title: Kinetics of Hg(II) exchange between organic ligands, goethite, and natural organic matter studied with an enriched stable isotope approach.
abstract: The mobility and bioavailability of toxic Hg(II) in the environment strongly depends on its interactions with natural organic matter (NOM) and mineral surfaces. Using an enriched stable isotope approach, we investigated the exchange of Hg(II) between dissolved species (inorganically complexed or cysteine-, EDTA-, or NOM-bound) and solid-bound Hg(II) (carboxyl-/thiol-resin or goethite) over 30 days under constant conditions (pH, Hg and ligand concentrations). The Hg(II)-exchange was initially fast, followed by a slower phase, and depended on the properties of the dissolved ligands and sorbents. The results were described by a kinetic model allowing the simultaneous determination of adsorption and desorption rate coefficients. The time scales required to reach equilibrium with the carboxyl-resin varied greatly from 1.2 days for Hg(OH)2 to 16 days for Hg(II)-cysteine complexes and approximately 250 days for EDTA-bound Hg(II). Other experiments could not be described by an equilibrium model, suggesting that a significant fraction of total-bound Hg was present in a non-exchangeable form (thiol-resin and NOM: 53-58%; goethite: 22-29%). Based on the slow and incomplete exchange of Hg(II) described in this study, we suggest that kinetic effects must be considered to a greater extent in the assessment of the fate of Hg in the environment and the design of experimental studies, for example, for stability constant determination or metal isotope fractionation during sorption.
authors.affiliation: Soil Chemistry, Institute of Biogeochemistry and Pollutant Dynamics (IBP), ETH Zurich , CH-8092 Zurich, Switzerland.
authors.forename: Martin
authors.initials: M
authors.lastname: Jiskra
authors.forename: Damian
authors.initials: D
authors.lastname: Saile
authors.forename: Jan G
authors.initials: JG
authors.lastname: Wiederhold
authors.forename: Bernard
authors.initials: B
authors.lastname: Bourdon
authors.forename: Erik
authors.initials: E
authors.lastname: Björn
authors.forename: Ruben
authors.initials: R
authors.lastname: Kretzschmar
mesh_terms: D000327:Adsorption; D005591:Chemical Fractionation; D004777:Environment; D058085:Iron Compounds / Q000737:chemistry*; D007553:Isotope Labeling; D007700:Kinetics; D008024:Ligands; D008628:Mercury / Q000737:chemistry*; D008629:Mercury Isotopes / Q000737:chemistry; D008903:Minerals / Q000737:chemistry*; D008962:Models, Theoretical; D009930:Organic Chemicals / Q000737:chemistry*; D012117:Resins, Synthetic / Q000737:chemistry
chemical_list: D058085:Iron Compounds; D008024:Ligands; D008629:Mercury Isotopes; D008903:Minerals; D009930:Organic Chemicals; D012117:Resins, Synthetic; C094886:goethite; D008628:Mercury