A theoretical and experimental physicochemical study of sulfur species in the anoxic lagoon of Aitoliko-Greece

A theoretical and experimental physicochemical study of sulfur species in the anoxic lagoon of Aitoliko-Greece

Chemosphere 74 (2009) 1011–1017 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere A theor...

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Chemosphere 74 (2009) 1011–1017

Contents lists available at ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

A theoretical and experimental physicochemical study of sulfur species in the anoxic lagoon of Aitoliko-Greece Ioannis T. Papadas, Lambros Katerinopoulos, Areti Gianni, Ierotheos Zacharias, Yiannis Deligiannakis * Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 30100 Agrinio, Greece

a r t i c l e

i n f o

Article history: Received 9 July 2008 Received in revised form 4 November 2008 Accepted 5 November 2008 Available online 27 December 2008 Keywords: Lagoon Anoxic Sulfide Sulfate Speciation pe-pH

a b s t r a c t The spatiotemporal changes of metals, inorganic ions and physiochemical parameters of Aitoliko lagoon, an anoxic wetland in Western Greece, were studied with special emphasis in sulfur species. Theoretical physicochemical modeling was performed for the sulfur speciation, based on experimental pH and redox potential data. Accordingly, the speciation of sulfur in the lagoon can be operationally divided in two domains: (a) for depths d = 0–10 m below the surface, the sulfur speciation can be described by equilibrium reactions between the aqueous species. (b) At depths d > 10 m a progressive decline for SO2 4 concentration is observed between theory and experiment. At the lagoon-bottom an elevated concentration of 19 ± 2 mM SO2 4 was measured, which cannot be described by physicochemical equilibrium based on the pH, Eh, O2 concentrations measured in situ. Accordingly, we suggest that additional biogeochemical processes, such as sulfur bacteria activity, have to be invoked. Of particular importance is that the exper imental pH-pe values cross the critical region where the interplay of SO2 4 /S2 /H2S occurs. This explains why a relatively small fluctuation of pH, pe values may result in a shift of the equilibrium over one sulfur species. This explains the, otherwise accidental, previously reported releases of H2S in the air over the lagoon. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Aitoliko lagoon is part of a complex of wetlands in Western Greece, rich in biodiversity (Fig. 1, map), belonging to the ‘‘Natura 2000” zone (code number GR2310002) and is protected under the Ramsar convention. The area of this meromictic lagoon is 1700 hectares, with a mean depth 12 m, maximum depth 30 m (Leonardos and Sinis, 1997). It is connected with the nearby Messolonghi lagoon in the south part through narrow openings, 1 m deep, under the bridges of Aitoliko (Dasenakis et al., 1994). Aitoliko lagoon has an atypical orientation and is tectonically formed (Leonardos and Sinis, 1997). The main physical characteristics of this lagoon are the permanent thermocline and halocline (Danielidis, 1991) and the anoxic conditions in the hypolimnion (Dasenakis et al., 1994). H2S release, which is the major environmental concern in the area, appears to be strongly related with the meteorological conditions. In the past, strong winds appeared to correlate with stratification destruction. Consequently the deep anoxic sulphide rich water is mixed with the surface layers, the water column becomes anoxic and H2S releases to the atmosphere. This column mixing * Corresponding author. Tel.: +30 2641074114. E-mail addresses: [email protected] (I.T. Papadas), [email protected] (L. Katerinopoulos), [email protected] (A. Gianni), [email protected] (I. Zacharias), [email protected] (Y. Deligiannakis). 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.11.009

