G7 (IV) Environmental Biogeochemistry: Lab and Experimental Approaches

Tuesday, 26 July, 2011

Mercury biogeochemistry: Lab and Experimental Approaches — 11:00-12:00 and 17:30-18:30
Authors: BLOOM, Nicolas1, GALLUP, Darrell2
(1) URS Corp. R&D , nbloom@ymail.com; (2) Thermochem Inc.

The solubility of elemental mercury (Hgo) was determined in 30 pure and mixed hydrocarbons (and water) from -65oC to +65oC, over the range where each sample remained a liquid. Measurements were made both of Hgo, and of total mercury (HgT)--demonstrating that previously reported Hgo solubility data (made by measuring HgT) may be severely biased high. We observed that oxygenated hydrocarbons react to form soluble oxidized species that result in HgT levels far higher than the true Hgo levels present. Aliphatic and aromatic hydrocarbons, on the other hand, often contain impurities that oxidize Hgo to fine particulates that can contaminate the measurements and introduce considerable variability if HgT is reported as Hgo. The oxidizing components of non-oxygenated hydrocarbons can be eliminated by first shaking the hydrocarbon with an aqueous solution of NH2OH.HCl or with pure Cu granules. The solubility of HgCl2 was quantified in a small subset of hydrocarbons, and was found to always be orders of magnitude more soluble than Hgo. No conversion of HgCl2 to Hgo was observed in solution. In most cases, multiple determinations at each of three or more temperatures were obtained, allowing the concentration to be expressed as an exponential function of temperature ([Hgo] = KeAT). The solubility of Hgo at any temperature varies by about a factor of five as a function of solvent type: lowest in polar compounds such as CH3OH or C2Cl3F3 (300-500 ng/g at 22oC) and highest in aromatic compounds (2,500-3,000 ng/g at 22oC). The solubility of HgCl2 spans 5 orders of magnitude, with the greatest solubility in polar solvents. Calculating the ratio of the solubility of Hgo in air and in the solvent of interest at each temperature allowed us also to determine the Henry’s Law distribution coefficient (KH) for Hgo in each solvent as a function of temperature. We continue to investigate the possibility of making KH calculations for HgCl2. The high solubility of Hgo in solvents such as xylene, without significant conversion of species, further allowed us to develop a means of quantifying Hgo concentrations in soils and sludges. Typically, a 0.25-10.0 gram aliquot of soil is extracted in xylene using air-free end-over-end agitation for 48 hours, or in CHCl3 with insertion of an ultrasonic probe for one hour. After allowing the solids to settle, the solvent phase is analyzed directly for Hgo, allowing it’s quantification in the range of 0.001µg/Kg to 5,000 µg/Kg.

TG7-P2 — 11:00-12:00 and 17:30-18:30
Authors: HUI, Christina1, CARPI, Anthony1
(1)John Jay College, The City University of New York, christina.hui@jjay.cuny.edu

Mercury emissions from soil have been shown to increase as a result of both artificial irrigation and natural precipitation. Mechanisms proposed in the past to explain the phenomenon have included soil gas displacement, the addition of mercury through precipitation, the desorption of soil-bound Hg2+ due to infiltrating water, and enhanced redox reactions of Hg2+ in the aqueous phase (Lindberg et al., 1999; Song & Heyst, 2005; Dustin & Stamenkovic, 2005; Gillis & Miller, 2000). The various proposed mechanisms demonstrate uncertainty in the actual mechanism(s) contributing to the observed phenomenon. To investigate the emission of mercury following soil irrigation, various volumes of water were applied to soil samples under both light and dark conditions. Mercury emissions were monitored using Teflon dynamic flux chambers connected to a Tekran Mercury Vapor Analyzer 2537A. Mercury emissions from irrigated soil were greater in light than in the dark, and the increase in emissions from irrigated soil in light was greater than the additive effect of either variable independently. The synergistic effect of light and water suggests at least some contribution from a chemical mechanism as opposed to a strictly physical one.

