ameliorate Cu toxicity in the receiving environment, and should not result in significant adverse effects (e.g., excessive Cu remobilization from sediments).
Key words: copper, mining, diffusive gradients in thin films, biotic ligand model