Mobility and phytoavailability of antimony in an area impacted by a former stibnite mine exploitation
Highlights
► Environmental assessment of an area impacted by a former stibnite mine exploitation. ► Severe pollution with high total and soluble Sb contents in the uppermost soil layers. ► Changes in environmental conditions moderately raise the current soil Sb mobility. ► Daphne gnidium L. seems a suitable species for Sb phytostabilization strategies.
Introduction
Antimony is a non-essential element which exhibits a high degree of toxicity, especially to mammals (McBride, 1994). In humans it causes a great variety of adverse health effects, including carcinogenic problems (Winship, 1987, Sundar and Chakravarty, 2010). Antimony is gaining interest as a global contaminant. Antimony inputs to the environment can take place by natural sources (Reimann et al., 2010, He et al., 2012), but its release has been greatly enhanced by human activities. The main anthropogenic Sb sources include combustion of fossil fuels, waste incineration, mining operations, smelting processes, shooting practice and road traffic (Scheinost et al., 2006, Kabata-Pendias and Mukherjee, 2007, Ackermann et al., 2009, Telford et al., 2009, Reimann et al., 2010, He et al., 2012). Amongst them, mining operations are assumed to be the greatest Sb source (Takahashi et al., 2010).
Antimony is used in the manufacture of many industrial products. Thus, it is employed as flame retardant, catalyst in plastic production, pigment in paints and lacquers, additive in glassware and ceramics, and hardening agent in alloys for the production of batteries, ammunitions and brake pads (Filella et al., 2002, Oorts et al., 2008, Ackermann et al., 2009). Such diverse industrial applications result in Sb being the ninth most mined metal worldwide (Scheinost et al., 2006, Okkenhaug et al., 2011). More than 100 minerals are known to contain Sb (Kabata-Pendias and Mukherjee, 2007, Reimann et al., 2010), with stibnite (Sb2S3) being the main ore mineral (Alloway, 1995). The exploitation of Sb deposits has induced a wide legacy of Sb in mine wastes, especially when early mining operations were involved owing to their low efficiency. In general, these wastes have been poorly managed, promoting the Sb dispersion to the surrounding ecosystem. Antimony is readily leached form stibnite-bearing wastes (Wilson et al., 2004). Dissolution of stibnite has been shown to produce up to 55 mg l− 1 of Sb (Ashley et al., 2003).
Antimony exists in environmental systems in predominantly two oxidation states: Sb(III) and Sb(V). Both are strongly sorbed on Fe, Al and Mn (oxyhydr)oxides (Thanabalasingham and Pickering, 1990, Leuz et al., 2006, Scheinost et al., 2006, Mitsunobu et al., 2010, Biver et al., 2011, Ilgen and Trainor, 2012). Antimony(III) binds to them over a wider pH range than Sb(V) for which sorption importantly decreases under neutral and alkaline conditions (Johnson et al., 2005, Leuz et al., 2006). Organic matter has been also reported to tightly bind Sb, acting as an important sink in soils, especially for Sb(III) (Buschmann and Sigg, 2004, Steely et al., 2007, Van Vleek et al., 2011, Sh et al., 2012). In any case, the sorption of Sb to organic matter strongly decreases with increasing pH from moderately acid values (Pilarski et al., 1995, Buschmann and Sigg, 2004, Tighe et al., 2005, Klitzke and Lang, 2009).
Studies performed on soils polluted with Sb using X-ray absorption spectroscopy (Mitsunobu et al., 2005, Mitsunobu et al., 2006, Takaoka et al., 2005, Scheinost et al., 2006, Okkenhaug et al., 2011) point out that Sb in soils exists dominantly as Sb(V), regardless of the Sb emission source and the soil redox state (Filella, 2011). The predominance of Sb(V) in soils is explained by the rapid oxidation of Sb(III) to Sb(V) by Fe and Mn (oxyhydr)oxides (Belzile et al., 2001, Leuz et al., 2006). Humic acids have been also proven to catalyze the oxidation of Sb(III) to Sb(V) (Buschmann et al., 2005, Steely et al., 2007, Ceriotti and Amarasiriwardena, 2009, Sh et al., 2012), and in some instances this oxidation process has been found to be relatively fast (Buschmann et al., 2005, Ceriotti and Amarasiriwardena, 2009). Hence, humic acids should also play an important role in the Sb(V) prevalence in soil environments. The effect of such co-oxidants could contribute to Sb mobilization and dispersion to other environmental components, especially in neutral and alkaline soils.
Concentrations of Sb are typically below 10 mg kg− 1 in unpolluted soils (Wilson et al., 2010), but in mining-affected areas Sb concentrations can be increased up to three orders of magnitude, mainly in the close environs of mine dumps or mineral processing facilities (Okkenhaug et al., 2011, Qi et al., 2011, Hiller et al., 2012). Such huge soil Sb concentrations can pose severe hazards to the surrounding ecosystem, particularly if environmental conditions favor Sb release. Therefore, this kind of scenarios require a strict study to establish the current and eventual environmental risks and to assess the right measures to be undertaken in order to minimize such risks.
The main objectives of the present work are: a) to perform the environmental characterization of soils affected by the former mine exploitation of a stibnite deposit, and b) to study the native plants growing in this area so as to evaluate the Sb phytoavailability and the consequent environmental implications.
Section snippets
Study area
The studied mining area is located 10 km southwest of Alburquerque village in the north-west of the Badajoz province (Spain) where the abandoned San Antonio mine is situated. This mine is located within the largest stibnite deposit in Spain, which was exploited extensively from 1940 to 1986. It is a hydrothermal vein deposit, with quartz–stibnite–scheelite (CaWO4) mineralization hosted in limestones and calc-schists. Mining activities produced huge amounts of wastes composed of barren rocks,
Soil mineralogical composition
As derived from the XRD analysis, soils in the studied mining area are mainly constituted by quartz, calcite and muscovite, together with some clay minerals such as illite, kaolinite and chlorite. Samples from the mine waste dump show similar mineralogical composition. Additionally, in samples from the WD-3 and WD-5 sampling sites the occurrence of Sb minerals is also detected, namely clinocervantite (β-Sb2O4) and hydroxycalcioroméite (CaSb2O5(OH)2). These secondary minerals are end members of
Conclusions
The environmental characterization of the mining area affected by the abandoned exploitation of a stibnite deposit reveals the presence of high Sb pollution levels. Total Sb concentrations in the range 585–3184 mg kg− 1 were found in the uppermost soil layer of the neighboring area of the mine dumps. Soil mobile Sb contents represent < 2% of the total concentrations, whereas the soil Sb contents more prone to be mobilized under different environmental conditions rise up to about 4–9%. In any case,
Acknowledgments
This study was carried out under the projects CGL2008-06357 and 200930I026 funded by the Ministry of Science and Innovation of Spain.
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