on Potential Radio Tracer

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ORIGINAL PAPER

Validation of methodology for determination of the mercury

methylation potential in sediments using radiotracers

Suzana iek& Sergio Ribeiro Guevara & Milena Horvat

Received: 21 December 2007 /Revised: 7 February 2008 /Accepted: 9 February 2008 /Published online: 3 March 2008# Springer-Verlag 2008

Abstract Experiments to determine the mercury methyla-

tion potential were performed on sediments from twolocations on the river Idrijca (Slovenia), differing in

ambient mercury concentrations. The tracer used was the

radioactive isotope 197Hg. The benefit of using this tracer is

its high specific activity, which enables spikes as low as

0.02 ng Hg2+ g1 of sample to be used. It was therefore

possible to compare the efficiency of the methylation

potential experiments over a range of spike concentrations

from picogram to microgram levels. The first part of the

work aimed to validate the experimental blanks and the

second part consisted of several series of incubation

experiments on two different river sediments using a range

of tracer additions. The results showed high variability in

the obtained methylation potentials. Increasing Hg2+ addi-

tions gave a decrease in the percentage of the tracer

methylated during incubation; in absolute terms, the spikes

that spanned four orders of magnitude (0.019190 pg g1 of

sediment slurry) resulted in MeHg formation between 0.01

and 0.1 ng MeHg g1 in Podroteja and Kozarska Grapa.

Higher spikes resulted in slightly elevated MeHg produc-

tion (up to a maximum of 0.27 ng g1). The values of

methylation potential were similar in both sediments. The

results imply that the experimental determination of

mercury methylation potential strongly depends on the

experimental setup itself and the amount of tracer added tothe system under study. It is therefore recommended to use

different concentrations of tracer and perform the experi-

ments in several replicates. The amount of mercury available

for methylation in nature is usually very small. Therefore,

adding very low amounts of tracer in the methylation

potential studies probably gives results that have a higher

environmental relevance. It is also suggested to express the

results obtained in absolute amounts of MeHg produced and

not just as the percentage of the added tracer.

Keywords Mercury . Methylmercury . Mercury

methylation . 197Hg radiotracer. Sediment

Introduction

Mercury, although naturally present in the Earths crust, is a

global pollutant mostly arising from human activities.

Therefore, the management of and policy regarding mer-

cury pollution is a global challenge [1]. It is well known

that the formation and bioaccumulation of monomethyl-

mercury (MeHg) is the most critical aspect of environmen-

tal quality regarding Hg pollution due to its accumulation

and biomagnification properties in the food chain. There-

fore the reduction of MeHg formation can be defined as the

priority in remediation options [2]. This applies in

particular to sites that are heavily contaminated with

mercury or sites with ecosystem characteristics that favour

methylation of mercury even at much lower concentration

levels, and are therefore identified as sensitive areas [3].

The present study was implemented in an area contam-

inated due to past mercury mining activities, in Idrija,

Slovenia (Fig. 1). Mercury concentrations have long been

Anal Bioanal Chem (2008) 390:21152122

DOI 10.1007/s00216-008-1968-1

S. iek (*) : M. Horvat

Department of Environmental Sciences, Joef Stefan Institute,

Jamova 39,

1000 Ljubljana, Slovenia

e-mail: [email protected]

S. Ribeiro Guevara

Laboratorio de Anlisis por Activacin Neutrnica,

Centro Atmico Bariloche,

8400 Bariloche, Argentina


2/8

monitored [4] in air [5, 6], soil [79] and the river Idrijca [4,

912]. The inorganic mercury that enters the river undergoes

various transformations [11]. The step in this biogeochemical

cycling that is of greatest concern is methylation. Because of

its toxicity and its potential to bioaccumulate and biomagnify,

methylmercury (MeHg) poses a threat not only to the local

community but also to locations downstream of Idrija and in

the Gulf of Trieste [13]. Therefore, it is important to be able tocorrectly estimate the methylation potential of the Idrijca river

system. The aim of this work was to assess the efficiency of

laboratory Hg methylation experiments on riverine sediments.

