Article pubs.acs.org/est
Mercury Isotope Fractionation during Precipitation of Metacinnabar (β-HgS) and Montroydite (HgO) Robin S. Smith,†,‡ Jan G. Wiederhold,*,†,‡ and Ruben Kretzschmar† †
Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CH-8092 Zurich, Switzerland Isotope Geochemistry Group, Institute of Geochemistry and Petrology, ETH Zurich, CH-8092 Zurich, Switzerland
‡
S Supporting Information *
ABSTRACT: To utilize stable Hg isotopes as a tracer for Hg cycling and pollution sources in the environment, it is imperative that fractionation factors for important biogeochemical processes involving Hg are determined. Here, we report experimental results on Hg isotope fractionation during precipitation of metacinnabar (β-HgS) and montroydite (HgO). In both systems, we observed mass-dependent enrichments of light Hg isotopes in the precipitates relative to the dissolved Hg. Precipitation of β-HgS appeared to follow equilibrium isotope fractionation with an enrichment factor ε202Hgprecipitate−supernatant of −0.63‰. Precipitation of HgO resulted in kinetic isotope fractionation, which was described by a Rayleigh model with an enrichment factor of −0.32‰. Small mass-independent fractionation was observed in the HgS system, presumably related to nuclear volume fractionation. We propose that Hg isotope fractionation in the HgS system occurred in solution during the transition of O- to Scoordination of Hg(II), consistent with theoretical predictions. In the HgO system, fractionation was presumably caused by the faster precipitation of light Hg isotopes, and no isotopic exchange between solid and solution was observed on the timescale investigated. The results of this work emphasize the importance of Hg solution speciation and suggest that bonding partners of Hg in solution complexes may control the overall isotope fractionation. The determined fractionation factor and mechanistic insights will have implications for the interpretation of Hg isotope signatures and their use as an environmental tracer.
■
from those of the corresponding bulk minerals.17 Previous experimental studies have determined isotope fractionation factors during several environmental Hg transformation processes, such as both biotic18,19 and abiotic20 methylation, redox transformations,21,22 sorption to goethite,23 binding of Hg(II) to thiol groups,24 fractionation during phase transfer,25,26 and Hg(0) diffusion through air.27 However, much remains unknown about fractionation during mineral precipitation. An experiment described in the framework of a previous study focusing on the investigation of a contaminated creek system28 reported that the heavy Hg isotopes were enriched in the dissolved phase after HgS formation. However, a fractionation factor was not derived, and there were no investigations conducted on the mechanisms for isotope fractionation or the mineral phase(s) formed. Stable metal isotope fractionation during sulfide formation has previously been observed for Fe29−31 and Cu.32 In the FeS system, isotope fractionation was reported to occur during inner-sphere ligand exchange in solution, before the formation of a FeS precipitate phase.29 In the precipitation of CuS (covellite), large Cu isotope fractionation occurred due to a
INTRODUCTION Stable Hg isotope signatures are increasingly used as a biogeochemical process and pollution source tracer.1−3 Hg isotopes fractionate by mass-dependent fractionation (MDF) and mass-independent fractionation (MIF), primarily observed on the two odd-mass isotopes.4 MDF and MIF signatures generate a two-dimensional tool that can be used to understand the Hg behavior and cycling in the environment. However, to exploit the potential of this tracing tool,5 it is necessary to understand Hg fractionation factors between reactants and products for specific reactions of Hg species in the environment. The mobility and bioavailability of Hg largely depends on Hg speciation and the high affinity of Hg(II) for both organic and inorganic reduced sulfur compounds.6−10 HgS phases play an important role in environmental Hg cycling, and they can act as both source and sink of Hg for natural ecosystems. On one hand, HgS minerals represent the most abundant form of Hg in the lithosphere. The mining of HgS minerals stands at the beginning of much of the anthropogenic Hg cycling. On the other hand, due to the extremely low solubility of HgS phases,11 the precipitation of Hg-sulfides can effectively remove Hg from further environmental cycling. HgS nanoparticle precipitation occurs in natural systems, for instance in the presence of DOM12−15 and with burial of Hg contaminated soils,16 and these nanoparticles can exhibit properties different © XXXX American Chemical Society
Received: January 26, 2015 Revised: March 16, 2015 Accepted: March 17, 2015
A
DOI: 10.1021/acs.est.5b00409 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology change in oxidation state from Cu(II) to Cu(I)32 during the formation of aqueous clusters in solution immediately prior to mineral precipitation.33 Furthermore, the authors also precipitated Cu(OH)2, i.e., in the absence of a redox reaction, and found a smaller isotope fractionation.32 Precipitates from these experiments were all enriched in the corresponding light isotopes. In this study, we precipitated Hg from aqueous solutions as either metacinnabar (β-HgS) or montroydite (HgO) such that only partial precipitation occurred, thus allowing for the study of the fractionation between Hg remaining in solution and the precipitated phases. In the sulfide system, the influence of aging after 6 days was also investigated. Metacinnabar is one of the primary Hg minerals found in the environment, as it is both a relatively common Hg ore mineral as well as the phase that is formed in anoxic soils and sediments, when Hg sulfide precipitation occurs.12,13,16 Montroydite, although not an important ore-forming mineral, has been found in processed Hg ore waste (calcine).34,35 It was studied as an example for mineral formation in the absence of a Hg coordination change from oxygen to another element. Thus, the objectives of this study were (i) to determine the Hg isotope fractionation factors for precipitation of β-HgS and HgO, and (ii) to discuss the results in terms of the mechanisms driving kinetic and/or equilibrium Hg isotope fractionation during mineral precipitation.
2− − Hg(O2 CCH3)2(aq) + S(aq) → HgS(s) + 2(O2 CCH3)(aq)
(1)
Initially, a series targeting 90% precipitation was also prepared, but it had to be discarded later for experimental reasons (precipitates did not spin down). The vials of the Aseries were immediately (
