S9 (I) Understanding mercury - organic matter interactions

Thursday, 28 July, 2011

RS9-O1 — 8:30-8:45
Authors: AIKEN, George1, BERGAMASCHI, Brian1, SHANLEY, James B.1
(1) US Geological Survey, graiken@usgs.gov

A number of biogeochemical processes that influence the fate, bioavailability and transport of mercury (Hg) in aquatic systems are mediated by the interactions of Hg with dissolved organic matter (DOM). Laboratory experiments using a variety of organic matter isolates from surface waters in the Florida Everglades indicate that DOM binds Hg very strongly and is the dominant ligand for Hg in the absence of sulfide. These experiments have also shown that the presence of DOM influences the geochemical behavior of cinnabar (HgS) through the stabilization of nanocolloidal HgS, resulting in relatively high Hg concentrations under supersaturated conditions with respect to HgS, a common condition in waters containing measurable sulfide concentrations. In field studies of rivers and streams across a range of watershed types, we have almost universally observed strong linear correlations between DOM and total dissolved Hg concentrations. Further, we have observed that the relationships between dissolved total Hg concentration and hydrophobic organic acid (HPOA) content (aquatic humic substances) were stronger than those observed between Hg and DOM, suggesting that the HPOA fraction drives the transport of dissolved Hg in aquatic systems. The relationships between methylmercury (MeHg) and DOM and HPOA content were often significant, but not as strong as those observed with Hg. In our field studies, we have also noted that UV absorbance measured at 254 nm correlated strongly with DOM, HPOA content, MeHg, and Hg concentrations. UV absorbance is a proxy both for the quantity of DOM, but also its quality, correlating with aromatic content. These observations suggest that inherent optical properties of DOM, such as UV absorbance, may be useful as proxies for DOM and HPOA concentrations within a given system. By extension, because of the strong relationships between Hg and DOM, these properties can also be used to derive relationships between DOM optical properties and Hg concentrations. These results support the hypothesis that DOM in general and HPOA in particular exerts a strong control on the concentration, transport, and reactions of dissolved Hg in aquatic systems. Further, the results suggest that using the optical properties of organic matter as proxies for mercury species may facilitate our understanding of mercury dynamics in complex environments. Optical measurements are relatively inexpensive to obtain, can be designed into in situ monitoring devices and, when combined with discharge data, can be used to tighten estimates of both DOM and Hg flux in streams and rivers.

RS9-O2 — 8:45-9:00
Authors: NAGY, Kathryn L.1, MANCEAU, Alain2, GASPER, Jarrod D.3, RYAN, Joseph N.4, AIKEN, George R.5
(1) University of Illinois at Chicago, klnagy@uic.edu; (2) ISTerre, Université J. Fourier and CNRS; (3) Integral Consulting Inc.; (4) University of Colorado at Boulder; (5) United States Geological Survey.

Bioavailability of environmental mercury depends on how mercury(II) binds to natural organic matter. Mercury(II)-sulfur (Hg-S) bonds are the strongest, but difficult to characterize. We observed two new Hg-S molecular structures using X-ray absorption spectroscopy in peats from the Florida Everglades with added Hg. The first new structure, observed at a mol ratio of organic reduced S to Hg (Sred/Hg) of 200 to 1,100 (equivalent to 59 to about 350 ppm adsorbed Hg), is a Hg4Sx cluster in which each Hg atom is bonded on average to two S atoms at 2.34 Å and one S at 2.53 Å, and is separated from three other Hg atoms by 4.12 Å. The cluster structure matches those reported for Cu metallothioneins, but not those of HgS minerals. Linear S-Hg-S structures with bond lengths of 2.34 Å dominated at Sred/Hg of about 10 to 20 (about 4,200 ppm adsorbed Hg). The second new structure containing one S atom at 2.34 Å and approximately six C atoms at 3 to 3.3 Å appeared at Sred/Hg of about 4 (18,400 ppm adsorbed Hg). At Sred/Hg of about 1 (99,1000 ppm adsorbed Hg) a more abundant five-membered chelate ring containing oxygen or nitrogen and carbon was the dominant structure. The modeled Hg4Sx cluster suggests strong binding to protein-like cysteinyl sulfur derived from swamp plants, algae, microbes, and microfauna. Such a structure might be strong enough to prevent this bound Hg from being methylated. The Hg-S structure with many nearest-neighbor C atoms occurred mixed with the linear S-Hg-S and five-membered chelate ring structures. The high number of nearest neighbor C atoms suggests a thiolated aromatic subunit that may explain reported accelerated dissolution of the mercuric sulfide cinnabar in the presence of dissolved organic matter. These Hg-S structures support a continuum in bond strength and consequent variable reactivity of natural organic matter with Hg species in the environment.