Methylation potential experiments usually involve spiking

the sample with inorganic mercury as a tracer and extracting

MeHg after an incubation period. The tracers used are either

enriched stable isotopes [1416] or radioactive isotopes,

usually203

Hg [11, 17, 18]. The amount of mercury spiked

also varies considerably. Ramlal et al. [19] added 203Hg as

HgCl at a concentration of 2 g g1 dry sediment. 203HgCl2was also used by Hines et al. [11] at concentrations of 40

100 ng mL1 of sediment slurry. Guimares et al. [17] spiked50100 mL of sediment with 2 g of 203HgCl2. Hintelmann et

al. [14] used enriched 199Hg(NO3)2 in additions that increased

the total mercury (THg) concentrations in sediments by 10

13%, which meant spikes of 1930 ng g1 dry weight. It

should be stated, however, that the natural concentrations of

inorganic mercury ions in sediments constitute only about 1%

of the total mercury concentration [8]. Rodrguez Martn-

Doimeadios et al. [15] increased ambient THg in estuarine

sediments by 0.498 nmol 199Hg g1 dry weight (66.7 ng g1)

by spiking with 199HgCl. The results for the mercury

methylation potential can be expressed as the percentage of

methylmercury formed per day [16], nanograms of MeHg

formed per day per gram of sediment [14], percentage of

spiked inorganic mercury methylated per gram per hour [17,

19] or the percentage of tracer converted to MeHg per day

[11]. In the report by Rodrguez Martn-Doimeadios et al.

[15], rate constants for methylation were calculated based onthe ratio of the different stable isotopes added.

The tracer used in the present study was the radioactive

isotope 197Hg (t1/2=64.14 h), obtained in an experimental

nuclear reactor by irradiating mercury 51.58% enriched in

the isotope196

Hg (the natural abundance of196

Hg is

0.15%). The benefit of using this tracer is its high specific

activity, which enables spikes as lowas 0.02 ng Hg2+ g1sample.

The present work was conducted as a continuation of the

experiments performed using 197Hg radiotracer [18] where

the feasibility of using this tracer was confirmed. Owing to

its high specific activity, it is possible to compare the

efficiency of methylation potential experiments over arange of spike concentrations from picogram to microgram

levels. These experiments were performed on sediments

from two locations on the river Idrijca, differing in ambient

mercury concentrations. This was done to establish how

these differences affect the determination of the methylation

potential. Net mercury methylation in riverine sediments is

very low compared with marine and estuarine sediments,

mostly due to the lower content of sulfate-reducing bacteria.

The estimation of net methylation should not be depen-

dent on the methodology or the amount of added tracer. The

present study was conducted to verify that the methodology

of tracer experiments is a valid way of estimating mercury

methylation potential.

Another consideration was addressed in connection with

methylation potential experiments, namely the way of

determining the experimental blanks. A certain amount of

the inorganic mercury spike can be carried over into the

organic solvent during extraction, which leads to overestima-

tion of MeHg formation. Another source of error could stem

from adding the tracer before inhibiting bacterial activity, thus

enabling biotic mercury methylation to occur. A separate set

of experiments was therefore designed in which bacterial

activity was inhibited either before or after the spike addition.

Methods

Sampling and sample preparation

Sediment samples were collected at two sites on the river

Idrijca (see Fig. 1). Sampling took place over four

consecutive weeks so that each experiment was performed

on a fresh batch of sediments. All sample manipulations

Podroteja

Kozarska Grapa

Fig. 1 Location of the river Idrijca and the sampling sites

2116 Anal Bioanal Chem (2008) 390:21152122


3/8

and tracer additions were performed in a glove box under anitrogen atmosphere in order to maintain the redox

potential of the samples, their anoxic nature and to disturb

their bacterial populations as little as possible. Organic and

water contents as well as total and methylmercury concen-

trations were measured (Table 1). To estimate the amount of

Hg(II) already present in the samples, reactive mercury

(RHg) was measured in pore water. THg and MeHg in pore

water had been measured in a previous research and were

shown to be 2001,800 ng L1 and 6200 ng L1, respec-

tively. THg in pore waters was measured using UV

digestion and cold vapour atomic absorption spectrometry

(CV AAS) detection [20] and MeHg was measured afterdistillation, derivatisation, gas chromatographic separation

and detection by CV atomic fluorescence spectrometry

(AFS) [21]. Sulfate concentrations in the water of river

Idrijca are 6.1936.4 mg L1 (Dr. Tjaa Kandu, 2007,

personal communication) and therefore the concentrations

in sediment are not expected to be high.

Experimental setup

A scheme of all the steps involved in the experimental work

is shown in Fig. 2.

