Colloidal Hg Research Articles

Mobilization, Methylation, and Demethylation of Mercury in a Paddy Soil Under Systematic Redox Changes.

Methylmercury (MeHg) contamination in paddy fields is a significant environmental issue globally since over half of the population of our planet consumes rice. MeHg is a neurotoxin produced by microorganisms in oxygen-limited environments. Microbial effect on MeHg production is a hotspot of research; however, it has been largely ignored how the oxidation-reduction potential (Eh) shapes MeHg formation. Here, we elucidated Hg (de)-methylation in a contaminated soil by increasing Eh stepwise from -300 to +300 mV using a sophisticated biogeochemical microcosm. At the Eh range from -300 to -100 mV, high MeHg concentration and dissolved total Hg (THg) concentration were found due to a high relative abundance of Hg-methylation bacteria (e.g., Desulfitobacterium spp.), acidification, and reductive dissolution of Fe(oxyhydr)oxides. At the Eh range from 0 to +200 mV, the formation of colloids leads to adsorption of Hg and as a result colloidal Hg increased. MeHg reduction with Eh (-300 to +200 mV) increase was mainly attributed to a reduced Hg methylation, as dissolved THg and relative abundance of Desulfitobacterium spp. decreased by 50 and 96%, respectively, at Eh of +200 mV as compared to Eh of -300 mV. Mercury demethylation might be less important since the relative abundance of demethylation bacteria (Clostridium spp.) also decreased over 93% at Eh of +200 mV. These new results are crucial for predicting Hg risks in paddy fields.

Mercury mobility, colloid formation and methylation in a polluted Fluvisol as affected by manure application and flooding–draining cycle

Abstract. Floodplain soils polluted with high levels of mercury (Hg) are potential point sources to downstream ecosystems. Repeated flooding (e.g., redox cycling) and agricultural activities (e.g., organic matter addition) may influence the fate and speciation of Hg in these soil systems. The formation and aggregation of colloids and particles influence both Hg mobility and its bioavailability to microbes that form methylmercury (MeHg). In this study, we conducted a microcosm flooding–draining experiment on Hg-polluted floodplain soils originating from an agriculturally used area situated in the Rhone Valley (Valais, Switzerland). The experiment comprised two 14 d flooding periods separated by one 14 d draining period. The effect of freshly added natural organic matter on Hg dynamics was assessed by adding liquid cow manure (+MNR) to two soils characterized by different Hg (47.3±0.5 or 2.38±0.01 mg kg−1) and organic carbon (OC: 1.92 wt % or 3.45 wt %) contents. During the experiment, the release, colloid formation of Hg in soil solution and net MeHg production in the soil were monitored. Upon manure addition in the highly polluted soil (lower OC), an accelerated release of Hg to the soil solution could be linked to a fast reductive dissolution of Mn oxides. The manure treatments showed a fast sequestration of Hg and a higher percentage of Hg bound by particulate (0.02–10 µm). Also, analyses of soil solutions by asymmetrical flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry (AF4–ICP–MS) revealed a relative increase in colloidal Hg bound to dissolved organic matter (Hg–DOM) and inorganic colloidal Hg (70 %–100 %) upon manure addition. Our experiment shows a net MeHg production the first flooding and draining period and a subsequent decrease in absolute MeHg concentrations after the second flooding period. Manure addition did not change net MeHg production significantly in the incubated soils. The results of this study suggest that manure addition may promote Hg sequestration by Hg complexation on large organic matter components and the formation and aggregation of inorganic HgS(s) colloids in Hg-polluted Fluvisols with low levels of natural organic matter.

