Simultaneously extracted metals/Acid-volatile sulfide (SEM-AVS) is an approach used in the field of
aquatic toxicology to assess the potential for metal ions found in
sediment to cause toxic effects in organisms dwelling in the sediment. In this approach, the amounts of several
heavy metals
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Heavy metals are generally defined as ...
in a sediment sample are measured in a laboratory; at the same time, the amount of acid-volatile sulfide (sulfide which can be liberated from the sediment by treatment with
hydrochloric acid) is determined. Based on the chemical interactions between heavy metals (SEM) and acid-volatile sulfide (AVS), the
concentrations of these two components can be used to assess the potential for toxicity to sediment-dwelling organisms.
Background
Metals
A number of heavy metals, such as cadmium and lead, are toxic to various forms of life, particularly when dissolved in water as metal ions. Toxic heavy metals are often present in surface water as a result of natural processes, such as the weathering of metal-containing rocks, or due to human activity, such as mining and smelting. Only the
ionic forms of most metals are soluble in water. These ionic forms have a high chemical affinity for the surfaces of most sediment particles, meaning they bind tightly to their surface. As a result, when water bearing heavy metal ions is in contact with sediment, the ions tend to accumulate in the sediment at elevated concentrations.
[Allen, H. E. Metal Contaminated Aquatic Sediments. (CRC Press, 1996).] This is an example of
partition equilibrium. If metal ions are present in great enough quantities, they may have toxic effects on organisms that are exposed to them by ingestion or absorption.
Sulfide
The sulfide ion (S
2−) is present in some
anoxic sediments as a result of bacterial activity. In environments containing little or no oxygen gas (O
2) but large amounts of sulfate ion (SO
42−),
sulfate-reducing bacteria use sulfate in their metabolism as an electron acceptor. This process creates sulfide as a product according to Equation 1.
The sulfide ion produced by this process is sensitive to biological or chemical oxidation in the presence of oxygen, so it only persists in sediments that are continuously anoxic.
Metal-Sulfide Interactions
When dissolved in water, sulfide has a high affinity for numerous heavy metal ions. That is, the
solubility-product constants (K
sp) for the sulfides of these metals are very small, meaning they will
precipitate as solids when a heavy metal ion and sulfide ion come into contact, as in Equation 2, where M
2+ is a generic metal in the +2 oxidation state.
In anoxic sediments uncontaminated by heavy metals, the associated metal (M in equation 2) is usually iron (Fe) or manganese (Mn).
[Di Toro, D. M. et al]
Acid volatile sulfide predicts the acute toxicity of cadmium and nickel in sediments
Environmental Science & Technology 26, 96–101 (1992). Iron (II) is abundant in anoxic sediment, and the K
sp for iron (II) sulfide is 10
−22.39 (with a comparable value for manganese (II) sulfide), so effectively all the sulfide in an uncontaminated sediment will be bound to Fe or Mn.
Several toxic heavy metals, however, have K
sp values substantially lower than those of the sulfides of iron and manganese - for example, cadmium (II) sulfide (CdS) has a K
sp equal to 10
−32.85.
This means cadmium binds sulfide with a much higher affinity than does iron. When water contaminated with cadmium ions comes into contact with sediment containing FeS, the cadmium displaces the iron according to Equation 3 and becomes tightly bound to the sulfide ion.
Due to the large difference in K
sp values for the two metal sulfides, this reaction proceeds effectively to completion, meaning that until all the sulfide in a sediment is used up, all the cadmium in that sediment will be present in the solid CdS form.
A number of other toxic heavy metals behave similarly, including lead, copper, zinc, mercury, and nickel.
Bioavailability
In order for toxic substances like heavy metals to cause effects in organisms, they must be
bioavailable. For organisms residing in contaminated sediments, the contaminants are most bioavailable when dissolved in the
pore water, as opposed to being precipitated as a solid or
sorbed to a sediment particle. Metals in the solid metal-sulfide form are thus considered non-bioavailable, and are unlikely to cause toxicity in sediment-dwelling organisms.
Thus, sediments with the same quantity of metals in them may have vastly different toxic effects, depending on the quantity of sulfide available to bind with them and render them non-bioavailable.
For this reason, the SEM-AVS approach was developed to account for differences in sulfide and refine methods for predicting heavy metal toxicity in sediments.
Methods
Sample Collection
Because sulfide is quickly degraded in the presence of oxygen, sediment samples for SEM/AVS analysis must be kept under rigorously anoxic conditions from the moment they are sampled.
[
"Guidance on Evaluating Sediment Contaminant Results"]
Ohio Environmental Protection Agency: Division of Surface Water
January 2010. In addition, samples should be kept at 4 °C to keep bacterial metabolism from altering sediment composition. The State of Ohio Environmental Protection Agency recommends storing samples for no longer than 14 days before analyzing them.