leads to fish mass mortalities. Reportedly, in a H2S release event observed in November 1990, 137 ton of fish were lost (Leonardos and Sinis, 1997). 0 Sulfur species, including thiosulfate S2 O2 3 , elemental sulfur (S ), 2 2 ), sulfide (S ) and sulfate (SO ) as well as intermediate sulfite (SO2 3 4 metastable oxidation states, are increasingly recognized as important species in ore deposit formation (Stoffreggen, 1986; Spirakis, 1991; Williamson and Rimstidt, 1992), sulfide oxidation kinetics (Goldhaber, 1983; Moses et al., 1987), volcanic eruption sequences (Takano, 1987; Takano and Watanuki, 1990), acid rain formation (Graedel and Goldberg, 1983), and sulfur cycling in sedimentary environments (Jorgensen, 1990). Because of their metastability, many aqueous sulfoxy intermediates do not participate in equilibrium which would allow accurate determination of their thermodynamic properties (Williamson and Rimstidt, 1992). Therefore, a detailed understanding of the role of sulfur species in aquatic environments requires a complete set of thermodynamically consistent physicochemical data recorded in situ. A minimal set should include sulfur species concentration, pH, Eh, O2 and temperature. In addition, a proper study of sulfur species should also include the role of metal species if present. Iron, is of immediate relevance due to its abundance, redox activity and most particularly due to its strong affinity for sulfur species (Sposito, 1989). As an example, iron–sulfur complexes are in equilibrium with Fe2+ and S2, possibly Fe2S2 (Merian, 1991). FeS aqueous clusters, of unknown stoichiometry have been found in a variety of marine and estuarine

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environments (Hebert and Morse, 2003) where dissolved sulfide is higher in concentration than or nearly equal to that of dissolved Fe(II) (Luther et al., 2003). Lakes and rivers contain iron in heterogeneous sediments (Merian, 1991). Buffle (1988) has pointed out the differences in a lake water between particulate iron measured in summer vs. colloidal and electroactive Fe(III), measured in autumn. Moreover Buffle et al. (1989) had described the role of colloidal iron phosphate species, formed at oxic–anoxic interfaces in lakes, exemplifying the need for a detailed account of redox and speciation phenomena. In the present work, the spatiotemporal changes of a wide range of metals, inorganic ions and physiochemical parameters of the Aitoliko lagoon were monitored for a period of 3 months. Specific emphasis was put on the iron and sulfur species. Our aim was (a) to determine the concentrations of sulfide, sulfate and sulfite in Aitoliko lagoon, (b) to determine Fe and O2 concentration, as well as pH and redox potential values, and (c) to develop a theoretical model for sulfur speciation under the measured physicochemical conditions.

2. Materials and methods 2.1. Study site The morphology of the Aitoliko lagoon, in combination with the fresh water inflows and the limited connection with the adjacent Messolonghi lagoon permits the development of a permanent pycnocline to its water column. This stratification pattern is controlled by salinity vertical distribution, while the role of temperature is secondary. According to (Danielidis, 1991) seasonal samplings, the surface layer of Aitoliko lagoon was characterized by salinity values equal to 13% while the bottom layer by values equal to 27%. A halocline layer is well developed throughout the sampling period from the depth of 10–20 m. The surface layer was always well oxygenated while the oxic/ anoxic interface was placed at the depth of 9 m during the summer months and 14 m during winter (Danielidis, 1991). Data about sulfur speciation in the lagoon’s water column are not available. Sulfide concentration in the maximum depth was determined by Hatzikakidis (1951) (28.8 mg L1), and Almpanakis et al. (1995), (45 mg L1). Voutsinou-Taliadouri et al. (1987) introduce data about the metal composition in the sediments of Aitoliko and Messolongi lagoons, while (Dasenakis et al., 1994) measured metal concentration in the water column and sediments of Aitoliko lagoon after a total mixing event. According to them, trace metals concentrations in Aitoliko lagoon sediments are elevated in comparison with other non-polluted Mediterranean areas (indicative values: Cu <30 ppm, Cr <40 ppm, Ni <50 ppm). Data about the vertical distribution of metals in Aitoliko lagoon water column, under normal conditions – water column stratification – are not available. 2.2. Reagents All solutions were prepared with analytical grade chemicals and ultra pure water (Milli-Q Academic system) with a conductivity of demineralised water 18.2 lS cm1. Zn(C2H3O2)22H2O, KI, starch (C6H10O5)n, KIO3 and NaHCO3 were obtained from Merck; I2, from Alfa Aesar, salicylic acid, NH2SO3H, ZnCl2 and Na2SO3 from Sigma-Aldrich and EDTA C10H14N2Na2O82H2O from Riedel-de Haën. 2.3. Sampling Three samplings were performed between February and May 2006. Water samples were collected from 10 sites at representative depths. However, the most interesting was the deepest part on the

