Pinus nigra bark from a mercury mining district studied with high resolution XANES spectroscopy

• Published in: Environmental science--processes & impacts 2022-10, Vol.24 (10), p.1748-1757
• Main Authors: Bardelli, Fabrizio, Rimondi, Valentina, Lattanzi, Pierfranco, Rovezzi, Mauro, Isaure, Marie-Pierre, Giaccherini, Andrea, Costagliola, Pilario
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Bark; Chemical Sciences; Food processing; High resolution; Mercury; Mercury (metal); Nanoparticles; Particulates; Pine trees; Pinus nigra; Principal components analysis; Soil mechanics; Speciation; Spectroscopy; Spectrum analysis; X ray absorption
• ISSN: 2050-7887; 2050-7895
• DOI: 10.1039/d2em00239f

Tree bark near former mercury (Hg) mines and roasting plants is known to have exceptionally high (up to several mg kg −1 ) Hg concentrations. This study explores the change of Hg speciation with depth (down to 25–30 mm from the outermost surface) in black pine ( Pinus nigra ) bark by means of high-resolution X-ray absorption near edge structure (HR-XANES) spectroscopy at the Hg L III -edge. Principal component analysis and linear combination fitting applied to the HR-XANES spectra suggested that in the outermost layer (∼0–2 mm from the surface), roughly 50% of Hg is in the form of nanoparticulate metacinnabar (nano-β-HgS). A progressive increase in Hg-organic species (Hg bound to thiol groups) is found in deeper bark layers, while nano-β-HgS may decrease below the detection limit in the deepest layers. Notably, bark layers did not contain cinnabar (α-HgS), which was found in the nearby soils along with β-HgS (bulk), nor Hg 0 , which is the main Hg species in the atmosphere surrounding the sampled trees. These observations suggested that nano-β-HgS, at least in part, does not originate from mechanically trapped wind-blown particulates from the surrounding soil, but may be the product of biochemical reactions between gaseous elemental Hg and the bark tissue.