Voltammetric determination of elemental sulfur in pore waters
Feiyue Wang, André Tessier1
INRS-Eau, Université du Québec, Sainte-Foy, Quebec GlV 4C7, Canada
Jacques Buffle
CABE, Sciences II, Université de Genève, 30 quai Ernest-Ansermet, CH 1211, Genève, Suisse
Abstract
We describe a new simple, reliable, and sensitive method that employs square-wave cathodic stripping voltammetry (SWCSV) for the measurement of dissolved elemental sulfur, S(0), in pore waters. The method, which requires
only small pore-water samples (~1.5 ml), shows a linear relationship over three orders of magnitude (~10-9-10-6
Eq S liter-1) between peak current or peak area and S(0) concentration; the practical detection limit (3 X 10-9 Eq
S liter-1) is below that required for most pore waters. Experiments carried out over a wide range of pH (2.5-11.5)
indicate that peak current and peak potential vary with pH and that thiosulfate, sulfite, and thiols (and probably
humic substances) would not interfere with the determination of S(0) over this pH range; interference from sulfide
is easily removed by acidifying and bubbling with N,. The slopes (nA/Eq S liter-l) obtained by standard additions
of S(0) to pore-water samples containing 10 mg liter-l dissolved organic carbon were not statistically different from
the slopes of calibration curves, indicating that fouling effects due to natural organic matter should be minimal.
Porewater concentration profiles obtained at an anoxic site of a Shield lake indicate that S(0) concentrations were
greater than those predicted from equilibrium with orthorhombic sulfur.
reliable analytical methods to measure the low concentrations of elemental sulfur in small volumes of pore waters
(Wilkin and Barnes 1996).
No method is available at present to measure individual
polysulfide species directly in water samples. Instead, individual polysulfide species concentrations have to be calculated from measured pH and total concentrations of dissolved elemental sulfur and sulfide, using appropriate
equilibrium constants for the formation of the polysulfide
species (Giggenbach 1972, 1974; Cloke 1963a). Methods are
available to measure total dissolved sulfides at the levels
found in natural waters (e.g., Cline 1969), but measurement
of total dissolved elemental sulfur at low levels (below 10-7
M) in small pore-water samples remains a challenge.
Several approaches have been reported for measuring total
dissolved elemental sulfur. One of these involves extracting
elemental sulfur in an organic solvent such as chloroform
and measuring UV absorbance (Boulegue et al. 1979; Gagnon et al. 1996); however, this method is not sensitive, requires relatively large volumes of water, and is prone to interferences from organic matter. Several other methods are
based on the transformation of dissolved elemental sulfur to
thiosulfate by the addition of sulfite according to the reaction
Elemental sulfur, S(O), is formed during the biotic and
abiotic oxidation of dissolved sulfide and solid metal monosulfides (Chen and Morris 1972; Hoffman 1977; Pyzik and
Sommer 1981; Troelsen and Jorgensen 1982; Moses et al.
1987). It is only sparingly soluble in water; however, its
solubility can be greatly increased in sulfidic waters by its
reaction with dissolved sulfide to form polysulfide species
(Giggenbach 1972; Boulegue 1975). Both elemental sulfur
and polysulfides are actively involved in the formation of
pyrite (Cutter and Velinsky 1988; Wilkin and Barnes 1996)
and in the transformation of sedimentary organic matter
(Aizenshtat et al. 1995). Polysulfides also form strong complexes with class B metal ions such as Au(I), Cu(I), Hg(II),
and Ag(I) (Holtje and Beckert 1935; Cloke 19633; Shea and
Helz 1988; Berndt et al. 1994; Thompson and Helz 1994;
Paquette and Helz 1997), which may affect solubility and
speciation of these metals (Gardner 1974; Huerta-Diaz et al.
1998) and hence their bioavailability
and cycling. Despite
the geochemical and environmental importance of elemental
sulfur and polysulfides in sedimentary environments, there
are only limited data available on their abundance in marine
pore waters (Boulegue et al. 1982; Luther et al. 1986, 1985;
Gagnon et al. 1996) and no data for pore waters of freshwater systems. This lack of data is the result of a paucity of
1Corresponding author.
