Journal of Oceanography, Vol. 62, pp. 473 to 480, 2006
Complexation of Dimethylsulfide with Mercuric Ion in Aqueous Solutions G UI-P ENG YANG1,2*, SHIZUO TSUNOGAI1 and S HUICHI WATANABE3 1
Laboratory of Marine and Atmospheric Geochemistry, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan 2 Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266003, China 3 Mutsu Institute for Oceanography, Japan Agency for Marine-Earth Science and Technology, Sekine, Mutsu 035-0022, Japan (Received 25 July 2005; in revised form 27 January 2006; accepted 30 January 2006)
The tendency of dimethylsulfide (DMS) to form complexes with heavy metal ions in aqueous solutions and the factors that influence it have been investigated. Among five heavy metal ions examined (Cu 2+, Cd2+, Zn2+, Pb2+ and Hg2+), only Hg2+ bound significantly with DMS in aqueous solutions in which Hg2+ concentration was increased to much higher levels than that of natural seawater. The complexation capacity of Hg2+ for DMS was influenced by pH and media. The affinity of Hg2+ for DMS was generally lower at high than at low pH, presumably due to the competition of hydroxide ion to form hydroxomercury species. In different solutions, the affinity of Hg2+ for DMS followed the following sequence: ultra-purified water > 35‰ NaCl solution > seawater. It seems apparent that chloride had a negative impact on the complexation of DMS by Hg 2+, owing to the competition of chloride with DMS for complexing Hg2+. In addition, the affinity of Hg2+ for DMS in the bulk seawater appeared to be higher than that in the surface microlayer seawater. The tendency of Hg2+ to form complexes with DMS in aqueous solution can be reduced by the presence of 2 mM amino-acid such as glycine, alanine, serine and cysteine, as these ligands give stable mercury complexes. However, the presence of 2 mM acetate in experimental solutions had no significant effect on the complexation of Hg2+ with DMS, even though this ligand has a relatively strong complexing capacity for Hg2+. Although mercury ions appeared to have a strong affinity for DMS, the concentration of mercury in seawater is too low to produce a great effect on the distribution of DMS in oceans.
Keywords: ⋅ Dimethylsulfide, ⋅ complexation, ⋅ heavy metal ions, ⋅ seawater, ⋅ aqueous solutions.
esses (Dacey et al., 1998; Kiene and Linn, 2000). There are several recognized mechanisms for the loss of DMS from the surface of the oceans. Historically, the main removal of DMS from the surface oceans was thought to be evasion into the marine atmosphere. Yet while the absolute quantity of DMS emitted to the atmosphere may be significant on a global basis, the atmospheric ventilation invariably accounts for a very small proportion of DMS that is turned over in the water column. Recent evidence shows that microbial consumption may be a significant sink for DMS in seawater and hence a dominant factor controlling the net flux of DMS to the atmosphere (Kiene and Bates, 1990). More recent studies have indicated that the photolysis of DMS in seawater accounted for 7–40% of the total turnover of DMS (Kieber et al., 1996), and might be an important sink of DMS in shallow coastal waters (Brugger et al., 1998).
1. Introduction A lot of progress has been made in the biogeochemical study of dimethylsulfide (DMS) in oceans, because it is a major biogenic, reduced sulfur compound emitted from the surface oceans to the atmosphere (Andreae, 1990; Turner et al., 1996; Uzuka et al., 1996; Simó and Pedrós-Alió, 1999; Aranami et al., 2001; Yang and Tsunogai, 2005; Yang et al., 2005a). Its sea-toair flux represents the most important natural source of reactive sulfur in the troposphere, about half of the global amount of biogenic sulfur. Distribution of DMS in the surface oceans is controlled by the interaction of complex production pathways with rapid consumption proc* Corresponding author. E-mail:
[email protected] Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer
