Chemical Dynamics of
SEDIMENTARY USFWS
Acid Volatile Sulfide J O H N W. M O R S E TEXAS A&M UNIVERSITY D AV I D R I C K A R D CARDIFF UNIVERSITY ( WALES)
T
he simultaneously extracted metal–acid volatile sulfide (SEM–AVS) method is widely used to estimate whether or
These measurements not metals in sediments are toxic. have implications for Although considerable attention has metal toxicity but lack
been given to metal–sulfide interactions and related toxicological re-
consistent definition. search, the complexities of the sedimentary sulfide system and how they may influence the SEM–AVS JOHN MORSE
relationship have received scant consideration. © 2004 American Chemical Society
APRIL 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 131A
In reviewing the many published AVS analyses of sediments, we were struck by the extreme variability in the results. The method rests on the basic concept that many toxic metals, such as cadmium, lead, zinc, mercury, and copper, readily precipitate as highly insoluble metal sulfide minerals when exposed to dissolved sulfide, S(–II). But what do the AVS and SEM values obtained from sediment analyses actually represent? Both are operational definitions. AVS and SEM values are obtained by adding acid to the sediment and measuring the amount of S(–II) evolved and the metal concentrations in the solution. We refer to the products of this analysis as AVS for brevity. The operationally defined volatile sulfides comprise a group of what are generally believed to be metastable iron sulfide minerals and dissolved S(–II) species that, when exposed to HCl, form H2S—which can subsequently be collected and analyzed. These volatile sulfides are presumed to provide a metalreactive source of S(–II). Thus, when the AVS concentration in sediments exceeds that of the metals under consideration, the metals precipitate and should not be toxic. Initial studies with organisms supported this concept (1, 2). However, the concept is confusing. Uncomplexed aqueous S(–II) is highly toxic to most organisms and even sulfate-reducing bacteria, which produce almost all sedimentary S(–II) and have a limited free S(–II) tolerance. More recent toxicological studies have raised serious questions about the general applicability of the earlier work (3, 4). In other words, toxicity is not only a function of the metal concentrations; the toxic effects of metals and S(–II) need to be deconvoluted as well. With occasional exceptions, AVS is confined to a relatively small portion ( Crystal growth Nucleation > Crystal growth ➝ Microcrystalline pyrite ➝ Microcrystalline pyrite (e) Low-sulfidation environments (IAP < Ks FeS) Nucleation > Crystal growth Nucleation < Crystal growth ➝ ➝ Microcrystalline pyrite Pyrite single crystals and overgrowths
is reduced when the AVS concentration is greater than the concentration of the simultaneously extracted metals. However, because these metals generally occur at relatively trace concentrations, even in contaminated sediments, nucleation of the pure phases is probably unlikely. It is more likely that these trace concentrations are removed via coprecipitation or metal-exchange reactions with the more dominant iron sulfide phases (26). The sorption properties of nanocrystalline mackinawite are very different from those of the bulk, and they change with time. As a consequence, studies of the sorption properties of mackinawite have yielded conflicting results, and a predictive theory for mackinawite solubility in sedimentary environments is not currently available. The presence of a solution stage in all sedimentary pyrite-forming processes (except possibly the marcaAPRIL 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 135A
AVS measurements in sediments are not readily interpretable and have no simple meaning, although they are very easy to carry out. site-to-pyrite transformation) has significant implications for the behavior of AVS minerals in sedimentary systems. It means that toxic metals initially adsorbed on or contained within mackinawite are released to solution during the pyrite formation process. The removal of toxic metals into pyrite then depends on the concentration of the metals in solution and their sorption characteristics relative to pyrite. The result is that, although metal toxicity can be reduced in environments in which the AVS concentration exceeds the SEM concentration, this is a temporary situation. As the metastable AVS minerals dissolve, any metals sequestered with them also dissolve and the final trace metal toxicity is determined by their behavior with respect to pyrite rather than AVS. We conclude that AVS measurements in sediments are not readily interpretable and have no simple meaning, although they are very easy to carry out.
Acknowledgments Support was provided by funds from the Louis and Elizabeth Scherck Endowed Chair in Oceanography for J. W. M and NERC grant NERLS200000611 to D. R. We thank the two anonymous reviewers for their insightful comments, which led to improving this paper. John W. Morse is a professor of oceanography at Texas A&M University. David Rickard is a professor at Cardiff University in Wales. Address correspondence regarding this article to Morse at
[email protected].
References (1) (2) (3) (4) (5)
FIGURE 4
Eh–pH diagram for the Fe–S–H2O system at 25 ºC The chemical dynamics of the Fe–S–H2O system determine the nature of the sulfide components of an AVS–pyrite system. Pyrite nucleation in AVS systems is kinetically related to the degree of supersaturation of the solution with respect to pyrite. The graded shading of the pyrite (FeS2) and FeS stability fields indicate relative supersaturation; the darker color indicates higher supersaturation. In suboxic systems, which are situated near the SO42–/S(–II) boundary, small changes in redox conditions (represented by the standard potential relative to the hydrogen half-cell, Eh) result in major changes in pyrite supersaturation. This means that the rate of formation of pyrite is extremely sensitive to the local physicochemical conditions. In some regions, pyrite formation will be kinetically hindered and other AVS components will dominate; in nearby regions, with slightly different redox conditions, pyrite formation may be fast and pyrite will dominate the system. (Adapted with permission from Ref. 27.)
(6) (7) (8) (9) (10) (11)
(12) (13) (14) (15) (16) (17)
(18)
0.4 2+ Fe(aq)
0.2
FeOOH (19)
0 Eh (V)
Because of the potential significance of the sulfide components to the ecology of anoxic environments, further research is required on methods to more precisely determine the composition of AVS.
2– SO4 /S(–II)
(20)
redox boundary
–0.2 –0.4 H O stability limit
FeS2
(21) (22)
FeS
(23) (24) (25) (26)
2
2+ Fe(aq)
–0.6 –0.8 2
3
4
5
6
7 pH
8
9
10
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11
12
(27)
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