The inorganic mercury tracer was produced fromelemental mercury 51.58% enriched in the 196Hg isotope

(Isoflex, CA, USA). Hg0 was dissolved in 2% HNO3 and

its concentration in the solution was determined by CV

AAS [20] to be 0.057 mg mL1. A 1-mL aliquot of this

solution was irradiated in a sealed quartz ampoule in the

central irradiation facility (th=11013 n cm2 s1) of the

TRIGA Mark II (250 kW) research reactor of the Joef

Stefan Institute, Slovenia. Irradiation times were 1015 h.

Fresh tracer was prepared every week for each set of

experiments for four consecutive weeks [18].

For the methylation potential experiments sediment

samples were mixed with river water from the same siteand 3 g of the slurry was subsampled into Teflon vials.

After spiking with 197Hg2+ , the samples were vortexed.

Incubation samples were left in the dark at room temper-

ature for 24 h. MeHg was then extracted into 10 mL of

toluene after adding 7 mL of 4 M KBr and 7 mL of 4 M

H2SO4, saturated with CuSO4. For control samples the

extractions took place immediately after spiking. The

activity of 197Hg in the toluene extracts was measured on

a well-type HPGe (high purity germanium) detector. All

experiments were performed in triplicate. A detailed

procedure is described elsewhere [18].

In order to approximate the amount of inorganic mercury

that is reduced during incubation, an experiment was

performed to measure Hg2+ reduction to Hg0 simultaneously

with the methylation experiments. This was done in one set

of experiments. The Hg0 vapour released during incubation

Table 1 Characteristics of the examined sediments and their mercury

concentrations

Parameter Sampling site

Podroteja

Sampling site

Kozarska Grapa

Organic content (LOI) 18.3 4.1% 0.35 0.05%

Water content 43.1 2.0% 35.80.7%

THg 182

38.7 ng g11760

123 ng g1

MeHg 0.18

0.07 ng g11.18

0.06 ng g1

Hg(II) in pore water 2.70

0.22 ng L16.02

0.30 ng L1

Hg(II)/THg 6.4104% 1.2104%

THg and MeHg are given on a wet weight basis. All the results are

averages of three replicate measurements.

LOI loss on ignition

sediment collection from Podroteja and

Kozarska Grapa in four consecutive weeks

sediment subsampling in glove-boxunder N2 atmosphere

radiotracer addition subsampling of standards radiotracer measurement

validation of experimental blanks

control runs extractionimmediately after tracer addition

incubation of samples for 24hours at room temperature

extraction

extraction

radiotracer measurementin toluene

radiotracer measurementin toluene

Fig. 2 Flow chart of the exper-

imental setup

Anal Bioanal Chem (2008) 390:21152122 2117


4/8

was flushed out by a flow of N2 from the incubation vessel

and trapped on selenium-coated paper [22], which was then

measured on a coaxial HPGe -ray and x-ray detector by

placing the glass tube on the detector cap [18].

The methylation experiments were performed on sedi-

ments from the two sites on the river Idrijca, one high and

the other low in ambient mercury concentrations. The

experiments were carried out with spikes containing 0.019,0.19, 0.94, 1.9, 4.7, 19, 47, 190 and 1,900 ng Hg g1

sediment slurry.

Results and discussion

The aim of this work was to verify the validity of radio-

tracer experiments for the assessment of mercury methyl-

ation potential in riverine sediments. In order to do this, the

first part of the work was to validate the experimental

blanks and the second part consisted of several series of

incubation experiments on two different sediments using arange of tracer additions.

Validation of experimental blanks

The method used for inhibiting bacterial activity after

adding the inorganic mercury tracer was to add the

extraction reagents which contain sulfuric acid and potas-

sium bromide. This method proved to be efficient in

stopping bacterial activity compared with other methods

used in similar works, namely flash freezing, heating or

gamma-ray irradiation [23]. In order to ascertain that no

biotic mercury methylation takes place in the time between

adding the tracer and starting the extraction (ca. 20 s), a

separate experiment was designed. A set of sediments was

taken from the Podroteja sampling site. Subaliquots of 3 g

were spiked with Hg2+ either immediately or after adding

the extraction reagents (KBr, H2SO4 and CuSO4). Two

levels of Hg2+ spike were used: 19 ng g1 wet sediment and

1.9 ng g1 wet sediment. To compare the results of the two

controls with biotic methylation, a separate set of samples

was incubated for 10 min after spiking. Each experiment

was done in three independent replicates. The results are

presented in Fig. 3 for each replicate separately and as an

average, and the standard deviation indicated by the error

bar. Figure 4 shows the absolute values of MeHg pro-

duction. There were no significant differences between the

two procedures, whereas after 10 min of incubation there

was a significant increase in the activity of 197Hg in the

toluene extracts. The blank experiments at two different

spike concentrations revealed that the amount of inorganic

mercury carried over is dependent on the amount of

inorganic mercury spiked into the sediment. This is in

agreement with data obtained previously [18].

