Mercury speciation in Pinus nigra barks from Monte Amiata (Italy): An X-ray absorption spectroscopy study☆
Graphical abstract
Introduction
Tree barks are, in principle, excellent adsorbents of airborne pollutants, including toxic metals (Panichev and McCrindle, 2004, Baltrénaité et al., 2014). Due to its porosity, and to the lack of metabolic processes, bark surface was generally considered chemically inert (e.g. Schulz et al., 1999), i.e. with little or no capability to react in the presence of inorganic and organic substances. In spite of these potential advantages, until recently the use of tree barks for environmental monitoring was not widespread. Possible drawbacks for the employment of barks include a) low levels of pollutant accumulation, making analysis comparatively difficult; b) superposition of direct uptake from the atmosphere and metabolic uptake from soil via the root-phloem-xylem system (Lodenius, 2013). However, Chiarantini et al. (2016) showed the use of Hg concentrations in tree bark as a reliable biomonitor for Hg in an area of past Hg mining. Alternatively, the use of lichens for environmental monitoring is more widespread (e.g., Szczepaniak and Biziuk, 2003, Grangeon et al., 2012). However, this technique also has some disadvantages, including patchy distribution, slow regeneration rates, and difficulty in differentiating between similar species (Pacheco et al., 2002). By contrast, tree barks offer advantages such as year-round and ubiquitous availability, simple species identification, and easy sampling. Specifically for Hg, barks are a potentially ideal trap to monitor airborne pollution, since this metal is not significantly taken up through plant roots due to its low bioavailability in soils (Boszke et al., 2008). Mercury is a global pollutant (Selin, 2009), characterized by long residence times in the atmosphere, and a corresponding ability to be transported over long distances. An effective monitoring of its distribution patterns at regional scale would benefit from simple, low-cost techniques such as tree bark sampling.
Our research group recently documented comparatively high (several mg kg−1) Hg concentrations in Pinus nigra barks from the Abbadia San Salvatore (Monte Amiata, Italy) Hg mine site (ASSM; Chiarantini et al., 2016). These high concentrations support the idea that the terrestrial vegetative biomass is, with soils, the primary reservoir of Hg derived from both natural and anthropogenic sources (Gamby et al., 2015). Leaves are considered to be a trap for Hg but, as suggested by Chiarantini et al. (2016), barks may also actively sequester this metal in significant amounts, contributing to its mass budget in forested areas. The mechanisms of Hg sequestration and the stability of its compounds in the bark tissues are key factors in controlling this budget, and for environmental biomonitoring purposes. To our knowledge, however, papers describing the speciation of Hg in barks are limited to the study conducted by Vázquez et al. (2002), who propose procyanidin as the most likely binder. Until recently, the reported Hg concentrations in barks (typically up to some tens of μg kg−1; Siwik et al., 2010) were too low to encourage specific investigation on Hg speciation in this type of matrix. The concentrations reported by Chiarantini et al. (2016) are high enough to allow the determination of Hg speciation using X-ray Absorption Spectroscopy (XAS), and this study was carried out on a subset of the samples examined in the Chiarantini et al. (2016) study.
The Monte Amiata area is highly anomalous in Hg because of the presence of the 3rd largest Hg mine district in the world (Rimondi et al., 2015). Over a century of mining and roasting caused Hg dispersion in the atmosphere and in the drainage network. Moreover, the area hosts a geothermal field, which is exploited for the production of energy; geothermal power plants are an additional source of Hg in the environment (Bravi and Basosi, 2014). In the study by Chiarantini et al. (2016), barks of Pinus nigra were used as biomonitors of airborne Hg pollution of the area. Nevertheless the fate and form of Hg in bark remain unknown, and a better knowledge of those is needed to successfully implement the biomonitoring approach.
The objective of this study is to improve our understanding of the uptake of Hg by barks. More specifically, we seek to determine the partitioning and fate of Hg in the surface and in deeper compartments of barks. To this end, we studied Pinus nigra barks from ASSM by X-ray absorption near-edge structure (XANES) spectroscopy. The study was conducted on a limited number of samples, and the results are therefore exploratory; nonetheless, we believe that they are useful for understanding the nature and dynamic of interaction between airborne Hg and tree barks, and to further use barks for Hg biomonitoring.
Section snippets
Materials and methods
Samples were recovered as five-centimeter thick cylinders using a hand-drill from trees close to the ASSM, to the geothermal power plants at Piancastagnaio, and from sites far from any known anthropogenic source, representing the local natural background (Fig. 1).
The choice of sites was aided by data on air Hg0 concentrations (ng m−3), as reported by Vaselli et al. (2013) and Cabassi et al. (2017), and summarized in Table 1.
Bark sampling was carried out at a height of about 1.5 m above the
Results and discussion
All samples spectra, including those with the lowest Hg concentrations, showed a recognizable Hg LIII absorption edge step. However, only for the five samples with Hg > 1 mg kg−1 the signal to noise ratio was sufficiently high to allow a meaningful LCF analysis. The samples spectra with the corresponding LCF curves and the reference spectra are displayed in Fig. 2, whereas the results of LCF analysis are presented in Table 2.
For all spectra, a meaningful fit can be obtained by considering only
Conclusions
This study investigates the speciation of Hg in Pinus nigra barks from the Monte Amiata region (Italy) by means of X-ray absorption spectroscopy (specifically, XANES). The barks are exceptionally rich in Hg because of the proximity to wastes of the former mines and production facilities. Linear combination fitting of experimental XANES spectra to those of selected relevant reference Hg compounds suggests that Hg in these barks is partly present as inorganic species (mainly metacinnabar, and
Acknowledgments
The research was funded by MIUR (PRIN 2010-2011 and 2010MKHT9B grant to PC), University of Firenze (“ex-60%” grants to PC and FDB), and Regione Sardegna (LR7/2007 grant to PL). XANES spectra collection was made possible by ESRF support (experiments EV-165 and ES-126).
We thank the BM30B (Fame) beamline staff for technical support, and Alejandro Fernandez-Martinez for having prepared the Hg-tannic acid reference. We acknowledge the constructive criticism of two anonymous referees.
ISTerre is part
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This paper has been recommended for acceptance by Prof. W. Wen-Xiong.