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Kinetic controls on metalloid speciation in seawater
Marine Chemistry, 40 (1992) 65-80
65
Elsevier Science Publishers B.V., Amsterdam
Kinetic controls on metalloid speciation in seawater Gregory A. Cutter Department of Oceanography, Old Dominion University, Norfolk, VA 23529-0276, USA (Accepted 27 March 1992)
ABSTRACT Cutter, G.A., 1992. Kinetic controls on metalloid speciation in seawater. Mar. Chem., 40: 65-80. Mechanisms for the production of thermodynamically unstable species of three metalloids, antimony, arsenic, and selenium, and the rates of their transformation to stable forms have been critically reviewed. The occurrence of thermodynamically unstable species of these metalloids falls into two categories: reduced species in oxic waters and oxidized species in anoxic water. In surface waters biotic processes produce reduced species (e.g. Sb(III), arsenite, selenite), while in deep anoxic waters lateral advection and sinking detritus deliver oxidized species (e.g. arsenate, antimonate) from surface waters. In both cases, slow rates of conversion allow unstable species to persist. Dissolved and particulate data for antimony, arsenic, and selenium speciation in the Black Sea are used to illustrate the processes involved in this kinetic stabilization.
INTRODUCTION
Amongst the trace elements dissolved in seawater, the metalloids (germanium, arsenic, selenium, antimony, tellurium) are distinguished by the number of chemical forms in which they can exist (i.e. their speciation). The chemical speciation of metalloids includes not only different oxidation states (e.g. As(Ill) and As(V)), but also different chemical forms within an oxidation state (e.g. dimethylarsinic and methylarsonic acids). Initial approaches to examining the speciation of metalloids in the ocean utilized equilibrium thermodynamic calculations such as those first proposed by Sillen (1961). In general, the use of this approach for the metalloids predicted that the fully oxidized states (e.g. As(V)) should be the only species in oxic water, while reduced species (e.g. As(Ill)) should only be found under anoxic conditions. Unlike many other trace elements, sensitive analytical techniques have been developed to determine the chemical speciation of metalloids (e.g. Andreae, Correspondence to: G.A, Cutter, Department of Oceanography, Old Dominion University, Norfolk, VA 23529-0276, USA.
0304-4203/92[$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
66
G.A. CUTTER
1977; Hambrick et al., 1984). The use of these techniques has shown that the biogeochemical cycles of metalloids in the ocean are more complicated than equilibrium thermodynamic calculations predicted. In particular, reduced species such as Se(IV) are found in oxic waters (e.g. Measures et al., 1980) and oxidized species such as Sb(VI) are found in anoxic waters (e.g. Bertine and Lee, 1983). For thermodynamically unstable species to be found in seawater, mechanisms for their production and slow rates of interconversion (i.e. kinetic stabilization) must exist. While the biogeochemical cycle of each metalloid is unique, one common process is the kinetic stabilization of thermodynamically unstable species. Indeed, studies of individual metalloids in the Pacific (e.g. Andreae, 1979), Atlantic (e.g. Measures and Burton, 1980), Baltic Sea (Andreae and Froelich, 1984), and the Saanich Inlet (e.g. Peterson and Carpenter, 1983) have all invoked such kinetic arguments to explain metalloid speciation. This review attempts to evaluate critically existing kinetic data for three of the metalloids, antimony, arsenic, and selenium. An emphasis is placed on the specific mechanisms that produce thermodynamically unstable species and the rates of their conversion to stable forms. To facilitate this review of the processes and rates affecting the chemical speciation of antimony, arsenic and selenium, previously published data from the central basins of the Black Sea (Cutter, 1991), supplemented by some newly obtained dissolved and particulate speciation results, are utilized. Surface waters in this environment are meso- to oligotrophic, and nutrient cycling in the oxic water column is similar to that in many ocean regimes (e.g. see data in Murray et al., 1989). Below approximately 100m, anoxic conditions are well developed (Murray et al., 1989) and in a relative steady state compared with other anoxic basins and t]ords in coastal regions (e.g. Saanich Inlet, Cariaco Trench). By possessing attributes of two regimes, mesotrophic surface waters and a well-defined anoxic zone, the central Black Sea affords an ideal environment in which to examine kinetic processes affecting metalloid speciation. S T U D Y SITE A N D M E T H O D S
Study site Water samples and sediment box cores were collected at two stations in the Black Sea (Station BS3-2, 42°50'N, 32°00'E, depth of 2086 m; Station BS3-6, 43°04'N, 34°00'E, depth of 2185m) during Cruise 3 of the 1988 Black Sea Expedition (3-16 June 1988). Relevant hydrographic parameters for these stations are thoroughly described in Murray et al. (1989, 1991), as well as by Cutter (1991).
METALLOID SPECIATION IN SEAWATER
67
Methods
Sampling and analytical methods are given in the original references, and only procedures used to obtain the new data for antimony, arsenic, and selenium will be reviewed here. Sediment samples were acquired with a box corer. After arrival on deck, subcores were obtained by carefully inserting acrylic tubes (5.6 cm i.d.) into the sediment and then sealing the ends of the tubes with polyethylene core caps. These subcores were sectioned at 1 cm intervals under nitrogen within 2 h of collection. Sections were placed in polyethylene bags, homogenized, and immediately frozen. The determination of dissolved selenium speciation used the selective hydride generation/atomic absorption methods outlined by Cutter and Bruland (1984). Three selenium fractions are directly determined on 0.4#m filtered samples that are preserved by acidification to pH 2: selenite (Se(IV)), selenite + selenate (Se(IV + VI)), and total dissolved selenium. Selenate is then calculated as the difference between Se(IV + VI) and Se(IV), while S e ( - I I + 0) is the difference between total selenium and Se(IV + VI). Since Se(0) is insoluble and would only be found in the S e ( - I I + 0) fraction if it passed through the 0.4 #m filter (i.e. as a colloid), the S e ( - I I + 0) fraction is considered to be largely dissolved organic selenide (Cutter, 1982; Cutter and Bruland, 1984). The detection limit for dissolved selenium species is 0.01 nmol 1-I, and precision (as relative standard deviation) is 5% at the 0.5 nmoll -~ level. For sediment samples, solid-phase selenium speciation was determined using the method of Cutter (1985) for total selenium and Se(IV + VI), and using the procedure of Velinsky and Cutter (1990) for the determination of elemental selenium. Total sedimentary arsenic and antimony were determined using the method of Cutter et al. (1991), while the 2 M HC1 leach technique given by Andreae and Froelich (1984) was used to quantify sedimentary arsenic and antimony speciation. DISCUSSION
Antimony, arsenic, and selenium have perhaps the most diverse chemistries of the metalloids. Dissolved antimony and arsenic can be found in the trivalent (Sb(III) and arsenite) and pentavalent (antimonate and arsenate) oxidation states, and as methylated forms of the pentavalent state (e.g. dimethylarsinic acid, methylstibonic acid). Selenium has three dissolved oxidation states ( - I I , selenide; IV, selenite; and VI, selenate), and may be found in organic compounds such as dissolved free and peptide-bound amino acids (e.g. selenomethionine). The occurrence of thermodynamically unstable species of these metalloids falls into two general categories: reduced species in oxic waters and oxidized
68
G.A. CUTTER
species in anoxic waters. The mechanisms and rates that are associated with both of these occurrences are reviewed in the sections to follow. In addition, data from the Black Sea are used to illustrate the net results of these kinetic processes. Reduced species in the oxic water column
