Biogenic Volatilization of Trace Elements from European Estuaries

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Chapter 12

Biogenic Volatilization of Trace Elements from European Estuaries Emmanuel Tessier, David Amouroux, and Olivier F. X. Donard Laboratoire de Chimie Analytique Bio-Inorganique et Environnement, CNRS U M R 5034, Université de Pau et des Pays de l'Adour, Pau, France

This paper describes the importance of volatile chemical forms of selected trace elements in the environment with emphasis on the estuarine aquatic environment. The occurrence, distribution and fluxes of gaseous species of trace elements such as Mercury (Hg), Selenium (Se), Tin (Sn) and Iodine (I), was investigated in three European macro-tidal estuaries in relation with the main biogeochemical parameters. Water and air samples were collected according to ultra-clean methods, in the Gironde, the Rhine and the Scheldt estuaries. On a seasonal basis, this approach allowed us to evaluate the potential pathways of these gaseous species in estuarine waters and to derive their seasonal fluxes to the atmosphere. We were then able to provide an estimation of the global contribution of European estuarine emission of these trace elements and to compare it with anthropogenic sources.

© 2003 American Chemical Society Cai and Braids; Biogeochemistry of Environmentally Important Trace Elements ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

151

152

Introduction Many elements are involved in the transfer of gaseous species between earth surface and the atmosphere. Most of the lighter elements (C, N , S etc) are involved in the basic life processes in the environment, including photosynthesis, respiration, and the production of radiatively important ("green house") gases (1). While there have been abundant studies and evidence involving these lighter elements, the atmospheric transfer of the heavier (trace) elements is less recognised. Heavier elements, as shown in Figure 1, can form volatile species mainly after reduction and/or alkylation processes (2). These pathways are a consequence of biological activity, or indirectly via abiotic reactions. This is important as it provides a transfer mechanism via the atmosphere into ecosystems with potentially harmful or nutritious effects.

Figure 1 : Trace elements forming volatile gaseous species under natural environmental conditions (elements in grey cells : clear = verified pathways, dark = suggested pathways). Several investigations made on iodine (I), selenium (Se), mercury (Hg) and tin (Sn) have demonstrated the significance of transfer processes of these heavy elements between earth surface and the atmosphere. As shown in Figure 2, investigation performed in marine and estuarine environments, but also in pristine or polluted continental ecosystems suggest that as for lighter elements,

Cai and Braids; Biogeochemistry of Environmentally Important Trace Elements ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

153 volatilisation of heavy elements through natural processes is a major pathway in their biogeochemical cycles (2).

Figure 2 : Main biogeochemical routes for trace elements volatilisation in aquatic environments. For example, studies in the productive estuarine and coastal waters suggest that trace element fluxes from these areas are equivalent to the open ocean. Like dimethylsulfide (Me S), all heavier elements (Se, Te, Po) in the same group of the periodic table have volatile alkyl species (2-5). In the case of ecosystems contaminated by inorganic Se and Hg, or tributylSn (TBT), formation and volatilisation of dimethyl selenide (Me Se), elemental mercury (Hg°) or methylated butylSn (Bu SnMe), respectively, can be an efficient process to remove and remobilize these potentially harmful contaminants via their transfer to the atmosphere (6-8). In this paper, we present an example of investigation that was achieved during the last four years in the framework of an EU Environment and Climate project focused on the emission of biogenic gasesfromWest-European estuarine waters (BIOGEST). This work presents a good illustration of the scientific strategy that we have developed in the recent years owing to investigation of the formation of transient labile and/or volatile trace element species in the aquatic environments and their impacts on the transfer of such trace elements between the various environmental compartments, including biological cells (bacteria, plankton, fishes...). 2