Acknowledgments
Financial support from the Natural Sciences and Engineering Research Council of Canada, the Québec Fonds pour la Formation de
Chercheurs et 1’Aide à la Recherche, and the U.S. Environmental
Protection Agency is acknowledged. F.W. was supported by a postdoctoral fellowship from the Institut National de la Recherche
Scientifique. R. Rodrigue is gratefully acknowledged for his
SCUBA diving skill, L. Rancourt for her assistance with the voltammetric techniques, and L. Hare and two anonymous reviewers
for critical comments on the manuscript.
where Sin2-,with n = 2-6, represents polysulfide species.
Thiosulfate generated by Eq. 1 is then measured by iodometry (Boulègue and Popoff 1979), by titration with mercuric
chloride using an Ag/Ag2S electrode (Boulègue and Popoff
1979; Boulègue et al. 1979), or by polarographic or voltammetric techniques (Renard et al. 1975; Luther et al. 1985).
The titrations are time consuming, require relatively large
volumes (20-50 ml) of water, and have detection limits
(~10-6M)
often above dissolved sulfur concentrations in
the pore waters of freshwater systems. Among the methods
proposed to measure the thiosulfate produced by Eq. 1, the
1353
1354
Wang et al.
differential pulse polarographic method (DPP) proposed by
Luther et al. (1985) is probably the most convenient for pore
waters; indeed, it requires small volumes (2-5 ml) and presents an acceptable detection limit. However, Clarke et al.
(1994) reported recently that the addition of sulfite to polysulfide standards gave variable and inconsistent results and
they claimed that increases of sulfide and thiosulfate did not
follow the stoichiometry of Eq. 1.
Taking advantage of the good performance (selectivity,
sensitivity, accuracy, low volume of sample required) of voltammetric techniques, researchers have suggested a few other voltammetric methods to measure S(0). For example, Batina et al. (1992) proposed that dissolved elemental sulfur
could be estimated by phase-sensitive alternating-current
(a.c.) voltammetry from the ratio of anodic to cathodic currents. They claimed that the technique was capable of determining simultaneously dissolved sulfide and elemental
sulfur without alteration of the sample. Recovery of sulfur
standards by this method was, however, not acceptable (Batina et al. 1992). A problem encountered when using polarographic or voltammetric techniques is that both dissolved
sulfide and elemental sulfur are measured at the same potential and thus interfere with each other’s determination. To
circumvent this problem, Buffle et al. (1987) suggested measuring dissolved elemental sulfur by cathodic sweep voltammetry after acidification and purging of the sample with
an inert gas to remove all sulfide; they reported that it was
necessary to add ethanol to the water sample to maintain the
elemental sulfur in dissolved form.
The present paper describes the development of a simple,
reliable, and sensitive voltammetric method for the measurement of dissolved elemental sulfur in pore waters; in
developing the method, we built on the work of Buffle et al.
(1987) and Zali (1983). The method is then applied to determine for the first time dissolved S(0) profiles in lake sediment pore waters at a l-cm vertical resolution.
Materials
and Methods
Instrumentation and operating conditions- The voltammetric measurements were performed by square-wave cathodic stripping voltammetry (SWCSV) with a computercontrolled BAS-100B (Bioanalytical Systems Inc.) polarographic analyzer connected to a 663 VA stand (Metrohm,
Switzerland)
in the hanging mercury drop electrode
(HMDE) mode. The medium size of a mercury drop was
used. The reference electrode was an Ag/AgCl (saturated
KCl) electrode. All potentials are reported relative to this
reference electrode. A glassy carbon rod served as the auxiliary electrode. Solutions to be analyzed had a total volume
of 4 ml comprising 2.0 ml H2O, 2.0 ml ethanol, and 0.04 ml
tetrahydrofuran (THF); they were contained in small Princeton Applied Research glass polarographic cups (25 ml) installed inside a Metrohm polarographic cell and were deoxygenated for 5 min with N2 bubbled through a bubbling
bottle containing Mini-Q water and THF and ethanol in the
same proportions as in the polarographic cup. When polysulfide solutions were analyzed, they were first acidified to
pH ~3 to decompose the polysulfides to elemental sulfur
and hydrogen sulfide, which was then removed by bubbling
with N2. It is necessary to use water-miscible organic solvents such as THF and ethanol to maintain zero-valent sulfur
in dissolved form because it has a low solubility in water
and has a tendency to coagulate and sorb on surfaces. Each
time a new solution was analyzed, the cup and electrodes
were carefully cleaned with THF to minimize carryover and
high blank values due to adsorption of sulfur on the vessel
walls and electrodes. All measurements were done at room
temperature (~22°C). After deposition of sulfur at -0.10 V
and a quiet time of 20 s, potential was scanned from -0.1
V to -0.8 V. To optimize the conditions for elemental sulfur
measurement, the following instrumental parameters were
systematically varied: pulse amplitude from 5 to 50 mV,
potential step amplitude from 1 to 10 mV; frequency (F)
from 5 to 100 Hz; deposition time from 0 to 240 s. Possible
interferences from sulfite, thiosulfate, thiols, and fulvic acids
were verified. Peak potential (Ep) and current (ip) for these
compounds and for elemental sulfur were measured at various pH values ranging from 2.5 to 11.5. Apparent pH was
measured in the water-ethanol-THF
solution with an Orion
Research (model 70) pH meter connected to a glass combination electrode calibrated with reference buffer solutions.
