Intertidal Sedim ents Exposedto
SolarRadiation: AFieldExperim ent
J O A ˜ O C A N A Ä R I O * A N D
National Institute for Agronomy and Fisheries Research,
IPIMAR, Av. Brasõ Â lia, 1449-006 Lisboa, Portugal
C A R L O S V A L E
There is increasing evidence of the primary importance of
to the atmosphere. Although mercury in aquatic sediments
is efficiently retained, resuspension and bioturbation in
to solar radiation. Field experiments were performed to
investigate these processes. Anoxic sediments fromtwo
areas in the Tagus estuary with different degrees of Hg
contamination (experiments I and II) were homogenized
and distributed into two sets of 36 uncovered Petri dishes.
The samples were placed on the intertidal sediments
and exposed to direct solar radiation and kept under dark
(control) for 6-8 h. The decrease rates of acid volatile
sulfides (abrupt in the first 3 h) and of pyrite (linear) were
the same in sediments under solar radiation and dark.
The total Hg concentrations were relatively constant in
sediments kept indark, but decreasedfrom17.6to7.65and
3.45to 1.35nmol g-1in experiments I and II, respectively.
In those exposed to solar radiation during the period of
higher UV intensity. Similar evolutions were found in
nonreactive Hg in pore waters (3.00-2.59 and 0.725-0.105
nM). On the contrary, reactive Hg was higher in pore
waters of the sediments exposed to solar radiation and
increased with time, from424to 845pM and 53to 193pM.
These results indicate that most mercury released in
pore waters was photochemicallyreducedina shortperiod
of time and escaped rapidly to the atmosphere. Episodes
of bottomresuspension and bioturbation in the intertidal
sediments enhance the transfer of gaseous mercury to the
Mercury entered the environmental history because it was
the first metal with a direct connection between concentra-
tions in water, bioaccumulation up the food chain, and a
heavy metals, theglobal cycleof natural and anthropogenic
mercury has the highest vapor pressure of all metals (3).
Significant quantities are released by volcanic activity (4),
vegetation (5), volatilization from land (6, 7) and ocean
surfaces (8, 9), smelting of minerals, and burning of fossil
fuels (10, 11).
Among the volatile forms of mercury, Hg0is the major
constituent ofthedissolved gaseousmercury in open ocean
waters (8), and several works have shown its transfer to the
atmosphere(9-11). In thepast decadetherewasincreasing
evidence of the primary importance of photochemical
ability ofmany Hg(II) compoundstoabsorb partofthesolar
radiation (15) and theimportanceofFe(III) in thereduction
processes (16) are mechanisms proposed for the reduction
of Hg(II). Even solid surfaces such as HgS can also be
photochemically reduced (17). Soils naturally enriched in
Hg havebeen considered important sourcescontributing to
indicate that these natural sources may be comparable to
the anthropogenic sources in their impacts on regional and
global atmospheric Hg pools (22).
Anthropogenic Hg is retained in sediments nearby the
sources, recording the historical evolution of the anthro-
pogenic inputs (23, 24). Mercury in sediments is mainly
precipitated as sulfides, associated with iron sulfides, or
incorporated in organic matter (25-27). In many coastal
are periodically inundated and exposed to the atmosphere.
