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

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 LIII-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-beta-HgS). A progressive increase in Hg-organic species (Hg bound to thiol groups) is found in deeper bark layers, while nano-beta-HgS may decrease below the detection limit in the deepest layers. Notably, bark layers did not contain cinnabar (alfa-HgS), which was found in the nearby soils along with beta-HgS (bulk), nor Hg0, which is the main Hg species in the atmosphere surrounding the sampled trees. These observations suggested that nano-beta-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.