Nucleation of mercury sulfide by dealkylation OPEN

International audience ; Metal sulfide minerals are assumed to form naturally at ambient conditions via reaction of a metallic element with (poly)sulfide ions, usually produced by microbes in oxygen-depleted environments. Recently, the formation of mercury sulfide (β-HgS) directly from linear Hg(II)-thiolate complexes (Hg(SR) 2) in natural organic matter and in cysteine solutions was demonstrated under aerated conditions. Here, a detailed description of this non-sulfidic reaction is provided by computations at a high level of molecular-orbital theory. The HgS stoichiometry is obtained through the cleavage of the S-C bond in one thiolate, transfer of the resulting alkyl group (R') to another thiolate, and subsequent elimination of a sulfur atom from the second thiolate as a thioether (RSR'). Repetition of this mechanism leads to the formation of RS-(HgS) n-R chains which may self-assemble in parallel arrays to form cinnabar (α-HgS), or more commonly, quickly condense to four-coordinate metacinnabar (β-HgS). The mechanistic pathway is thermodynamically favorable and its predicted kinetics agrees with experiment. The results provide robust theoretical support for the abiotic natural formation of nanoparticulate HgS under oxic conditions and in the absence of a catalyst, and suggest a new route for the (bio)synthesis of HgS nanoparticles with improved technological properties. Nucleation of metal sulfide solids typically occurs when solubility is exceeded by elevated concentration of reduced sulfur, metal cation, or both components 1,2. In environmental aquatic systems, metal ions are commonly complexed with natural organic matter or inorganic anions, including sulfide, and free sulfide ions (S(-II)) produced mainly by dissimilatory sulfate reducing microbes 3,4 are considered necessary for solid nucleation. Sulfide can also be generated in the laboratory from intracellular cysteine by photosynthetic aerobic microorganisms 5,6 and from decomposition of sulfur compounds, such as thioglycolic acid, thioglycerol, ...