The topmost layer, which in many cases coincides with the
oxic layerofthesediments, isfrequently resuspendedby the
and mixed with suboxic and anoxic layers by macrofauna
activity (29).When resuspension orbioturbation isstronger,
anoxic sedimentsfrom theintertidalzoneareexposedtothe
atmosphere during the ebb tide. Moreover, dredging opera-
tions in harbors and navigational channels cause exposure
of anoxic sediments to the atmosphere. This paper reports
the effect of exposing anoxic sediments to the atmosphere,
under direct solar radiation and dark conditions, on the
alterations of mercury concentrations in solids and pore
waters. The field experiments were performed during the
atmospheric exposure of intertidal sediments in two areas
of the Tagus estuary with contrasting degree of contamina-
Experim ental Section
Description of theField Experiment. Thefield experiments
consisted of exposing anoxic sediments to the atmosphere
during 6-8 h, under direct solar radiation and dark condi-
tions. Theexperimentswereperformed in thetwo intertidal
areas in the Tagus estuary (A1 and A2), during the period
between two high tides when sediments were air-exposed,
in May and July 2003, respectively (Figure 1). The area A1 is
are heavily contaminated (30), while the area A2 is in the
Tagus Natural Park with sediments containing low Hg
contamination (31).Approximately 5kgofsuboxic toanoxic
sediments (between 5 and 10 cm depth) were collected in
each area, transferred into a decontaminated recipient,
homogenized, and distributed into 36 glass uncovered Petri
dishes (diameter 9.5cm). Transferred sediment covered the
entiresurfaceof thedish, and theheight wassuch (between
0.5 and 1 cm) as to facilitate the exposure of the sediment
to solar radiation. At each site 18 dishes were exposed to
direct solar radiation and another 18 were covered with a
black box in order to maintain the darkness inside. Every
hour two dishes exposed to solar radiation plus two kept in
temperature and the intensity of UV radiation (UV pronde,
The experiments ran between (GMT) 10:00 and 16:00 (A1,
experiment I) and between 9:00 and 17:00 (A2, experiment
II) under similar conditions of clear sky and sunshine. The
intensity of UV radiation increased pronouncedly by 11:00
+351.21.3015948; e-mail: email@example.com.
Environ. Sci. Technol. 2004, 38, 3901-3907
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Published on Web 06/10/2004
2004 American Chemical Society VOL. 38, NO. 14, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY93901
(654 times in experiment I and 440 in experiment II) and
then started decreasing by 15:00 (Figure 2). In both experi-
cycle, reaching a maximum of 39 °C in experiment I and 27
°C in experiment II around 14:00. As a result of these
conditions, the water content of sediments in the Petri dish
I). The temperature of light exposed sediments never
exceeded the values registered in dark sediments by 2 °C.
Analytical Methods. In the laboratory, samples were
filtered through 0.45 µm polycarbonate membranes (MSI,
Micron Separations Inc.) in a N2chamber. This procedure
was used in previous works, and errors associated with iron
and sulfur determinations were less than 6% (32). The
obtained pore waters were acidified to pH < 2 with HNO3
(Merck, mercury-free) and used for dissolved mercury
determinations. The solid fraction was oven dried at 40 °C,
disaggregated, homogenized, and stored in polyethylene
bottles for future analysis. Fresh portions of each sample
were frozen before centrifugation for acid volatile sulfide
used for determination of the water content by weight loss
at 105 °C.
Total Analysis of Solid Sediment. Total determinations
of the sediment samples with a mixture of acids (HF, HNO3,
and HCl) according tothemethod described by Rantalaand
Loring (33). Metal concentrations were obtained by flame-
AAS (Perkin-Elmer Aanalist 100) using direct aspiration into
N2O-acetylene flame (Al, Si, Ca, and Mg) or air-acetylene
flame (Fe and Mn). For total Hg determinations, sediment
wasdigested overnight at room temperaturewith 4M HNO3
in borosilicate glass Erlenmeyer flasks and then heated
(60-70 °C) for 2 h in a sand bath (21). Mercury was
determined by cold vapor AAS (Perkin-Elmer FIMS-FIAS-
100) using SnCl2‚2H2O as reduction agent. The precision
expressedasrelativestandarddeviation waslessthen 4%for
all metal investigate (p < 0.05). International certified
sample was stirred for 6 h with an NH2OH‚HCl (0.04 M)
solution in CH3COOH (25%) according to the method
described by Chester and Hughes (34). The extraction was
performed in leak-proof tubes sealed with Parafilm to
minimize possible losses of Hg0. The supernatant solution
was removed by centrifugation at 3000 rpm for 10 min and
filtered through 0.45 µm membranes. Iron and Mn in the
CV-AAS asdescribedabove.Detection limitsforFe, Mn, and
Hg were 0.80 and 0.60 µmol g-1and 0.02 nmol g-1, and
precision errors were 7, 6, and 3% (p < 0.05), respectively.