Of all the metalloids, perhaps the greatest attention has been focused on arsenic. As early as 1962 arsenite was reported in oxygenated seawater (Sugawara et al., 1962), in disagreement with the equilibrium thermodynamic predictions of Sillen (1961). Considerable effort was then devoted to elucidating the production mechanisms for arsenite in oxic seawater and its rate of oxidation to thermodynamically stable arsenate. Potential sources of arsenite include the in situ reduction of arsenate (biotic or abiotic), and delivery to surface waters by atmospheric deposition and riverine inputs. Johnson and Pilson (1975) estimated that the riverine and atmospheric contributions of arsenite are probably minor. Indeed, recent direct measurements of atmospheric (e.g. Andreae, 1980; Scudlark and Church, 1988) and riverine/ estuarine inputs (e.g. Andreae, 1978; Waslenchuk and Windom, 1978) show that arsenite is generally less than 10% of the total arsenic in these sources. The in situ production of arsenite in oxic waters has been thoroughly examined by a number of investigators (e.g. Andreae, 1979; Andreae and Klumpp, 1979; Sanders and Windom, 1980). These studies concluded that arsenite is largely derived from a planktonic detoxification mechanism, where arsenate is reduced to arsenite under low phosphate concentrations. In addition to arsenite, methylarsonic and dimethylarsinic acids are produced during the uptake of arsenate by phytoplankton, with the ratio of methylated acids to arsenite varying with the species (Andreae and Klumpp, 1979). Overall, the production mechanism for arsenite in oxic seawater is controlled by biotic processes. Kinetic studies of arsenite oxidation suggest that both biotic (e.g. Sanders and Windom, 1980; Scudlark and Johnson, 1982) and abiotic (e.g. Peterson and Carpenter, 1983) factors can be involved. Most of these studies have found that the oxidation rates are pseudo first order in arsenite, and this behavior will be assumed for the rest of this paper; a summary of rate constants for arsenite oxidation is contained in Table 1. The early work of Johnson and Pilson (1975) used Sargasso Sea water to obtain a rate constant (at 41°C) of 0.009-0.04 day ~ (Table 1). If this value is representative of oceanic conditions (in spite of the elevated temperature), then rate constants (and rates) in coastal waters are clearly much higher (Table 1). Scudlark and Johnson (1982) suggested an average rate constant of 2.2 day -~ for 'productive' coastal waters, and demonstrated that arsenite oxidation can be microbially catalyzed. Other determinations of the rate constants for arsenite oxida-
69
METALLOID SPECIATION IN SEAWATER TABLE 1 Pseudo first order rate constants Reaction ARSENIC As(llI) ~ As(V)
k (day i)
Notes
Reference
0.009-0.04 0.1 2.2
Sargasso; 41°C Culture; 16°C 'Productive' coastal waters Saanich; oxic; 30°C Saanich; oxic; 6°C Saanich; Mn oxide max. Estuary Estuary Black Sea; 65-85 m Saanich; 6°C Avg. anoxic Black Sea
Johnson and Pilson (1975) Sanders and Windom (1980) Scudlark and Johnson (1982)
0.04-0.3 0.005-0.03 0.07-0.3
As(V) ~ As(III)
1.9 0.5-2.2 0.01 0.06-0.08 1.6 × 10 -5
Peterson and Carpenter (1983) Peterson and Carpenter (1983) Peterson and Carpenter (1983) Knox et al. (1984) Andreae and Andreae (1989) This work Peterson and Carpenter (1983) Cutter (1991)
ANTIMONY Sb(III) ~ Sb(V) Sb(V) --, Sb(III)
0.008 1.1 x 10 -6
Black Sea; 65-85 m Avg. anoxic Black Sea
This work Cutter (1991)
SELENIUM S e ( - II) --} Se(IV)
1.1 x 10 -4
Suzuki et al. (1979)
Se(IV) ~ Se(VI)
1.1 × 10 -5
Surf. and deep Pacific Surf. and deep Pacific Surf. Pacific Deep Pacific Deep Pacific Sediments, brackish ponds Water column, anoxic lakes
Se(IV, VI) ~ Se(O)
2.1 x 10 -3 0.4-2.9 × 10 -6 2.4 x 10 -6 1-30 0.009
Suzuki et al. (1979) Measures et al. (1980) Measures et al. (1980) Cutter and Bruland (1984) Oremland et al. (1990) Cutter (unpublished data)
tion in estuaries (Table 1) are very similar to the Scudlark and Johnson (1982) value. In another coastal environment, the Saanich Inlet, Peterson and Carpenter (1983) demonstrated both strong temperature dependence (Table 1) and a biotic role in the oxidation of arsenite. Moreover, they found that manganese oxides increase the oxidation rate, a factor that would be important in waters overlying an oxic/anoxic surface. The arsenic speciation data for the Black Sea can be used to illustrate the net effect of these mechanisms and rates. Figure 1 presents the depth profiles for dissolved arsenic species in the upper 100 m of Station BS3-2, taken from the results of Cutter (1991). Arsenate displays the surface-water depletion that
70
G.A. CUTTER
SfafIon B S 3 - 2 As(V) ( n m o l / L ) 0
As(Ill) ( n m o l / L )
1o
20
30
40
]
I
i
,
0 1 2 3 4 5 .,.,.,.,.rJ-,
DIVAs ( n m o l / L )
20
40 .
,
0
1 .
,
2 .
,
3 .
,
4 ,
,
0
20
40 E
t
t-
Q. 0
"~
60
80
100
Fig. 1. Depth profiles of dissolved arsenate (As(V)), arsenite (As(III)), and dimethylarsinic acid (DMA) in the upper 100m at Station BS3-2 in the Black Sea (data from Cutter, 1991). The water column becomes anoxic at 90 m. Note concentration scale break for arsenite.
is typical of many ocean regimes (e.g. Andreae, 1979). Correspondingly, non-equilibrium concentrations of arsenite are also present in the surface waters of BS3-2 (Fig. 1). This arsenite is most likely derived from bioreduction, and the presence of dimethylarsinic acid in these same waters (Fig. 1) is strong evidence for arsenate uptake by biota (Andreae and Klumpp, 1979; Sanders and Windom, 1990). Thus, the biotic production of thermodynamically unstable arsenic species found in most oceanic regimes also appears to operate in surface waters of the Black Sea. Unlike other ocean regimes, arsenite in the oxic waters of the Black Sea might also be transported from anoxic waters (i.e. below 90m at BS3-2). However, this source appears to be limited since arsenite is almost completely removed by 65 m (Fig. 1). Such removal of arsenite from anoxic waters was also found in the Saanich Inlet by Peterson and Carpenter (1983), who ascribed it primarily to oxidation in the presence of manganese oxides (measured rate constant of 0.07-0.3 day-l). Significantly, the depth range
METALLOID SPEC1ATION IN SEAWATER
71
over which arsenite is removed at Station BS3-2 also coincides with the particulate manganese maximum (Lewis and Landing, 1991). In order to quantitatively compare these Black Sea results with those of Peterson and Carpenter (1983), as well as those in Table l, the rate constant for arsenite oxidation is needed. Using a vertical advection-diffusion model for a non-conservative species (e.g. Craig, 1969; Craig and Weiss, 1970) over the depth range where the steep arsenite gradient is found (65-85m), the removal rate for arsenite can be estimated. Most recently, this type of model has been used to examine Black Sea results by Lewis and Landing (1991) for trace metals, and by Cutter (199 l) to determine arsenate reduction rates in the anoxic zone. To accurately use this model, horizontal inputs must be minimized, and an examination of the temperature-salinity diagram for Station BS3-2 shows that this is possible over the 40-250 m depth interval (Lewis and Landing, 1991). Unfortunately, horizontal effects preclude using an advection-diffusion model for the upper 40 m where arsenite is also found. For this application of the model, the vertical eddy diffusion coefficient (K) is first obtained by fitting the salinity profile to the solution of the advectiondiffusion equation for a conservative species (i.e. the removal term, J, is set to zero), assuming a vertical upwelling rate (w) of 3.6 m year-1 (Murray et al., 1991). In this manner an eddy diffusion coefficient of 115 m 2 year-~, identical to that determined by Lewis and Landing (1991) for the same depth interval, is obtained. Fitting the 65-85 m arsenite data to the non-conservative solution (constant J with depth) to the model gives a removal rate (J) of 75 pmol 1day -~ . Using the average arsenite concentration from 65 to 85 m (7.6 nmol 1-1), a first-order rate constant of 0.01 day -~ is calculated. The removal of arsenite in the 65-85 m depth range includes not only oxidation, but also scavenging by particles (e.g. metal oxides). However, the close agreement between the rate constant calculated here (0.01 day -~) and those in the Saanich Inlet (0.07-0.3day-l; Peterson and Carpenter, 1983), as well as others in Table l, suggests that arsenite from the anoxic zone is largely removed by oxidation to arsenate. Nevertheless, the production of arsenite via bioreduction of arsenate and its slow removal by oxidation (residence time with respect to oxidation 1/k, ranges from 0.5 to 100 days; Table 1) allow arsenite to persist in oxygenated surface waters. While numerous investigators have examined the kinetic processes affecting arsenic, relatively little is known about the other Group V metalloid, antimony. In surface waters of Saanich Inlet (Bertine and Lee, 1983), the Baltic Sea (Andreae and Froelich, 1984), and the North Atlantic (Middelburg et al., 1988) thermodynamically unstable Sb(III) has been detected. Moreover, Andreae and Froelich (1984) found methylstibonic acid in the Baltic Sea, and these workers proposed that Sb(III) and methylstibonic acid, like their arsenic analogs, may be produced by phytoplankton. Indeed, Sb(III) has been detected in phytoplankton and macroalgae (Kantin, 1983; Andreae and
72
G.A. CUTTER Station B S 3 - 2
SbCY) (nmol/L) 0 . . . . . . . . .
Sb(lll)(nmol/L)
1
2
i . . . . . . . . .
i
MSb (nmol/L)
0.0 0.1 0.2 0.3 0.4 0.5 •
i
•
i
,
't
•
i
•
I
0.00 i
0.05 .
.
.
.
I
,
0.10 '
•
'
1
20
.-'--,
40
E
~=~
(.-
u)
a
6o
80
100
Fig. 2. Depth profiles of dissolved antimonate (Sb(V)), Sb(IlI), and methylstibonic acid (MSb) in the upper 100 m at Station BS3-2 in the Black Sea (data from Cutter, 1991). The water column becomes anoxic at 90m.
