Nucleation of mercury sulfide by dealkylation OPEN

International audienceMetal 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, dithiocarbamate, thio-acetamide, and cystine, by hydrothermal, solvothermal, and biomimetic synthesis routes, sonochemical reaction, microwave irradiation, and hydrolysis 7–18. Recently it was shown that sulfide ions were not required to form a metal sulfide solid 19. Metacinnabar (β-HgS) precipitated directly from linear Hg-thiolate complexes (Hg(SR) 2) in natural organic matter (NOM) and from Hg-dicysteinate complexes (Hg(Cys) 2) in aerated and deaerated aqueous solutions in the dark without a catalyzing agent. These results are relevant to soil and aquatic systems, especially in cases where organo-sulfide is the dominant sulfide source. The reaction was rather slow and took several days for Hg(II) complexed to NOM at a concentration of 30–200 mg of Hg/kg of NOM dry weight (ppm). A global reaction pathway was proposed that has similarities to one suggested for β-HgS precipitation in sodium hydrosulfide (NaHS) solution 20,21. In its reaction with NaHS, Hg(II) initially forms an unstable low coordination chain-type complex (–S-Hg-S-Hg-S-) that rapidly transforms to a four-coordinate mercury sulfide with the short range ordering of β-HgS. The disordered β-HgS nanostructures eventually yield β-HgS crystals. In the case of thiolate as the source of reduced sulfur, the starting reactant is the linear Hg(SR) 2 complex (RS-Hg-SR), which is the most stable coordination of mononu-clear Hg with thiolate ligands at neutral and acidic pH 22,23. Because β-HgS nanostructures appear rapidly once –S-Hg-S-Hg-S-chains are formed in sulfidic solution 20 , we infer that formation of the chain structure limits the rate of formation of β-HgS from Hg(SR) 2. The pathway proposed 19 for chain formation in natural organic matter is the cleavage of the S-R bond according to the reaction: + → + RS Hg SR RS Hg SR RS Hg S Hg SR R S R, (1) followed by growth of the chain through the addition of new Hg(SR) 2 complexes