2

3


154

Material and Methods A multi-element analytical method, involving cryogenic trapping, gas chromatography and inductively coupled plasma-mass spectrometry (CT-GCICP/MS) for the speciation of dissolved and atmospheric volatile metal compounds allowed us to perform simultaneous investigations on several trace elements such as mercury (Hg), selenium (Se), tin (Sn) and iodine (I) (9-10). Water and atmospheric samples were collected during several cruises on the Gironde (Nov 1996, June 1997, Sept 1997, Feb 1998), the Scheldt (July 1996, Dec 1997, May 1998, October 1998) and the Rhine (Oct 1996, July 1997, Nov 1997, Apr 1998) estuaries along with hydrological parameters. On each estuary, sampling stations were chosen to collect surface waters every 2.5 salinity unit increments with salinity ranging between 0 and 34 (practical salinity scale, PSS). Surface waters were sampled at a 3m depth using a PTFE coated Go-Flo non-metallic sampler (General Oceanic) to avoid ship contamination and microlayer surface water contamination. After sampling, collected water was immediately transferred through a silicone tubing into a gastight, PTFE-lined IL Pyrex bottle until shipboard treatment. Water sample treatment including a cryofocusing step was detailed elsewhere by Amouroux et al. (10) and Tseng et al. (11). Within 30 minutes after collection, 1 Litre of sample is purged for 1 hour with 600 to 700 ml min" He flow. Water vapour is removed from the gas stream through a moisture trap maintained at -20°C. Volatile compounds are subsequently cryo-trapped into glass tube filled with glass wool and immersed in liquid nitrogen (-196°C). Most of the volatile compounds extracted from the aqueous sample can, therefore, be concentrated and stabilised into the cryogenic trap. Those traps are then sealed, stored and transported into a dry atmosphere cryogenic container (-190°C) until analyzed. The general operating parameters for the whole analytical procedure are described in detail elsewhere by Amouroux et al. (10). The analytical set-up developed for simultaneous determination of volatile species using ICP/MS detection is described in Figure 3. In the laboratory clean room, the cryogenic traps are flash desorbed by a fast-heating furnace. The volatile species are then cryofocused onto the head of the chromatographic column, immersed in liquid nitrogen (-196°C), before gas chromatographic (GC) separation. The chromatographic column is packed with Chromosorb W-HP (60/80 mesh size) coated with 10% Supelco SP-2100 and is then silanized with about 200μ1 of Hexamethyldisilazane. The desorption and separation of the analytes (non-polar organometals) can then be operated by applying a temperature gradient (from 1


155 100°C to 250°C) allowing the volatile compounds to elute according to their boiling point. In order to avoid plasma disturbances due to the presence of desorbed gas or carbon-containing compound, such as carbon dioxide, a PTFE 3-way valve is positioned at the output of the chromatographic column to allow to vent the gas excess. Furthermore the transfer lines between the desorption cell and the chromatographic column on the one hand and between the column and the detector on the other hand are heated during the desorption steps allowing us to avoid any water condensation in the analytical chain. Finally, the detection is performed by inductively coupled plasma mass spectrometry (ICP/MS). This detector allows one to identify and quantify the volatile compounds of up to 30 isotopes of different trace metals and metalloids with very low detection limits and high sensitivity and selectivity (9). Absolute detection limit, defined as 3a , ranges from 0.4 to 10 fmol for the different trace elements analysed. noise

Figure 3: Experimental set-up for the simultaneous determination of volatile species of several elements in the environment using ICP/MS detection. Air samples were collected using a similar method described by Pécheyran et al. (9). Air is pumped during V4 hour at c.a. 800 ml min" from the top or the bow of the ship to avoid any contamination. The aerosols and water vapour are removed from the gas stream using on-line 0.1 μπι quartz filter (Millipore) and moisture trap (-20°C), respectively. The volatile species are then trapped onto a cryogenic trap (-170°C) and stored in cryogenic container as described below. In the laboratory, the cryogenic traps are desorbed and analysed similarly to the purge and trap samples. 1


156

Results and Discussion

Identification of Volatile Trace Element Species The analytical method described below has been applied for the determination of volatile metal and metalloid species in estuarine waters and the atmosphere. During the Biogest cruises, we focussed our attention on four trace elements, mercury (Hg), selenium (Se), tin (Sn) and iodine (I). Examples of chromatogram obtained from estuarine water samples are presented in Figures 4 and 5. We observed and identified several ubiquitous chromatographic peaks from the various samples collected in the Gironde, the Rhine and the Scheldt estuaries. The major volatile compounds encountered are outlined in Table I.