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In addition to these sinks it appears likely that DMS is lost via other processes such as complexing with heavy metal ions, as suggested by Brimblecombe and Shooter (1986). A few previous studies have demonstrated that DMS can form a stable HgCl2 complex with composition 2DMS-3HgCl 2 and stability constant of logK = 14.5 (McAllan et al., 1951; Wagner et al., 1967; Yang, 1996). The formation of 2DMS-3HgCl2 complex is the basis of a method developed by Nguyen et al. (1978) for the separation of DMS in seawater, followed by gas chromatographic determination after regeneration with 6 M hydrochloric acid. As yet there has been little study of the reactions of DMS with metal ions in seawater. Through headspace analysis of the gas above solutions of DMS, Shooter and Brimblecombe (1989) found no detectable difference ( seawater. This result is apparently due to the effect of media and will be discussed in Subsection 3.4. However, at lower pH range ( 35‰ NaCl solution > seawater. This observation may be attributed to the diversity in the composition of media, which can be elucidated as follows. Mercury is known to form stable complexes with various organic and inorganic ligands. Recent study has shown that organically associated Hg appears to be the predominant species in aquatic environments (Allard and Arsenie, 1991; Mierle and Ingram, 1991; Watras et al., 1995). Besides dissolved organic matter, inorganic anion in seawater may also strongly compete with DMS to complex Hg2+. From the published diagram that showed predominance regions of different chloro- and hydroxomercury species as a function of pCl and pH (Lockwood and Chen, 1973), it can be seen that HgCl42– ion should be the principal soluble inorganic form of mercury in seawater containing about 0.55 M chloride. Therefore, the affinity of Hg for DMS in seawater would be influenced by the presence of various organic and inorganic ligands. Compared with the situation in UPW, the affinity of Hg for DMS in seawater, on average, decreased by about 30% when the concentration of Hg was lower than 20 mM (Fig. 5). In the absence of dissolved organic matter, the speciation of mercury in aqueous solution is mostly controlled by chloride [HgCl+, HgCl20, HgCl3–, HgCl42–] and pH [Hg2+, Hg(OH)+, Hg(OH)20] (Dyrssen and Wedborg, 1991; Schuster, 1991). HgCl42– complex will predominate in 35‰ NaCl solution at pH 8.0. As expected, an average 20% decrease in the complexation ability of Hg2+ for DMS was found in sodium chloride solution relative to UPW due to the competition of chloride (Fig. 5), which suggests that increasing salinity of the water had a negative effect on the complexation of Hg2+ with DMS.
25
3.5 Complexation of Hg 2+ with DMS in the microlayer seawater The sea surface microlayer plays a critical role in air-sea interactions and biogeochemical processes. Although many studies have focused on the chemistry of
35
Seawater
y = 35e -0.09x
Microlayer
y = 0.0172x - 1.5306x + 34.187
2
DMS (nmol/l)
30
20 15 10 5 0 0
10
20
30
40
50
Hg (mmol/l)
Fig. 6. Variation of DMS concentrations with increasing Hg level in bulk seawater and surface microlayer sample.
the microlayer, no research has been carried out on the extent or nature of the reactions between metal ions and DMS within the microlayer. Our study has shown that the complexing ability of Hg2+ for DMS in the microlayer is somewhat different from that in bulk seawater (Fig. 6). It is clear from Fig. 6 that in the bulk seawater the concentration of DMS decreased faster than in the microlayer seawater in the presence of Hg2+, suggesting that affinity of Hg2+ for DMS in the bulk seawater is higher than that in the surface microlayer. This may be due to the difference in aqueous constituents between bulk seawater and the surface microlayer: the surface microlayer is frequently enriched in organic compounds at concentrations greater than those found in the underlying bulk water (Liss and Duce, 1997). The ability of dissolved organic matter to form stable complexes with heavy metals in the microlayer has been observed (Shine and Wallace, 1996). Because surface microlayer usually contains a higher concentration of dissolved organic matter relative to the bulk seawater, the association between Hg2+ and DMS in the microlayer would be weakened due to the completion of coexistent organic matter. This point is especially relevant when the mercuric concentration is lower than 20 mM. 3.6 Effect of organic matter on the complexation of Hg2+ for DMS In order to further demonstrate the effects of coexistent organic matter on the complexation of Hg 2+ with DMS in seawater, a series of organic ligands possessing varying binding strengths with mercury was chosen, including acetate and aminoacids. Aminoacids are common constituents of dissolved organic matter in seawater.