TG7-P3 — 11:00-12:00 and 17:30-18:30
Authors: RUTTER, Andrew P.1, SCHAUER, James J.2, GRIFFIN, Robert J.3, SHAKYA, Kabindra M.3, LEHR, Rebecca4, PARMAN, Andrew2
(1) Rice University & University of Wisconsin-Madison, andrew.p.rutter@rice.edu; (2) UW-Madison; (3) UNH and Rice University; (4) UNH;

Estimates for the atmospheric lifetime of mercury depend upon a complete understanding of the oxidation and reduction processes which occur in dry and deliquesced aerosols. Previous research provides reaction kinetics of GEM oxidation by halogens, ozone and the hydroxyl radical in homogeneous reaction systems studied in small halocarbon coated reactors. In order to more closely represent complex atmospheric reaction systems we conducted oxidation experiments during the formation of secondary organic aerosol, to explore the effects of reactive organic species on the reaction in both the gas and particle phases. The reactions were performed in a 9-m3 Teflon smog chamber irradiated with UV lights, generating both homogeneous and heterogeneous photochemical reaction systems consisting of volatile organic compounds (VOC), ozone, and secondary hydroxyl radicals. The kinetics of the ozone and GEM reaction were measured to provide a consistency check with previous publications. Then VOCs were added to the chamber to study the impact of reactive species produced during the reactions between ozone and isoprene, alpha-pinene, toluene, and propene. In some experiments OH scavenger was added to observe the effect secondary OH had on the net oxidation.

The net conversion of GEM to RM during the formation of SOA was decreased when compared to the ozone-only case. In each case, net GEM conversion to RM deviated away from model predictions which used the classically studied Hg-ozone reaction, although the time at which this deviation occurred varied between cases. Numerical model schemes of the VOC oxidation did not identify a specific species or class of species by which the net oxidation is decreased. This result suggests that the decreased net oxidation may be caused by some aspect of the particle surface composition or gas-phase VOC chemistry which is not yet included in the model.

Si, L. and Ariya, P. A. (2008). Reduction of oxidized mercury species by dicarboxylic acids (C-2-C-4): Kinetic and product studies. Environmental Science & Technology 42, 5150-5155.

TG7-P4 — 11:00-12:00 and 17:30-18:30
Authors: SUN, Rongguo1, ZHANG, Yutao1, MA, Ming1
(1)1. Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment (Ministry of Education), College of Resources and Environment, Southwest University, Chongqing ,400715, China, sunrongguo88@163.com

The overall impact of methylmercury photodegradation on the aquatic Hg cycle is unclear. While many research have focused on biological methylation and demethylation, photodemethylation of methylmercury are likely very important to reduction of methylmercury in aquatic systems. Here we report the results of laboratory experiments designed to identify the light irradiation conditions (including dark, natural light, UVA (365 nm), UVB (310nm), UVC (254nm) and control) with the highest rate of demethylation. Furthermore, we could hypothesize the mechanism of this progress in aquatic systems. The identified products are Hg0, CH3HgCl and Hg2+. No change in Hg0 and CH3HgCl was observed in dark control, while a significant demethylation was observed by UV radiation. The rate of this reactions is first-order with respect to intensity, wavelength and methylmercury concentration (K=0.0119-0.0159min-1). We also demonstrate the effect of Cl-, NO3- and Fe2+ on photodemethylation. In the presence of Cl-, the rate of photodemethylation was reduced, and the photodegradation was increased with NO3- and Fe2+. These results indicate that mercury demethylation by UV is a photolytic process that accelerate the cleavage of carbon-mercury bond. Hg2+ produced in photolytic decay is subsequently reduced to Hg0. We assumed that Cl- could inhibit the photodemethylation of Hg, while NO3- and Fe2+ could contributes to this process in nature water phase. More researches are needed to understand the process and the end-products of photodemethylation in fresh water and sea water.