Evaluating the transport of Hg(II) in the presence of natural organic matter through a diffusive gradient in a thin-film passive sampler

The effects of a model natural organic matter (NOM) on the transport of Hg(II) into diffusive gradient in thin-film devices (DGTs) was evaluated in order to better understand their ability to measure colloidal Hg species in porewater. The presence of NOM significantly reduced the diffusivity of the Hg(II) species and the reduction was dependent upon NOM to Hg(II) ratio. This relationship was modeled by determining the Hg(II) partition coefficients (Kd) of size fractionated NOM obtained by ultrafiltration and estimating the Hg diffusivity through the DGT for the different NOM size fractions across a range of Hg-NOM ratios. The estimated diffusivities were consistent with experimental observations of uptake into the DGT. Overall, this study indicated that Hg(II) associated with NOM passes into a DGT, however the transport is slowed in accordance with the diffusivity of the NOM to which the Hg(II) is associated. Thus, the Hg—NOM association and complex diffusivities need to be considered when relating DGT uptake to Hg porewater concentration. The results also suggest that Hg(II) associated with colloidal or larger particles of negligible diffusivity are unlikely to contribute significantly to DGT measurements.

Source tracing of natural organic matter bound mercury in boreal forest runoff with mercury stable isotopes.

Terrestrial runoff represents a major source of mercury (Hg) to aquatic ecosystems. In boreal forest catchments, such as the one in northern Sweden studied here, mercury bound to natural organic matter (NOM) represents a large fraction of mercury in the runoff. We present a method to measure Hg stable isotope signatures of colloidal Hg, mainly complexed by high molecular weight or colloidal natural organic matter (NOM) in natural waters based on pre-enrichment by ultrafiltration, followed by freeze-drying and combustion. We report that Hg associated with high molecular weight NOM in the boreal forest runoff has very similar Hg isotope signatures as compared to the organic soil horizons of the catchment area. The mass-independent fractionation (MIF) signatures (Δ199Hg and Δ200Hg) measured in soils and runoff were in agreement with typical values reported for atmospheric gaseous elemental mercury (Hg0) and distinctly different from reported Hg isotope signatures in precipitation. We therefore suggest that most Hg in the boreal terrestrial ecosystem originated from the deposition of Hg0 through foliar uptake rather than precipitation. Using a mixing model we calculated the contribution of soil horizons to the Hg in the runoff. At moderate to high flow runoff conditions, that prevailed during sampling, the uppermost part of the organic horizon (Oe/He) contributed 50-70% of the Hg in the runoff, while the underlying more humified organic Oa/Ha and the mineral soil horizons displayed a lower mobility of Hg. The good agreement of the Hg isotope results with other source tracing approaches using radiocarbon signatures and Hg : C ratios provides additional support for the strong coupling between Hg and NOM. The exploratory results from this study illustrate the potential of Hg stable isotopes to trace the source of Hg from atmospheric deposition through the terrestrial ecosystem to soil runoff, and provide a basis for more in-depth studies investigating the mobility of Hg in terrestrial ecosystems using Hg isotope signatures.

Ionic strength reduction and flow interruption enhanced colloid-facilitated Hg transport in contaminated soils

The effects of ionic strength (IS) reduction (5–0.05mM) and flow interruption (FI, flow stopped for 7d) on colloid and Hg release in the leachate were examined in column experiment. Two Hg contaminated soils (13.9 and 146mg/kg) were used, with Hg concentrations in colloids being 2–4 times greater than bulk soils. Based on sequential extraction, Hg concentrations in organic matter (OM) fraction were the most abundant in soils (31–48%). Column leaching after IS reduction and FI released large amounts of colloidal Hg, accounting for 44–48% of released Hg. The highest colloidal Hg concentrations at 27.8 and 360μg/L were observed at ∼1 pore volume after FI. Concentration distribution of colloidal OM and colloidal Fe was similar to colloidal Hg in the leachate, showing peak concentrations after IS reduction and FI. Most of the released colloidal Hg was in OM fraction (37–53%), with some in Fe/Mn oxide fraction (11–19%). Based on composition of released colloids and Hg fractionation in soils and colloids, colloidal OM could serve as an important carrier for Hg transport in soils.