Reagents- Standard stock solutions of elemental sulfur
(10 mM) were freshly prepared by dissolving 32 mg of
99.99% pure orthorhombic sulfur powder (Alfa Products)
into 10 ml THF and diluting to 100 ml with double-distilled
ethanol; this solution was stable for at least 1 week at 4°C.
Sulfide stock solutions were freshly prepared from anhydrous Na2S (Aldrich) and standardized by iodometric titration. Polysulfide standards were prepared by dissolving
known amounts of elemental sulfur in alkaline sulfide solutions (pH 8-10); the solutions were allowed to stand for
about 2 d to be sure that all sulfur was dissolved, and in all
cases, they were undersaturated with respect to rhombic sulfur. The sulfide and polysulfide solutions were prepared and
stocked in a glove box (Anaerobic System model 1025, Proforma Scientific
Inc.) filled with ultrapure nitrogen
(99.996%; 02 < 0.0001%). The ionic strength (0.1 M) of
voltammetric working solutions was adjusted with 1 M Suprapur® sodium nitrate (Merck) (Luther et al. 1985, 1986),
and pH was adjusted with 1 M environmental-grade nitric
acid (Anachemia); a high purity of the sodium nitrate and
nitric acid reagents is desirable to minimize introduction into
the working solution of contaminants such as trace metals
that may react with sulfur species (Florence 1979). The thiols, cysteine and 3-mercaptopropionic acid, were obtained
from Sigma, thiosulfate (ACS grade) from ACP sulfite (Baker analyzed reagent) from Baker, and THF from Fisher
fulvic acid solution was prepared by dissolving IHSS Suwannee River fulvic acid standard (1S101F) in Mini-Q water. Water used for rinsing and preparing all solutions was
Mini-Q water (>18 MR cm); it was deoxygenated before
preparing the solutions.
Field sampling and analyses-- Overlying and pore-water
samples were obtained with in situ dialysis samplers (peepers) of the type described by Carignan et al. (1985) in July
1355
Sulfur in pore waters
0
-0.33
,
2
4
6
a
10
12
2
4
6
8
10
12
I
Fig. 1. Effect of scan rate on the general shape (A), peak current
(B), and peak potential (C) of SWCSV voltammograms of elemental sulfur obtained for [S(0)] = 1 X 10-7 Eq S liter-1 and the
following instrumental conditions: pulse amplitude = 25 mV, potential step amplitude = 2-10 mV, F = 5-100 Hz; deposition at
- 100 mV for 60 s; quiet time of 20 s. In A, the potential step was
2 mV and frequencies were 5 Hz (a), 10 Hz (b), 25 Hz (c), and 50
Hz (d). In B and C, the potential step amplitude was kept constant
at 2 mV and F was varied from 5 to 50 Hz (0), or frequency was
kept constant at 50 Hz and the potential step amplitude was varied
from 2 to 10 mV (Cl). The solution contained milli-Q water, ethanol,
and THF (ratio 1 : 1 : 0.02); ionic strength was made 0.1 M with
NaNO3; and pH was adjusted to 2.9 with HNO3.
Fig. 2. Effect of apparent pH on the general shape (A), peak
potential (B), and peak current (ip; 0) or peak area (Ap; 0) (C) of
SWCSV voltammograms of elemental sulfur obtained at apparent
pH 2.4-11.6 for [S(0)] = 1 X 10-7 Eq S liter-1 and the following
instrumental conditions: pulse amplitude = 25 mV, potential step
amplitude = 2 mV, F = 50 Hz; deposition at - 100 mV for 60 s.;
quiet time of 20 s. In A, pHs were 2.4 (a), 3.8 (b), 5.1 (c), 8.8 (d),
10.3 (e), and 11.6 (f). Solution composition and ionic strength were
as in Fig. 1.