TG7-P5 — 11:00-12:00 and 17:30-18:30
Authors: ZHANG, Yutao1, SUN, Rongguo1, WANG, Dingyong1
(1) 1. Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment (Ministry of Education), College of Resources and Environment, Southwest University, Chongqing 400715, China, zhangyutao790108@126.com

Photochemistry is of great importance to mercury reduction in aquatic environment. Series of laboratory experiments were conducted with mercury nitrate and mercury chloride solutions under different light irradiation conditions, including dark, natural light, UVA (365 nm), UVB (310nm) and UVC (254nm). The result of experiments with mercury chloride certified the opinion that mercury reduction was mediated by light irradiation. However, an unexpected result was obtained from experiments with mercury nitrate. Compared with the mercury reduction rate under natural light , rate of reduction was significantly reduced by all the UVA, UVB and UVC; while, dark induced the fastest mercury reduction . It was deduced that hydroxyl radical (·OH), which could be produced through direct photolysis of NO3- in water, re-oxidized Hg0 to Hg2+. To certify this hypothesis, KNO3 and methanol, as a NO3- provider and a hydroxyl radical scavenger respectively, was introduced into this study. The result showed that methanol did not increased reduction of mercury chloride and even decreased this reaction in dark; however, reduction of mercury nitrate was significantly increased by methanol under all light conditions besides dark. KNO3 decreased reduction of both the mercury chloride and mercury nitrate under natural light and UV irradiation except in dark. The result indicated that light irradiation did not always mediate mercury reduction as that had been firmly believed previously, and the coexisting negative radicals is of considerable importance to mercury reduction in aquatic environment. The experiments of mercury nitrate under natural light were also performed at 10 ℃, 20 ℃ and 30 ℃ respectively and the result showed that temperature had no significantly effect on the mercury reduction. In addition, kinetic study of mercury nitrate reduction under natural light was conducted, and the first-order kinetics was proved through both concentration gradients and trial method.

TG7-P6 — 11:00-12:00 and 17:30-18:30
Authors: YAO, luo1, LEI, duan1
(1) Tsinghua University, y-luo09@mails.tsinghua.edu.cn

Flue gas desulfurization gypsum (FGDG), the by-product of desulfurization processes in coal-fired power plants, has been used as an acid soil amendment in recent years. This study evaluated the fate of high concentration mercury (908µg/kg) in FGDG and the change of mercury distribution in forest when FGDG was spread on acid forest soil surface (63.4g/m2). Throughfall, soil water (at the soil depth of 0cm, 10cm, 20cm, and 30cm) and litterfall were collected monthly for mercury analysis. Moreover, mercury in mineral soil, vegetation components, litter, and air in forest were quantified after one year FGDG treated. As a result, with FGDG treated, mercury contained in air and dicraepteris decreased significantly (p<0.05), and the average mercury concentration of litterfall and the soil water of 0cm was lower than the control plot except the first two months after spreading FGDG, but not significantly (p>0.05). In addition, almost all of the mercury contained in FGDG was reserved in litter layer. The results suggested that FGDG enhanced the capacity of organic matter on mineral soil about reserving mercury, and newly deposited mercury was easily bound to litter layer. Consequently, amending acid forest soil with FGDG didn’t increase the output of mercury in this study.

TG7-P7 — 11:00-12:00 and 17:30-18:30
Authors: DUONG, Van Hieu1, HAN, Seunghee1
(1) Gwangju Institute of Science and Technology, hieu@gist.ac.kr

Microorganisms may play a major role in the production of elemental mercury in wetland sediments. In the current study, the reduction of divalent mercury to elemental mercury was followed using slurries enriched with iron-reducing bacteria (FeRB) or sulfate-reducing bacteria (SRB). Slurries enriched with FeRB showed a dramatic increase in Hg(0)/THg, from 7% to 37%, in the solid phase during 20 days of culture, while SRB enrichment showed %Hg(0)/THg variation from 14% to 28%. We also cultured microbenthic algal communities to test their ability to reduce divalent mercury to elemental mercury. Microbenthic algae clearly contributed to the production of Hg(0) in oxic sediments. The overall results suggest that anoxic bacteria and microbenthic algae both play important roles in the reduction of Hg in wetland sediments.