We also concluded that bacterial mercury methylation

does not take place within the time it takes to add the

reagents after spiking the samples. The activity observed in

the extracts is therefore probably inorganic mercury carried

over during extraction. However, one cannot completely

exclude the process of abiotic methylation, as it is well

known that in sediments and soils a number of methyl

group donors are present, such as methyl cobalamine

(CH3CoB12) and humic and fulvic acids [24]. Among these

compounds humic matter is the most likely methylating

agent, since it is ubiquitous in aquatic environments, is

associated with mercury circulation, complexes mercury,

and methylates Hg2+ in model studies [25]. Nagase et al.

[26] demonstrated that abiotic methylation of divalent

mercury can occur through humic substances. In fresh-

waters, oxidised mercury is to a large extent bound to sulfur

groups (thiols) in humic molecules. However, humic matter

contains several different kinds of functional groups and,

besides coordination to sulfur, mercury is probably addi-

tionally coordinated to neighbouring carboxylic groups

[27]. In this sense, Weber [25] emphasised that abiotic

methylation, which includes methylation by chemicals

released to the environment by biotic processes, may be a

primary driver of CH3Hg+ production. In addition, at

polluted coastal sites, sediments may contain organometal-

lic compounds (e.g. organotins) that may also be donors of

0,000

0,002

0,004

0,006

0,008

0,010

Hg2+

addition: 19 ng.g-1

relative

197HgafterMethyl-Hgextractio

n(%.g

-1W

S)

fast incubationreal kill

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

Hg2+

addition: 0.19 ng.g-1

repl. 1

repl. 2

repl. 3

average

reagents

BEFORE

tracer

reagents

AFTER

tracer

relative

197HginMeHgextracts(%g

-1W

S)

10 min. incubation

Fig. 3 Comparison of tracer recoveries in MeHg extracts when

adding reagents before the tracer or after the tracer and after 10 min of

incubation. Hg2+ additions: 0.19 and 19 ng g1 wet sediment



5/8

organic ligands, resulting in formation of organomercury

compounds [28]. It is therefore suggested that at all studysites these possibilities be verified in order to assess biotic

versus abiotic methylation and the amount of inorganic

mercury carried over to the organic phase. The quantity

related to the carryover of inorganic mercury constitutes a

blank that needs to be subtracted from the activities

obtained in the extracts after incubation.

Reduction experiments

In one incubation experiment on Podroteja sediment,

selenium traps were included in order to estimate the

amount of mercury that was reduced and volatilised from

the samples during incubation. The results are shown in

Fig. 5a. Volatilisation of Hg0 was very low and constant at

about 0.02% of the added tracer. At very high tracer

additions, however, the evaporation of mercury from the

sample increased to approximately 0.1%. Hg volatilization

measurement was used to estimate the mass balance of the

system under study. It was determined that approximately

99% of Hg2+ added to the sample remained in the inorganic

form or underwent the reverse transformations of methyl-

ation and demethylation.

If the results are expressed as the amount of mercury

recovered as Hg(0), as shown in Fig. 5b, we may conclude

that the amount of evaporated mercury increases linearly

with amount of Hg2+ spikes.

Methylation experiments

Figures 6 and 7 show the dependence of the blanks on the

amount of added tracer, both as a percentage and in

absolute terms. There is a linear relationship between the

amount of tracer in the spike and in the extracts (linear

fitting gives =0.91 for Podroteja sediments and =0.92

for Kozarska Grapa). It can be concluded from the control

samples that a certain amount of the inorganic tracer is

always carried over into the organic solvent and this is

measured in the extracts. This is in agreement with the

results obtained earlier [18], where it was shown that

mercury carried over in the blank experiment is only

inorganic mercury. This was confirmed by the use ofvalidated analytical methods for mercury speciation in

aqueous samples based on derivatisation, gas chromato-

graphic separation and detection by CV AFS [21].

