The extraction and refining of natural resources from the subsoil through mining and metallurgical activities has produced much of the wealth of today’s society. However, concurrently immense volumes of waste material from these activities have been and are continuously generated, deposited, and exposed to environmental conditions worldwide.
Evidently, these large amounts of waste frequently cause large negative environmental and ecological impacts, making mining and metallurgy some of the largest polluting activities of natural environments. Their effects are present to a greater or lesser extent in all areas of the planet where deposits of solid minerals (metallic and non-metallic), liquid, and gaseous materials are exploited.
Potentially toxic trace elements are usually concentrated through mining and metallurgy. Among them, thallium, although found in much lower contents than most other trace metals, is the most toxic regulated inorganic pollutant, more toxic than mercury, cadmium, lead, copper, and arsenic, which have been more intensively investigated.
The geochemical behavior of metals, in general, depends mainly on the chemical forms (species) in which they occur, and in turn, this influences their solubility, toxicity, and bioavailability, and in particular their binding mode to the soil matrix. Metals in uncontaminated soils are mainly present in primary minerals, such as silicates, resulting in relatively insoluble forms, while those in contaminated soils tend to be more soluble due to their weaker binding to other soil components. The two main geochemical sources of Tl in the environment are from weathering of rocks rich in potassium and from sulfide minerals.
We investigated, using a 6-step extraction experimental procedure, the solubility behavior of Tl on the surface of residues and contaminated soils from three mining zones of Mexico and two mining zones of Spain, spanning samples with acidic to alkaline pH values. Surprisingly, we found a non-negligible fraction of completely water-soluble Tl in most samples of all areas, a fact that has not been reported before in these environments and which raises concern for potential health risks not previously identified.
This Tl occurs in the +1 oxidation state and readily dissolves in water, which means that it is not bound to any mineral surface with a negative charge, such as clays, which are very abundant in soils. This highly-soluble phase surpasses the drinking water standards of several countries and may easily be transported to groundwater if poor Tl retention occurs throughout the soil column and the initial content at the surface is relatively high, as was found in some sites. It also poses an important health risk associated with breathing small dust particles of efflorescent salts containing Tl, which rise to the soil surfaces in (semi-)arid climates during the dry seasons.
Most of the soil samples from a metallurgical area showed high levels of relatively available Tl, and a strong correlation was obtained between extracted manganese and Tl in one of the extraction steps of the procedure followed, suggesting a strong association of Tl to poorly crystalline manganese oxides.
In contrast, in the majority of contaminated soil samples from purely mining environments, most of the Tl was found in the residual fraction in more refractory minerals, most probably bound to alumino-silicate minerals, either of primary (feldspars and micas) or of secondary (clays and jarosites) origins. Nevertheless, even low values of relatively available Tl fractions in these mine residues may represent potential health hazards that need to be further evaluated.
These findings are described in the article entitled, Fractionation and mobility of thallium in areas impacted by mining-metallurgical activities: Identification of a water-soluble Tl fraction, recently published in the journal Environmental Pollution.