Understanding contaminants associated with mine wastes

Both historical and modern metal mining areas and accompanying waste material related to the extraction and processing of ore very often contain considerable amounts of toxic heavy metals and metalloids. These sites represent one of the most important latent environmental hazards for waters, soils, biodiversity and the food chain. Investigations of mineral-dissolution and precipitation reactions in mine-waste material provide insight into weathering processes of such material and sources of metal concentrations in solutions leached from it. The project UMWELt - Understanding contaminants associated with mine wastes of the Lojane AsSbCr and the Alsar AsSbTlAu deposits, FYR Macedonia, aims at a detailed characterization of phases containing arsenic (As), antimony (Sb), thallium (Tl) and chromium (Cr) (primary and secondary minerals) and the processes of their formation. The phases containing the toxic elements, which are often very fine-grained, will be studied by a combination of mineralogical methods involving X-ray, spectroscopic and electron-microscopic techniques. Furthermore we will quantify the distribution of the As, Sb, Tl and Cr contaminants in the waste dumps, and define the geochemical controls on the dissolved As, Sb, Tl and Cr in the waste material. This will be achieved by the analysis of sampled pore waters and by performing selective extractions on solid samples. In conclusion, we will combine the results from detailed mineralogical studies, selective extractions and measurements of pore waters in order to model the evolution of pore waters in the studied sites and assess the potential of these sites to contaminate the local environments. The research scope of this project lies at the intersection of mineralogy, geochemistry and environmental chemistry. The results of this research will be transferable to any of these fields. We do not focus on the specific local conditions at the investigated deposits. Although they were selected as excellent natural laboratories, the central question of the project is the environmental behaviour of the selected toxic elements (As, Sb, Cr, and Tl). The results of this project can thus be used to model, understand and predict the pollution development at any site worldwide. Furthermore, the results can set the stage for further research directions at similarly polluted sites in terms of the solubility and mobility of the toxic elements, and the nature and role of secondary phases in the cycling of these toxic elements in the environment.

Secondary minerals could be effective scavengers of toxic arsenic (As), antimony (Sb) and thallium (Tl). In the environments polluted by mining, these elements are abundant both in acid rock/mine drainage scenarios, as well as in carbonate-buffered environments. Within our project "UMWELt", we have investigated the behavior of As, Sb, Tl and partly Cr during weathering in mine waste dumps, tailings and associated soils from two abandoned mines in North Macedonia (Lojane As-Sb-Cr mine and the Allchar As-Sb-Tl-Au mine). The multi-technique characterization of As-, Sb- and Tl-bearing secondary minerals improves our understanding of their role in immobilizing As, Sb and Tl in acid mine drainage systems, temporarily avoiding its dispersion in the environment. The main result from our investigations connected to the mining waste material at Lojane deposit is that the weathering of primary As- and Sb-bearing sulphides in the mining wastes and flotation tailings of Lojane proceeded under mostly oxidizing, acidic and temporarily wet conditions. Highly acidic conditions on the surface of the concentrate body imply dissolution of arsenolite (As2O3) and scorodite (FeAsO4H2O), thus causing contamination of the environment and high mobility of As. Under the present conditions, the confirmed secondary minerals are not ultimate sinks. In particular, As and to a lesser extent, Sb and Ni could be re-released into the environment. The mining waste dumps and especially realgar-rich tailings are of the most serious concern. The pollutants and contaminants are distributed by wind, surface water and groundwater. By these pathways, they pollute sources of drinking water, agricultural soils and reach the food chain of humans and other organisms. Hence, a comprehensive remediation concept for all mining waste dump of Lojane is required to avoid distributing pollutants by anthropogenic sources in the future. At Allchar deposit, besides the recognition of Tl-storage in several already known secondary phases, we provided the evidence of Tl uptake by several previously unknown thallium phases (mostly Tl-arsenates with Tl:As ratios 2 and 4, as well as Tl-Sb- and Tl-Mn-oxides), detected in carbonate-buffered (near-neutral pH) As- and Tl-rich technosols and waste dumps. Since the treatment of highly As-, Sb- and Tl-contaminated soils depends on the character of the As-, Sb- and Tl-bearing phases, it is necessary to fully characterize As-, Sb- and Tl-retention through secondary minerals, in order to build a sound basis for successful remediation and better environmental management of As- and Tl-contaminated soils. These results indicate the need for further studies on Tl-speciation in extremely As-, Sb- and Tl-rich environments.