TG7-P8 — 11:00-12:00 and 17:30-18:30
Authors: ZOH, Kyung-Duk1, KIM, Moon-Kyung1, JUNG, Se-Young1, OH, Sehee1
(1) Seoul National University, zohkd@snu.ac.kr

Mercury, especially methylmercury (MeHg), causes neurological and developmental disorders in humans and biomagnified through the food chain in aquatic ecosystem. The photo-chemical degradation of MeHg in natural surface water is an important pathway for reducing the bioavailability of MeHg to aquatic biota. Hydroxyl radical (OH·) is one of the most powerful oxidative reagents of the reactive oxygen species, and produced by solar irradiation in natural surface waters. There are many sources of OH· photo-formation in natural waters, such as the photolysis of hydrogen peroxide (H2O2), dissolved organic matter (DOM), and the photo-Fenton reaction. It is reported that bicarbonate (HCO3-) and chloride ions (Cl-), which are common ionic species in water, and act as OH· scavengers with higher reaction rate with OH·. The objectives of this study were to elucidate the kinetics and mechanism of MeHg degradation in natural water especially mediated by OH·.in the presence of various ions and organic matters. The results showed that the addition of Fe(III) or Cl- in the solution enhanced the photodecomposition of MeHg. However, the addition of natural organic matter (NOM) or H2O2 in the solution inhibited the MeHg decomposition rate. These results imply the competitive or hindering effect of NOM, H2O2, and Cl- with MeHg when reacting with hydroxyl radical. Further study regarding kinetics and mechanism of MeHg photo-degradation is needed in order to predict the fate of MeHg in an aquatic environment, and preventing bioaccumulation of MeHg in food webs.

TG7-P9 — 11:00-12:00 and 17:30-18:30
Authors: LONG, Stephen1, HENDRICKS, Jay1, SCHAEDLICH, Frank2
(1) NIST, selong@nist.gov; (2) Tekran Inc.

An apparent difference between the historical mercury vapor concentration equations used by the mercury atmospheric measurement community, and a new vapor pressure correlation proposed by NIST, has generated significant controversy. Several mercury vapor concentration data sets/equations (mostly based on the work of Knudsen in1909) have been in use by the mercury atmospheric measurement community over the last several decades. These predict the equilibrium vapor concentration of mercury as a function of temperature, and have been validated over the years by numerous chemical measurements. Recently a new NIST vapor pressure correlation was proposed by Huber et. al. [1], based on a survey of published data on high-quality mercury vapor pressure measurements. This has caused controversy in the mercury measurement community because the mercury vapor concentration calculated from the new correlation with the assumption of the ideal gas law, is some 7-9 % higher than that predicted using the accepted equations/data sets. This has critical implications for the mercury measurement community, because it brings into question the validity of historical atmospheric mercury measurements made over the last several decades.

In an attmept to resolve this situation, independent ambient measurements of the equilibrium saturated mercury vapor concentration, using isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS), and equilibrium saturated vapor pressure measurements using a calibrated ultrasonic interferometer manometer (UIM) have been made. The vapor concentration measurements employed a commercial calibration vapor saturation instrument (Tekran Inc. 2505). The data from these measurements indicate that the existing equations predicting the mercury concentration as a function of temperature are valid. However, the UIM measurements on the saturated vapor pressure of mercury also provide experimental evidence supporting the validity of the new NIST correlation. The reason for the remaining difference between the two sets of measurements is being investigated.

[1] Huber, M.L., Laesecke, A.., Friend, D.G., Correlation for the Vapor Pressure of Mercury, Ind. Eng. Chem., 45:7351-7361 (2006).