RS9-O3 — 9:00-9:15
Authors: MYNENI, satish1, MISHRA, Bhoopesh2, SHOENFELT, Elizabeth1, FEIN, Jeremy3
(1) princeton university, smyneni@princeton.edu; (2) Argonne National Labs; (3) University of Notre Dame.

Bacteria are ubiquitous in a wide-range of low temperature aqueous systems, and strongly influence the distribution and transport of mercury in the environment. However, the role of mercury adsorption onto bacteria, via the reactive cell wall functional groups, and its influence on Hg speciation has been largely overlooked. We examined the reactions of mercury with several nonmetabolizing bacteria that are common to aquatic systems, as a function of pH, and in the presence of inorganic (Cl-), and organic (DOM) ligands. Our studies suggest that mercury sorption is significant on all bacterial species examined, and the sorption increased strongly with increases in pH. Presence of Cl- modified this sorption pattern, and the adsorption edge shifted to higher pH. The EXAFS spectroscopy studies at the Hg K-edge indicated that the coordination environment of adsorbed Hg on cell surfaces was significantly altered by the concentration of mercury in the system (or the Hg:microbial biomass ratio). When the mercury concentration was low, mercury interacted with the thiols on bacterial surfaces and formed Hg-(thiol)3 complexes, and the Hg:thiol ratio in the complex increased from 1:3-1:1 as the Hg:biomass ratio increased. At elevated Hg concentrations, mercury coordinates to carboxyl groups. This type of mercury speciation was found to be similar in all cases of bacteria examined, however, the mercury concentration at which this speciation was found was different between different species. Our current studies are focusing on the concentration and chemistry of bacterial cell wall bound thiols, and the role of DOM on Hg-speciation and interactions with bacterial cell walls. A summary of these results will be presented.

RS9-O4 — 9:15-9:30
Authors: HSU-KIM, Helen1, DEONARINE, Amrika1, GONDIKAS, Andreas1, ZHANG, Tong1, LAU, Boris2
(1)Duke University, hsukim@duke.edu; (2) Baylor University.

