EAGER: GOALI: Novel Anaerobic Nitrogen Transformations in Gold-Cyanide Metallurgical Processing Tailings

Colorado School Of Mines, Golden CO

Investigators

Abstract

1464496 Landkamer EAGER: GOALI: Novel anaerobic nitrogen transformations in gold-cyanide metallurgical processing tailings Harnessing anaerobic microbial processes for control of cyanide release from heap-leach, milling and other industrial operations would transform how waste cyanide material is managed. Anaerobic incubations of sediment and water collected from gold-cyanide heap-leach and milling operations containing cyanide, nitrate and sulfate produced data that suggest a previously undocumented transformation of cyanide and denitrification mechanism. Two important nitrogen transformations are hypothesized for the observed transformations in the microcosms. The native heap-leach bacteria appeared to convert cyanide to formate and ammonia and subsequently consume the formate under anaerobic conditions in the absence of added carbon. Secondly, nitrate was reduced in the absence of added organic carbon. Reduced iron minerals present in the mill residues (waste rock) may have acted as electron donors for nitrate reduction along with the formate produced from cyanide degradation. Harnessing these novel anaerobic nitrogen transformations in the management of cyanide at mining ore processing operations has the potential to significantly reduce treatment cost. The utilization of autotrophic anaerobic cyanide degradation to prevent environmental cyanide releases at gold mining ore processing sites represents the potential for significant monetary savings because there are no (or minimal) chemical or aeration requirements. Total nitrogen discharge limits also result in expensive treatment scenarios that could be replaced by inexpensive in-situ autotrophic denitrification. This university/ industry partnership will facilitate the applicability and dissemination of the research outcomes to the industrial community. Autotrophic (no added organic carbon) anaerobic cyanide biodegradation has not been documented previously. To find an organism(s) that can utilize cyanide as both an electron donor and a carbon source in the absence of oxygen would represent an entirely new metabolic pathway for supporting microbial growth that has not been explored. Secondly, denitrification driven by solid phase reduced iron as an electron donor has not been investigated in mining site environments such as heap leach operations. Accordingly, the effect of pH, temperature on these microbes and associated metabolisms will also represent new knowledge. Experiments that control for potentially competing reactions are required to provide clearer evidence for these transformations. Planned work involves anaerobic, autotrophic microcosm studies to validate the hypothesized mechanisms. Experimental variables will include pH, temperature and nitrate concentration to elucidate their effect on cyanide degradation and denitrification. This will be accomplished by incubating anaerobic batch microcosms containing heap leach residues, cyanide and sulfate at different pH and temperatures in the presence and absence of nitrate. Liquid samples will be withdrawn from the microcosms periodically and analyzed for cyanide species, sulfate, sulfide, nitrate, nitrite and ammonia. Headspace gas phase samples will be analyzed for the appearance of nitrogen gas. Additionally, microbial DNA from key microcosms will be sequenced to identify members of the microbial consortium responsible for the transformations. The role of solid phase reduced iron in denitrification will also be studied by including microcosms that contain a chemically pure reduced iron solid phase rather than heap leach residues.