Figures 8 and 9 show the methylation potential in

sediments following 24-h incubation after the control

values (which include real blank, carryover of inorganic

mercury and any abiotic formation of MeHg) were sub-

tracted. The subtraction was performed on average values.

Methylation was considered significant if the result was

1 10 100 1000

0.00

0.05

0.10

Hg2+

spikes (ng.g-1

WS)

Hg

2+ redu

cedtoHg

0 after24hincubation(%.g

-1W

S)

replicate 1

replicate 2

replicate 3

average

1 10 100 1000

1E-4

1E-3

0,01

0,1

1

10

Hg2+

addition (ng.g-1

wet sediment)

Hg

0p

roduction(ng.g

-1W

S)

a

b

Fig. 5 a Hg2+ reduction to Hg0 after 24-h incubation in sediments

from Podroteja. b Absolute Hg0 production after 24-h incubation in

sediments from Podroteja (log fitting, =0.9997)

0,000

0,001

0,002

0,003

0,004

0,005

Total197HgafterM

ethyl-Hgextraction(ng.g

-1W

S)

10 min. incubationfast incubationreal kill

Hg2+

addition: 0.19 ng.g-1

Hg2+

addition: 19 ng.g-1

reagentsBEFOREtracer

reagentsAFTERtracer

Total197Hgin

MeHgextracts(ng.g

-1W

S)

Fig. 4 Comparison of absolute MeHg production when adding

reagents before the tracer or after the tracer and after 10 min of

incubation. Hg2+ additions: 0.19 and 19 ng g1 wet sediment



6/8

higher by more than two standard deviations from the mean

blank value. In cases where the variability of the incubation

results was high, but the incubation values were higher than

the controls, three standard deviations from the mean

control values were regarded as the criteria for significant

methylation.

The results obtained show high variability in the esti-

mated methylation potentials. Increasing Hg2+ additions

results in a decrease in the percentage of the tracer that is

methylated during incubation, but in absolute terms the

production of MeHg increases slightly. The values of

methylation potential are similar in both sediments. In our

study area this suggests that inhomogeneity of samples

plays a more important role in the determination of mercury

methylation potential than ambient mercury concentrations.

The distribution of bacterial colonies in sediments isscattered and the microhabitats are only a few micrometres

in size. These factors strongly influence the potential of the

sediment to methylate mercury.

The results imply that the experimental determination of

mercury methylation potential strongly depends on the

experimental setup itself and the amount of tracer added to

the system under study. It is therefore recommended to use

different concentrations of tracer and perform the experi-

ments in several replicates. The amount of mercury avail-

able for methylation in nature is usually very small.

Therefore, adding very low amounts of tracer in methyla-

tion potential studies probably gives results that have higher

environmental relevance. It is also suggested to express the

results obtained in absolute amounts of MeHg produced, as

well as a percentage of the added tracer. This gives more

information and enables estimations of the mercury mass

balance in the environment. Moreover, if the results are

expressed as the amount of mercury methylated, the

variability between the experiments using different spikes

is much smaller compared with the results expressed as a

percentage of mercury added to the sediment sample. As

shown in Figs. 8 and 9, spikes that span four orders of

magnitude (0.019

190 pg g

1

sediment slurry) result inMeHg formation between 0.01 and 0.1 ng MeHg g1 in

Podroteja and Kozarska Grapa. Higher spikes seem to result

in slightly elevated MeHg production (up to a maximum of

0.27 ng g1). If the results are expressed as the percentage

of mercury spiked, the variability seems to be much higher,

with a very significant trend to a lower percentage as the

spike concentrations increase. In the literature, however,

0,1 1 10 100 1000

1E-3

0,01

0, 1

run 1

run 2

0.1 1 10 100 1000

0.01

0.1

run 1

run 2

Hg2+

addition (ng.g-1

wet sediment)

197Hgrelativemeasurements

(%.g

-1W

S)

Hg2+

addition (ng.g-1

wet sediment)

197Hg

2+c

arryover(ng.g

-1

WS)

Fig. 7 Average relative and average absolute 197Hg measurements in control samples from Kozarska Grapa

0.01 0.1 1 10 100 1000

1E-3

0.01

0.1

197Hgrelative

measurements(%.g

-1W

S)

Hg2+

addition (ng.g-1

wet sediment)

run 1

run 2

run 3

single spike

0,01 0,1 1 10 100 1000

1E-4

1E-3

0,01

197Hg

2+ carryover(ng.g

-1W

S)

Hg2+

addition (ng.g-1

wet sediment)

run 1

run 2

run 3

Fig. 6 Average relative and average absolute 197Hg measurements in control samples from Podroteja



7/8

most researchers express their results as the percentage of

spiked mercury methylated. In order to be able to compare

results between various studies, a normalized and harmo-

nized way of expressing results of tracer experiments needsto be agreed upon.