Froelich, 1984), suggesting that biotic production is possible. While these studies suggest a mechanism for the production of Sb(III) in oxic waters, its persistence under oxic conditions requires kinetic stabilization. However, no rate measurements for Sb(III) oxidation in seawater are available; only the rate of antimonate reduction in anoxic waters has been estimated (Cutter, 1991). The depth profiles for antimony species in the upper 100 m of the Black Sea (Station BS3-2; Cutter, 1991) shown in Fig. 2 allow a further examination of processes and rates affecting antimony. Like the Baltic Sea (Andreae and Froelich, 1984) antimonate displays a surface maximum and decreases with depth. In contrast to the previous studies, no detectable Sb(III) is found in the upper 60 m (Fig. 2); the highest Sb(III) concentrations are found in the anoxic zone (below 90 m), with a steep concentration gradient between the oxic and anoxic waters. However, methylstibonic acid is present in surface waters (Fig. 1), suggesting the biotic uptake of antimonate proposed by Andreae and Froelich (1984). If the production mechanism for Sb(III) is equivalent to that
METALLOID SPEC1ATION IN SEAWATER
73
of arsenite, the absence of Sb(III) may be due to a more rapid oxidation rate. Since rates for Sb(III) oxidation are not reported in the literature, the results in Fig. 2 (and Cutter, 1991) can be used to estimate the Sb(III) removal rate (oxidation) via the same advection-diffusion model used for arsenite (i.e. fitting the Sb(III) gradient from 65 to 85 m). The loss rate for Sb(III) obtained in this manner is 0.65 pmol l-~ day -I , and using the average Sb(III) concentration (0.08nmol 1-~), a pseudo first order rate constant of 0.008 day -~ is calculated. This rate constant includes all forms of removal since Sb(III) may also be scavenged by adsorption to particles. However, this constant can serve as a guide for future studies, and suggests that rapid removal alone cannot explain the lack of Sb(III) in Black Sea surface waters. Indeed, Sb(III) is as kinetically stable as arsenite (residence time of Sb(III) with respect to removal, 1/k, is 125 days). If Sb(III) is produced during the biotic uptake of antimonate, then the production rates in the Black Sea may be relatively slow or species dependent such as those observed for arsenic. Like arsenic, dissolved selenium also has thermodynamically unstable species in oxic waters, including selenite and organic selenides. One of the first reliable reports of selenite in seawater was that of Sugimura and coworkers (1976), who found concentrations ranging from 0.6 to 1.1 nmol l-1 in vertical profiles from the western North Pacific. These investigators suggested that 'biochemical reactions' might control the speciation of dissolved selenium. Later work by Measures and Burton 0980) and Measures et al. (1980) presented oceanographically consistent depth profiles for selenite and selenate in the Atlantic and Pacific Oceans. In their Atlantic study Measures and Burton 0980) suggested that selenite may be produced via the regeneration of particulate organic selenide, while Measures et al. (1980) used box and advection-diffusion models to explain how selenite could be produced by selenate reduction and transported by sinking detritus. Dissolved organic selenium (presumably selenide) has been reported in the upper water column of the Pacific (Suzuki et al., 1979; Cutter and Bruland, 1984), in the Saanich Inlet (Cutter, 1982), and in the Gulf of Mexico (Takayanagi and Wong, 1985). Using freshly collected biogenic particles, Cutter (1982) showed that selenium undergoes a multistep regeneration, with the first step being the release of dissolved organic selenide from particulate matter, which then oxidizes to selenite, and then to selenate. This multistep regeneration provides a simple mechanism for the production of thermodynamically unstable species in the oxic water column. In addition to process studies, numerous investigators have estimated the transformation rates for selenium species; Table 1 contains a compilation of these rate constants. Using a two-layer box model Suzuki and coworkers (1979) estimated a rate constant for the oxidation of dissolved organic selenide to selenite of 1.1 x l0 -4 day -~, and a rate constant of 1.1 × 10 -5 day -1 for the oxidation of selenite to selenate (Table 1). Although organic selenide was
74
G.A.
CUTTER
Station B S 3 - 6
S*(IV) (nmol/L)
S*(Vl) (nrnol/L) 0.0 I
0.5 .
.
.
.
?
o
i
. . . .
1.0
0.0
i
I
0.5 . . . .
I
S,(-ll+O) (nmol/L) 1.0
. . . .
i
0.0
0.5 . . . .
I
1.0 . . . .
i
oxic
lOO arloxic
200
v
E
300
..c
/
500
1000
1500
2000
Fig. 3. Depth profiles of dissolved selenate (Se(VI)), selenite (Se(IV)), and S e ( - I I + 0) at Station BS3-6 in the Black Sea. Waters above the oxic line have detectable oxygen concentrations and waters below the anoxic line have detectable hydrogen sulfide concentrations. Note that the upper 300 m are plotted on an expanded scale.
not considered, Measures et al. (1980) used a two-layer box model and advection-diffusion calculations to estimate a rate constant for selenite oxidation of 2.1 x 10 -3 day -~ in surface waters and 0.4-2.9 x 10 - 6 day -j in deep waters (Table 1). Cutter and Bruland (1984) used a kinetic rate model in which selenite is produced by organic matter oxidation (i.e. respiration) and removed by oxidation to selenate to determine a rate constant of 2.4 x 10 - 6 day-~ for the oxidation of selenite in deep waters (Table 1). Perhaps the most significant feature in Table 1 is the close agreement between the rate constants for selenite oxidation, in spite of the fact that completely independent data sets and modeling approaches were used. The results in Table 1 clearly show that selenite, and to a lesser extent organic selenide, are kinetically stable in oxic seawater. The newly obtained results for dissolved selenium speciation at Station BS3-6 in the Black Sea (Fig. 3) are similar to previous observations in the open ocean (e.g. Cutter and Bruland, 1984) and the Saanich Inlet (Cutter, 1982).
M E T A L L O I D SPECIATION IN SEAWATER
75
The primary reduced species in the oxic water column is organic selenide ( S e ( - I I + 0)), while selenite concentrations are very low (average of 0.03 nmol 1-~), but detectable in the upper 50 m. Like the Pacific results of Cutter and Bruland (1984), the surface maximum of organic selenide probably results from organic matter regeneration, and the 25 year residence time based on the oxidation rate constant of Suzuki et al. (1979) explains how organic selenide can persist in oxic surface waters. Oxidized species in the anoxic water column
The occurrence of oxidized arsenic and antimony species in anoxic waters has been reported in the Saanich Inlet (Bertine and Lee, 1983; Peterson and Carpenter, 1983), the Baltic Sea (Andreae and Froelich, 1984), and the Black Sea (Cutter, 1991). A variety of mechanisms to explain these results have been postulated, including delivery of As(V) and Sb(V) on sinking detritus from oxic waters (Andreae and Froelich, 1984; Cutter, 1991), formation of thiocomplexes with the pentavalent elements (Bertine and Lee, 1983; Andreae and Froelich, 1984), and advection of surface waters with high concentrations of arsenate and antimonate (Cutter, 1991). All of these mechanisms must then be coupled with relatively slow rates of reduction. Rate constants for arsenate reduction under anoxic conditions have only been reported for the Saanich Inlet (Peterson and Carpenter, 1983) and the Black Sea (Cutter, 1991), while the reduction rate constant for antimonate has only been estimated using Black Sea data (Cutter, 1991); these values are listed in Table !. In the Black Sea, Cutter (1991) found arsenate and antimonate to be widely distributed in the anoxic zone (average arsenate, 7.3 nmol 1-~; average antimonate, 0.5nmol 1-t) and speculated that one source of these dissolved species could be their transport on detritus from surface waters. The newly obtained data in Table 2 for arsenic and antimony speciation in Black Sea sediments provide strong evidence for a detrital source of antimonate and arsenate to anoxic waters, as first proposed by Andreae and Froelich (1984) for the Baltic. Surficial sediments underlying the anoxic water column contain As(V) and Sb(V), with the concentrations decreasing with depth (Table 2). Interestingly, the fractions of sedimentary arsenic and antimony that can be recovered as As(III + V) and Sb(III + V) also decrease with depth (e.g. for 1-2cm, As(III + V) and Sb(III + V) are equal to total As and Sb, but at 11-12cm only 50% of the total As and Sb are As(III + V) and Sb(III + V)). This suggests that the arsenic and antimony are being incorporated into a more refractory phase such as pyrite (e.g. Huerta-Diaz and Morse, 1990). Overall, investigations of several anoxic marine systems, and the data in Tables 1 and 2, demonstrate that the abundance of dissolved arsenate and antimonate in anoxic waters can be attributed to detrital fluxes, advection from surface waters (Black Sea), and slow reduction rates.
4.76 4.30 4.25
1-2 6 7 11-12
N.D., non-detectable.
Org. C (mmol g i )
Depth (cm)
51.3 42.4 31.6
Tot. Se (nmol g i ) 5.1 3.4 N.D.