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Table 1. Stability constants for complexes between Hg2+ and DMS, acetate and aminoacids. Ligand
logK
Stoichiometry
Reference
DMS Acetate
14.5 6.1 10.1 18.25 22.0 41.6 10.9 20.1 17.34
2L-3HgCl 2 HgL HgL2 HgL2 HgL HgL2 HgL HgL2 HgL2
Wagner et al. (1967); Yang (1996) Stumm and Morgan (1996)
Glycine Serine
Among them, glycine, alanine, and serine are most important and account for a large proportions of total free aminoacids in seawater (Buffle, 1988). Cysteine, as first organosulfur metabolite produced by assimilation of sulfate in seawater, serves as the starting compound for the biosynthesis of all other sulfur-containing organic matter, including dimethylsulfoniopropionate (DMSP), the precursor of DMS (Andreae, 1990). A literature summary of the stability constants of mercury complexes with the organic ligands used in this study is given in Table 1. Figure 7 shows the influence of organic compounds on the extent of complexation of DMS by Hg 2+ in seawater. Mercury is considered to be a soft acid and thus may form strong complexes with some soft bases such as sulfur and nitrogen. In particular, Hg2+ has an extremely high affinity for sulfhydryl groups, with the formation constants of Hg2+ and the anionic form of a sulfhydryl group, R-S–, being over 10 orders of magnitude higher than Hg 2+ affinity constants for amino group or acetate (Table 1). Since the formation constants of mercury-thiol complexes are so high, Hg2+ will bind first to any free thio compounds present. Therefore, at a mercuric concentration lower than 10 mM, no formation of Hg(II)DMS complex was expected in the solution, due to strong competition by L-cysteine. Between a mercuric concentration of 10 and 20 mM, the concentration of DMS declined slightly with increasing mercuric level, indicating that a minor part of DMS in the solution was bound to mercury. As the mercuric concentration was greater than 20 mM, owing to the decrease in the competitive ability of L-cysteine, a larger proportion of the DMS in the solution was associated with mercury. Mercuric affinity for glycine, serine and alanine was not so strong as for L-cysteine, so these three amino-acids had a much smaller effect on mercuric complexation with DMS than L-cysteine (Fig. 7). However, it is worth noting that alanine revealed a smaller effect on the complexation of DMS by Hg 2+ than serine, despite the
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Sillén and Martell (1971) Dyrssen and Wedborg (1991) Stumm and Morgan (1996) Sillén and Martell (1971)
45 Seawater Seawater Seawater Seawater Seawater Seawater
40 35
DMS (nmol/l)
Alanine L-cysteine
+ 2 mM Cysteine + 2 mM Glycine + 2 mM Serin e + 2 mM Alanine + 2 mM Acetate
30 25 20 15 10 5 0 0
10
20
30
40
50
Hg (mmol/l)
Fig. 7. Effect of organic ligands on complexation of Hg2+ with DMS in seawater. Regression equations of the data: 䉱 Seawater: y = 38e–0.0712x; 䉬 2 mM Cysteine: y = 0.0005x 3 – 0.0361x2 + 0.2371x + 38; 䊏 2 mM Glycine: y = 38e–0.0332x; 䊐 2 mM Serine: y = 38e –0.0393x ; × 2 mM Alanine: y = 38e–0.0445x; 䉭 2 mM Acetate: y = 38e–0.0638x.
alanine affinity constant for Hg2+ being somewhat greater than that of serine (Table 1). This disagreement may be caused by the fact that some variation would occur in the stability constant of the complex between Hg2+ and organic ligand due to change in the composition of solution. Among all the organic ligands used in this study, acetate did not produce a significant effect on mercuric complexation with DMS. This might be a result of the relatively weak binding strength between mercury and this ligand as compared with DMS (Table 1).
3.7 Effect of mercury on the distribution of DMS in natural seawater Many efforts have been directed to the measurement of mercury in the oceans, but there has been a very slow development in oceanographic information on the amounts and distribution of mercury in seawater, primarily due to difficulties in obtaining uncontaminated seawater collections. At the early stage of oceanographic investigation, reported concentrations of mercury in open ocean were generally high and spread over a surprisingly broad range, from 0–>1000 ng/l (Nriagu, 1979). Recent advances in analytical techniques and improved sampling methods have produced more reliable data showing that mercury concentrations in seawater usually vary over a small range between 1–10 pM (Gill and Fitzgerald, 1987; Guentzel et al., 1996). Although mercuric ion is demonstrated to be capable of complexing DMS, mercuric concentrations in natural seawater are too low to sequester significant amounts of the DMS. This finding contradicts the previous suggestion of Brimblecombe and Shooter (1986) that complexing with heavy metal was a possible sink for DMS in seawater. Our study suggests that the removal of DMS through complexing with heavy metal ions such as Hg 2+ is much less important than other removal pathways such as biological degradation, photochemical oxidation and sea-to-air exchange. 4. Conclusions This study provides some insight into the complexation reaction between DMS and heavy metal ions in aqueous solutions. Some important conclusions were drawn, as follows. 1. Among five heavy metal ions examined in this study, only Hg 2+ can strongly bind DMS in aqueous solution. The complexation capacity of Hg 2+ for DMS was influenced by pH and media. The affinity of Hg2+ for DMS was generally smaller at high than at low pH. In different solutions, the affinity of Hg2+ for DMS followed the following sequence: UPW > 35‰ NaCl solution > seawater. Moreover, the affinity of Hg 2+ for DMS in the bulk seawater was somewhat higher than that in the surface microlayer. 2. The tendency of Hg2+ to form complexes with DMS in aqueous solution can be reduced by the presence of other organic ligands such as aminoacids. However, the presence of acetate in experimental solutions had no significant effect on the complexation of Hg2+ with DMS, even though this ligand has a relatively strong complexing capacity for Hg2+. 3. According to the present study, the complexation of DMS with Hg is of little significance for affecting the distribution of DMS in seawater. Although this process would be unlikely to have environmental significance, given the low concentrations of the metals, it is only by
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