Periphyton communities as biological polishing agents in mine waste waters and the precipitation process in tailings : final report.

Abstract

Ecological Engineering as a decommissioning technology for acid-generating waste material from base metal mine sites is being developed on a site in northern Ontario. A zinc/copper concentrator operated 65 km northeast of Ear Falls for 10 years, producing about 760,000 short tons of tailings, covering 20 ha, and containing 41 % pyrite and 4 % pyrrhotite. The tailings are situated above a lake which has acidified during the life of the mine. Since 1986 the site has been investigated intensively. This report represents the last detailed investigation of the hydrology and geochemistry, concentrating on the oxidation and iron precipitation rates which occur in the tailings mass. These rates define the rate at which the contaminants are released. The released contaminants entering the acidified lake are removed in this Ecological Engineered system mainly by different groups of attached algae. A literature review was carried out to provide a technical framework for biological removal processes taking place on the site. Water samples collected from piezometers in 1990 were used to confirm that several precipitation processes could take place in the tailings mass or in the surface water receiving AMD seepage. All piezometer samples show super-saturation with respect to several iron minerals, including ferric hydroxides, aoethite, hematite, letidocrocite, and maanetite, which may be precipitated. Super-saturation with respect to iarosite is shown only by samples from 2 piezometers and Mill Pond. Samples from piezometers M4 and M10 (and from Mill Pond and Boomerang Landing) show super-saturation with respect to several aluminum minerals, including alunite, boehmite, diawore, gibbsite, and several clay minerals. Samples from piezometers M7B, MI 0, M25, M30, M47, M55, Mill Pond, and Dave's Dam show super-saturation with respect to quartz; those from M7B, MI 0, and M47 also show super-saturation with respect to gypsum. Revised sulphide-oxidation rates for the South Bay tailings range from 0.0001 to 0.0427 mol.kg" .yi' (overall average 0.0033 mol.kg-'.yi'). Revised Fe-precipitation rates range from 0.000009 to 0.001 135 mol.kg-'.yr-' (overall average 0.00025 mol.kg-'.yr-'). The literature review on periphytic algae indicated that algal growth rate data in acidic metal rich waters are scarce. However, the tolerance mechanisms of periphytic algal groups which facilitate growth and metal removal from the water were evident. Extracellular carbon in the form of sheaths or jelly is produced in response to metal and acid stress by the algal population. Extracellular carbon production is not only related to metal and acid stress, but also to low nutrients (P and N) in the water or plants. It was concluded that periphytic algae, given their biological polishing characteristics, will indeed prove to be an important contaminant removal process within the Ecological Engineering technology. Research is required to define the conditions which facilitate growth of periphytic algae, and which of the contaminant removal processes (biosieving, uptake, ad/absorption) is the main contaminant removal process. With this information, predictions on the contaminant removal capacity of the system can be made. With the contaminant release rates determined here, the overall effectiveness of biological polishing as a process can be determined.