Mercury Mobilization in a Flooded Soil by Incorporation into Metallic Copper and Metal Sulfide Nanoparticles

Mercury is a highly toxic priority pollutant that can be released from wetlands as a result of biogeochemical redox processes. To investigate the temperature-dependent release of colloidal and dissolved Hg induced by flooding of a contaminated riparian soil, we performed laboratory microcosm experiments at 5, 14, and 23 °C. Our results demonstrate substantial colloidal Hg mobilization concomitant with Cu prior to the main period of sulfate reduction. For Cu, we previously showed that this mobilization was due to biomineralization of metallic Cu nanoparticles associated with suspended bacteria. X-ray absorption spectroscopy at the Hg LIII-edge showed that colloidal Hg corresponded to Hg substituting for Cu in the metallic Cu nanoparticles. Over the course of microbial sulfate reduction, colloidal Hg concentrations decreased but continued to dominate total Hg in the pore water for up to 5 weeks of flooding at all temperatures. Transmission electron microscopy (TEM) suggested that Hg became associated with Cu-rich mixed metal sulfide nanoparticles. The formation of Hg-containing metallic Cu and metal sulfide nanoparticles in contaminated riparian soils may influence the availability of Hg for methylation or volatilization processes and has substantial potential to drive Hg release into adjacent water bodies.

Transport and interactions of kaolinite and mercury in saturated sand media

To evaluate the potential of Hg release and co-transport by colloids, it is important to understand how Hg, colloids and Hg-loaded colloids migrate in soils. Hg sorption by kaolinite and sand were nonlinear and fit the Langmuir model, with the maximum Hg sorption capacity being 1.2mg/g kaolinite and 0.11mg/g sand. Co-transport of Hg and kaolinite was evaluated using: (1) 1 or 100mg/L Hg or 100mg/L kaolinite, (2) 1 or 100mg/L Hg mixed with 100mg/L kaolinite, (3) 1 or 100mg/L Hg presorbed onto kaolinite, and (4) 250mg/L kaolinite in Hg-loaded sand columns. The presence of kaolinite (100mg/L) reduced Hg's mobility through sand column by increasing deposition rate of Hg-loaded kaolinite. At 100mg/L Hg, soluble Hg dominated Hg transport; but at 1mg/L Hg, colloidal Hg (Hg sorbed on kaolinite) affected Hg transport. Preloading 100mg/L Hg onto kaolinite (0.43mg/g) reduced kaolinite's mobility with low recovery rate (78%), with Hg retardation (R=1) in Hg-loaded kaolinite being lower than Hg retardation at 100mg/L Hg (R=1.287). The Hg recovery rate (93%) from Hg-loaded kaolinite at 1mg/L Hg was higher compared to 22% from 1mg/L Hg. Kaolinite can serve as a carrier to enhance Hg transport in porous media, with 250mg/L kaolinite mobilizing ∼2.4% Hg presorbed onto sand media. Correlation analysis revealed that desorbed Hg was significantly correlated with kaolinite (r=0.81, P<0.0001). Hence kaolinite enhanced Hg transport in the sand media serving both as a carrier (Hg was loaded before transport) and as mobile colloids stripping Hg off the sand media (Hg was loaded during transport).

Interaction of Macroaggregates and Hg in Coastal Waters (Gulf of Trieste, Northern Adriatic Sea)

Since a great quantity of dissolved Hg in natural waters was shown to be bonded onto colloidal organic matter (COM), Hg speciation, transport, biological availability and cycling are affected by interactions between Hg and COM. They are in large proportion dependent on the chemical composition and structure of COM. Macroaggregates (macrogels), produced episodically in the northern Adriatic by agglomeration of dissolved organic macromolecules—mostly heteropolysaccharides—of prevalently phytoplankton (diatom) origin, offer a rare opportunity to study these topics. The chemical speciation of macroaggregate Hg in the Gulf of Trieste (northern Adriatic Sea) was studied using filtration and centrifugation discriminating between matrix (water insoluble) and interstitial water (water soluble) colloidal fraction. The colloidal fraction was subsequently ultrafiltered through membranes with a nominal pore size of 30, 10 and 5 kDa cutoff sequentially (in a cascade fashion), and fractions analyzed for carbohydrate, Corg. and Ntot. contents, and FTIR. The highest carbohydrate content and the lower Corg./Ntot. ratio were associated with higher molecular weight (MW) fraction (>30 kDa) suggesting that aminopolysaccharides and glycoproteins can be important constituents of this fraction. Analyses of MW of permeates, assayed by high-pressure size exclusion chromatography (HPSEC), revealed that they are composed of macromolecules of a rather narrow MW distribution. Analyses of Hg concentrations revealed the highest concentration in high MW fraction in connection with the highest content of organic N constituents. This scenario was confirmed during a degradation experiment showing the preservation of Hg associated with higher MW fraction containing organic N constituents. Conversely, Hg in lower MW fractions, composed mostly of carbohydrates and subjected to degradation, is released into solution. The higher Hg concentration and lower Hg/Corg. ratio observed in matrix, characterized by the important presence of lipids in addition to polysaccharides and proteins, suggests the possible interaction between macrogel and HgS particles originating from Idrija (NW Slovenia) mining district.