FIGURE 1. Map of the Tagus estuary with the site location of the
field experiments (A1 and A2).
FIGURE 2. Evolutions of UV radiation intensity (mW cm-2), sediment temperature (°C), and water content (%) during experiments I and
39029ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 14, 2004
Extraction with 1 M HCl. Acid volatile sulfides, mainly
amorphous iron sulfides and poorly crystallized iron oxides
were extracted with 1 M HCl (35, 36). Sulfide was trapped in
polarography (DPP) using a Metrohm apparatus equipped
with a 693 VA processor and a 694 VA stand. Fe, Mn, and Hg
weredetermined in theextracted solution by flame-AAS (Fe
and Mn) and CV-AAS (Hg) as described above. Recovery of
standard sulfidesolutionswas97% (32).Detection limitsfor
sulfide, Fe, Mn, and Hg were 0.01, 0.14, and 0.36 µmol g-1
and 0.04 nmol g-1and precision errors were 5, 2, 3, and 6%
(p < 0.05), respectively.
Determination of Pyrite and Elemental Sulfur. Pyrite
was determined in a 100 mg sediment sample using
the chromium reduced sulfur (CRS) method described
by Canfield (37). This method has proven to be specific
for inorganic sulfur (AVS + FeS2 + S0) with an accuracy
that is not affected by the presence of organic sulfur.
Before analysis, S0was extracted from the sample by 16 h of
stirring with 20 mL of acetone followed by centrifugation
(3000 rpm/10 min) and filtration through 0.45-µm mem-
branes. The residue was then placed in the reaction vessel
with 10 mL of 1 M HCl and purged with N2for 20 min to
release AVS. Finally, the CRS method was used in the last
residue to analyze the pyrite content. Elemental sulfur was
determined using the same CRS method in the acetone
extracts. The measurements of the released H2S were made
Dissolved Mercury in Pore Waters. Reactive dissolved
mercury was measured directly from the filtered acidified
solutions by cold vapor atomic fluorescence spectroscopy
(CV-AFS) using a cold vapor generator (PSA, model 10.003)
and weakly bound organic complexes easily reducible by
sameequipment afteraUV oxidation step with a1000W UV
lamp following the method described by Mucci et al. (39).
Nonreactive dissolved mercury was calculated by the dif-
ference between total and reactive mercury. The detection
limits and precision errors for Hgdisswere 0.01 nM and 4.0%
(p < 0.05), respectively.
the computer software Statistic. The normality of all data
was assessed by a Kolmogorov-Smirnoff test. Mean con-
centrationsofthechemical parametersused tocharacterize
way). For all other analysis, nonparametric tests Mann-
Whitney and Krustal-Wallis were performed.
of fine material with an absence of macrofauna. The mean
values of Al concentrations and of Si/Al, Fe/Al, and Mn/Al
ratios showed no significant differences (p < 0.05) between
the two sediments (Table 1). Low Si/Al ratios confirm that
sediments consist of fine particles. Sediments used in
experiment I were extremely rich in sulfur, the maximum
concentrations of AVS (663 µmol g-1) and pyrite (236 µmol
g-1) being 2 orders of magnitude higher than those in
Evolution of the Parameters during the Experiments.
Extractable Fe and Mn. The concentrations of Fe and Mn
extracted from the sediments with the hydroxylamine
solution did not vary significantly (p < 0.05) during the
experiments, although slight differences were observed
between solar radiation and dark conditions (Table 2).