Se(1V + VI) (nmol g l) 15.6 11.4 6.8
Se(0) (nmol g - 1)
Antimony, arsenic and selenium in Black Sea sediments (BS3-2)
TABLE 2
209 195 155
Tot. As (nmol g i ) 12.4 5.9 4.2
As(Ill) (nmol g L) 197 86.8 78.8
As(V) (nmol g-~ )
10.6 3.9 7.9
Tot. Sb (nmol g i )
1.5 0.4 0.5
Sb(III) (nmol g i )
9.1 3.1 3.0
Sb(V) (nmol g - t )
METALLOID SPECIATION IN SEAWATER
77
The chemical speciation of dissolved selenium in anoxic waters differs markedly from that of arsenic and antimony. With the exception of low selenite concentrations found in the anoxic brines of the Orca Basin (Takayanagi and Wong, 1985), dissolved selenite and selenate have not been detected in anoxic waters. Like arsenic and antimony, oxidized selenium species may be delivered to anoxic waters by sinking detritus or by advection of surface waters. With respect to the former, Cutter and Bruland (1984) found low concentrations of selenate and selenite in fluxing particles from the North Pacific. However, the rate of reduction (most likely to insoluble Se(0); Cutter, 1982; Oremland et al., 1990) must be sufficiently rapid to limit the concentration of oxidized selenium in anoxic waters. Indeed, studies of selenate reduction in the sediment pore waters of several brackish evaporation ponds (Oremland et al., 1990) suggested reduction rate constants of up to 30 day -1 (Table 1). Additionally, Cutter (unpublished data) has estimated a reduction rate constant for selenite + selenate of 0.009 day-~ (Table 1) in the seasonally anoxic bottom waters of several lakes. Thus, it would appear that although mechanisms for the transport of oxidized selenium to anoxic waters exist, high rates of reduction limit the concentrations of selenite and selenate. The data for the anoxic water column of the Black Sea (Fig. 3) indicate that no oxidized forms of dissolved selenium (Se(IV + VI)) are found in these waters, and that the only detectable form is organic selenide. These data are generally consistent with those from the anoxic Saanich Inlet (Cutter, 1982) and the anoxic brines of the Orca Basin (Takayanagi and Wong, 1985). The data in Table 2 support the assumption that oxidized selenium, like arsenic and antimony, is delivered from oxic surface waters to the anoxic zone via detrital fluxes. However, the rate constants given in Table 1 suggest that any particulate selenate or selenite released into the anoxic water column would be rapidly reduced (i.e. a maximum residence time with respect to reduction, l/k, of 111 days; Table 1). Unfortunately, the apparent removal of dissolved selenate at Station BS3-6 (30-75 m; Fig. 3) occurs at a depth where horizontal effects and limited data points preclude the use of advection-diffusion modeling to estimate a reduction rate constant. CONCLUSIONS
The existence of thermodynamically unstable metalloid species can be explained by a variety of production mechanisms and slow rates of conversion to stable forms. In oxygenated seawater biological processes produce reduced oxidation states. For selenium, the biotic uptake of selenium produces particulate organic selenide. During the regeneration of organic matter this particulate selenide is oxidized through a series of intermediates to selenate. The net result is the ubiquitous presence of selenite and organic selenide in oxic waters. In contrast, arsenite is produced by the direct bioreduction of
78
G.A. CUTTER
arsenate, presumably as a detoxification mechanism. In anoxic waters arsenate and antimonate have slow rates of reduction, and their input from organic detritus or water transport results in relatively high steady-state concentrations. While oxidized forms of selenium such as selenate are similarly delivered to anoxic waters, the rates of reduction are sufficiently fast to prevent any accumulation. Therefore, if the biogeochemical cycles of elements such as the metalloids are to be quantitatively described, a kinetic rather than equilibrium thermodynamic approach is required. However, rate constants determined under realistic conditions (e.g. pH, temperature, salinity) are needed to ensure the accuracy of the kinetic approach; for antimony, arsenic, and selenium, further studies should be undertaken to complement existing data. ACKNOWLEDGMENTS
I sincerely thank Ed Goldberg for getting me started in chemical oceanography, and for providing insight and suggestions that will last throughout my career. I also thank L. Cutter and M. San Diego-McGlone for the laboratory analyses and thorough manuscript reviews, and D. Burdige for his review of the manuscript. This work was supported by the Chemical Oceanography program of the National Science Foundation, Grant OCE-8608823. REFERENCES Andreae, M.O., 1977. Determination of arsenic species in natural waters. Anal, Chem., 49: 820-823. Andreae, M.O., 1978. Distribution and speciation of arsenic in natural waters and some marine algae. Deep-Sea Res., 25: 391-402. Andreae, M.O., 1979. Arsenic speciation in seawater and interstitial waters: the influence of biological-chemical interactions on the chemistry of a trace element. Limnol. Oceanogr., 24: 440-452. Andreae, M,O., 1980. Arsenic in rain and the atmospheric mass balance of arsenic. J. Geophys. Res., 85: 4512-4518. Andreae, M.O. and Andreae, T.W., 1989. Dissolved arsenic species in the Schelde estuary and watershed, Belgium. Estuarine Coastal Shelf Sci., 29: 421-433. Andreae, M.O. and Froelich, P.N., 1984. Arsenic, antimony, and germanium biogeochemistry in the Baltic Sea. Tellus, 36:101-117. Andreae, M.O. and Klumpp, D.W., 1979. Biosynthesis and release of organoarsenic compoun~dsby marine algae. Environ. Sci, Technol., 13: 738-741. Bertine, K.K. and Lee, D.S., 1983. Antimony content and speciation in the water column and interstitial waters of Saanich Inlet. In: C.S. Wong et al. (Editors), Trace Elements in Seawater. Plenum, New York, pp. 21-38. Craig, H., 1969. Abyssal carbon and radiocarbon in the Pacific. J. Geophys. Res., 74: 5491-5506. Craig, H. and Weiss, R.F., 1970. The Geosecs 1969 intercalibration station: introduction, hydrographic features, and total CO2-O2 relationships. J. Geophys. Res., 75: 7641-7647. Cutter, G.A., 1982. Selenium in reducing waters. Science, 217: 829-831. Cutter, G.A., 1985. Determination of selenium speciation in biogenic particles and sediments. Anal. Chem., 57: 2951-2955.
METALLOID SPECIATION IN SEAWATER
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Cutter, G.A., 1991. Dissolved arsenic and antimony in the Black Sea. Deep-Sea Res., 38 (suppl.):5825-5843. Cutter, G.A. and Bruland, K.W., 1984. The marine biogeochemistry of selenium: a re-evaluation. Limnol. Oceanogr., 29:1179-1192. Cutter, L.S., Cutter, G.A. and San Diego-McGlone, M,L.C., 1991. Simultaneous determination of inorganic arsenic and antimony species in natural waters using selective hydride generation with gas chromatography-photoionization detection. Anal. Chem., 63:1138-1142. Hambrick, G.A., Froelich, P.N., Andreae, M.O. and Lewis, B.L., 1984. Determination of methylgermanium species in natural waters by graphite furnace atomic absorption spectrometry with hydride generation. Anal. Chem., 56: 421-424. Huerta-Diaz, M.A. and Morse, J.W., 1990. A quantitative method for determination of trace metal concentration in sedimentary pyrite. Mar. Chem., 29:119-144. Johnson, D.L. and Pilson, M.E.Q., 1975. The oxidation of arsenite in seawater. Environ. Lett., 8: 157-171. Kantin, R., 1983. Chemical speciation of antimony in marine algae. Limnol. Oceanogr., 28:165-168. Knox, S., Langston, W.J., Whitfield, M., Turner, D.R. and Liddicoat, M.I,, 1984. Statistical analysis of estuarine profiles: II Application to arsenic in the Tamar Estuary (S.W. England). Estuarine Coastal Shelf Sci., 18: 623-638. Lewis, B.L. and Landing, W.M., 1991. The biogeochemistry of manganese and iron in the Black Sea. Deep-Sea Res., 38 (suppl.): 5773-5803. Measures, C.I. and Burton, J.D., 1980. The vertical distribution and oxidation states of dissolved selenium in the northeast Atlantic Ocean and their relationship to biological processes. Earth Planet. Sci. Lett., 46: 385-396. Measures, C.I., McDuff, R.E. and Edmond, J.M., 1980. Selenium redox chemistry at the GEOSECSI re-occupation. Earth Planet. Sci. Lett., 49: 102-108. Middelburg, J.J., Hoede, D,, van der Sloot, H.A., van der Weijden, C.H. and Wijkstra, J., 1988. Arsenic, antimony and vanadium in the North Atlantic Ocean. Geochim. Cosmochim. Acta, 52: 2871-2878. Murray, J.W., Jannasch, H.W., Honjo, S., Anderson, R.F., Reeburgh, W.S., Top, Z.., Friederich, G., Codispoti, L.A. and Izdar, E., 1989. Unexpected changes in the oxic/anoxic interface in the Black Sea. Nature, 338: 411-413. Murray, J.W., Top, Z. and Ozsoy E., 1991. Temperature and salinity distributions in the Black Sea. Deep-Sea Res., 38 (suppl.): 5663-5689. Oremland, R.S., Steinberg, N.A., Maest, A.S., Miller, L.G. and Hollibaugh, J.T., 1990. Measurement of in situ rates of selenate removal by dissimilatory bacterial reduction in sediments. Environ. Sci. Technol., 24:1157-1164. Peterson, M.L. and Carpenter, R., 1983. Biogeochemical processes affecting total arsenic and arsenic species distributions in an intermittently anoxic fjord. Mar. Chem., 12: 295-321. Sanders, J.G. and Windom, H.L., 1980. The uptake and reduction of arsenic species by marine algae. Estuarine Coastal Mar. Sci., 10: 555-567. Scudlark, J.R. and Church, T.M., 1988. The atmospheric deposition of arsenic and association with acid precipitation. Atmos. Environ., 22: 937-943. Scudlark, J.R. and Johnson, D.L., 1982. Biological oxidation of arsenite in seawater. Estuarine Coastal Shelf Sci., 14: 693-706. Sillen, L.G., 1961. The physical chemistry of seawater. In: M. Sears (Editor), Oceanography. American Association for the Advancement of Science (AAAS), Washington, DC, pp. 549-581. Sugawara, K., Terada, K., Kanamori, S., Kanamori, N. and Okabe, S., 1962. Arsenate and arsenite in environmental waters of Japan. J. Earth Sci., 10: 34-50. Sugimura, Y., Suzuki, Y. and Miyake, Y., 1976. The content of selenium and its chemical form in sea water. J. Oceanogr. Soc. Jpn, 32: 235-241.