1356
Wang et al.
;
Fig. 3. Typical calibration curve obtained for standards prepared by dissolving S(0) in THF (0) or in alkaline sulfide solution
(0). Solution composition, ionic strength, and apparent pH were as
in Fig. 1 and instrumental conditions were pulse amplitude = 25
mV, potential step amplitude = 2 mV; F = 50 Hz; deposition at
-100 mV for 60 s; and quiet time of 20 s.
1997 at a hypolimnetic site (depth of 7 m) in Lake Creche
(45°25’N, 74°3O’W), a circumneutral productive Shield lake
that develops an anoxic hypolimnion during the summer
months. The Plexiglas components of the peepers were kept
under an N2 atmosphere for a minimum of 15 d prior to
filling the cells with Milli-Q water and covering them with
a 0.2-µm pore size hydrophilic polysulfone membrane (Gelman HT-200). It is critical to remove 0, from the Plexiglas
to avoid its slow release into the sampler compartments during in situ equilibrium, as this can significantly alter the
shape of the pore-water profiles of redox-sensitive species
(Carignan et al. 1994). The assembled samplers were replaced under a nitrogen atmosphere again for at least 5 d
prior to placement in the lake sediments by divers. After 2
weeks, the three peepers were retrieved from the lake by
divers and sampled immediately. Samples (3 ml) for total
dissolved sulfide (2 S(-II)) and total dissolved sulfur (2
S(0)) analysis were collected first within 2 min from the
compartments with N2-purged polypropylene syringes; a volume of 1.5 ml (for s S(-II) analysis) was injected through
a Teflon septum into pre-evacuated amber vials containing
0.0027 M N,N-dimethyl-p-phenylenediamine
sulfate (Eastman Kodak) and 0.0055 M FeCl3 (60 µl of each); the remaining 1.5 ml (for x S(0) measurement) was injected
through a Teflon septum into preweighed amber vials containing 2 ml of ethanol, 0.4 ml of 1 M NaN03, 40 µl of
THE and 10 µl of 1 M HNO3 (which gives an apparent final
pH of 2.9). Addition of reagents to the latter vials was done
in a glove box under a nitrogen atmosphere to minimize
contamination with oxygen. These vials containing the reagents for S(0) analyses were weighed before and after injection of pore-water samples in order to determine the exact
volume of pore water injected in the field. Blanks were prepared in the field by injecting Mini-Q water into the vials
containing
the reagents for x S(-II)
and X S(0) determinations. Samples and blanks were stored at 4°C in the dark
until return to the laboratory where 2 S(-II) was measured
0.0
Fig. 4. Typical SWCSV voltammograms of various S-containing compounds (A) and effect of pH on their peak potentials (B).
Instrumental conditions were pulse amplitude = 25 mV, potential
step amplitude = 2 mV; F = 50 Hz; deposition at - 100 mV for
60 s; and quiet time of 20 s. Voltammograms in A were obtained
at pH 2.9 for 0.5 µmol liter-1 thiosulfate (a), 1 µmol liter-1 cysteine
(b), 1 µmol liter-1 3-mercaptopropionic acid (c), and 30 mg liter-1
fulvic acid (d). In B, pH was varied from 2.5 to 10.7 and concentrations of compounds were as in A. Solution composition and ionic
strength were as in Fig. 1.
immediately (within 4-6 h of collection); X S(0) was measured the next day. Samples for pH measurements were
taken using l-ml syringes and pHs were measured within
10-30 min in the field with a combination microelectrode
(Microelectrode Inc., model MI-710).
Z S(-II) was determined with a Technicon autoanalyzer,
using the methylene blue method (Cline 1969), whereas 2
S(0) was analyzed by SWCSV, using the method developed
in this study. The contents of the vials for S(0) analyses were
poured into the polarographic cup and deoxygenated for 5
min. The instrumental conditions used were: scan from -0.1
to -0.8 V at a scanning rate of 100 mV s-l; quiet time 20
s; pulse amplitude 25 mV, potential step amplitude 2 mV;
frequency 50 Hz; deposition time at a potential of -0.10 V
from 60 to 120 s, depending on concentration. After processing of each sample, the cup and electrodes were carefully cleaned with THE
1357
Sulfur in pore waters
To verify if the reagents used for the pore-water samples
could affect the concentration of S(O), by either oxidation of
sulfide or polysulfides or decomposition of thiosulfate during
the storage (1 d; dark and 4°C), we injected 2 ml of standard
solutions of sulfide (10 µM), polysulfides (1.22 µEq S liter-1), or thiosulfate (25 µM) with syringes into amber vials
containing the reagents for S(0) analyses (pH 2.9 with
HNO3; 0.1 M NO3-; ethanol and THF), as in the field for the
pore-water samples; these samples were stored (dark; 4°C).
