March 2021 – Mercury Came from Outer Space – But Who Put it in My Site?

The 6^th century alchemists who named mercury were convinced that the metal transcended both the solid and liquid states, both earth and heaven, both life and death. It turns out they were right on every count. It is formed in dying low-mass stars and merging neutron stars. Like all the elements heavier than iron, it made its way to earth through interstellar travel. Through various reactions with sulfur and halides, it is stabilized into solid geologic formations, but also exists as Hg(II) in solutions, as Hg(O) vapor, and as methylmercury (MeHg) – a toxic contaminant that attaches to and modifies the functions of biological tissues. We know its ancient origins, and through environmental forensics, we can identify more earth-bound sources.

Environmental mercury (Hg) backgrounds and hotspots may contain mixtures of Hg from atmospheric, industrial, geological, and watershed-derived inputs. Industrial process equipment, oil pipelines and many environmental media may contain Hg mixtures from oil formations or other geologic sources.

The good news is that Environmental Standards Chemists have been able to differentiate Hg from sources that have distinctive ratios of the seven stable isotopes of Hg (¹⁹⁶Hg, ¹⁹⁸Hg, ¹⁹⁹Hg, ²⁰⁰Hg, ²⁰¹Hg, ²⁰²Hg, ²⁰⁴Hg); and these isotopic ratios can be determined with high precision and accuracy by isotope ratio mass spectrometry (IRMS). Hg IRMS is a highly specialized technique that is not widely available in commercial laboratories, but Environmental Standards has identified laboratory application where this analysis has been shown to be highly effective.

Ratio measurements in the range of the 198-202 amu isotopes have been demonstrated to be most reliable; therefore, the mass-dependent fractionation (MDF) ratios ¹⁹⁹Hg/¹⁹⁸Hg, ²⁰⁰Hg/¹⁹⁸Hg, ²⁰¹Hg/¹⁹⁸Hg, and ²⁰²Hg/¹⁹⁸Hg have been successfully used for source identification. Sources that Environmental Standards has identified include: intentionally produced Hg from cinnabar, inadvertently produced Hg from mining mine tailings, and combustion-related releases, which can be distinguished, tracked, and assigned loadings.

Mass-independent fractionation (MIF) of Hg can occur as a result of environmental processes such as photoreduction of aqueous Hg (II), photodegradation of MeHg, elemental Hg evaporation, and equilibrium Hg–thiol complexation. This type of fractionation is monitored using odd-numbered, normalized isotopic ratios. Environmental Standards uses these measures to provide information on whether Hg has migrated via surface, or subsurface transport mechanisms.

The not-so-good news is that methods for isolating the Hg for IRMS without affecting the ratios, for ensuring that data of sufficient quality are obtained, and for proper interpretation, are currently not codified in US EPA methods or any other consensus methods compendium, but live in the scientific literature, and in the hands-on expertise of experienced Chemists. Over the last decade Environmental Standards has performed, or provided oversight on, several large Hg isotope forensic studies of groundwaters, soils, and sediments in landfills and river systems, and has participated in successful method refinement for the determination of stable isotopes in oils, and oil sludges, all in support of forensic studies. Environmental Standards has forged relationships with key research laboratories in the field and have verified, proprietary workflows to prepare and analyze the samples for stable isotopes of Hg.

Environmental Standards  also has significant expertise in the proper sampling analysis of environmental Hg and MeHg in human tissue, fish and sediments, which can help provide multiple lines of evidence that drive home a good forensics case. If you are wondering what kind (or kinds) of mercury you have, contact us.