Imaging and characterization of H g S nanoclusters in natural organic matrix: challenges and results

Abstract

Methylmecury is the environmental form of neurotoxic mercury that is biomagnified in the food chain. Methylation rates are reduced when the metal is sequestered in crystalline mercury sulfides or bound to thiol groups in macromolecular natural organic matter. Mercury sulfide minerals are known to nucleate in anoxic zones, by reaction of the thiol‐bound mercury with biogenic sulfide, but not in oxic environments. However, recent work, using indirect characterization methods such as EXAFS spectroscopy, provided evidence that mercury sulfide forms from thiol‐bound mercury alone in aqueous dark systems in contact with air. To confirm such new result, proof of mercury sulfide precipitation using direct characterization methods was needed. Such direct imaging, using TEM techniques, was however challenging due to the scarce occurrence and tininess of mercury sulfide particles in heterogeneous an beam‐sensitive natural soil samples. First results of HRTEM imaging of HgS nanoclusters of particles in Hg‐contaminated humic acid from Elliott reference soil, combined with diffraction technique to assess the of HgS polymorph, will be presented. Specific challenges due to the nature of the samples will be discussed.