Heavy metal distribution in mine-soils and plants growing in a Pb/Zn-mining area in NW Spain
Introduction
There is worldwide concern regarding the high inputs and flow of heavy metals in the biosphere, elements which on a global perspective should normally be present in very low concentrations. Although some trace elements are essential for organisms, when present in concentrations exceeding those normally found in nature they can lead to serious problems by disrupting metabolic cycles. Elevated concentrations of heavy metals can be found in soils as a result of natural geochemical processes, but human activities can significantly accelerate these processes. Mining activities and mineral processing generate large volumes of metal-rich waste materials and are considered the principal cause of soil contamination. After closure of the mine and the processing facilities the mine-spoils and ore processing wastes persist, representing a source of potentially toxic elements in the environment (Perez and Calvo, 1992, Monterroso et al., 1999, Clark et al., 2001, Khorasanipour et al., 2011, Miler and Gosar, 2012).
Heavy metal mobility and bioavailability is determined by both the total metal concentration and its geochemical forms. These factors are therefore crucial when evaluating the short- and long-term environmental impacts associated with mine wastes (McBride, 1994, Pueyo et al., 2008) and tend to vary both vertically and superficially across mine tailings as a result of the heterogeneity caused by the movement of substrates during mineral extraction and dumping. Simple soil extractions and chemical fractionation analyses have been increasingly applied to mine waste samples. The most important metal pools in mine wastes include exchangeable, metal sulphides, metals associated with manganese and iron oxi-hydroxides and metals incorporated within mineral structures. Mobility and bioavailability are governed by dynamic equilibria between these fractions, rather than by the total metal content.
Areas affected by mining activities typically support a sparse natural vegetation cover, principally as a result of an elevated concentration and bioavailability of heavy metals combined with adverse growth conditions (such as low pH values, low water retention, high compaction, low organic matter and nutrients). Despite these unfavourable conditions, plant metallophytes have evolved biological mechanisms permitting them to resist and tolerate toxic concentrations of metals, and colonise this type of substrate (Whiting et al., 2004, Batty, 2005). Plants have developed different strategies by which they either (hyper)accumulate metals in non-toxic forms within their tissues, or restrict the entry of metals into the root and/or transport to the shoots (metal-excluders) (Baker, 1981). Metal-tolerant populations of common plant species (pseudometallophytes) are able to resist higher concentrations of metals compared with members of the same species growing on uncontaminated soils. On the other hand, true metallophytes have evolved over time on substrates derived from weathered mineral deposits (Whiting et al., 2004). For example, hyperaccumulators can accumulate extreme amounts of trace metals in their aboveground biomass when growing in metal-enriched habitats (van der Ent et al., 2012). Many studies have focused on these plants due to their potential use in the rehabilitation of metal-contaminated land or, more recently, due to their possible application in phytoremediation (Barrutia et al., 2011, Becerra-Castro et al., 2012, Moreno-Jimenez et al., 2009). Phytoextraction aims to remove trace metals from the soil through their uptake and accumulation by plants; whereas phytostabilisation aims to establish a vegetation cover and promote in situ inactivation of trace metals. These gentle remediation techniques can potentially restore soil quality and functions, leading to the recovery of vital biogeochemical cycles.
Phytoremediation techniques offer cost-effective in situ alternatives to conventional technologies for remediation of low- to medium-contaminated substrates. However, it is generally accepted that these techniques are highly site-specific (Mench et al., 2010). Native or indigenous plant species are more likely to cope with the adverse growth conditions of each zone and are therefore recommended when implementing any phytoremediation strategy (Batty, 2005, Chehregani et al., 2009, Frérot et al., 2006). Geobotanical surveys of mine-soils and plant screenings in different climatic regions are therefore useful tools for the identification of candidate metal-tolerant plant species. The aim of this study was to evaluate metal-tolerant plants colonizing an abandoned Pb/Zn mining area for their potential application in phytoremediation strategies in the humid temperate region. For this we studied the bioavailability and chemical fractionation of heavy metals in the mine-soils, and their accumulation or exclusion by native plant species.
Section snippets
Description of study site and sampling procedure
This study was carried out in the abandoned Pb/Zn mine of Rubiais in the Lugo province of NW Spain (UTM 29T 660781/4726800). The altitude is 1070 m and the climate of the region is Oceanic, with a mean annual precipitation of 2000 mm and annual temperature of 8–9 °C (Carballeira et al., 1983). Alumi-umbric Leptosols and Regosols (WRB, 2006) are the most frequent natural soils in the area.
The Rubiais ore deposit is situated within the Westasturian-Leonese zone of the Variscan Chain (Julivert et
Soil physicochemical properties
Table 1 presents the mean values (and range) for the main properties of the mine-soils. Soil pH ranged from slightly acid to alkaline (pHH2O = 5.8–8.8, pHKCl = 4.6–8.9), although the majority of samples presented pH values close to neutrality (mean values of 7.3 ± 0.8 and 7.0 ± 1.1 for pHH2O and pHKCl, respectively). In general, the effective cation exchange capacity (CECe) was low, varying from 1.5 to 27.9 cmolc kg−1 (9.8 ± 7.2 cmolc kg−1). The Ca2+ cation was dominant (ranging from 0.3 to 25.8 cmolc kg−1),
Discussion
The mine-soils presented a large degree of heterogeneity and generally showed unfavourable conditions for plant growth: very low nutrient availability, low cation exchange capacity, limited organic matter content and elevated concentrations of some heavy metals. According to soil standards proposed by Buol et al. (1975), practically the entire area presented an absolute deficiency in K and N. In addition, at more than 70% of the sampling points K was unbalanced compared to the rest of the base
Conclusions
The mine-soils generated after mineral extraction from the Rubiais orebody presented high concentrations of Cd, Pb and Zn. Most of these metals were found in the non-residual geochemical fractions (principally Fe/Mn oxy(hydr)oxides, sulphides and carbonates). Environmental conditions inducing important changes in redox potential (oxidation or reduction) or acidification will therefore significantly influence the liberation of these metals into their surroundings.
Despite the unfavourable
Acknowledgements
This research was supported by the Ministerio de Ciencia e Innovación (CTM2009-14576-C02-01/02) and FP7 EC project “Gentle remediation of trace element contaminated land” (Greenland 266124; Theme 2:Food, Agriculture and Fisheries, and Biotechnology).
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