Natural organic matter (NOM) plays a critical role for complexation of dissolved Hg, precipitation and dissolution of Hg-bearing minerals, aggregation and deposition of particles, photochemical reactions, and bioavailability to bacteria. The identity and reactivity of geochemical species containing Hg, organic matter, and sulfide are of particular interest. These compounds are the most relevant forms of mercury in anaerobic environments where uptake to bacteria is a moderator of methylmercury production. Here, we present our research investigating the mechanisms of metal sulfide (HgS and ZnS) nanoparticle formation during heterogeneous precipitation with dissolved NOM. We utilized dynamic light scattering to monitor relative growth rates of HgS and ZnS nanoparticle as they precipitate in the presence of NOM. We tested nine different NOM isolates that were derived from several different surface waters and represented a wide range of NOM composition. Our results indicated that stabilization of nanoparticles occurred mainly with NOM fractions of the greatest molecular weight and aromatic carbon content. The early stages of metal-sulfide polymerization were further investigated by studying how cysteine, a low molecular weight analogue for thiol ligands in NOM, influenced the size and surface properties of ZnS clusters formed during the first few hours of precipitation reactions. We utilized a combination of methodologies (photon scattering, X-ray absorption, X-ray diffraction) to demonstrate that cysteine altered both the growth and aggregation rates of ZnS clusters. Cysteine coordinated to the surface of clusters, resulting in slower aggregation rates. Furthermore, in solutions with molar ratios of 2:1 cysteine:ZnS, we observed larger cluster subunits compared to solutions with 1:1 cysteine:ZnS. These observations indicated that in the presence of excess cysteine, sorption of this amino acid on ZnS clusters prevented aggregation of the clusters, but did block growth sites on the surface of monomer subunits. The mechanisms of ZnS-cysteine polymerization are analogous to precipitation processing involving nanoparticulate mercuric sulfides and mercury associated with other metal sulfides (e.g. ZnS, FeS). Nanoscale particles and clusters of HgS are expected to play key roles in the mobilization of mercury in aquatic environments and bioavailability to microorganisms. Overall, our results highlight that nanoscale products are formed from reactions between Hg, sulfide and NOM and that these entities would not be predicted by conventional chemical equilibrium models.

RS9-O5 — 9:30-9:45
Authors: GERBIG, Chase A.1, KIM, Christopher S.2, STEGEMEIER, John P.2, RYAN, Joseph N.1, AIKEN, George 3
(1) University of Colorado Boulder, gerbig@colorado.edu; (2) Chapman University; (3) US Geological Survey.

Understanding mercury (Hg) speciation in systems containing dissolved organic matter (DOM) and sulfide is necessary for predicting the fate and transport of mercury in the environment. We have developed a method to directly determine the speciation of mercury at concentrations more environmentally relevant than those in previous studies. The Hg:DOM ratio was low enough in most experiments so that only the strongest mercury binding sites in the DOM participate in mercury speciation. Mercury from aqueous solutions containing DOM with and without sulfide was concentrated on C18 resin and extended X-ray absorption fine structure (EXAFS) spectroscopy was used to examine the local mercury binding environment. In sulfidic systems we tested the role of varying Hg:DOM ratio, sulfide concentration (1-100 µM), equilibration times, and the DOM aromatic content by using a variety of DOM isolates obtained from different sources. Systems with the highest sulfide concentration and highest Hg:DOM ratio (> 5 µg mg-1; above the strong binding capacity of the DOM) showed a mercury binding environment comprised of 3.3 sulfur atoms at a distance of 2.53 Å, which is similar to metacinnabar (ß-HgS(s); four sulfur atoms at 2.53 Å) and distinctly different from mercury-sulfide and mercury-thiol aqueous complexes (Hg-S distance of 2.3-2.4 Å). As the Hg:DOM ratio was decreased at a constant sulfide concentration the number of sulfur atoms coordinating mercury decreased, but the Hg-S bond distance remained constant. At two different Hg:DOM ratios (one above and one below the strong binding site capacity of the DOM) the sulfur coordination number around mercury decreased with decreasing sulfide concentration, but the Hg-S bond distance still resembled metacinnabar. Systems with different DOM isolates showed different sulfur coordination numbers but the Hg-S bond distance was consistently indicative of a metacinnabar-like particle. Kinetics studies indicate that the metacinnabar-like particles forms rapidly, and is insensitive to Hg-DOM equilibration time. These results suggest that a nanocolloidal metacinnabar-like species is stabilized by interactions with DOM, even under conditions where both mercury and sulfide concentrations are relatively low. The metacinnabar-like particle becomes either smaller or less ordered with decreasing Hg:DOM ratio and decreasing sulfide concentration.