north side of the lagoon, marked as sampling point A9 in the map in Fig. 1. This is because (a) it is located away from the inhabited area of the Aitoliko town, see map in Fig. 1, thus suffering minimal anthropogenic perturbation (b) is located in the deepest part of Aitoliko lagoon and (c) is the only site where the hypolimnion was fully developed and anoxic conditions prevailed during the sampling period. Thus we focus here on concentration and physicochemical parameters referring to sampling point A9. Water samples were collected with a free flow sampler (HYDRO-BIOS) and transported to the laboratory within 2 h. In addition, all the physicochemical parameters such as pH, dissolved oxygen, salinity, conductivity and redox potential were measured in situ using a portable meter. A recording current meter, RCM 9 Mk II was used in the first two samplings and a multi-parameter TROLL 9500 in the third. 2.4. Metals analysis The calibration working standard solutions were prepared from the metal standard solutions of 1000 ± 3 mg L1 (AAS Standard, Merck) and kept in polyethylene bottles, at pH <2, using 0.5% HNO3 (Merck). The water samples were filtered through Whatman 70 mm filters, acidified with concentrated nitric acid (pH < 2) and stored at 4° C. Total concentrations of Fe, Cu, Cr and Mn were determined by atomic absorption spectroscopy with a Perkin–Elmer AAS-700 spectrophotometer. 0.2% CaCl2 was added in the samples for the measurements of Fe and Mn to minimise chemical interference (Standard Methods, 1995). 2.5. Sulfur species determination Water samples for sulfide (S2) and sulfite (SO2 3 ) determination were always the first taken from the sampling bottle immediately after the sampler was back on board being careful to take samples with minimum aeration. To preserve sulfide samples, zinc acetate and sodium hydroxide solutions were put into the bottles before filling it with the sample. Water samples for sulfide and sulfite analyses were measured unfiltered and freshly within 24 h using the iodometric methods (Standard Methods, 1995). The iodometric titration method is suitable for waters with concentrations above 2 mg L1 (SO2 3 ). The presence of other oxidizable materials as sulfide, thiosulfate and Fe2+ ions can cause apparently high results for sulfite. Some metal ions, such as Cu2+, may catalyze the oxidation 2 of (SO2 3 ) to (SO4 ) when the sample is exposed to air, thus leading to low results. The use of iodometric method is accurate for determining sulfide concentrations above 1 mg L1 (Standard Methods, 1995). Sulfate (SO2 3 ) measurements were performed by ion chromatography, using a Dionex ICS-1500 ion chromatograph. Samples were first filtered through Whatman 70 mm filters and then diluted one hundred times, in the cases where concentrations of sulfate were too high.

3. Theoretical analysis 3.1. The sulfate–sulfide system In anoxic waters, sulfate may become the primary oxidant. Although important intermediate redox sulfur compounds exist (e.g. such as S0), the reaction usually proceeds all the way to the formation of sulfides and can be written as

1 2 10 þ 1 1 SO þ H þ e ¼ H2 S þ H2 O pe0 ¼ 5:1: 8 4 8 8 2

ð1Þ

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Fig. 1. Map of the Aitoliko lagoon. A9 is the deepest sampling point where the spatiotemporal data presented here were collected. Dotted-shadowed areas are breccias of limestone and gypsum.

Thus a simplified aqueous sulfur solution includes only sulfate, 2 , species (Morel and Hering, 1993). (SO2 4 ), and sulfide, S In addition to reaction (1) the pertinent pH-dependent reactions are,

6 HSO 4

HSO4 ¼ Hþ þ SO2 4

ð2Þ

0

pK ¼ 7:0;

ð3Þ

-2

pK ¼ 13:9:

ð4Þ

H2 S ¼ H þ HS 

2

þ

HS ¼ H þ S

pK ¼ 2:0;

In a pe-pH diagram (Sposito, 1989; Morel and Hering, 1993), reactions 2–4 provide three vertical lines separating the various 2 acid-base species (see Fig. 2) i.e. HSO 4 from (SO4 ) at pH 2.0, H2S   2 from HS at pH 7.0, and HS from S at pH 13.9 (Morel and Hering, 1993). By defining,

pe ¼  log½e:

ð4aÞ

(Morel and Hering, 1993), the pe value can be conveniently derived from the redox potential, Eh (V):

F pe ¼ Eh 2:3RT

ð4bÞ

where F = 9648 C/M is the Faraday constant. At 25 °C the factor 2:3RT F has a value of 59 mV. Equilibrium constants for redox reactions can be expressed as: 0

log K ¼ npe ;