We found, after the 4-d storage, that elemental sulfur was
quantitatively recovered (99 ± 6%; n = 5) from the polysulfide standards, that the sulfide standards did not produce
any measurable S(0) (n = 5), and that only 0.4 ± 0.04% (n
= 5) of the thiosulfate was decomposed to S(0). To verify
if Fe(II) present in the pore-water samples could be oxidized
by nitrate to Fe(III), which in turn could oxidize sulfide to
S(0), we prepared two solutions of Fe(II) (10 and 20 µM)
adjusted to pH 2.9 with HNO3 and adjusted to an ionic
strength of 0.1 M with NaNO3; measurement of Fe(II) with
ferrozine (Stookey 1970) at various times did not indicate
any decrease of Fe(II) for at least 1 d. This is in agreement
with scientific literature which indicates that neither 02
(Stumm and Morgan 1970), H2O2 (Millero and Sotolongo
1989), or NO3- (Pound 1939) can oxidize Fe(II) at this low
pH. All of these results indicate that the reagents used
(NO3-, THF low pH) should not affect the concentration of
S(0) present in the pore-water samples during the l-d storage
of these samples.
Results and discussion
Because the form of elemental sulfur in the THE ethanol,
and water solution used is unknown, S(0) concentrations are
expressed herein in terms of Eq S liter-1.
Optimization of instrumental parameters- The behavior
of elemental sulfur at the mercury electrode can be explained
by the following mechanism (Buffle et al. 1987; Davison et
al. 1988; Batina et al. 1992; Ciglenecki and Cosovic 1996)
according to which S(0) electrochemically
oxidizes the
Hg(0) of the working electrode at positive potentials and the
HgS formed adsorbs at the surface of the electrode:
S(0) + Hg(0) + HgS(ads).
I
0.0
I
I
I
I
I
0.2
0.4
0.6
0.8
1.0
1.2
(2)
Accumulation of HgS(ads) should be proportional to the
elemental sulfur concentration and to deposition time. During a cathodic stripping scan toward negative potentials, the
adsorbed HgS is reduced irreversibly to elemental mercury
and sulfide:
HgS(ads) + H+ + 2e- + Hg(0) + HS-.
(3)
Typical peaks obtained by SWCSV at various frequencies
for a given S(0) concentration are displayed in Fig. 1A.
These peaks are asymmetric. Figure 1A also shows that peak
current (ip) increased and peak potential (Ep) shifted negatively with increasing frequency; the same effects on ip and
EP were observed for increasing potential step amplitude.
There is presently no detailed theory for explaining the behavior of redox systems in SWCSV where reduced (Red)
and/or oxidized (Ox) species are adsorbed at the electrode.
1358
Wang et al.
16
6-
6
Fig. 6. Pore-water profiles of pH (A), z S(-II) (B), z S(0) (C), and [S(0)aq] (D) at a hypolimnetic site in Lake Creche. It should be noted that S(0)aq is the nonpolysulfidic component of the
total dissolved elemental sulfur in Eq. 7; for calculating its concentration, we used the formation
constants of Giggenbach (1974) given in Table 1. The various symbols are for profiles 1 (O), 2
(O), and 3 (0). Th e h orizontal dashed line indicates the sediment-water interface, and the vertical
dashed line in D indicates the [S(0)aq] for saturation with orthorhombic sulfur obtained from Boulègue (1978b) (Table 1).
The behavior of such systems has, however, been studied in
detail for cathodic stripping voltammetry (CSV) and cyclic
voltammetry. For example, Wopschall and Shain (1967)
have studied the influence of scan rate in CSV on redox
electrode processes in which Ox was adsorbed; they have
shown that, for strongly adsorbed Ox, iP increased linearly
with scan rate, v, and that EP shifted negatively with log v.