RS9-O6 — 9:45-10:00
Author: GU, Baohua1
(1)Oak Ridge National Laboratory, gub1@ornl.gov

Oxidized mercuric ion, Hg2+, has been generally considered as the species that form complexes with naturally dissolved organic matter (DOM) such as humic acid (HA), a key component that affects mercury chemical and biological transformation and cycling in aquatic environments. Dissolved elemental mercury, Hg(0), is also widely observed in sediments and water. However, reactions between Hg(0) and DOM have rarely been studied in anoxic environments. In this study, DOM isolates of various origins were pre-reduced either chemically or biologically to simulate natural anoxic environments where both microbial reduction of DOM and methylation occur, and subsequently studied for their reactions with either Hg2+ or Hg(0). We report that DOM, particularly the reduced humic acid (HA), can strongly interact with dissolved Hg(0) through thiolate-ligand induced oxidative complexation with an estimated binding capacity about 3.5 mmol Hg/g and a partitioning coefficient is greater than 106 mL/g. The binding capacity varied with different DOM isolates and diminished when DOM became oxidized. The reduced DOM isolates are also capable of rapidly reducing Hg(II) species to gaseous Hg(0) under anaerobic conditions. The purgeable Hg(0) was found to increase with increasing DOM initially, reach an peak value, then decrease at higher levels of DOM due to the formation of Hg-DOM complexes. This phenomenon is explained by the dual functional role played by DOM in the reduction and complexation of Hg since the reduced DOM usually contains a higher reducing than binding (thiol ligands) equivalent. These observations are likely widespread in anoxic sediments and water and may be expected to significantly influence the mercury species transformations and biological uptake that leads to the formation of toxic methylmercury.

RS9-O7 — 10:00-10:15
Authors: ZHONG, Huan1, WANG, Wen-Xiong2
(1) Trent University, zhonghuan1982@hotmail.com; (2) Hong Kong University of Science and Technology.

This study aimed at exploring the controls of different sorption among mercury (including inorganic mercury and methylated mercury), organic matters (OM, principally one kind of fulvic acid or FA), and inorganic particles, as well as bacteria within sediments on the partitioning and bioavailability (quantified by gut juice extraction) of mercury in sediments when the contact time among mercury, OM, and inorganic particles increased. Radioactive inorganic mercury (Hg) and methylated mercury (MeHg) isotopes were used to spike into artificial sediments with different sorption (FA-Particle, Mercury-FA and Mercury-Particle), and the strength of the sorption was changed by changing the contact time among mercury, FA and inorganic particles. By determining the partitioning and gut juice extraction of Hg or MeHg in those different artificial sediments, we found that all the three sorptions were important in controlling the partitioning of Hg or MeHg in the sediments: all sorptions became stronger and contributed to the increase of mercury partitioning in the sediments when the contact time was increased. Extraction of Hg or MeHg by gut juices was mainly controlled by the Mercury-Particle sorption while FA-Particle and Mercury-FA sorption were not as important in decreasing the extraction efficiencies with the increased contact time, especially for MeHg. Bacteria could also increase the partitioning of Hg and MeHg in the sediments, especially for MeHg. The growth of bacteria could be favored by the presence of organic matters, especially in the OM-Particle+Mercury sediments. However, the complexation between Hg or MeHg and organic matters (humic and fulvic acids) may inhibit the sorption of Hg or MeHg by bacteria and thus the partitioning of Hg or MeHg in sediments, while in contrast the Mercury-Cysteine complexes could be more available for bacteria sorption. Bioavailability of Hg or MeHg quantified by gut juice extraction was slightly influenced by the activities of bacteria. This study highlighted the importance of different sorption as well as bacteria in controlling the partitioning and bioavailability of Hg or MeHg in sediments.

