The role of phytotechnology in the rehabilitation of the BHPBilliton Cannington Ag-Pb-Zn mine

Phytotechnology utilises the unique biochemical processes of plants to manage and remediate contaminants such as heavy metals, hydrocarbons, radionuclides and pesticides from soil and water. The use of in situ biological systems to rehabilitate large volumes of contaminated soil has enormous potential for application around the globe, particularly in the mining and metal production industries. This study investigated the use of two phytotechnologies (pastoral vegetation covers and chemically-assisted phytoextraction) as environmental tools to manage mine tailings and soil contaminated with mine tailings at the BHPBilliton Cannington Ag-Pb-Zn Mine. The study was conducted in accordance with the mine's Environmental Management Overview System (EMOS) and employed the Australian and New Zealand Environment and Conservation Council (ANZECC) Investigation Guidelines for heavy metal and metalloid contamination of industrial and commercial soil. Selected pasture plant species (Chloris gayana, Crotalaria novae-hollandiae, Cymbopogon ambiguus, Cymbopogon bombycinus, Cyperus victoriensis, Gomphrena canescens and Triodia molesta) were cultivated in soil contaminated with mine tailings (60 micrograms Ag g ⁻', 2039 micrograms As g ⁻', 30 micrograms Cd g ⁻', 11950 micrograms Pb g ⁻' and 4150 micrograms Zn g ⁻'). The addition of 5 wt% to 35 wt% mine tailings to uncontaminated soil significantly improved the biomass production of Chloris gayana. In contrast, the biomass production of the remaining species (all native pasture plants) was significantly reduced on soil contaminated with 5 wt% to 35 wt% mine tailings. The pasture plant species accumulated low concentrations of heavy metals and metalloids from soil contaminated with mine tailings, indicating their suitability for the revegetation of pastoral lands. In addition, limestone amendments to soil contaminated with mine tallings effectively improved the revegetation potential of Cymbopogon ambiguus, Cymbopogon bomhycinus and Crotalaria novae-hollandiae on soil contaminated with mine tailings, in addition to reducing the uptake of heavy metals and metalloids by the plants. The chemically-assisted phytoremediation of soil contaminated with mine tailings was investigated using Chloris gayana, Crotalaria novae-hollandiae, Cymbopogon bombycinus and Cyperus victoriensis and soil amendments of EDTA, DTPA, EDDS, ammonium thiosulphate, ammonium thiocyanate and thiourea. Plant uptake of heavy metals and metalloids resulting from the application of the soil amendments indcated that, based upon published models for the technology, no pasture plant species would be suitable for the chemically-assisted phytoremediation of contaminated soil at the Cannington mine. Crotalaria novae-hollandiae and Cypcrus victoriensis, however, did tolerate the effects of ongoing soil treatments with EDTA and EDDS, while accumulating modest quantities of heavy metals and metalloids, suggesting that vegetation covers with these plants could be used to phytoremediate low levels of soil contamination. The leaching of Ag, Pb and Zn from mine tailings using weekly amendments of low-ionic-strength solutions of EDTA, ammonium thiosulphate, ammonium thiocyanate, thiourea and sodium cyanate was investigated over a three-month period. EDTA, ammonium thosulphate and ammonium thiocyanate leached significant quantities of metals from the mine tailings over an approximate eight-week leaching period. EDTA solutions were found to dissolve large quantities of Pb (28.1%) and Zn (12.6%) from the mine tailings. Zinc dissolution was also high using a solution of ammonium thiosulphate (12.1%) and Ag dissolution was only notable using an ammonium thocyanate solution (83.7%). The data indicate that chemical leaching of the Cannington mine tailings using low-ionic-strength solutions may remove a large proportion of the wastes contained heavy metals thus increasing metal production at the site, in addition to decontaminating a hazardous mine waste material. This research project concludes that the pasture plant species investigated are highly suited to the revegetation of soil contaminated with mine tailings. In addition, the study concludes that the native pasture plant species that were deemed appropriate for phytotechnology applications at the Cannington mine are not suitable for the chemically-assisted phytoremedation of soil contaminated with mine tailings. The study also concludes that periodic leaching of the mine tailings using chemical reagents employed for phytoextraction applications has the potential to elevate metal production by reprocessing the waste while also reducing its toxicity and environmental risk