This work was performed on riverine sediments, since the

study area was the Idrijca river system. However, estuarine,

marine and lake sediments have different characteristics,

more abundant bacterial communities and therefore higher

mercury methylation potentials [24]. Similar experiments on

those sediment types would probably give different results

and are recommended for further work.

The method using 197Hg2+ as the tracer is extremely

sensitive and enables experiments in the range of concen-

trations from picogram to microgram levels. The detection

limit was approximately 0.001% of the added tracer, whichenabled the detection of even the slightest transformation.

However, unlike stable isotope methodologies, where the

measurements are performed on an ICP-MS where it is

possible to determine individual compounds, the drawback

of using radioactivity as the measure of transformations is

that there is a possibility of undetected contamination of

extracts with inorganic mercury. In order to prevent this, it

is helpful to perform a backextraction of MeHg from the

extracts and ascertain that there is no inorganic contamina-tion by measurement with standard analytical methods such

as CV AFS.

Conclusions

It has been shown that mercury methylation potential

experiments using tracer additions can successfully be

applied with extremely low spikes to prevent unnecessary

perturbation of the samples. Experiments with various

concentrations of spiked mercury showed that if the results

are expressed as the amount MeHg formed the variabilitiesfound in spikes lower than 190 ng g1 are mostly related to

the inhomogeneity of the samples. At higher spikes slightly

higher MeHg formation was found. However, if the results

for MeHg are expressed as percentage mercury used to

spike the samples, the variabilities are significant, showing

1 10 100 1000

0,01

0,1

1

run 1

run 2

1 10 100 1000

1E-3

0,01

0,1

run 1

run 2

Hg2+

addition (ng.g-1

wet sediment)Hg2+ addition (ng.g-1 wet sediment)

MeHgrelativeproduction

(%.g

-1W

S)

MeHgproduction(ng.g

-1W

S)

Fig. 9 Average relative 197Hg measurements and average MeHg production after incubation in samples from Kozarska Grapa

1 10 100 10001E-3

0,01

0,1

1

1 10 100 10001E-4

1E-3

0,01

0,1run 1run 2run 3

run 1

run 2

run 3

Hg2+ addition (ng.g-1 wet sediment)Hg2+ addition (ng.g-1 wet sediment)

MeHgrelativeproduction(%.g

-1WS)

MeHgpro

duction(ng.g

-1W

S)

Fig. 8 Average relative 197Hg measurements and average MeHg production after incubation in samples from Podroteja



8/8

a strong decreasing trend of MeHg with increasing spikes.

This indicates that the sediments have a capacity for MeHg

formation, regardless of the spikes used in this study. In

other worlds, the amount of mercury that is subjected to

methylation is rather small. It is very important that the

results are correctly expressed. As in all experiments with

sediments, sample inhomogeneity is also a large source of

variability.In environmental studies on areas such as the mercury-

polluted site in the Idrija region, where it is important to

know how the system will react to a potential additional

input of mercury into the water system, the results of such

laboratory experiments have some limitations and should

be compared with the real environmental changes. Con-

clusions based on laboratory experiments can be misleading

unless carefully done and interpreted.

The use of high specific activity 197Hg2+ radiotracer

enables laboratory tracer experiments to follow mercury

methylation and reduction processes over a wide range of

concentrations. It was therefore possible to perform aninvestigation on how these different concentrations affect

the estimation of mercury methylation potential. The results

have shown that care is needed when estimating methyla-

tion potential and interpreting the results in their environ-

mental context.

Acknowledgements This work was implemented in the framework

of the bilateral cooperation between Slovenia and Argentina entitled

The production and the use of radiotracers in the biogeochemistry of

mercury, the young researchers programme, the programme P1

0143 Environmental cycling of nutrients and contaminants, mass

balance and modelling of environmental processes and risk assess-ment and the project L17407 Biological methods as an early

warning system in mercury contaminated sites. The authors also wish

to express their gratitude to Dr. Anthony Byrne for his constructive

remarks and help with the English language.

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