It is interesting to note that the same dependence of ip, and
Ep,on v, where v in SWCSV is the product of frequency and
potential step amplitude, is observed here (Fig. lB,C).
Peak current for a given S(0) concentration was also found
to increase linearly with the square wave of pulse amplitudes
from 5 to 100 mV and with potential step amplitudes from
1 to 10 mV, however, at potential step amplitude = 10 mV,
resolution of the peak deteriorated. From these results and
those shown in Fig. 1, the following instrumental conditions
were considered to be optimal and were thus adopted in the
further experiments: F = 50 Hz, pulse amplitude = 25 mV,
potential step amplitude = 2 mV.
Effect of pH on peak potential, peak current, and peak
area- Voltammograms obtained at various apparent pH values between 2.4 and 11.6 for an elemental sulfur concentration of 10-7 Eq S liter-1 show that peak potential and peak
current are very sensitive to pH changes (Fig. 2A). For a
given S(0) concentration, peak potential shifts to negative
values (Fig. 2A,B) and peak current decreases (Fig. 2A,C)
when apparent pH (i.e., pH measured in the aqueous solu-
1359
Sulfur in pore waters
Table 1. Equilibrium
constants (I = 0, T = 25°C) relevant to the sulfide and polysulfide system.
NIST 1997
Boulègue 1978b
Giggenbach 1974
Giggenbach 1974
Cloke 1963a
Giggenbach 1974
Cloke 1963a
Giggenbach 1974
Cloke 1963a
Cloke 1963a
Schwarzenbach and
Schwarzenbach and
Schwarzenbach and
Schwarzenbach and
Schwarzenbach and
Schwarzenbach and
Schwarzenbach and
Schwarzenbach and
Fischer
Fischer
Fischer
Fischer
Fischer
Fischer
Fischer
Fischer
1960
1960
1960
1960
1960
1960
1960
1960
* Values at 20°C.
tions containing ethanol and THF) is increased. Zali (1983)
also reported an important shift of EP toward negative potentials when he analyzed S(0) by DPP It should be noted
that despite the important decrease in ip, with an increase in
apparent pH, peak area remained nearly constant (Fig. 2C).
tain reduced sulfur (Luther et al. 1986; Zali 1983) and give
rise to voltammetric signals (Fig. 4A). Important processes
involving these compounds are the oxidation of Hg(0) with
formation of thiosulfate, sulfite, and thiol complexes at the
mercury electrode (Luther et al. 1986; Davison et al. 1988):
Range of concentration and detection limit-Figure
3
shows that the relation between iP and elemental sulfur concentrations is linear between 5 X 10-9 and 5 X 10-6 Eq S
liter-l for a deposition time of 60 s; for this deposition time,
linearity is broken at concentrations greater than 5 X 10-6
Eq S liter-l and peak widening is observed. We also found
that peak current (or peak area) increases linearly with deposition times between 0 and 240 s for low concentrations
of elemental sulfur (e.g., 0.1 µEq S liter-1). As expected,
peak potential was shifted toward more negative values
when either S(0) concentration or deposition time increased.
Figure 3 shows no significant difference between standards
prepared by dissolving elemental sulfur directly in THF and
ethanol or by dissolving it in alkaline sulfide solutions to
form polysulfide species; these results indicate that the procedure used (acidification and bubbling) insures complete
recovery of S(0) from polysulfides. From the measurement
of seven replicate aliquots of a 5 X 10-9 Eq S liter-1 solution
of S(0) with a deposition time of 60 s, we calculate a detection limit of 3 X 10-9 Eq S liter-1. Multiple peaks for
S(0) were observed by DPP at S(0) concentrations greater
than 5 X 10-6 Eq S liter-1 (Zali 1983) and by a.c. polarography at concentrations greater than 1 X 10-5 M (Batina
et al. 1992); appearance of additional peaks was attributed
to the formation of additional layers of mercury sulfide at
the mercury electrode. This situation should occur when
large S(0) concentrations are analyzed or long deposition
times are used. In all of our measurements of S(0), save one,
we observed a single peak for S(0).