TG7-P10 — 11:00-12:00 and 17:30-18:30
Authors: MASSEY, Delia1, AMIRBAHMAN, Aria1, TRIPP, Carl1, MERRITT, Karen2
(1) University of Maine, delia.massey@umit.maine.edu; (2) Environ.

Mercury-specific thin films were synthesized and tested for the assessment of mercury (Hg) and methylmercury (MeHg) lability and bioavailability in porewater and sediment. A thin film is a reactive layer of material deposited on a suitable substrate. Thin films were prepared by dip-coating glass microscope slides with a mercapto-functionalized polydimethylsiloxane copolymer ([4-6% mercaptopropyl]methylsiloxane-dimethylsiloxane copolymer) dissolved in hexane. Parameters including the copolymer concentration, dip rate, coating period, removal rate, and number of coats were optimized with respect to the Hg adsorption capacity. IR analysis was used to determine the thiol functional group density. Hg percent recovery and compatibility with analytical instruments was assessed by elution of the Hg bound to the thin film with acidified thiourea. Loss of copolymer material from the slides during deployment in water was negligible and was only observed during the final elution step with acidic thiourea. Hg and MeHg experiments were conducted in the presence of fulvic acid to show the effectiveness of the thin films in the presence of strong competing ligands. The coated slides were placed in a holder made of rigid yet flexible Delrin® plastic with a protective polysulfone dialysis membrane to allow diffusive porewater flux to the thin film. The slides were deployed in estuarine sediments for different periods of time, after which they were removed, extracted, and analyzed for total Hg and MeHg. The sorbed Hg and MeHg concentrations were compared to those in the porewater to determine the lability of these species in the sediment.

TG7-P11 — 11:00-12:00 and 17:30-18:30
(1) LCABIE, UMR 5254 and LFC-R, UMR 5150, r.feniou@etud.univ-pau.fr; (2) LCABIE, UMR 5254; (3) Total-CSTJF; (4) TIGF; (5) LFC-R, UMR 5150; (6) TOTAL-GEN.

Mercury is an ubiquitous transition element which can be naturally found in atmospheric, aquatic and terrestrial systems. However some human activities can lead to an artificial contamination of natural resources. Due to the mercury’s toxicity, the impact of these contaminations need to be surveyed and particularly for atmospheric ones.

In order to better understand the transfer of metallic mercury in gas phase, an experimental system has been developed for the determination of mercury’s solubility in different gases. The device is able to work at temperatures up to 200°C in the pressure range 1 to 200 bar. The gas flow is controlled by a mass flowmeter and can reach up to 105 mL/min. The experimental set up is constituted of two main elements: a permeation cell of 8 ml where is leading the gas contamination with mercury and a melting cell that can be used to dilute the contaminated gas or to work simultaneously with two different gases. In order to avoid the phenomenon of mercury’s adsorption on the surfaces (tubes, cells, valves, pressure gauges), the stainless steel has been chemically treated.

The mercury quantification in the gas has been done using a preconcentration by double amalgamation on gold traps followed by measurement with atomic fluorescence spectrometry. The apparatus used for the analysis is the Sir Galahad II developed by PS Analytical.

The experimental set up and the obtained data are presented and discussed in detail. Pressure, temperature and gas flow velocity effects on mercury’s solubility in Ar and N2 are discussed. These first results permit to validate the experimental procedure and device and demonstrate its interest in studying mercury’s solubility in other gases such as CO2, in the application of its underground storage for example.

TG7-P12 — 11:00-12:00 and 17:30-18:30
Authors: KOLKA, Randy1, SEBESTYEN, Stephen1, MITCHELL, Carl2, NATER, Ed3, BRANFIREUN, Brian4, HANSON, Paul5
(1) USDA Forest Service Northern Research Station, rkolka@fs.fed.us; (2) University of Toronto Scarborough; (3) University of Minnesota; (4) University of Western Ontario; (5) Oak Ridge National Lab.

