Competitive ligand exchange reveals time dependant changes in the reactivity of Hg–dissolved organic matter complexes

Abstract

Environmental context. Mercury, a globally important pollutant, undergoes transformations in the environment to form methylmercury that is toxic to humans. Naturally occurring dissolved organic matter is a controller in these transformations, and we demonstrate that its strength of interaction with mercury is time dependent. These changes in complexation with dissolved organic matter are likely to affect mercury's reactivity in aquatic systems, thereby influencing how mercury is methylated and bioaccumulated.Abstract. Mercury interactions with dissolved organic matter (DOM) are important in aquatic environments but the kinetics of Hg binding to and repartitioning within the DOM remain poorly understood. We examined changes in Hg-DOM complexes using glutathione (GSH) titrations, coupled with stannous-reducible Hg measurements during Hg equilibration with DOM. In laboratory prepared DOM solutions and in water from a Hg-contaminated creek, a fraction of the Hg present as Hg-DOM complexes did not react to GSH addition. This unreactive Hg fraction increased with time from 13 % at 1 h to 74 % after 48 h of equilibration with a Suwannee River DOM. In East Fork Poplar Creek water in Oak Ridge, Tennessee, ,58 % of the DOM-complexed Hg was unreactive with GSH 1 h after the sample was collected. This timedependent increase in unreactive Hg suggests that Hg forms stronger complexes with DOM over time. Alternatively the DOM-complexed Hg may become more sterically protected from the ligand exchange reactions, as the binding environment changes within the DOM over time. These results have important implications to understanding Hg transformations in the natural environment, particularly in contaminated aquatic systems due to non-equilibrium interactions between Hg and DOM.

Photochemical reactions between mercury (Hg) and dissolved organic matter decrease Hg bioavailability and methylation

“…After 6 h, only ~7% (under UV) and 36% (under solar) of the Hg was reducible by Sn(II). In the dark control experiment, HgR decreased by ~20%, as observed previously with HA at the same DOC concentration due to strong binding between Hg and certain fractions of DOM (Miller et al, 2013;Miller et al, 2009). Similar results were also obtained for Hg reactivity changes in PBS (Fig.…”

“…As stated earlier, this fraction of reducible Hg was operationally defined as the reactive Hg (HgR) (Lamborg et al, 2003;Miller et al, 2013;Miller et al, 2009). For mass balance analysis, the remaining aliquot (0.5 mL) was mixed with BrCl (5% v/v), allowed to react overnight at 4°C, and then determined for total non-purgeable Hg (Hg NP ) (Hu et al, 2013;Lin et al, 2015;Lin et al, 2014).…”

“…Decreases in Hg reactivity (defined as Sn(II)-reducible Hg) (Lamborg et al, 2003;Miller et al, 2013;Miller et al, 2009) and bioavailability (Barkay et al, 1997;Jonsson et al, 2014) by DOM in the dark have been attributed to Hg binding with reduced sulfur or thiol (-S) functional groups in DOM. However, formation of Hg-thiol complexes does not necessarily prevent Hg from being taken up by microorganisms for methylation (Chiasson-Gould et al, 2014;Graham et al, 2012b;Schaefer et al, 2011), and Hg-cysteine complexes are known to enhance microbial methylation (Graham et al, 2012b;Lin et al, 2015;Schaefer et al, 2011).…”

Mercury Isotope Signatures in Contaminated Sediments as a Tracer for Local Industrial Pollution Sources

“…The relatively higher temperatures and estimated biological productivity of the Mar site compared with the other elemental Hg sites (Table S1) Hg during the extraction can be potentially explained by different bonding characteristics of the two NOM materials and different aging times of the Hg-NOM materials before conducting the experiment of the Hg-spiked NOM material compared with the peat sample to which naturally abundant Hg had been added. It has been shown before that longer reaction times between Hg(II) and NOM may decrease the mobility and bioavailability, 39,55,56 presumably due to the formation of more stable bonding environments of Hg(II) in NOM over time. Obviously, not all NOM-bound Hg is extracted during the 6 M HNO 3 procedure and this probably depends on the properties of the NOM material as well as the reaction time of Hg(II) with the NOM material.…”

Bench-Scale Flushing Experiments for Remediation of Hg-Contaminated Groundwater

“…It is also possible that other complexing agents in the treated water, such as dissolved organic matter (DOM), are responsible for the enhanced Hg recovery. Hg forms strong complexes with organic matter due to its high binding affinity [24,25]. Also, common complexing ligands, such as chloride and sulfate, can affect the sorption of Hg on the mineral surfaces [26].…”