(4)
Interferences- Several other compounds in pore waters,
such as thiosulfate, sulfite, thiols, and fulvic acids, also con-
(5)
(6)
Figure 4A shows typical SWCSV peaks obtained for fulvic acid, thiosulfate, and two thiols-cysteine
and 3-mercaptopropionic acid. These two thiols were chosen because
they were reported to be among the most important thiols in
pore waters (Mopper and Delmas 1984; Shea and MacCrehan 1988). The voltammetric behavior of these compounds needs to be studied as a function of pH because they
have the potential to cause interference in the SWCSV determination of S(0). Figure 4B shows the pH dependence of
peak potential for the various compounds studied; comparison of Figs. 2B and 4B indicates that S(0) can be distinguished easily from thiosulfate, sulfite, and thiols. The only
possible interference could be from fulvic acids, but, given
the large difference in peak width between S(0) and fulvic
acids, particularly at low pH values, it should be possible to
quantify S(0). In addition, iP increases with deposition time
for S(0) contrary to fulvic acids. Consequently, the peak of
S(0) can be distinguished from those of the other compounds
tested. It should be noted also that sensitivity (peak current/
concentration) is greater for S(0) than for S,Oz- (~4X), thiols (~10X) and SO;- (~20X).
Measurement of S(0) in lake pore waters- Elemental sulfur concentrations in the pore waters from Lake Creche were
measured in all cases at an apparent pH 2.9 to facilitate
complete removal of sulfide and to obtain sharp peaks for
S(0) (Fig. 2A) that could be distinguished from the large
1360
Wang et al.
peaks of fulvic acids (Fig. 4A). It is important to remove
sulfide completely because it would be measured at the same
peak potential as S(0); we verified that at the low apparent
pH used, all sulfide was removed to unmeasurable levels by
bubbling nitrogen for 5 min.
Figure 5 shows typical peaks obtained in pore-water samples of Lake Creche at depths above 5.5 cm where S(0)
concentrations were in the 10-6 Eq S liter-1 range (Figs. 5A,
6C) and below 5.5 cm where S(0) concentrations were in
the 10-7 Eq S liter-1 range (Figs. 5B, 6C). According to Fig.
5B, the S(0) peak is superimposed on a wider peak that
might be ascribed to natural organic matter. In no case did
we measure any thiosulfate, sulfite, or thiols in the pore waters of this lake.
Figure 5C shows standard additions of S(0) to a porewater sample at depth 9.5 cm, where S(0) concentration was
minimal and the dissolved organic carbon concentration was
10 mg C liter-1; the slope obtained for the standard additions
(193.2 nA/µEq S liter-l) is not significantly different at the
95% confidence interval to that of the calibration curve obtained with S(0) standards prepared in Mini-Q water (204.2
nA/µEq S liter-1). Another comparison of calibration curve
and standard additions made in a pore-water sample from a
depth of 0.5 cm, where S(0) concentration was maximum
(Fig. 6C) and dissolved organic carbon concentration was 10
mg C liter-1, also indicated nonsignificantly different slopes.
These results suggest that fouling effects due to natural organic matter were minimal.
The three vertical profiles of pore-water pH, sulfide, and
elemental sulfur obtained at an anoxic hypolimnetic site of
Lake Creche are shown in Fig. 6. Saturation state of pore
waters with respect to rhombic sulfur were calculated with
the computer code HYDRAQL (Papelis et al. 1988), after modifying its thermodynamic database to include the equilibrium
constants given in Table 1. The concentration of S(O),, in
pore waters was calculated from the measurements of pH
(Fig. 6A), 2’ S(-II) (Fig. 6B), and 2 S(0) (Fig. 6C), and the
equilibrium constants given in Table 1; the following mass
balance equations were used in HYDRAQL for the calculation:
and
plains why concentrations of dissolved elemental sulfur correlate closely with those of total sulfide for each vertical
profile (Fig. 6B,C; r = 0.82-0.90; n = 15; P < 0.01). Indeed, provided that solid elemental sulfur is present in the
sediments, concentrations of polysulfides should depend on
dissolved sulfide concentration and pH; Fig. 6A shows that
pH does not vary much with depth at the site that we studied.
We did not measure solid elemental sulfur in these sediments, but other studies have shown that it can be present
in the upper 10 cm of freshwater (Chen et al. 1997) and
coastal marine (Troelsen and Jorgensen 1982) sediments. Supersaturations of waters with respect to rhombic sulfur have
been reported for various environments including salt marsh
pore waters (Luther et al. 1985; Boulègue et al. 1982),
groundwaters (Boulègue 1975, 1977), and brines (Boulègue
1978a).
Sulfur in pore waters
1361
Received: 18 March 1998
Accepted: 24 June 1998