Estuarine mixing behavior of colloidal organic carbon and colloidal mercury in Galveston Bay, Texas

Mercury (Hg) in estuarine water is distributed among different physical phases (i.e. particulate, colloidal, and truly dissolved). This phase speciation influences the fate and cycling of Hg in estuarine systems. However, limited information exists on the estuarine distribution of colloidal phase Hg, mainly due to the technical difficulties involved in measuring it. In the present study, we determined Hg and organic carbon levels from unfiltered, filtered (<0.45 μm), colloidal (10 kDa-0.45 μm), and truly dissolved (<10 kDa) fractions of Galveston Bay surface water in order to understand the estuarine mixing behavior of Hg species as well as interactions of Hg with colloidal organic matter. For the riverine end-member, the colloidal fraction comprised 43 ± 11% of the total dissolved Hg pool and decreased to 17 ± 8% in brackish water. In the estuarine mixing zone, dissolved Hg and colloidal organic carbon showed non-conservative removal behavior, particularly in the low salinity (<15 ppt) region. This removal may be caused by salt-induced coagulation of colloidal matter and consequent removal of dissolved Hg. The particle-water interaction, K(d) ([particulate Hg (mol kg(-1))]/[dissolved Hg (mol L(-1))]) of Hg decreased as particle concentration increased, while the particle-water partition coefficient based on colloidal Hg and the truly dissolved Hg fraction, K(c) ([colloidal Hg (mol kg(-1))]/[truly dissolved Hg (mol L(-1))]) of Hg remained constant as particle concentration increased. This suggests that the particle concentration effect is associated with the amount of colloidal Hg, increasing in proportion to the amount of suspended particulate matter. This work demonstrates that, colloidal organic matter plays an important role in the transport, particle-water partitioning, and removal of dissolved Hg in estuarine waters.

Mercury speciation in the colloidal fraction of a soil polluted by a chlor-alkali plant: a case study in the South of Italy

Mercury (Hg) speciation in different size fractions of a soil sample collected near an industrial area located in the South of Italy, which had been polluted by the dumping of Hg-containing wastes from a chlor-alkali plant, was investigated by XANES spectroscopy. In particular, a special procedure has been developed to study the soil colloidal fraction, both for sample preparation and for XANES data collection. In this soil, Hg was speciated in quite insoluble inorganic forms such as cinnabar (alpha-HgS), metacinnabar (beta-HgS), corderoite (Hg(3)S(2)Cl(2)), and some amorphous Hg, S and Cl-containing species, all derived from the land-disposal of K106 Hg-containing wastes. The contribution of the above-mentioned chemical forms to Hg speciation changed as a function of particle size. For the fraction <2 mm the speciation was: amorphous Hg-S-Cl (34%) > corderoite (26%) > cinnabar (20%) = metacinnabar (20%); for the fraction <2 microm: amorphous Hg-S-Cl (40%) > metacinnabar (24%) > corderoite (20%) > cinnabar (16%); and for the fraction 430-650 nm, where most of the colloidal Hg was concentrated: amorphous Hg-S-Cl (56%) > metacinnabar (33%) > corderoite (6%) > cinnabar (5%). From these data it emerged that, even if Hg was speciated in quite insoluble forms, the colloidal fraction, which is the most mobile and thus the most dangerous, was enriched in relatively more soluble species (i.e. amorphous Hg-S-Cl and metacinnabar), as compared with cinnabar. This aspect should be seriously taken into account when planning environmental risk assessment, since the small particle size in which Hg is concentrated and the changing speciation passing from millimetre to nanometre size could turn apparently safe conditions into more hazardous ones.