Scientists at the Oak Ridge National Laboratory and the USDA Forest Service are developing a large-scale ecosystem study in which temperature and CO2 will be experimentally increased to quantify effects of climatic forcing on ecological, hydrological, and biogeochemical processes in a northern peatland. Here we provide an overview of the mercury research directions within the Spruce and Peatland Responses Under Climate and Environmental change (SPRUCE) Experiment that will occur at the Marcell Experiment Forest in northern Minnesota. We plan on heating peatland soils up to 9oC down to 3 m depth and elevate CO2 up to 900 ppm in 12-m diameter chambers on a black spruce-Sphagnum bog.

Each chamber will have a nest of piezometers, runoff collector, and a pair of sippers to measure routine chemistry and total Hg (THg) and methyl Hg (MeHg). Piezometer nests will include samples from near the soil surface to near the peat-mineral interface at depth. A belowground corral is planned to isolate the chamber from outside influences of hydrology and allow us to sample surface and near-surface runoff events. In addition, we will insert a pair of Teflon sippers in each chamber to sample distinct soil water zones focusing on samples just above and below the water table.

We anticipate that the soil warming treatments alone or in combination with elevated CO2 will increase primary productivity and lower water tables as a result of more uptake and higher evaporation. We hypothesize that lowering of water tables will result in a decline of THg and MeHg in soil pore water as deeper organic soils with lower mercury burdens contribute to pore water chemistry. However, when we have a significant event such as major storm or snowmelt, we hypothesize that treatments with lower initial water tables will have more Hg available for methylation and thus higher THg and MeHg concentrations, especially under heated conditions. These responses will be also controlled by how water table variation and heating influence carbon and sulfate cycles. We seek interaction with the mercury community on the design and hypotheses of the SPRUCE experiment.

TG7-P13 — 11:00-12:00 and 17:30-18:30
Authors: FRITSCHES, Johannes 1, NILSSON, Mats B.2, ALEWELL, Christine1, BISHOP, Kevin3, ÅKERBLOM, Staffan2, OSTERWALDER, Stefan4, SAGERFORS, Jörgen2
(1) Univeristy of Basel, Johannes.Fritsche@unibas.ch; (2) SLU; (3) Uppsala University & SLU; (4) University of Basel;

We investigated the effect of increased sulfur deposition on the evasion of TGM (total gaseous mercury) from a high-latitude peatland using dynamic flux chambers during August 2009. For this purpose we used a long-term field manipulation site on a boreal peatland with plots of low and high levels of sulfate deposition (i.e. 3 kg/ha/yr “low” ambient deposition and 20 kg kg/ha/yr of “high” deposition). The emission of TGM from the peatland surface was generally low at all plots. Nonetheless, we found that emission to the atmosphere from plots with ambient S deposition was significant (1.40 +/- 0.78 ng m2/hr; mean +/- sd), but that TGM evasion was completely suppressed (‑0.01 +/- 0.38 ng m2/hr) by 15 years of S addition at rates of 20 kg ha/yr. Sampling of the soils on these plots also revealed significant differences in the amount of total Hg in the peat. The quantity of Hg-tot in plots subjected to ambient S-deposition was (2.9±0.2 µg dm-3) while there was significantly less Hg-tot on the plots subjected to the high level of S deposition (2.4±0.2 µg/dm3). The results suggest that the S deposition to peatlands not only influences the formation of MeHg, but also alters the long-term balance of Hg in- and outputs.

TG7-P14 — 11:00-12:00 and 17:30-18:30
Authors: MIRANDA, Marcio R.1, GUIMARAES, Jean Remy D.2, MARINHO, Claudio3, COELHO SOUZA, Sergio A.1, CARVALHO, DP de 4, BASTOS, Wanderley R.5
(1) Inpetam, topo@biof.ufrj.br; (2) IBCCF/Universidade Federal do Rio de Janeiro; (3) IB/UFRJ; (4) UNIR; (5) .