Acid Volatile Sulfide, Pyrite, and Elemental S. The AVS
and pyriteconcentrationsin thesedimentsarepresented in
Figure3. Despitethedifferencesin sulfur content, thetime-
course evolutions of AVS were practically the same in the
sediments of the two experiments. The AVS decreased
abruptly in the first 3 h, the evolutions being independent
of the light conditions and described by polynomial curves
[AVS] ) 103(18t2- 21t + 6) (r2) 0.93) and [AVS] ) 45t2-
55t + 17 (r2) 0.88) in experiments I and II, respectively.
Pyrite concentration decreased linearly with time (r2better
than 0.91) in both experiments, although values were lower
in solar-radiation-exposed sediments. The slope was more
accentuated in experiment I (11.2 µmol g-1h-1) than in
experiment II (0.06 µmol g-1h-1). Elemental sulfur showed
no differences between light and dark conditions, both in
experiment I (34.7-36.8 µmol S g-1) and in experiment II
(0.38-0.41 µmol S g-1).
Total Hg in Solid Sediments. The evolutions of total Hg
in the two experiments are presented in Figure 4: concen-
trations were relatively uniform in sediments kept in dark,
but decreased considerably in those exposed to solar
despitethedifferenceson Hgconcentrations.In experiment
I, Hg decreased abruptly from 17.6 to 10.0 nmol g-1during
the first 3 h and to 7.65 nmol g-1at the end. The decrease
in experiment II was more gradual, from 3.45 to 1.35 nmol
g-1in 8 h.
Reactiveand NonreactiveHg in Sediment PoreWaters.
Evaporation differedin thesedimentsmaintainedatthetwo
experimental conditions. Mercury concentrations in sedi-
ment pore waters were thus corrected to the decrease of
TABLE1. M eanandStandardDeviations of Water Content (% ); Concentrations of Al (m m ol g-1), AVS, andPyrite (µm ol g-1S);
andSi/Al, Fe/Al andM n(×10-4)/Al M olar Ratios of Sedim ents fromExperim ents I andII
(%)(mmol g-1)(µmol g-1)
experiment I (n)6)
experiment II (n)6)
75 ( 2.2
84 ( 1.3
2.90 ( 0.02
2.30 ( 0.04
663 ( 12
2.41 ( 0.34
236 ( 3.34
0.91 ( 0.01
2.15 ( 0.07
2.59 ( 0.06
2.06 ( 0.04
1.89 ( 0.03
2.91 ( 0.05
2.30 ( 0.06
TABLE2. M eanConcentrations andStandardDeviations of Fe andM n(µm ol g-1) Sim ultaneously Extractedwitha H ydroxylam ine
SolutioninExperim ents I andII under Light (solar radiation) andDark Conditions
Fe (µmol g-1)Mn(µmol g-1)
experiment I (n)6)
experiment II (n)6)
384 ( 30.4
67.1 ( 13.3
412 ( 32.6
81.8 ( 11.6
5.64 ( 0.16
1.32 ( 0.18
5.41 ( 0.17
1.44 ( 0.19
VOL. 38, NO. 14, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 93903
water content and reported to the initial moisture. The
corrected concentrations of reactive mercury (HgR) and
nonreactive mercury (HgNR) are presented in Figure 5. A
similar pattern was observed in the two experiments: HgR
was higher in pore waters of the sediments exposed to solar
radiation and increased with the time, 424-845 pM (experi-
ment I) and 53-193 pM (experiments II). On the contrary,
HgNR concentrations decreased from 3.00 to 2.59 nM in
experiment I and from 0.725 to 0.105 nM in experiment II.
Extractable Hg in Solids. The mercury extracted by the
hydroxylamine solution was only detectable in the highly
contaminated sediments(experimentI) keptin dark (Figure
limit (0.02 nmol g-1) to 0.75 nmol g-1after 4 h. Mercury was
undetected (<0.04 nmol g-1) in the HCl extracts in all
sediments from the two experiments.