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Suzuki, Y., Miyake, Y., Saruhashi, K. and Sugimura, Y., 1979. A cycle of selenium in the ocean. Pap. Meteorol. Geophys., 30: 185-189. Takayanagi, K. and Wong, G.T.F., 1985. Dissolved inorganic and organic selenium in the Orca Basin. Geochim. Cosmochim. Acta, 49: 539-546. Velinsky, D.J. and Cutter, G.A., 1990. Determination of elemental selenium and pyrite-selenium in sediments. Anal. Chim. Acta, 235: 419-425. Waslenchuk, D.G. and Windom, H.L., 1978. Factors controlling the estuarine chemistry of arsenic. Estuarine Coastal Mar. Sci., 7: 455-464.
65
Elsevier Science Publishers B.V., Amsterdam
Kinetic controls on metalloid speciation in seawater Gregory A. Cutter Department of Oceanography, Old Dominion University, Norfolk, VA 23529-0276, USA (Accepted 27 March 1992)
ABSTRACT Cutter, G.A., 1992. Kinetic controls on metalloid speciation in seawater. Mar. Chem., 40: 65-80. Mechanisms for the production of thermodynamically unstable species of three metalloids, antimony, arsenic, and selenium, and the rates of their transformation to stable forms have been critically reviewed. The occurrence of thermodynamically unstable species of these metalloids falls into two categories: reduced species in oxic waters and oxidized species in anoxic water. In surface waters biotic processes produce reduced species (e.g. Sb(III), arsenite, selenite), while in deep anoxic waters lateral advection and sinking detritus deliver oxidized species (e.g. arsenate, antimonate) from surface waters. In both cases, slow rates of conversion allow unstable species to persist. Dissolved and particulate data for antimony, arsenic, and selenium speciation in the Black Sea are used to illustrate the processes involved in this kinetic stabilization.
INTRODUCTION
Amongst the trace elements dissolved in seawater, the metalloids (germanium, arsenic, selenium, antimony, tellurium) are distinguished by the number of chemical forms in which they can exist (i.e. their speciation). The chemical speciation of metalloids includes not only different oxidation states (e.g. As(Ill) and As(V)), but also different chemical forms within an oxidation state (e.g. dimethylarsinic and methylarsonic acids). Initial approaches to examining the speciation of metalloids in the ocean utilized equilibrium thermodynamic calculations such as those first proposed by Sillen (1961). In general, the use of this approach for the metalloids predicted that the fully oxidized states (e.g. As(V)) should be the only species in oxic water, while reduced species (e.g. As(Ill)) should only be found under anoxic conditions. Unlike many other trace elements, sensitive analytical techniques have been developed to determine the chemical speciation of metalloids (e.g. Andreae, Correspondence to: G.A, Cutter, Department of Oceanography, Old Dominion University, Norfolk, VA 23529-0276, USA.
0304-4203/92[$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
66
G.A. CUTTER
1977; Hambrick et al., 1984). The use of these techniques has shown that the biogeochemical cycles of metalloids in the ocean are more complicated than equilibrium thermodynamic calculations predicted. In particular, reduced species such as Se(IV) are found in oxic waters (e.g. Measures et al., 1980) and oxidized species such as Sb(VI) are found in anoxic waters (e.g. Bertine and Lee, 1983). For thermodynamically unstable species to be found in seawater, mechanisms for their production and slow rates of interconversion (i.e. kinetic stabilization) must exist. While the biogeochemical cycle of each metalloid is unique, one common process is the kinetic stabilization of thermodynamically unstable species. Indeed, studies of individual metalloids in the Pacific (e.g. Andreae, 1979), Atlantic (e.g. Measures and Burton, 1980), Baltic Sea (Andreae and Froelich, 1984), and the Saanich Inlet (e.g. Peterson and Carpenter, 1983) have all invoked such kinetic arguments to explain metalloid speciation. This review attempts to evaluate critically existing kinetic data for three of the metalloids, antimony, arsenic, and selenium. An emphasis is placed on the specific mechanisms that produce thermodynamically unstable species and the rates of their conversion to stable forms. To facilitate this review of the processes and rates affecting the chemical speciation of antimony, arsenic and selenium, previously published data from the central basins of the Black Sea (Cutter, 1991), supplemented by some newly obtained dissolved and particulate speciation results, are utilized. Surface waters in this environment are meso- to oligotrophic, and nutrient cycling in the oxic water column is similar to that in many ocean regimes (e.g. see data in Murray et al., 1989). Below approximately 100m, anoxic conditions are well developed (Murray et al., 1989) and in a relative steady state compared with other anoxic basins and t]ords in coastal regions (e.g. Saanich Inlet, Cariaco Trench). By possessing attributes of two regimes, mesotrophic surface waters and a well-defined anoxic zone, the central Black Sea affords an ideal environment in which to examine kinetic processes affecting metalloid speciation. S T U D Y SITE A N D M E T H O D S
Study site Water samples and sediment box cores were collected at two stations in the Black Sea (Station BS3-2, 42°50'N, 32°00'E, depth of 2086 m; Station BS3-6, 43°04'N, 34°00'E, depth of 2185m) during Cruise 3 of the 1988 Black Sea Expedition (3-16 June 1988). Relevant hydrographic parameters for these stations are thoroughly described in Murray et al. (1989, 1991), as well as by Cutter (1991).