ð4cÞ

Ox

SO42-

2

pe



þ

4

3 4 5 6

-4 H2S

-6

1 2

-8

Red

HS-

-10 -12

S2-

0

2

4

6

8

10

12

14

pH Fig. 2. Theoretical pe-pH diagram for a sulfate–sulfide system under the pH and redox conditions measured for the Aitoliko lagoon. Points 1–6 correspond to experimental values measured at consecutive sampling depths in Aitoliko lagoon. Dashed line represents the SO2 4 ¼ H2 S interface. Dotted line represents the  2 SO2 interface. The dash-dot line represents the SO2 4 ¼ HS 4 ¼ S interface. Solid line represents the HSO 4 = H2S interface.

where n = the number of electrons transferred in the half redox reaction and pe0 is derived from the standard redox potential, E0h (Morel and Hering, 1993).

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3.2. Construction of a predominance pe-pH diagram In Fig. 2 a pertinent pe-pH plot is presented, calculated for pH and pe i.e. redox potential values, measured in Aitoliko lagoon. The corresponding reactions and pe-pH equations were obtained by combining reactions 1–4 and expressing the mass laws,

1 9 1 1 HSO4 þ Hþ þ e ¼ H2 S þ H2 O pe0 ¼ 4:85 8 8 8 2 8pe þ 9pH ¼ 38:8 for ½HSO4  ¼ ½H2 S; 1 2 10 þ 1 1 SO þ H þ e ¼ H2 S þ H2 O pe0 ¼ 5:1 8 4 8 8 2  ¼ ½H2 S; pe þ 10pH ¼ 40:8 for ½SO2 4 1 2 9 þ 1 1 SO þ H þ e ¼ HS þ H2 O pe0 ¼ 4:23 8 4 8 8 2  8pe þ 9pH ¼ 33:8 for ½SO2 4  ¼ ½HS ; 1 2 1 1 SO þ Hþ þ e ¼ S2 þ H2 O pe0 ¼ 2:49 8 4 8 2 2 pe þ pH ¼ 2:5 for ½SO2 4  ¼ ½S :

ð5Þ

ð6Þ

ð7Þ

ð8Þ

Alternative, scenarios including polynuclear sulfur species were also tested, however the obtained theoretical results were in clear disagreement with the measured values. This is fully justified since based on the measured sulfur concentrations, Eh and pH values for Aitoliko lagoon polynuclear species are not expected to be formed (Morel and Hering, 1993). Thus we focus on the species described in Fig. 2. In each of the regions of Fig. 2, the free concentration of any sulfur species can be calculated from that of the dominant species (which is equal to the total sulfur concentration) by expressing the appropriate mass law. For example, the free sulfide concentration [S2], which is important for assessing the solubility of metal sulfide, is given by:

pS2 ¼ 19:9 þ pST þ 8pe þ 8pH ðin the SO2 4 regionÞ;

ð9Þ

pS2 ¼ 20:9 þ pST  2pH ðin the H2 S regionÞ;

ð10Þ

pS2 ¼ 13:9 þ pST  pH ðin the HS regionÞ:

ð11Þ

If we consider the presence of hydrogen sulfide gas, the relevant redox reaction is obtained by combination of reactions 1 and 12:

H2 SðgÞ ¼ H2 S pK H ¼ 1:0;

ð12Þ

therefore,

1 2 10 þ 1 1 SO þ H þ e ¼ H2 SðgÞ þ H2 O pe0 ¼ 5:23 8 4 8 8 2 log½SO2 4  ¼ 41:8 þ 8pe þ 10pH þ log½PH2 S :

ð13Þ

This equation gives the sulfate concentration at redox equilibrium for any partial pressure of hydrogen sulfide. Fig. 2 displays the characteristic features of the derived pe-pH diagram, pertinent for the case studied in this work. The boundaries represent the pH-pe conditions where half the element of interest is in each of the two major species on either side of the line. It is noteworthy that the domains of dominance of various species meet at common corners; typically three boundaries intersect at the same point. This is because the equations of the boundaries (which are the logarithmic expressions of mass laws) are linear combinations of each other. Most importantly, the solid squares in Fig. 2 crosses the critical  region where the interplay of SO2 4 =S2 =H2 S occurs. This shows that an eventual fluctuation of pH or/and pe may result is a shift of the equilibrium over one species. Thus Fig. 2 shows that a decrease of pH by no more 0.5 pH units combined with a decrease of pe by no more than 1 pe unit would favor the formation of H2S which could

