The environmental mobility of critical elements in mine waste: rare earth elements, antimony, tungsten, and cobalt

Very little is known about the environmental mobility of many critical metals, particularly how they are leached from primary host minerals, mobilized under different conditions of pH and redox, and potentially sequestered in secondary phases. We have conducted field and laboratory studies of weathered wastes from operating or formerly operating Canadian mines to study mineral-water interaction of rare earth elements (REEs), antimony (Sb), tungsten (W), and cobalt (Co). For REE, concentrations are expected to increase substantially as pH decreases in water associated with mining activity. Antimony is mobile over a range of pH values but can be sequestered in secondary minerals, although their stability is variable. Tungsten is relatively mobile at alkaline pH values and is commonly sorbed to Fe-oxyhydroxide minerals at lower pH. Cobalt is leached relatively easily from mixtures of primary sulfide minerals and secondary weathering products in the presence of near-neutral water. In Canada, no mine effluent regulations currently exist for any of these elements and, for REE and W, there are no guidelines for either drinking water or ecological protection. Expertise from geochemists and toxicologists needs to be combined to guide future mine waste management at critical mineral mines.