METALLOID SPECIATION IN SEAWATER
67
Methods
Sampling and analytical methods are given in the original references, and only procedures used to obtain the new data for antimony, arsenic, and selenium will be reviewed here. Sediment samples were acquired with a box corer. After arrival on deck, subcores were obtained by carefully inserting acrylic tubes (5.6 cm i.d.) into the sediment and then sealing the ends of the tubes with polyethylene core caps. These subcores were sectioned at 1 cm intervals under nitrogen within 2 h of collection. Sections were placed in polyethylene bags, homogenized, and immediately frozen. The determination of dissolved selenium speciation used the selective hydride generation/atomic absorption methods outlined by Cutter and Bruland (1984). Three selenium fractions are directly determined on 0.4#m filtered samples that are preserved by acidification to pH 2: selenite (Se(IV)), selenite + selenate (Se(IV + VI)), and total dissolved selenium. Selenate is then calculated as the difference between Se(IV + VI) and Se(IV), while S e ( - I I + 0) is the difference between total selenium and Se(IV + VI). Since Se(0) is insoluble and would only be found in the S e ( - I I + 0) fraction if it passed through the 0.4 #m filter (i.e. as a colloid), the S e ( - I I + 0) fraction is considered to be largely dissolved organic selenide (Cutter, 1982; Cutter and Bruland, 1984). The detection limit for dissolved selenium species is 0.01 nmol 1-I, and precision (as relative standard deviation) is 5% at the 0.5 nmoll -~ level. For sediment samples, solid-phase selenium speciation was determined using the method of Cutter (1985) for total selenium and Se(IV + VI), and using the procedure of Velinsky and Cutter (1990) for the determination of elemental selenium. Total sedimentary arsenic and antimony were determined using the method of Cutter et al. (1991), while the 2 M HC1 leach technique given by Andreae and Froelich (1984) was used to quantify sedimentary arsenic and antimony speciation. DISCUSSION
Antimony, arsenic, and selenium have perhaps the most diverse chemistries of the metalloids. Dissolved antimony and arsenic can be found in the trivalent (Sb(III) and arsenite) and pentavalent (antimonate and arsenate) oxidation states, and as methylated forms of the pentavalent state (e.g. dimethylarsinic acid, methylstibonic acid). Selenium has three dissolved oxidation states ( - I I , selenide; IV, selenite; and VI, selenate), and may be found in organic compounds such as dissolved free and peptide-bound amino acids (e.g. selenomethionine). The occurrence of thermodynamically unstable species of these metalloids falls into two general categories: reduced species in oxic waters and oxidized
68
G.A. CUTTER
species in anoxic waters. The mechanisms and rates that are associated with both of these occurrences are reviewed in the sections to follow. In addition, data from the Black Sea are used to illustrate the net results of these kinetic processes. Reduced species in the oxic water column
Of all the metalloids, perhaps the greatest attention has been focused on arsenic. As early as 1962 arsenite was reported in oxygenated seawater (Sugawara et al., 1962), in disagreement with the equilibrium thermodynamic predictions of Sillen (1961). Considerable effort was then devoted to elucidating the production mechanisms for arsenite in oxic seawater and its rate of oxidation to thermodynamically stable arsenate. Potential sources of arsenite include the in situ reduction of arsenate (biotic or abiotic), and delivery to surface waters by atmospheric deposition and riverine inputs. Johnson and Pilson (1975) estimated that the riverine and atmospheric contributions of arsenite are probably minor. Indeed, recent direct measurements of atmospheric (e.g. Andreae, 1980; Scudlark and Church, 1988) and riverine/ estuarine inputs (e.g. Andreae, 1978; Waslenchuk and Windom, 1978) show that arsenite is generally less than 10% of the total arsenic in these sources. The in situ production of arsenite in oxic waters has been thoroughly examined by a number of investigators (e.g. Andreae, 1979; Andreae and Klumpp, 1979; Sanders and Windom, 1980). These studies concluded that arsenite is largely derived from a planktonic detoxification mechanism, where arsenate is reduced to arsenite under low phosphate concentrations. In addition to arsenite, methylarsonic and dimethylarsinic acids are produced during the uptake of arsenate by phytoplankton, with the ratio of methylated acids to arsenite varying with the species (Andreae and Klumpp, 1979). Overall, the production mechanism for arsenite in oxic seawater is controlled by biotic processes. Kinetic studies of arsenite oxidation suggest that both biotic (e.g. Sanders and Windom, 1980; Scudlark and Johnson, 1982) and abiotic (e.g. Peterson and Carpenter, 1983) factors can be involved. Most of these studies have found that the oxidation rates are pseudo first order in arsenite, and this behavior will be assumed for the rest of this paper; a summary of rate constants for arsenite oxidation is contained in Table 1. The early work of Johnson and Pilson (1975) used Sargasso Sea water to obtain a rate constant (at 41°C) of 0.009-0.04 day ~ (Table 1). If this value is representative of oceanic conditions (in spite of the elevated temperature), then rate constants (and rates) in coastal waters are clearly much higher (Table 1). Scudlark and Johnson (1982) suggested an average rate constant of 2.2 day -~ for 'productive' coastal waters, and demonstrated that arsenite oxidation can be microbially catalyzed. Other determinations of the rate constants for arsenite oxida-
69
METALLOID SPECIATION IN SEAWATER TABLE 1 Pseudo first order rate constants Reaction ARSENIC As(llI) ~ As(V)
k (day i)
Notes
Reference
0.009-0.04 0.1 2.2
Sargasso; 41°C Culture; 16°C 'Productive' coastal waters Saanich; oxic; 30°C Saanich; oxic; 6°C Saanich; Mn oxide max. Estuary Estuary Black Sea; 65-85 m Saanich; 6°C Avg. anoxic Black Sea
Johnson and Pilson (1975) Sanders and Windom (1980) Scudlark and Johnson (1982)
0.04-0.3 0.005-0.03 0.07-0.3
As(V) ~ As(III)
1.9 0.5-2.2 0.01 0.06-0.08 1.6 × 10 -5
Peterson and Carpenter (1983) Peterson and Carpenter (1983) Peterson and Carpenter (1983) Knox et al. (1984) Andreae and Andreae (1989) This work Peterson and Carpenter (1983) Cutter (1991)
ANTIMONY Sb(III) ~ Sb(V) Sb(V) --, Sb(III)
0.008 1.1 x 10 -6
Black Sea; 65-85 m Avg. anoxic Black Sea
This work Cutter (1991)
SELENIUM S e ( - II) --} Se(IV)
1.1 x 10 -4
Suzuki et al. (1979)
Se(IV) ~ Se(VI)
1.1 × 10 -5
Surf. and deep Pacific Surf. and deep Pacific Surf. Pacific Deep Pacific Deep Pacific Sediments, brackish ponds Water column, anoxic lakes
Se(IV, VI) ~ Se(O)
2.1 x 10 -3 0.4-2.9 × 10 -6 2.4 x 10 -6 1-30 0.009
Suzuki et al. (1979) Measures et al. (1980) Measures et al. (1980) Cutter and Bruland (1984) Oremland et al. (1990) Cutter (unpublished data)
tion in estuaries (Table 1) are very similar to the Scudlark and Johnson (1982) value. In another coastal environment, the Saanich Inlet, Peterson and Carpenter (1983) demonstrated both strong temperature dependence (Table 1) and a biotic role in the oxidation of arsenite. Moreover, they found that manganese oxides increase the oxidation rate, a factor that would be important in waters overlying an oxic/anoxic surface. The arsenic speciation data for the Black Sea can be used to illustrate the net effect of these mechanisms and rates. Figure 1 presents the depth profiles for dissolved arsenic species in the upper 100 m of Station BS3-2, taken from the results of Cutter (1991). Arsenate displays the surface-water depletion that
70
G.A. CUTTER
SfafIon B S 3 - 2 As(V) ( n m o l / L ) 0
As(Ill) ( n m o l / L )
1o
20
30
40
]
I
i
,
0 1 2 3 4 5 .,.,.,.,.rJ-,
DIVAs ( n m o l / L )
20
40 .
,
0
1 .
,
2 .
,
3 .
,
4 ,
,
0
20
40 E
t
t-
Q. 0
"~
60
80
100
Fig. 1. Depth profiles of dissolved arsenate (As(V)), arsenite (As(III)), and dimethylarsinic acid (DMA) in the upper 100m at Station BS3-2 in the Black Sea (data from Cutter, 1991). The water column becomes anoxic at 90 m. Note concentration scale break for arsenite.
is typical of many ocean regimes (e.g. Andreae, 1979). Correspondingly, non-equilibrium concentrations of arsenite are also present in the surface waters of BS3-2 (Fig. 1). This arsenite is most likely derived from bioreduction, and the presence of dimethylarsinic acid in these same waters (Fig. 1) is strong evidence for arsenate uptake by biota (Andreae and Klumpp, 1979; Sanders and Windom, 1990). Thus, the biotic production of thermodynamically unstable arsenic species found in most oceanic regimes also appears to operate in surface waters of the Black Sea. Unlike other ocean regimes, arsenite in the oxic waters of the Black Sea might also be transported from anoxic waters (i.e. below 90m at BS3-2). However, this source appears to be limited since arsenite is almost completely removed by 65 m (Fig. 1). Such removal of arsenite from anoxic waters was also found in the Saanich Inlet by Peterson and Carpenter (1983), who ascribed it primarily to oxidation in the presence of manganese oxides (measured rate constant of 0.07-0.3 day-l). Significantly, the depth range
METALLOID SPEC1ATION IN SEAWATER
71
over which arsenite is removed at Station BS3-2 also coincides with the particulate manganese maximum (Lewis and Landing, 1991). In order to quantitatively compare these Black Sea results with those of Peterson and Carpenter (1983), as well as those in Table l, the rate constant for arsenite oxidation is needed. Using a vertical advection-diffusion model for a non-conservative species (e.g. Craig, 1969; Craig and Weiss, 1970) over the depth range where the steep arsenite gradient is found (65-85m), the removal rate for arsenite can be estimated. Most recently, this type of model has been used to examine Black Sea results by Lewis and Landing (1991) for trace metals, and by Cutter (199 l) to determine arsenate reduction rates in the anoxic zone. To accurately use this model, horizontal inputs must be minimized, and an examination of the temperature-salinity diagram for Station BS3-2 shows that this is possible over the 40-250 m depth interval (Lewis and Landing, 1991). Unfortunately, horizontal effects preclude using an advection-diffusion model for the upper 40 m where arsenite is also found. For this application of the model, the vertical eddy diffusion coefficient (K) is first obtained by fitting the salinity profile to the solution of the advectiondiffusion equation for a conservative species (i.e. the removal term, J, is set to zero), assuming a vertical upwelling rate (w) of 3.6 m year-1 (Murray et al., 1991). In this manner an eddy diffusion coefficient of 115 m 2 year-~, identical to that determined by Lewis and Landing (1991) for the same depth interval, is obtained. Fitting the 65-85 m arsenite data to the non-conservative solution (constant J with depth) to the model gives a removal rate (J) of 75 pmol 1day -~ . Using the average arsenite concentration from 65 to 85 m (7.6 nmol 1-1), a first-order rate constant of 0.01 day -~ is calculated. The removal of arsenite in the 65-85 m depth range includes not only oxidation, but also scavenging by particles (e.g. metal oxides). However, the close agreement between the rate constant calculated here (0.01 day -~) and those in the Saanich Inlet (0.07-0.3day-l; Peterson and Carpenter, 1983), as well as others in Table l, suggests that arsenite from the anoxic zone is largely removed by oxidation to arsenate. Nevertheless, the production of arsenite via bioreduction of arsenate and its slow removal by oxidation (residence time with respect to oxidation 1/k, ranges from 0.5 to 100 days; Table 1) allow arsenite to persist in oxygenated surface waters. While numerous investigators have examined the kinetic processes affecting arsenic, relatively little is known about the other Group V metalloid, antimony. In surface waters of Saanich Inlet (Bertine and Lee, 1983), the Baltic Sea (Andreae and Froelich, 1984), and the North Atlantic (Middelburg et al., 1988) thermodynamically unstable Sb(III) has been detected. Moreover, Andreae and Froelich (1984) found methylstibonic acid in the Baltic Sea, and these workers proposed that Sb(III) and methylstibonic acid, like their arsenic analogs, may be produced by phytoplankton. Indeed, Sb(III) has been detected in phytoplankton and macroalgae (Kantin, 1983; Andreae and
72
G.A. CUTTER Station B S 3 - 2
SbCY) (nmol/L) 0 . . . . . . . . .