Macroscopic and Microscopic Observations of Particle-Facilitated Mercury Transport from New Idria and Sulphur Bank Mercury Mine Tailings

Mercury (Hg) release from inoperative Hg mines in the California Coast Range has been documented, but little is known about the release and transport mechanisms. In this study, tailings from Hg mines located in different geologic settings--New Idria (NI), a Si-carbonate Hg deposit, and Sulphur Bank (SB), a hot-spring Hg deposit--were characterized, and particle release from these wastes was studied in column experiments to (1) investigate the mechanisms of Hg release from NI and SB mine wastes, (2) determine the speciation of particle-bound Hg released from the mine wastes, and (3) determine the effect of calcinations on Hg release processes. The physical and chemical properties of tailings and the colloids released from them were determined using chemical analyses, selective chemical extractions, XRD, SEM, TEM, and X-ray absorption spectroscopy techniques. The total Hg concentration in tailings increased with decreasing particle size in NI and SB calcines (roasted ore), but reached a maximum at an intermediate particle size in the SB waste rock (unroasted ore). Hg in the tailings exists predominantly as low-solubility HgS (cinnabar and metacinnabar), with NI calcines having >50% HgS, SB calcines having >89% HgS, and SB waste rock having approximately 100% HgS. Leaching experiments with a high-ionic-strength solution (0.1 M NaCl) resulted in a rapid but brief release of soluble and particulate Hg. Lowering the ionic strength of the leach solution (0.005 M NaCI) resulted in the release of colloidal Hg from two of the three mine wastes studied (NI calcines and SB waste rock). Colloid-associated Hg accounts for as much as 95% of the Hg released during episodic particle release. Colloids generated from the NI calcines are produced by a breakup and release mechanism and consist of hematite, jarosite/alunite, and Al-Si gel with particle sizes of 10-200 nm. ATEM and XAFS analyses indicate that the majority (approximately 78%) of the mercury is present in the form of HgS. SB calcines also produced HgS colloids. The colloids generated from the SB waste rockwere heterogeneous and varied in composition according to the column influent composition. ATEM and XAFS results indicate that Hg is entirely in the HgS form. Data from this study identify colloidal HgS as the dominant transported form of Hg from these mine waste materials.

Distribution of particulate, colloidal, and dissolved mercury in San Francisco Bay estuary. 1. Total mercury

Surface water samples were collected from the San Francisco Bay estuary in September‐October 2000 (low flow) and March 2001 (high flow). Total mercury (Hg) concentrations were measured in unfiltered, filter‐passing (&lt;0.45 µm), colloidal (1 kDa‐0.45 µm), and dissolved (&lt;1 kDa) fractions. Particulate Hg was the dominant phase (88 ± 7%, n = 29) in unfiltered water. Suspended particulate matter (SPM) explained most particulate Hg concentrations in the northern reach. A significant portion of filter‐passing Hg was associated with colloidal Hg, accounting for 38 ± 18% (n = 9) in the fall and 57 ± 10% (n = 12) in the spring. Seasonal variability of the filter‐passing Hg concentration observed in the upper estuary was attributed to the temporal change in the riverine colloidal Hg. The strong correlation observed between Hg and organic carbon in the filter‐passing fraction indicates that organic material is an important transport medium of Hg in San Francisco Bay. Assessment of various forms of the particle‐ water distribution coefficient revealed that Hg was preferentially associated with SPM during the low flow period but that colloidal material played as important a role in Hg phase speciation as SPM during the high flow condition. A steady‐state nonconservative estuarine mixing model suggested that the northern reach had an internal source of colloidal Hg in September‐October, possibly from resuspended sediments, and a sink of colloidal Hg within the estuary in March 2001.

