Kinetics
of Hg(II) Exchange between Organic Ligands,
Goethite, and Natural Organic Matter Studied with an Enriched Stable
Isotope Approach
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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)<sub>2</sub> 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