Table I. Volatile species of the investigated trace elements identified in estuarine water and atmosphere Elements Isotopes Water

Se 78/82

Hg° Me Hg (MeHgCl)

Me Se MeSSeMe Me Se

Me SnH4_ Me„SnBu . (n=l-4)

Mel (R-D (R= Et, Pr, Bu)

Hg° (MeHgCl)

Me Se

Me SnH4_„ (n=0-4)

Mel (R-D (R= Et, Pr, Bu)

2

Atmosphere

/

Sn 118/120

Hg 200/202

n

2

2

127

n

4 n

2

n

2

Compounds in parenthesis are subject to further investigations; Me=Methyl group, Et=Ethyl group, Pr=Propyl group, Bu=Butyl group. We identified the potential molecular species by comparison with the retention time obtained with standards solutions (10). The sample quantification for the volatile species was then based on aqueous standards, commercially available, and screening the mass spectrum of the observed compounds (i.e. Me Sn, EttSn and Bu Sn; Hg°, Me Hg and Et Hg, Me Se and Me Se ; Mel for volatile tin, mercury, selenium and iodine species, respectively). 4

4

2

2

2

2

2


157

Figure 4: Example of chromatograms for I and Se, obtained from a surface water sample collected in estuarine environment

Figure 5: Example of chromatograms for H g and Sn, obtained from a surface water sample collected in estuarine environment.


158 Saturation Ratio and Flux Calculation In a second step, air and water measurements of these volatile species allowed us to calculate fluxes between estuarine water and the atmosphere. First, the saturation ratio (SR) estimation is necessary to validate the transfer of volatile species at natural interfaces. For the water to air exchange, the saturation ratio (SR) is expressed as: SR = (C χ H) / C . Where C and C are the concentrations measured in the water column and in overlying air, respectively. Η represents the dimensionless Henry's law constant of the considered volatile compound. Η values used in our calculations were obtained from literature available constants for volatile Hg and I compounds and from a quantitative structure-activity relationship (QSAR) between Henry's law constants available in the literature for alkylated organometals species (12) and molecular total surface area (13) for volatile Se and Sn compounds. This simple method was found to provide a very good approximation of Henry's constant for volatile alkylated compounds (14). In order to apply these calculations to real environmental processes, we have previously postulated that the volatile species investigated exhibit a low solubility in water and are likely to be unreactive in the aqueous substrate (15). Flux calculations are then based on the Fick's first law of diffusion with the assumption that the concentrations of the studied compounds in the water column are in steady state compared to their transfer velocities at the interface. The flux density (mol m" d" ) is expressed as: w

2

a

w

a

1

c -— Γ"

H.

1

where Κ is the transfer velocity (cm h ) at the air-water interface. Considering the low water solubility of the studied compounds we can assume that Κ is equal to k , the water gas transfer velocity (16). We can then calculate the k for the various volatile compounds with the model proposed by Clark et al. (77), developed for the tidal estuarine system of the Hudson bay. The transfer velocity is given by the following expression: w

w

ι k

w

=

f i L f x( + 0.24u ) 2

2


159

where Se is the solute Schmidt number, inversely proportional to the diffusion coefficient (D ) and u is the wind speed (m/s) recorded at a height of 10 m. The free-solution diffusion coefficient D (m s" ) was obtained from the Wilke & Chang equation (18) and was then corrected for the temperature and salinity according to Saltzman et al. (79). We then implemented the Clark's model with in situ wind speed values measured during each sampling point. The application of the gaz exchange model was performed assuming that the half-life of the volatile species in the aqueous phase is greater than their respective transfer time through the air-water interface. 0

2

1

0

Overall Distribution and Air-Water Exchange of Volatile Trace Element Species Seasonal concentration profiles versus salinity, for the major volatile species observed in the three estuaries investigated, are presented in Figures 6 and 7. Volatile Hg was mainly found as elemental Hg (Hg°) in both aquatic and atmospheric compartments (Hg° range 50-1500 fmol L" and 1-13 ng m for water and air respectively). In the Gironde and the Scheldt estuaries, dimethyl mercury (Me Hg) and probably monomethyl mercury chloride (MeHgCl) were also evidenced. Dissolved Hg° variations in estuarine waters could be negligible such as in the Gironde estuary, or display significant ranges like in the Scheldt or Rhine estuaries. Concentrations in water are increasing at high salinity (20-35) during the summer period, and concentration maxima were observed at lower salinity (0-5). Hg° distribution was found to be well correlated to the potential "active chlorophyll" during the warmer seasons (i.e. ratio [chlorophyll a]/([chlorophyll a]+[Phaeopigments])). Moreover, seasonal variations of the Hg° mean concentrations present the same pattern in the three estuaries with higher values in spring and summer. These results suggest that biological turnover and seasonal pattern control Hg° production in surface water involving the reduction of inorganic mercury under light-induced biological or chemical processes. Hg° was then found to be supersaturated in water compared to the atmosphere during the warmer seasons, and near to the saturation during colder seasons. Estimated fluxes to the atmosphere are then seasonally dependent and range between 5 to 7500 pmol m" d" . Volatile Se in estuarine waters was mainly detected as dimethylselenide (Me Se) and concentrations were also found significantly higher in the Scheldt and the Rhine estuaries (ca. 200-25000 fmol L" ) than in the Gironde estuary (ca. 300-5000 fmol L" ) (20). Amounts of dimethyldiselenide (Me Se ) and dimethylselenide sulphide (Me^SSe) have also been observed in the three estuaries investigated. In the whole air samples the concentrations measured were near to the method detection limit (ca. < 0.2 ng m" ). Me Se in surface 1