Sb(lll)(nmol/L)
1
2
i . . . . . . . . .
i
MSb (nmol/L)
0.0 0.1 0.2 0.3 0.4 0.5 •
i
•
i
,
't
•
i
•
I
0.00 i
0.05 .
.
.
.
I
,
0.10 '
•
'
1
20
.-'--,
40
E
~=~
(.-
u)
a
6o
80
100
Fig. 2. Depth profiles of dissolved antimonate (Sb(V)), Sb(IlI), and methylstibonic acid (MSb) in the upper 100 m at Station BS3-2 in the Black Sea (data from Cutter, 1991). The water column becomes anoxic at 90m.
Froelich, 1984), suggesting that biotic production is possible. While these studies suggest a mechanism for the production of Sb(III) in oxic waters, its persistence under oxic conditions requires kinetic stabilization. However, no rate measurements for Sb(III) oxidation in seawater are available; only the rate of antimonate reduction in anoxic waters has been estimated (Cutter, 1991). The depth profiles for antimony species in the upper 100 m of the Black Sea (Station BS3-2; Cutter, 1991) shown in Fig. 2 allow a further examination of processes and rates affecting antimony. Like the Baltic Sea (Andreae and Froelich, 1984) antimonate displays a surface maximum and decreases with depth. In contrast to the previous studies, no detectable Sb(III) is found in the upper 60 m (Fig. 2); the highest Sb(III) concentrations are found in the anoxic zone (below 90 m), with a steep concentration gradient between the oxic and anoxic waters. However, methylstibonic acid is present in surface waters (Fig. 1), suggesting the biotic uptake of antimonate proposed by Andreae and Froelich (1984). If the production mechanism for Sb(III) is equivalent to that
METALLOID SPEC1ATION IN SEAWATER
73
of arsenite, the absence of Sb(III) may be due to a more rapid oxidation rate. Since rates for Sb(III) oxidation are not reported in the literature, the results in Fig. 2 (and Cutter, 1991) can be used to estimate the Sb(III) removal rate (oxidation) via the same advection-diffusion model used for arsenite (i.e. fitting the Sb(III) gradient from 65 to 85 m). The loss rate for Sb(III) obtained in this manner is 0.65 pmol l-~ day -I , and using the average Sb(III) concentration (0.08nmol 1-~), a pseudo first order rate constant of 0.008 day -~ is calculated. This rate constant includes all forms of removal since Sb(III) may also be scavenged by adsorption to particles. However, this constant can serve as a guide for future studies, and suggests that rapid removal alone cannot explain the lack of Sb(III) in Black Sea surface waters. Indeed, Sb(III) is as kinetically stable as arsenite (residence time of Sb(III) with respect to removal, 1/k, is 125 days). If Sb(III) is produced during the biotic uptake of antimonate, then the production rates in the Black Sea may be relatively slow or species dependent such as those observed for arsenic. Like arsenic, dissolved selenium also has thermodynamically unstable species in oxic waters, including selenite and organic selenides. One of the first reliable reports of selenite in seawater was that of Sugimura and coworkers (1976), who found concentrations ranging from 0.6 to 1.1 nmol l-1 in vertical profiles from the western North Pacific. These investigators suggested that 'biochemical reactions' might control the speciation of dissolved selenium. Later work by Measures and Burton 0980) and Measures et al. (1980) presented oceanographically consistent depth profiles for selenite and selenate in the Atlantic and Pacific Oceans. In their Atlantic study Measures and Burton 0980) suggested that selenite may be produced via the regeneration of particulate organic selenide, while Measures et al. (1980) used box and advection-diffusion models to explain how selenite could be produced by selenate reduction and transported by sinking detritus. Dissolved organic selenium (presumably selenide) has been reported in the upper water column of the Pacific (Suzuki et al., 1979; Cutter and Bruland, 1984), in the Saanich Inlet (Cutter, 1982), and in the Gulf of Mexico (Takayanagi and Wong, 1985). Using freshly collected biogenic particles, Cutter (1982) showed that selenium undergoes a multistep regeneration, with the first step being the release of dissolved organic selenide from particulate matter, which then oxidizes to selenite, and then to selenate. This multistep regeneration provides a simple mechanism for the production of thermodynamically unstable species in the oxic water column. In addition to process studies, numerous investigators have estimated the transformation rates for selenium species; Table 1 contains a compilation of these rate constants. Using a two-layer box model Suzuki and coworkers (1979) estimated a rate constant for the oxidation of dissolved organic selenide to selenite of 1.1 x l0 -4 day -~, and a rate constant of 1.1 × 10 -5 day -1 for the oxidation of selenite to selenate (Table 1). Although organic selenide was
74
G.A.
CUTTER
Station B S 3 - 6
S*(IV) (nmol/L)
S*(Vl) (nrnol/L) 0.0 I
0.5 .
.
.
.
?
o
i
. . . .
1.0
0.0
i
I
0.5 . . . .
I
S,(-ll+O) (nmol/L) 1.0
. . . .
i
0.0
0.5 . . . .
I
1.0 . . . .
i
oxic
lOO arloxic
200
v
E
300
..c
/
500
1000
1500
2000
Fig. 3. Depth profiles of dissolved selenate (Se(VI)), selenite (Se(IV)), and S e ( - I I + 0) at Station BS3-6 in the Black Sea. Waters above the oxic line have detectable oxygen concentrations and waters below the anoxic line have detectable hydrogen sulfide concentrations. Note that the upper 300 m are plotted on an expanded scale.
not considered, Measures et al. (1980) used a two-layer box model and advection-diffusion calculations to estimate a rate constant for selenite oxidation of 2.1 x 10 -3 day -~ in surface waters and 0.4-2.9 x 10 - 6 day -j in deep waters (Table 1). Cutter and Bruland (1984) used a kinetic rate model in which selenite is produced by organic matter oxidation (i.e. respiration) and removed by oxidation to selenate to determine a rate constant of 2.4 x 10 - 6 day-~ for the oxidation of selenite in deep waters (Table 1). Perhaps the most significant feature in Table 1 is the close agreement between the rate constants for selenite oxidation, in spite of the fact that completely independent data sets and modeling approaches were used. The results in Table 1 clearly show that selenite, and to a lesser extent organic selenide, are kinetically stable in oxic seawater. The newly obtained results for dissolved selenium speciation at Station BS3-6 in the Black Sea (Fig. 3) are similar to previous observations in the open ocean (e.g. Cutter and Bruland, 1984) and the Saanich Inlet (Cutter, 1982).