The exposure of anoxic sediments to solar radiation caused
both in solid sediments and in pore waters and an increase
of reactive dissolved forms. The transformations, which
occurred in theshort period of timethat solar radiation was
more intense, point to the possibility of mercury escaping
to the atmosphere when anoxic sediments are exposed to
solar radiation. This possibility is real in the intertidal flats
oftheTagusestuary, dueto strongercurrentsat spring tides
of strong wind. The escape may occur independent of the
degree of sediment contamination, as can be inferred from
the high proportion (> 50%) of mercury that escapes from
low- to high-contaminated sediments used in the field
experiments. To our knowledge, this is the first time that
release of mercury from intertidal sediments during their
temporary exposure to direct solar radiation is reported.
However, these observations are in concordance with the
exchanges on ocean surface. These emissions influence the
importantcompartmentin theglobal cycleofmercury (8,9).
The Mercury-Sulfur Association. The pronounced de-
creases of AVS and pyrite concentrations observed in all
sediments (Figure 3) indicate a rapid oxidation of sulfur
compounds during the experiments. The oxidation rates
apparently result from anoxic sediments being in contact
with the air, and no significant differences (p < 0.05) were
found between light and dark conditions. The decrease of
from the different oxidation kinetics (41, 44). Mercury is
Hg sulfides (26, 27) or being incorporated into iron sulfides
due to their poor solubility in 1 M HCl (44, 45), the
not excludetheexistenceof Hg in thoseforms. On theother
hand, it indicates that the amount of Hg incorporated into
also be associated with pyrite, as observed in other environ-
FIGURE 3. Evolutions of acidvolatile sulfides (AVS) andpyrite concentrations (µmol g-1S) insediments underlight(solarradiation) and
dark conditions in experiments I and II.
FIGURE 4. Evolutions of total mercury concentrations (nmol g-1,
dry weight) in sediments under light (solar radiation) and dark
conditions in experiment I (heavily contaminated sediment) and
experiment II (moderately contaminated sediment).
39049ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 14, 2004
ments (43), since the degree of trace metal pyritization
(DTMP) for mercury isoneofthehighest for all tracemetals
dark conditions, the production rates of reactive and non-
reactive mercury in pore waters (expressed on dry weight
basis) wererelatively constant: 0.021and0.138pmol g-1h-1
in experiment I and 0.002 and 0.004 pmol g-1h-1in
experiment II. These linear evolutions over the time had no
relation to the drastic decrease of AVS occurring in the first
2 h of the experiments. Indeed, mercury production in pore
waters persisted beyond the consumption of the entire AVS
in experiments I and II: 11.2 and 0.06 µmol g-1h-1. This
as pyrite is oxidized. The proportion of the oxidation rate of
pyriteandtheproduction rateofmercurydifferedin thetwo
in pyrite in the two sediments. Although oxidation of pyrite
results in the increase of reactive mercury, its variation was
pore waters (Figure 5). This similarity probably reflects an
equilibrium between dissolved species included in the two
mercury was also observed in vertical profiles of sediment
pore waters in previous studies (30, 46).
Hg in Pore Waters Exposed to Solar Radiation. Un-
doubtedly, solar irradiation of sediments led to additional
Furthermore, the considerable changes of reactive and
These changes are better illustrated by the evolution of the
([HgR]PW) and of total mercury in solids ([HgT]S), and of the
total dissolved mercury ([HgT]PW) presented in Figure7. The
is transferred from solids to pore waters, was greater under
light conditions, both in lowly and highly contaminated
sediments, and extended beyond theoxidation ofmostAVS.
The increase of the ratio [HgR]PW/[HgT]PW represents the
destruction of mercury complexes in pore waters and the
increase of simpler forms of Hg.