3

2

2

1

2

1

1

2

3

2


2

160

waters exhibits concentration maxima in the 0-10 salinity range and a decrease by estuarine mixing processes for higher salinity. Me Se distribution in all estuaries investigated was found closely anticorrelated with "active chlorophyll", showing that its formation may be related to plankton biomass degradation. Formation of volatile Se compounds is then mainly due to riverine and marine plankton inputs and the associated microbial activity. Concerning Me SSe, this compound was found to be produced under abiotic conditions (21). In all seasons for all estuaries the water column was found supersaturated with Me Se and estimated fluxes to the atmosphere are ranging between 0.4 and 160 nmol m d" . 2

2

2

2

1

1000

Scheldt estuary ζΓ*

800

Ο

600

ε

A

July 96

o

Dec. 96

{ M

400

I 200

m—Θ-

0

Rhine estuary

30

Ο

Ε α φ ω

CM

20

•

March 98

Δ

Oct. 96

10

Φ

0

5

10

15

20

25

30

35

Salinity

Figure 6: Typical seasonal profiles of volatile Hg and Se species concentrations as function of water salinity in estuarine environment.

Significant concentrations of volatile tin compounds have been discovered by Amouroux et al. in the different estuaries investigated (8). These species are probably originating from both methylation of natural or anthropogenic inorganic tin and tributyltin (TBT) released by ship antifouling "paintings.


161 Tributylmethyl tin (MeBu Sn) is the most ubiquitous compound observed in water and exhibits significantly higher concentrations in the Scheldt (ca. 75-2000 fmol L" ) than in the Rhine and the Gironde (ca. 5-125 fmol L" and 5-90 fmol L" \ respectively) (22). In air samples, only tetramethyl tin (Me Sn) was detectable in the overlying atmosphere of the Scheldt and the Rhine estuaries with concentrations ranging between 1 pg m" and 20 pg m' . Nevertheless MeBu Sn overall distribution in all seasons and all estuaries always displays the same pattern with concentration maxima between salinity 10 and 15 which then decrease in lower and higher salinity. Volatile organotin compounds seem then to be produced in the estuary at intermediate salinity, within the estuarine maximum turbidity zone characterised by a strong sedimentation rate and potential remobilization of the particulate material. The occurrence of these compounds seems also to be directly related to the anthropogenic load of the investigated area. The Scheldt estuary appears, then, to be contaminated by these species compared to the Rhine and the Gironde. Moreover, concentrations encountered in anoxic estuarine sediments of the Scheldt estuary are a thousand times higher than in the water above. Methylated butyl-Sn probably originates from the chemical and/or biological methylation in the sediment of anthropogenic butyl-Sn released in the estuarine environment. These results suggest that microbially mediated methylation mechanisms are likely to produce volatile organotin tin species into the estuary (8). These processes represent a significant remobilization pathway into the water column and then to the atmosphere of toxic volatile tin compounds accumulated in the sediment. Bu MeSn was found supersaturated in all seasons for all estuaries. Estimated fluxes are then strongly dependent on the contamination levels of the estuaries and range from 5 to 2400 pmol m" d" (22). Volatile Iodine was mainly detected as iodomethane (Mel) and numerous other volatile iodine compounds (iodo-alkanes) were also observed in both air and water samples (20). Mel represents around 50 % of the total volatile iodine species encountered. Concentrations in estuarine waters are ranging from 1 to 15 pmol L" for the Rhine and the Gironde and from 1 to 100 pmol L for the Scheldt estuary. Atmospheric sample concentrations display more homogeneous values (ca. 1-20 ng m ) in the three investigated areas. Here again the contamination level in Mel of natural waters seems to be dependent on the anthropogenic load of the studied environment. Water concentrations increase along the salinity gradient, with higher values during the warmer seasons and particularly in spring for the three estuaries. We also observed a significant spike of Mel concentrations at the mouth of the Scheldt estuary during spring phytoplanktonic bloom suggesting that plankton or algae primary productivity is directly involved in Mel production. Moreover the Mel distribution within the three estuaries shows a strong correlation with "active chlorophyll" and particularly during spring and summer. Biological light induced methylation 3