M E T A L L O I D SPECIATION IN SEAWATER
75
The primary reduced species in the oxic water column is organic selenide ( S e ( - I I + 0)), while selenite concentrations are very low (average of 0.03 nmol 1-~), but detectable in the upper 50 m. Like the Pacific results of Cutter and Bruland (1984), the surface maximum of organic selenide probably results from organic matter regeneration, and the 25 year residence time based on the oxidation rate constant of Suzuki et al. (1979) explains how organic selenide can persist in oxic surface waters. Oxidized species in the anoxic water column
The occurrence of oxidized arsenic and antimony species in anoxic waters has been reported in the Saanich Inlet (Bertine and Lee, 1983; Peterson and Carpenter, 1983), the Baltic Sea (Andreae and Froelich, 1984), and the Black Sea (Cutter, 1991). A variety of mechanisms to explain these results have been postulated, including delivery of As(V) and Sb(V) on sinking detritus from oxic waters (Andreae and Froelich, 1984; Cutter, 1991), formation of thiocomplexes with the pentavalent elements (Bertine and Lee, 1983; Andreae and Froelich, 1984), and advection of surface waters with high concentrations of arsenate and antimonate (Cutter, 1991). All of these mechanisms must then be coupled with relatively slow rates of reduction. Rate constants for arsenate reduction under anoxic conditions have only been reported for the Saanich Inlet (Peterson and Carpenter, 1983) and the Black Sea (Cutter, 1991), while the reduction rate constant for antimonate has only been estimated using Black Sea data (Cutter, 1991); these values are listed in Table !. In the Black Sea, Cutter (1991) found arsenate and antimonate to be widely distributed in the anoxic zone (average arsenate, 7.3 nmol 1-~; average antimonate, 0.5nmol 1-t) and speculated that one source of these dissolved species could be their transport on detritus from surface waters. The newly obtained data in Table 2 for arsenic and antimony speciation in Black Sea sediments provide strong evidence for a detrital source of antimonate and arsenate to anoxic waters, as first proposed by Andreae and Froelich (1984) for the Baltic. Surficial sediments underlying the anoxic water column contain As(V) and Sb(V), with the concentrations decreasing with depth (Table 2). Interestingly, the fractions of sedimentary arsenic and antimony that can be recovered as As(III + V) and Sb(III + V) also decrease with depth (e.g. for 1-2cm, As(III + V) and Sb(III + V) are equal to total As and Sb, but at 11-12cm only 50% of the total As and Sb are As(III + V) and Sb(III + V)). This suggests that the arsenic and antimony are being incorporated into a more refractory phase such as pyrite (e.g. Huerta-Diaz and Morse, 1990). Overall, investigations of several anoxic marine systems, and the data in Tables 1 and 2, demonstrate that the abundance of dissolved arsenate and antimonate in anoxic waters can be attributed to detrital fluxes, advection from surface waters (Black Sea), and slow reduction rates.
4.76 4.30 4.25
1-2 6 7 11-12
N.D., non-detectable.
Org. C (mmol g i )
Depth (cm)
51.3 42.4 31.6
Tot. Se (nmol g i ) 5.1 3.4 N.D.
Se(1V + VI) (nmol g l) 15.6 11.4 6.8
Se(0) (nmol g - 1)
Antimony, arsenic and selenium in Black Sea sediments (BS3-2)
TABLE 2
209 195 155
Tot. As (nmol g i ) 12.4 5.9 4.2
As(Ill) (nmol g L) 197 86.8 78.8
As(V) (nmol g-~ )
10.6 3.9 7.9
Tot. Sb (nmol g i )
1.5 0.4 0.5
Sb(III) (nmol g i )
9.1 3.1 3.0
Sb(V) (nmol g - t )
METALLOID SPECIATION IN SEAWATER
77
The chemical speciation of dissolved selenium in anoxic waters differs markedly from that of arsenic and antimony. With the exception of low selenite concentrations found in the anoxic brines of the Orca Basin (Takayanagi and Wong, 1985), dissolved selenite and selenate have not been detected in anoxic waters. Like arsenic and antimony, oxidized selenium species may be delivered to anoxic waters by sinking detritus or by advection of surface waters. With respect to the former, Cutter and Bruland (1984) found low concentrations of selenate and selenite in fluxing particles from the North Pacific. However, the rate of reduction (most likely to insoluble Se(0); Cutter, 1982; Oremland et al., 1990) must be sufficiently rapid to limit the concentration of oxidized selenium in anoxic waters. Indeed, studies of selenate reduction in the sediment pore waters of several brackish evaporation ponds (Oremland et al., 1990) suggested reduction rate constants of up to 30 day -1 (Table 1). Additionally, Cutter (unpublished data) has estimated a reduction rate constant for selenite + selenate of 0.009 day-~ (Table 1) in the seasonally anoxic bottom waters of several lakes. Thus, it would appear that although mechanisms for the transport of oxidized selenium to anoxic waters exist, high rates of reduction limit the concentrations of selenite and selenate. The data for the anoxic water column of the Black Sea (Fig. 3) indicate that no oxidized forms of dissolved selenium (Se(IV + VI)) are found in these waters, and that the only detectable form is organic selenide. These data are generally consistent with those from the anoxic Saanich Inlet (Cutter, 1982) and the anoxic brines of the Orca Basin (Takayanagi and Wong, 1985). The data in Table 2 support the assumption that oxidized selenium, like arsenic and antimony, is delivered from oxic surface waters to the anoxic zone via detrital fluxes. However, the rate constants given in Table 1 suggest that any particulate selenate or selenite released into the anoxic water column would be rapidly reduced (i.e. a maximum residence time with respect to reduction, l/k, of 111 days; Table 1). Unfortunately, the apparent removal of dissolved selenate at Station BS3-6 (30-75 m; Fig. 3) occurs at a depth where horizontal effects and limited data points preclude the use of advection-diffusion modeling to estimate a reduction rate constant. CONCLUSIONS
The existence of thermodynamically unstable metalloid species can be explained by a variety of production mechanisms and slow rates of conversion to stable forms. In oxygenated seawater biological processes produce reduced oxidation states. For selenium, the biotic uptake of selenium produces particulate organic selenide. During the regeneration of organic matter this particulate selenide is oxidized through a series of intermediates to selenate. The net result is the ubiquitous presence of selenite and organic selenide in oxic waters. In contrast, arsenite is produced by the direct bioreduction of
78
G.A. CUTTER
arsenate, presumably as a detoxification mechanism. In anoxic waters arsenate and antimonate have slow rates of reduction, and their input from organic detritus or water transport results in relatively high steady-state concentrations. While oxidized forms of selenium such as selenate are similarly delivered to anoxic waters, the rates of reduction are sufficiently fast to prevent any accumulation. Therefore, if the biogeochemical cycles of elements such as the metalloids are to be quantitatively described, a kinetic rather than equilibrium thermodynamic approach is required. However, rate constants determined under realistic conditions (e.g. pH, temperature, salinity) are needed to ensure the accuracy of the kinetic approach; for antimony, arsenic, and selenium, further studies should be undertaken to complement existing data. ACKNOWLEDGMENTS
I sincerely thank Ed Goldberg for getting me started in chemical oceanography, and for providing insight and suggestions that will last throughout my career. I also thank L. Cutter and M. San Diego-McGlone for the laboratory analyses and thorough manuscript reviews, and D. Burdige for his review of the manuscript. This work was supported by the Chemical Oceanography program of the National Science Foundation, Grant OCE-8608823. REFERENCES Andreae, M.O., 1977. Determination of arsenic species in natural waters. Anal, Chem., 49: 820-823. Andreae, M.O., 1978. Distribution and speciation of arsenic in natural waters and some marine algae. Deep-Sea Res., 25: 391-402. Andreae, M.O., 1979. Arsenic speciation in seawater and interstitial waters: the influence of biological-chemical interactions on the chemistry of a trace element. Limnol. Oceanogr., 24: 440-452. Andreae, M,O., 1980. Arsenic in rain and the atmospheric mass balance of arsenic. J. Geophys. Res., 85: 4512-4518. Andreae, M.O. and Andreae, T.W., 1989. Dissolved arsenic species in the Schelde estuary and watershed, Belgium. Estuarine Coastal Shelf Sci., 29: 421-433. Andreae, M.O. and Froelich, P.N., 1984. Arsenic, antimony, and germanium biogeochemistry in the Baltic Sea. Tellus, 36:101-117. Andreae, M.O. and Klumpp, D.W., 1979. Biosynthesis and release of organoarsenic compoun~dsby marine algae. Environ. Sci, Technol., 13: 738-741. Bertine, K.K. and Lee, D.S., 1983. Antimony content and speciation in the water column and interstitial waters of Saanich Inlet. In: C.S. Wong et al. (Editors), Trace Elements in Seawater. Plenum, New York, pp. 21-38. Craig, H., 1969. Abyssal carbon and radiocarbon in the Pacific. J. Geophys. Res., 74: 5491-5506. Craig, H. and Weiss, R.F., 1970. The Geosecs 1969 intercalibration station: introduction, hydrographic features, and total CO2-O2 relationships. J. Geophys. Res., 75: 7641-7647. Cutter, G.A., 1982. Selenium in reducing waters. Science, 217: 829-831. Cutter, G.A., 1985. Determination of selenium speciation in biogenic particles and sediments. Anal. Chem., 57: 2951-2955.
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Suzuki, Y., Miyake, Y., Saruhashi, K. and Sugimura, Y., 1979. A cycle of selenium in the ocean. Pap. Meteorol. Geophys., 30: 185-189. Takayanagi, K. and Wong, G.T.F., 1985. Dissolved inorganic and organic selenium in the Orca Basin. Geochim. Cosmochim. Acta, 49: 539-546. Velinsky, D.J. and Cutter, G.A., 1990. Determination of elemental selenium and pyrite-selenium in sediments. Anal. Chim. Acta, 235: 419-425. Waslenchuk, D.G. and Windom, H.L., 1978. Factors controlling the estuarine chemistry of arsenic. Estuarine Coastal Mar. Sci., 7: 455-464.