then be released to the atmosphere. Although pertinent records for pH-pe values are lacking, the pH-pe plot shows that the reported non-periodic release of H2S over the lagoon can be attributed to small fluctuations of pH-pe. 4. Results Fig. 3A shows the experimental vertical distribution of total Fe at sampling point A9. Fe concentration was increasing and reached its maximum value i.e. 1.2 lM, at the depth of 15 m and then was gradually decreasing at a final concentration of 0.43 lM at the bottom i.e. 25 m depth. Other metals measured, with non-zero concentrations, are listed in Table 1. Copper and chromium concentrations were low, in the 106 M range, with a monotonous increase towards the bottom of the lagoon. Manganese concentrations showed a vertical distribution profile with a maximum at d = 15 m i.e. a vertical distribution profile resembling that of iron. Inorganic ions i.e. Cl and Br showed a limited increase towards the bottom, Table 1. Sulfur species: SO2 3 concentration was found at low concentration i.e. near zero until d = 15 m, followed by a steep increase up was to 164 lM at the bottom (Table 1). On the other hand SO2 4 consistently measured at high concentrations, in the 103 M range, was measee Fig. 3B. Even at the surface nearly 15 mM of SO2 4 sured and this was constant until the depth of 10 m. From this was steep towards the bottom, point down, the increase of SO2 4 exceeding the value of 19 mM at 25 m. Overall the data show a heterogeneous vertical distribution, mainly for iron and sulfur species. A characteristic discontinuity is observed in the zone 10–15 m which is matching the temperature and pH gradient (Table 1). The influx of nutrients, from highly cultivated agricultural areas in the vicinity of the lagoon, causes an increase in primary productivity but also a high oxygen demand during decomposition of the organic matter (Leonardos and Sinis, 1997; Avramidou-Kalitsi and Koutsoukos, 1990). Our data reveal that an anticorrelation between sulfide, S2, and dissolved oxygen, D.O., concentrations in the lagoon become obvious based on the experimental data, see Fig. 4. We have chosen to plot data from the samplings of April and May because of their characteristic high sulfide values. Non-zero concentrations of sulfide appear in the point where D.O. vanishes and from this point down, the concentration of sulfide is sharply increasing. When we compare Figs. 3B and 4 we observe a parallel trend between  SO2 4 and S2 and in the lagoon. Before we proceed to the detailed theoretical calculations, we underline that this concomitant inand S crease of both SO2 4 2 species is not obvious to explain. In in the anoxic layer of the lagoon, addition, the increase of SO2 4 Fig. 3B, is a striking finding which does not concur with the zeroing of D.O., Fig. 4. At this point we notice that the vertical distribution of the redox potential, Eh, inset in Fig. 3B, parallels that of D.O. At the top 10 m from the surface, the Eh values were 150–160 mV. At d > 10 m a steady decrease of the Eh values was measured approaching strongly reducing potentials i.e. nearly 290 mV at the lagoon’s bottom.

5. Theoretical analysis The experimental results were modelled by assuming the redox and pH-dependent reactions, listed in Table 2. All calculations and the ensuing speciation analysis, of the aqueous system, was performed based on the calculations made using the software FITEQL (Herbelin and Westall, 1999) using activities i.e. taking ionic strength effect into account, based on the Davies equation (Morel

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A

B

0

0

0

Depth (m)

5

Depth (m)

5

5

10

10 15 20 25

10

30 -300

15

15

20

20

25

25 0.2

0.4

0.6

0.8

1.0

1.2

-200

-100

0

100

200

Eh (mVolts)

1.4

14

15

Concentration Fe (uM)

16

17

Concentration

18

[SO42-]

19

20

(mM)

Fig. 3. (A) Vertical distribution of Fe at sampling location A9 of Aitoliko lagoon. (B) Vertical distribution of SO2 4 at sampling location A9 of Aitoliko lagoon. Inset: Vertical distribution of redox potential.