1

1

4

3

3

3

3

2

1

1

1

3


162 mechanisms are probably involved in Mel production in estuarine waters. Mel was found supersaturated in all seasons and all estuaries with estimated fluxes to the atmosphere ranging between 0.5 and 170 nmol m" d" . 2

Scheldt estuary

120

Ο

80

•

May 98

Δ

Oct. 98

ε Φ

1

f

40

\

-* ^ 0

120

Ο

«ε+_

80

Gironde estuary Ί

* Sept. 97 • Oct. 96

Φ

c CO

J

40

°

5

- K

CO

3 10

15

20

25

30

35

Salinity

Figure 7: Typical seasonal profiles of volatile I and Sn species concentrations as function of water salinity in estuarine environment. We then extrapolated our flux estimations to the whole European estuaries in order to evaluate on a more global scale the potential contribution of European estuarine emission of Mercury, Selenium, Tin and Iodine compared to natural and anthropogenic sources (see Table II). European estuarine flux estimations were based on the average flux densities of the three investigated estuaries, and were then extrapolated to the total European estuarine surface area estimated to be 111,200 km (23). Our results suggest that estuaries are a significant source of those trace elements to the atmosphere, when compared to anthropogenic sources such as fossil fuel combustion. 2


163

2

1

Table II. Averagefluxdensity (FD, in nmol m" d") and Water to airflux(in t yr") of the investigated trace elements 1

Hp FD

TVS? Flux

FD

TVSr?

Mel (TVIf

Scheldt

0.5

0.01

32

Flux FD (t/yr) 0.25 0.99

Gironde

0.7

0.03

3

0.06

0.04

0.0011

Rhine

0.7

0.01

13

0.07

0.04

0.0004

(t/yr)

European Estuaries

5

52

Flux

FD

0.0114

2

Flux (t/yr)

Mr)

16 (46) 5 (11) 6 (11)

0.20 (0.60) 0.10 (0.30) 0.05 (0.09) 45 (116)

Flux calculations based on: TVSe concentrations ( Total sum of the volatile Selenium species observed). TVSn concentrations ( Total sum of the volatile Tin species observed). extrapolated TVI concentrations ( Total sum of volatile Iodine species extrapolated from the relative abundance of iodomethane). a

b

c

Conclusion The three European estuaries investigated were found to be a source of gaseous species of Hg, Se, Sn and I. Seasonal results have been integrated and annually derived emission rates indicate that estuaries could contribute to a significant input of Hg and Se into the atmosphere. Since the larger uncertainty on flux estimations comes from the transfer velocity determination, our final flux calculations provides an order of magnitude of the natural evasion process. Available literature data for anthropogenic emission of trace elements are obtained within the same range of accuracy. Thus, the comparison between natural and anthropogenic emissions is suitable for biogeochemical assessments. In our estimation, volatile mercury and selenium emitted from European estuaries to the atmosphere represent 4 and 14% of European emissions from fossil fuel combustion, respectively [(132 tons of Hg in 1992 (24), 373 tons of Se in 1979 (25)]. We did not find available anthropogenic emission data concerning Mel, in order to evaluate the significance of Iodine evasion from European estuaries. For tin, fluxes do not appear to have a consequent impact on atmospheric input (26), but the importance of these volatile tin species has to be reassessed in term of contaminant remobilization and risk assessment in aquatic environments. In order to understand the biogeochemical pathways involved and


164 predict trace elements volatilisation processes, we have now to focus on turbid and low oxygen areas of the estuary (sediments, maximum TZ), which probably represent privileged sites for microbial methylation (e.g. Bu SnMe, Me Se). Estuarine plumes with large exchange surface with the atmosphere exhibit intense photobiological and photochemical processes which should also be investigated to accurately estimate fluxes from the whole estuarine system (e.g. Hg°, Mel). 3