RS9-O8 — 10:15-10:30
Authors: JEREMIASON, Jeff D.1, SEBESTYEN, Stephen D2, TSUI, Martin T.K.3, FINLAY, Jacques C.4, NATER, Edward A.5, COTNER, James B.4, JACOBSON, Meghan4, CARLSON, Benjamin R.1
(1)Gustavus Adolphus College, jjeremia@gustavus.edu; (2) USDA Forest Service; (3) University of Michigan; (4) University of Minnesota; (5) Univerisity of Minnesota;

Relationships between dissolved organic matter (DOM), trace metals including mercury and major cations were examined in the S2 peatland of the Marcell Experimental Forest in northern Minnesota, USA. DOM is known to bind trace metals and is a key component controlling trace metal transport from wetlands. However, the complex and largely unknown character of DOM prevents the development of reliable models to predict delivery of toxic metals such as arsenic, lead, and mercury from wetlands. As several studies have reported increased DOM transport from multiple wetlands in recent decades, a better understanding of complex DOM-metal relationships is critical. In this study, we utilize the heavily instrumented and studied S2 peatland and an array of sampling locations within S2 to further understand the complex relationship between DOM and trace metals. Samples were collected in 2010 from the outflow weir, lagg and bog porewaters, and in subsurface flow from the uplands. More frequent samples were collected from the S2 weir and one lagg location, while ~monthly samples from June to September were collected from multiple upland-to-lagg-to-bog transects. DOM was characterized by measuring total and dissolved organic carbon and by UV and fluorescence spectroscopy. Distinct spatial contrasts were found between Hg and many of the other metals, helping us to better understand Hg dynamics in the S2 system. For example, Hg and several metals were found in significant quantities in the subsurface runoff. Metals that bind to soil organic carbon, such as Pb and As, and redox sensitive metals such as Mn and Fe, were low in the subsurface runoff, relative to the weir and lagg porewaters. Hg, which also has high affinity for soil carbon, had higher or similar concentrations in the subsurface runoff as the weir and lagg porewaters, demonstrating different binding affinities for soil and/or dissolved organic carbon. In general, Hg and other metals that were found at higher concentration in the subsurface flow (relative to the weir and lagg), were highest in concentration in the upland and lagg porewaters, and then decreased moving further towards the bog. Pb and As, on the other hand, both generally increased moving from upland to bog porewater. This study highlights the complex nature of metal/DOM interactions and insights that can be gained about the environmental system and Hg dynamics by also examining other metals.

RS9-O15 — 16:30-16:45
Authors: MOINGT, Matthieu1, LUCOTTE, Marc1, PAQUET, Serge1, GHALEB, Bassam1, BEAULNE, Jean-Sébastien2
(1) GEOTOP-UQAM, matthieumoingt@yahoo.fr; (2) .

Mercury dynamics in boreal lakes is of special concern as human populations may be exposed to the contaminant through fish consumption. To evaluate the impact of watershed perturbations on the mercury dynamics in boreal aquatic systems, we studied sediment cores retrieved in six large boreal lakes of Québec (Canada) with watersheds disturbed by anthropogenic activities such as logging or mining, and in two undisturbed lakes chosen as reference lakes. Sediment rates estimated by 210Pb dating ranged from 0.03 to 0.33 cm/yr. Total mercury profiles (THg) and lignin biomarkers (indicators of both quantity and quality of terrigenous organic matter) were obtained in all eight cores whereas watershed modifications over the last three decades were determined by analysis of remotely sensed images using GIS. In each core, THg concentrations gradually increased over recent years with maximum values comprised between 70 and 370 ng/g, the lowest mercury concentrations corresponding to the reference lake cores. Anthropogenic Sedimentary Enrichment Factor (ASEF) is comprised between 2 and 15 and surprisingly, not all the lakes with disturbed watersheds present an ASEF superior to the value of 3.5 usually reported in the literature. Terrestrial mercury fluxes, thus appear buffered by watershed characteristics such as average slopes near the lake shores., In one of the eight lakes, the sedimentation rate was high enough to provide a satisfactory resolution of the history of Hg and terrigenous organic matter inputs to the lake since the onset of the industrial era. In this lake, variations of total amount of lignin were correlated with variations of THg in sediments suggesting that newly deposited atmospheric mercury is easily transported by terrestrial organic matter to aquatic sytems.

Thursday, 28 July, 2011