The Amazon Basin is one of the most studied regions in South America regarding mercury contamination from anthropogenic and natural sources. However, very little is known about the mercury methylation process in worldwide terrestrial environments. Here we show mercury methylation potentials in non-flooded rhizospheres and soils from Madeira River Basin evergreen Terra Firme (upland) Forest. This forest is a species-rich mixed vegetation with a peculiar root mat (rhizosphere) composed of a tangled net of fine, hairless roots growing outside the soil, strongly attached to decomposing organic matter. The aim of this study was to investigate stimulation of the methylmercury production through simulated Terra Firme flooding experiment. Samples were collected at the Rondônia Federal University Campus forest and incubated with 203HgCl2 for 24h to evaluate methylmercury production after different lengths of flooding. The physical-chemical parameters (pH, redox potential, dissolved oxygen, conductivity, temperature and suspended solids) were related to the radioactive methylmercury formed in inundation experiments in order to evaluate their influence on methylmercury formation. After 20 days of flooding, mercury methylation potential increased 38 fold in rhizosphere (1.49±0.55 %MeHg/g dw/24h) and 44 fold in soil (3.98±1.11 %MeHg/g dw/24h) when compared to non-flooded controls (0.04±0.03 % MeHg/g dw/24h, rhizosphere; 0.09±0.07%MeHg/g dw/24h, soil). Methylmercury formation was related to the flooding time (days), increased conductivity, pH and suspended solids and decreased redox potential. These results indicate that the area that will be affected by the construction of the Santo Antonio reservoir, Madeira River, has an enormous potential for methylmercury formation. This potential is related with several environmental variables. Among these, the input of terrestrial organic matter to the aquatic system and its subsequent degradation are likely to be the most important ones.

TG7-P15 — 11:00-12:00 and 17:30-18:30
Authors: GUIMARAES, Jean1, CORREIA, Raquel Rose S.2, OLIVEIRA, Diane CM3
(1) IBCCF/Universidade Federal do Rio de Janeiro, jeanrdg@biof.ufrj.br; (2) IBCCF/UFRJ; (3) .

Aquatic macrophytes, such as Eichhornia crassipes, are known as sites for accumulation of Hg and methylmercury formation, the latter being conducted mainly by sulphate-reducing bacteria (SRB) present in the dense root periphyton. Recently, Molina et al. (2010) showed that in the Bolivian Amazon, periphyton supported longer macroinvertebrate trophic chains than sediment, leading to higher MMHg biomagnification. The use of radiotracer techniques on model ecosystems offers a unique possibility to better understand the Hg cycling in various environmental conditions. This approach allows a great simplification of the experimental set-up in studies of the distribution and transformation of a contaminant, such as Hg, in different compartments, and its interaction with the biota. Therefore, the objectives of this research were to observe MMHg formation and distribution among water, particles and whole plants of the macrophyte E. crassipes in microcosms with and without SRB inhibitor; and to compare the in vivo results with in vitro root incubations in the course of 17 days. MMHg formation was evaluated using the 203Hg radioisotope and measured through Liquid Scintillation after the organic form extraction protocol. Two experiments, with and without macrophytes, were made and the compartments analyzed for the presence of MMHg were 0.2 µm-filtered water, suspended and settled particles and roots. MMHg was detected on all compartments analyzed and the highest distribution values were found in roots and water, with up to 41% and 61% of all MMHg detected in the experiment with and without macrophytes, respectively. The sum of all MMHg found in the mesocosms with and without macrophytes reached 2.3% and 0.4% of total added Hg respectively. Hg methylation was partially inhibited in the presence of the SRB inhibitor sodium molibdate, and inhibition was lessened at the end of the experiment. MMHg formation was higher in in vitro incubations than in in vivo ones, mostly on long incubation times (> 48 hours). The results indicate a change in the pattern of MMHg formation in the presence of the macrophyte E.crassipes and the participation of SRBs in Hg methylation.