Possible Mechanisms for Hg Escape. The substantial
decrease of Hg in sediments coincided with the period of
higher UV radiation around 14:00 (Figures 2 and 4). Since
radiation had no effect on the pyrite oxidation, other
waters, namely the oxidation of particulate organic matter
by UV radiation. The direct photoreduction from the solid
sediments is not to be excluded, since HgS can be photo-
chemically reduced and semiconductor compounds (e.g.
metal oxides) can promote Hg photoreduction in providing
charge carriers and valence band holes (17). The increases
ofreactivemercury in porewatersat theend ofexperiments
I and II, under light conditions (0.29 and 0.05 pmol g-1,
respectively), can be considered negligible compared to the
decrease in solids (10 000 and 2100 pmol g-1, respectively).
reduced in short periods of time and escaped rapidly to the
Hg, CH3Hg+, CH3HgCl, CH3HgOH, CH3HgSH) can absorb
thehighlyenergetic UV ofthesolarspectrum (17,47,48) and
consequently be reduced in its excited state. Besides Hg(0),
theproduction and subsequent releaseof dimethylmercury
Mercury evasion was shown to correlate with temperature
(50, 51), and thus it could have accelerated the reactions
occurring in the exposed sediments during the periods of
FIGURE 5. Evolutions of reactive (HgR) andnonreactive mercury (HgNR) concentrations (pM andnM) insediment pore waters under light
(solar radiation) and dark conditions in experiments I and II.
FIGURE 6. Evolution of Hg concentrations (nmol g-1) extracted
with a hydroxylamine solution fromsediments under light (solar
radiation) and dark conditions in experiment 1.
VOL. 38, NO. 14, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 93905
higher temperature. Photoreduction by H2O2, which is
of DOC (16) could also be considered in experiment I due
to the high Fe(III) concentrations. According to Nriagu (17),
the photolysis rate is rather fast, occurring on the order of
seconds to hours. This explains therapid releaseof Hg from
sedimentsobserved in theexperiments.Apparently reactive
step of the following simplified scheme (Scheme 1). Interest-
was trapped in iron oxides and extracted by the hydroxy-
lamine solution (Figure 6). The incorporation has been
of iron oxides (30). This trapping was not observed under
light conditions. Two reasons may be invoked: the rapid
photoreduction of Hg(II) by the energetic UV radiation (17)
and the use of Fe(III) in the reduction process (16) offering
no possibilities to incorporate mercury. The trapping effect
was not detected in sediments used in experiment II that
contained lower Fe concentrations (Table 2).
Environmental Implications. The results obtained in
these field experiments suggest that the transfer of gaseous
dominated estuaries and coastal lagoons with extensive
intertidal areas. The thickness of oxic sediment in these
coastal environments is generally of millimeter scale (52),
and strongercurrents(28), winds, orbenthic organisms(29)
can resuspend this layer or transfer anoxic sediments to the
radiation. This transfer may occur in intertidal sediments
with different mercury contamination and moves forward
until Hg complexes susceptible of being destroyed by UV
radiation are present. Simple calculations permit us to
compare the magnitude of this transfer with mercury
concentration in the topmost sediments. In heavily con-
taminated sediments (e.g., area A1) with [Hg] ) 10 nmol g-1
and a flux of 235 nmol m-2h-1, the proportion of mercury
transferred to the atmosphere when exposed to solar
radiation during 6h is 56% of theamount existing in 0.5-cm
A2, [Hg] ) 3.5 nmol g-1and a flux of 37.5 nmol m-2h-1),
virtually all mercury is transferred to the atmosphere. The
during the period that anoxic sediments remained exposed
to direct solar radiation. All theseprocessesshould betaken
into consideration when trying to examine the regional and
global mercury budget.
The authors which to thank colleagues Eduarda Pereira,
Mo Ânica Va Âlega, Pedro Brito, and Joa Äo Lavrado for the help
from the comments of the anonymous reviewers. This work
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Received for review December 19, 2003. Revised manuscript
received May 5, 2004. Accepted May 10, 2004.
VOL. 38, NO. 14, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 93907