Production and Loss of Dissolved Gaseous Mercury in Coastal Seawater

The formation of dissolved gaseous mercury (DGM, mainly composed of elemental mercury, Hg0) in the surface ocean and its subsequent removal through volatilization is an important component of the global mercury (Hg) cycle. We studied DGM production and loss in the coastal waters of the Gulf of Mexico using 4−26 h in situ incubation experiments. DGM production was only induced in the presence of sunlight. Once produced, DGM was rapidly lost from solution (with a first order rate constant of k = 0.1 h-1), apparently as a result of oxidation. Furthermore, labora tory experiments showed that dissolved gaseous Hg0 could be rapidly oxidized in the presence of chloride. In the field, most DGM production (about 60%) was associated with the dissolved and colloidal Hg(II) phases. Spiking of samples with inorganic Hg(II) prior to in situ incubation greatly increased DGM production rates, suggesting that photoreducible Hg(II) complexes were limiting DGM production. Diurnally, DGM seems to be formed through photoproduction in the morning; DGM production halts when substrate is exhausted, and DGM levels decrease afterwards, presumably by oxidation of Hg0.

Mercury associated with colloidal material in an estuarine and an open-ocean environment

We have used traditional filtration and tangential flow ultrafiltration to isolate mercury into 4 size fractions: particulate (> 0.4 μm), high molecular weight (>10 kD, 1kD = 1000 daltons), medium molecular weight (1–10 kD), and low molecular weight ( 1 kD) represented 35–87% of the total dissolved Hg within the estuary and 10–50% of the total dissolved Hg in the North Atlantic. Equilibrium Hg speciation modeling supports speculation that colloidal Hg is bound by thiol-type functional groups associated with the colloidal organic carbon.

Mercury phase speciation in the surface waters of three Texas estuaries: Importance of colloidal forms

Surface‐water samples were collected from Galveston Bay, Corpus Christi Bay, Sabine Lake, and Laguna Madre and analyzed for Hg in the &lt;0.45‐µm, 0.45‐µm‐1‐kDa, and &lt;1‐kDa fractions. Colloidal material was isolated with an Amicon cross‐flow ultrafiltration system which was rigorously tested. Filter‐passing Hg concentrations (&lt; 0.45 µm) ranged from 0.12 to 13.6 pM, with the highest values in Corpus Christi Bay. Within Galveston and Corpus Christi Bays, filter‐passing Hg exhibited nonconservative estuarine mixing behavior. The colloidal Hg fraction ranged from 12 to 93% of the filter‐passing pool and averaged 57 ± 20%, indicating that a major portion of the operationally defined “dissolved” Hg is associated with submicron particles and macromolecules &gt; 1 kDa. Colloidal Hg covaries with colloidal organic C concentrations in Galveston Bay waters, suggesting that most of the filter‐passing Hg pool is associated with large organic macromolecules. The log of the particle‐water partition coefficient (Kd) ranged from 4.6 to 5.2, and covaried with suspended particulate matter concentrations, likely due to the presence of colloidal Hg.

Bimetallic Colloids: Silver and Mercury

Mercury ions, Hg2+, are radiolytically reduced in an aqueous silver sol. The plasmon absorption band of silver is blue-shifted, which is explained by the formation of a solid mercury layer around the silver particles. The metal deposition stops when the silver particles carry about two monolayers of mercury and the additionally reduced mercury forms colloidal Hg particles. Chemisorbed SH- and I- ions damp the plasmon absorption band of silver and silver−mercury particles, the effect being less pronounced with increasing mercury content of the particles. It is concluded that the collective excitation of the electron gas in the silver particles still strongly contributes to the optical absorption, even when they carry a mercury shell. The chemisorption equilibrium is shifted to the desorption side upon the deposition of excess electrons on the silver−mercury particles.

The reduction of HgCl 2 in gamma-irradiated aqueous-organic systems

In the presence of a variety organic solutes, mercuric chloride is reduced to mercurous chloride and HCl in aqueous solutions. G(HCl) values in dilute solutions (0.001 M HgCl 2 and 0.01 M organic) for a series of alcohols, acetaldehyde, acetone and acetonitrile lie within the approximate range of 3–6 molecules per 100 eV. The value depends upon the reactivity toward HgCl 2 of the radical product formed by reaction of OH with organic component. The results show a decreasing reactivity as the radical site is farther displaced from the α-carbon atom. At high concentrations (0.1 M HgCl 2 and 1 M organic) chain reactions occur in most of the systems with G(HCl) values increasing with decreasing dose rate. Evidence is presented for an involvement of colloidal Hg 2Cl 2 in the reaction chains.