Table 1 Experimentally measured concentrations of metals, ions and physicochemical parameters. Cu (lM)

Mn (lM)

Cr (lM)

Fe (lV)

Br (mM)

Cl (mM)

0 5 10 15 20 25 % Error

0.46 0.49 0.55 0.61 0.66 0.68 5

0.30 0.70 3.20 12.50 6.20 5.10 5

0.96 1.08 1.17 1.19 1.29 1.48 5

0.32 0.34 0.43 1.22 0.54 0.45 5

0.36 0.40 0.42 0.49 0.55 0.56 10

354 367 339 401 468 473 10

SO2 3 (lV)

Depth (m) 0 5 10 15 20 25 % Error

SO2 4 (mV)

0 0 5 67 111 164 10

14.43 15.42 15.50 17.47 18.71 19.10 10

S 2 (mV) 0 0 0.09 0.34 1.03 1.28 10

Depth (m)

pH

Conductivity (mS cm1)

Salinity (ppt)

Temperature (°C)

Turbidity (NTU)

5 10 15 20 25 % Error

8.54 8.48 7.84 7.61 7.31 2

27 26 25 27 32 10

19.90 20.20 21.20 23.50 26.90 10

16.50 14.60 11 11.20 12.60 10

1.74 3.30 1.34 2.34 1.63 10

and Hering, 1993). The experimental data of pH, pe and the sulfate (SO2 4 ) concentration, measured at the surface water of the lagoon, were used as input parameters in FITEQL. Accordingly, based on the measured pH and pe i.e. Eh values the theoretical predictions for the various species are displayed in Fig. 5A. In Fig. 5A, we observe that, according to the theoretical speciation of sulfur species under the measured pe-pH values at Aitoliko, the concentration of sulfate is predicted to be practically constant until the depth of 15 m, while the values of sulfide are zero. From this point down to the bottom of the lagoon, there should be a sharp decrease of sulfate with a simultaneous production of sulfide (H2S plus HS in Fig. 5A). According to the theory, HS which is the

0

100

200

300

400

500

0 5

Depth (m)

Depth (m)

D.O. (uM)

D.O. (April)

10

D.O. (May)

15

S2- (May) 20

S2- (April) 25 30 0

250

500

750

1000

1250

1500

S2- (uM) Fig. 4. Vertical distribution of S2 (d,o) and D.O. (j, h) for two samplings on April (solid symbols) and May (open symbols), respectively.

dominant of these two sulfide species, reaches a maximum value concentration, at the depth of 20 m, which is then decreasing, as it is converted to H2S (Fig. 5A). Finally, the values of S2 and SO2 3 remain practically zero under all the pe-pH range (Fig. 5A). Including the presence of gypsum at the lagoon’s bottom (open symbols in Fig. 5A) has little influence on the speciation profile. A resolvable difference of the dissolution of gypsum is that influences the sulfide’s abundance near the bottom (Fig. 5A). This is a direct effect of the reducing potentials 290 mV see Fig. 3B, prevailing at the bottom of the lagoon. Comparison with experiment: in Fig. 5B, the theoretical values for the measured species are compared to the experimental measurements. From the surface down to a depth of 10 m, good match between theoretical and experimental data is observed. However, from this point down to the bottom of the lagoon, the observed increase of sulfate is opposite to the theory. Under the reducing anoxic conditions measured at the lagoon-bottom Eh = 290 mV, should drop steeply towards zero (open circles in Fig. 5B). SO2 4