TG7-P16 — 11:00-12:00 and 17:30-18:30
Authors: SILVA, Raquel Rose C.1, GUIMARAES, Jean Remy D.2, OLIVEIRA, Diane CM2
(1) IBCCF/Universidade Federal do Rio de Janeiro, raquelrose10@gmail.com; (2) IBCCF/UFRJ;

Aquatic macrophytes, such as Eichhornia crassipes, are known as sites for accumulation of Hg and methylmercury formation. Aside its capacity to absorb and concentrate Hg, it is expected that the presence of this macrophyte alters Hg volatilization rates in the system since Hg would tend to form complexes with organic matter and particulates present in the plant roots and become trapped. The use of radiotracer techniques on model ecosystems offers a unique possibility to better understand the Hg cycling in various environmental conditions. This is particularly true if a gamma emitter isotope is used, as gamma spectrometry is non-invasive and non-destructive, allowing in-vivo experiments in a micro/mesocosm scale. The objectives of this research were to observe Hg distribution among air, water and whole plants of the macrophyte E. crassipes during 17 days. The distribution of a single 203Hg spike was evaluated by gamma spectrometry. Two experiments, with and without macrophytes, were made and the compartments analyzed for the presence of Hg were air, 0.2 µm-filtered water, suspended and settled particles, roots, leafs, petioles and adsorption on the desiccator walls. 203Hg was detected in all analyzed compartments and the highest total Hg concentrations were found in the roots and particles of the incubations with and without macrophytes which retained in average 68 and 34 % of added Hg respectively. On the other hand, the lowest concentrations were found in air for both incubations, with higher volatilization (up to 2.5 % of added Hg) in the absence of macrophytes and lower (up to 0.3%) in the presence of the macrophyte. Lower Hg values found in leafs (up to 1%) and petioles (up to 2.6%) suggest this plant has mechanisms of Hg exclusion in the roots. The results of these experiments suggest that the studied macrophyte promotes changes in the Hg cycle since it attracts most of the Hg present in the water and suspended particulate to its roots and settled particles underneath. The presence of the macrophyte also reduces Hg volatilization.

TG7-P17 — 11:00-12:00 and 17:30-18:30
Authors: SI, Lin1, ARIYA, Parisa A.2
(1) University of Western Ontario, lsi3@uwo.ca; (2) Department of Chemistry & Department of Atmospheric and Oceanic Sciences, McGill University.

Studies of the photochemistry of various Hg compounds are crucial for understanding Hg cycling in the environment and potential toxicity to living organisms. It has been suggested that strong natural ligands in low concentrations and with high conditional stability constants, possibly sulfides and thiols, could be primarily responsible for dissolved mercury complexation in estuarine waters. Thioglycolic acid (TGA) has been detected in various natural water systems. It is used as a model compound in several studies on the complicated interaction between mercury and ill-defined, polyfunctional Dissolved Organic Matter. We hereby present the first kinetic and product study on the photochemistry of oxidized mercury species with thioglycolic acid. Aqueous TGA reacts with Hg2+ to form, in the absence of excess thioglycolic acid, the Hg2+-TGA complex, i.e. Hg(OOCCH2S). UV-visible (UV-vis) spectrophotometry was used to determine the rate constant and the branching ratio of the reaction. Cold vapour atomic fluorescence spectrometry was applied to confirm Hg0 as one of the reaction products and to calculate the rate constant. The effects of pH and ionic strength were also investigated. Transmission electron microscopy coupled with energy dispersive spectrometry (TEM-EDS) and X-ray diffraction, were used to investigate the solid products formed after the photolysis. The apparent first order rate constant was reported at 296K, pH4. The rate constant was independent of ionic strength. The formation of elemental mercury and black metacinnabar were observed. The environmental implications of our studies will be also discussed.

Tuesday, 26 July, 2011