2

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Global Biogeochemical Cycles; Butcher, S.S.; Charlson, R.J.; Orians, G.H.; Wolfe, G.V., Eds.; Academic Press; London, 1992; 379 pp. Thayer, J.S. Environmental Chemistry of the Heavy Elements: Hydrido and Organo Compounds; VCH Publishers: New York, 1995; 145 pp. Amouroux, D.; Donard, O.F.X. Mar. Chem. 1997, 58, 173-188. Kim, G.; Hussain, N.;Church, T.M. Tellus 2000,52B,74-80. Amouroux, D.; Liss, P.S.; Tessier, E.; Hamren-Larsson, M.; Donard, O.F.X. Earth Planet. Sci. Let. 2001, 189, 277-283. Cooke, T.D.; Bruland, K.W. Environ. Sci. Technol. 1987, 21, 1214-1219. Mason, R.P.; Fitzgerald, W.F.; Morel, F.M.M. Geochim. Cosmochim. Acta 1994, 58, 3191-3198. Amouroux, D.; Tessier, E.; Donard, O.F.X. Environ. Sci. Technol. 2000, 34, 988-995. Pécheyran, C.; Quétel, C.R.; Martin, F.M.; Donard, O.F.X. Anal. Chem. 1998, 70, 2639-2645. Amouroux, D.; Tessier, E.; Pécheyran, C.; Donard, O.F.X. Anal. Chim. Acta, 1998, 377, 241-254. Tseng, C.M.; De Diego, Α.; Pinaly, H.; Amouroux, D.; Donard, O.F.X. J. Anal. At. Spectrom. 1998, 13, 755-764. De Ligny, C.L.; Van Der Veen N.G. Rec. Trav. Chim. 1971, 90, 984-1001. Craig, P.J. In Organometallic compounds in the environment; Craig, P.J., Ed.; Longman: London, 1986; pp 37-64. Amouroux, D. Ph.D. thesis, University of Bordeaux I, Bordeaux, France, 1995. Duce, R.A; Liss, P.S.; Merrill, J.T.; Atlas, E.L.; Buat-Menard, P.; Hicks, B.B.; Miller, J.M.; Prospero, J.M.; Arimoto, R.; Church, T.M.; Ellis, W.; Galloway, J.N.; Hansen, L.; Jickells, T.D.; Knap, A.H.; Reinhardt, K.H.; Schneider, B.; Soudine, Α.; Tokos, J.J.; Tsunogai, S.; Wollast, R.; Zhou, M . Global Biogeochem. Cycles 1991, 5, 193-259.


165 16. Liss, P.S.; Merlivat, L. In The role of air-sea exchange in geochemical cycling; Buat-Ménard P., Ed.; D. Reidel Publishing Company: Dordrecht, Netherlands, 1986; pp 113-127. 17. Clark, J.F.; Schlosser, P.; Simpson, H.J.; Stute, M . ; Wanninkof, R.; Ho, D.T. In Air-water gas transfer; Jaehne B. and Monahan E., Eds.; Aeon Verlag: Hanau, Germany, 1995; pp 785-800. 18. Wilke, C.R.; Chang, P. AlChE J. 1955, 1, 264-270. 19. Saltzman, E.S.; King, D.B. J. Geophys. Res. 1993, 98, 16481-16486. 20. Tessier, E.; Amouroux, D.; Abril, G.; Etcheber, H.; Donard, O.F.X. Biogeochem. 2001, in press. 21. Amouroux, D.; Pécheyran, C.; Donard, O.F.X. Appl. Organomet. Chem. 2000, 14, 236-244. 22. Tessier, E.; Amouroux, D.; Donard, O.F.X. Biogeochem. 2001, in press. 23. Frankignoulle, M.; Abril, G.; Borges, Α.; Bourge, I.; Canon, C.; Delille, B.; Libert E.; Théate, JM. Science (Washington D.C.) 1998, 282, 434-436. 24. Pirrone, N.; Keeler, G.J.; Nriagu, J.O. Atmos. Envir. 1996, 30, 2981-2987. 25. Pacyna, J.M. Atmos. Envir. 1984, 18, 41-50. 26. Nriagu, J.O.; Pacyna, J.M. Nature (London) 1988, 333, 134-139.


Biogenic Volatilization of Trace Elements from European Estuaries

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