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2006), Gotland (Dyrssen and Kremling, 1990), Lake La Cruz (Rodrigo et al., 2001) and Rogoznica Lake (Ciglenecˇki et al., 1996, 1998, 2005; Baric´ et al., 2003) have many similarities with the Aitoliko lagoon. However, the basin which is closer to Aitoliko, regarding the concentrations of sulfide, is Framvaren basin an anoxic fjord in South Norway. Framvaren is a permanently anoxic fjord (Yao and Millero, 1995; Dyrsenn, 1999) with a sill depth of 2 m and a basin depth of 180 m (Skei, 1988). In Framvaren, the anoxic part of the water and the first sign of sulfide, appear at a depth of 20 m (Anderson et al., 1988). Sulfide reaches its maximum value (8 mM) at the bottom of the fjord (Anderson et al., 1988). The vertical distribution of sulfide in Framvaren is similar to the distribution of sulfide in Aitoliko. The difference between the two basins is the distribution of sulfates. In the anoxic part of the Framvaren fjord, the concentration of sulfate is decreasing towards the bottom of the fjord (Anderson et al., 1988), which is exactly opposite of what is happening in the Aitoliko lagoon. As a result we observe an increase of total sulfur in the Aitoliko, towards the bottom of the lagoon, while in Framvaren, the sum of sulfate and sulfide is almost constant (Anderson et al., 1988). In this respect, Aitoliko lagoon appears to be unique. The theoretical methodology used here shows that it can describe successfully the sulfur speciation under the conditions where physicochemical equilibrium is the only determining factor i.e. at all depths down to 10 m from the bottom. Other factors, in addition to physicochemical equilibrium, influence decisively the sulfur species speciation near the bottom of the lagoon. Anaerobic sulfur bacteria populations, that have been found to abound at the bottom of the lake (data not shown) are most likely determining the local concentrations of sulfur species near the lake bottom. This will be analysed in detail in a forthcoming publication. Of particular importance is that the experimental pH-pe values (solid squares in Fig. 2) cross the critical region where the interplay  of SO2 4 /S2 /H2S occurs. This shows that an eventual fluctuation of pH, pe - or their combination- may result is a shift of the equilibrium over one species. This explains the, otherwise accidental, reported releases of H2S in the air over the lagoon. The predominance

Table 2 Reactions and the corresponding stability constants (log K) used to fit the experimental data. Reactions

log k = n pe0

Reference

Solution reactions H+ + OH = H2O 2 1 10 þ  1 1 8 SO4 þ 8 H þ e ¼ 8 H2 S þ 2 H2 O 2  1 9 þ  1 1 SO þ H þ e ¼ HS þ 4 8 8 8 2 H2 O 2 þ 1  1 2 þ 12 H2 O 8 SO4 þ H þ e ¼ 8 S 2 2 þ 1 1 1  2 SO4 þ H þ e ¼ 2 SO3 þ 2 H2 O

14.00 5.10 4.23 2.49 1.65

Morel Morel Morel Morel

Surface reactions CaSO4  2H2 O ¼ Ca2þ þ SO2 4 þ 2H2 O

4.62

McBride (1994)

and and and and

Hering Hering Hering Hering

(1993) (1993) (1993) (1993)

However the experimental values (solid circles in Fig. 5B) clearly show an opposite trend. Given that the precipitation-dissolution of gypsum has been taken into account, we observe a characteristic deviation between experiment and theory, which is maximized at the bottom of the lagoon. More elaborate reaction schemes i.e. including interactions between iron and sulfur caused even more pronounced deviation of theory from the experimental data (data not shown). 6. Discussion The present data show that pH and Eh have a characteristic vertical distribution which in turn influences the speciation of sulfur species in Aitoliko lagoon. However, near the bottom of the lagoon a striking deviation occurs in the concentration of sulfate and sulfides vs. the theoretically predicted values. Anoxic conditions in natural waters, which usually arise when water is isolated from the atmosphere and oxygen is consumed by the decomposition of organic matter, are characterized by low concentrations of sulfate and high concentration of sulfide species (Ciglenecˇki et al., 1996). Anoxic basins such as Cariaco Trench (Zhang and Millero, 1993; Astor et al., 2003), Black Sea (Haraldsson and Westerlund, 1988; Zhang and Millero, 1994; Clazer et al.,

Depth (m)

Concentration (mM)

A

25

20

Depth (m)

15 10

5 0

B

20

20

18

18

16

16

14

10

5 0

exp [SO42-] theor [SO42-]

12

[SO42-]

10

10

[H2S]

8

8

6

2

15

14

[HS-]

12

4

25 20

6 23

[SO ]

[S2-]

4

theor [H2S+HS ]

2

0

0

-2

-2 7.2

-

-

exp [H2S+HS ]

7.4

7.6

7.8

8.0

pH

8.2

8.4

8.6

8.8

7.2

7.4

7.6

7.8

8.0

8.2

8.4

8.6

8.8

pH

Fig. 5. (A) Theoretical speciation analysis of sulfur species for the pH and redox values measured in the Aitoliko lagoon. No participation (solid symbols), or with participation (open symbols) of the substrate of gypsum at the bottom of lagoon was assumed. (B) Theoretical speciation (open symbols) and experimental distribution of sulfur species (solid symbols) vs. pH, with participation of the substrate of gypsum at the bottom of the lagoon.

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