Extraction
Sediment samples to be analyzed are first purged with argon or nitrogen gas to ensure they are anoxic. The sample is placed in a flask connected to an apparatus for trapping hydrogen sulfide gas (H
2S). Oxygen-free water and hydrochloric acid (HCl) are added, and the sediment is stirred for one hour while argon or nitrogen gas is bubbled through.
["Draft Analytical Method for Determination of Acid Volatile Sulfide in Sediment". H.E. Allen, G. Fu, W. Boothman, D. DiToro, J.D. Mahony. US Environmental Protection Agency. December 1991.]
Sulfide Determination
When HCl is mixed with metal sulfides in the sediment, a reaction occurs that generates H
2S and liberates the metal ion into aqueous form, as shown in Equation 4. The gas formed by this process accumulates in the trap connected to the flask. By weighing the trap before and after the extraction process, the amount of H
2S produced by the reaction can be calculated.
A few important things should be noted about this reaction:
* Not all of the sulfide present in a sediment sample will undergo this reaction when exposed to HCl. Some may be buried deep inside sediment grains and be unavailable to react. Thus, the method is a measurement of the "acid-volatile sulfide," rather than the total sulfide.
* The
stoichiometric ratio
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
of MS to H
2S is 1:1, meaning for every sulfide ion present as a metal sulfide compound, one molecule of H
2S is generated. Thus, the amount (in moles) of H
2S measured by weighing the trap is equal to the amount of AVS originally present in the sediment.
* The metal (M) is transformed from solid form to the dissolved form. In uncontaminated sediment, M will mostly be a combination of Fe and Mn. In contaminated sediments, toxic heavy metals such as Cd, Pb, etc. will also be liberated by the reaction.
Once the quantity (in moles) of AVS has been determined in this way, it is divided by the dry mass of the sediment to obtain the AVS concentration.
In addition to the gravimetric method described here, other methods, such as
colorimetry, may be used.
[Allen, H. E., Fu, G. & Deng, B]
Analysis of acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) for the estimation of potential toxicity in aquatic sediments
Environmental Toxicology and Chemistry 12, 1441–1453 (1993).
Metals Determination
As noted above, treating metals-containing sediments with HCl liberates metal ions into the acid solution that were previously bound up with AVS. After treatment these are present in solution, along with any metals that were initially unbound to AVS (due to insufficient AVS in the sediment to "mop up" all the metal ions). The concentrations of metals in the acid solution can be measured by a number of
analytical chemistry methods, including
atomic emission spectrometry,
atomic absorption spectrometry (AAS), or
mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
.
These are known as "simultaneously extracted metals" because they are the metals liberated from the sediment ''while the volatilization of sulfide is occurring''. Metals extracted from sediment by digestion, or with a different acid than HCl are ''not'' simultaneously extracted metals. By correcting appropriately for dilution, the SEM concentration in the sediment can be determined.
In initial versions of the SEM-AVS approach, six metals were measured: nickel, zinc, cadmium, copper, lead, and mercury.
More current methods call for the measurement of silver in addition to these metals.
Toxicity
Theory
In applying the SEM-AVS approach, two concentrations are determined: the total concentrations of all toxic heavy metals of interest, represented as
EM and the acid-volatile sulfide concentration, represented as
VS[Ankley, G. T. et al]
Acid-volatile sulfide as a factor mediating cadmium and nickel bioavailability in contaminated sediments
Environmental Toxicology and Chemistry 10, 1299–1307 (1991). From these concentrations, the
EM VSratio can be obtained, summarizing the results in a single value.
Based on the extremely low K
sp values for heavy metal sulfides, if
EM VSis less than 1 (
EM < [AVS, then all the extractable metals in the sediment should be in their solid sulfide form and non-bioavailable; above this value, the pool of sulfide is "exhausted" and heavy metals are more likely to be present in the sediment pore water, their bioavailable form.
In theory, then, an
EM VSvalue of 1 represents a cutoff between a "no-effects" range and a "possible effects" range.
Spiked-Sediment Studies
A 1996 study of the predictive power of the
EM VSapproach employed laboratory
toxicity testing
Toxicology testing, also known as safety assessment, or toxicity testing, is the process of determining the degree to which a substance of interest negatively impacts the normal biological functions of an organism, given a certain exposure durati ...
of spiked sediments.
[Berry, W. J. et al]
Predicting the toxicity of metal-spiked laboratory sediments using acid-volatile sulfide and interstitial water normalizations
Environmental Toxicology and Chemistry 15, 2067–2079 (1996). Toxic heavy metals, alone and in combination, were added to clean sediments with varying concentrations of AVS. Benthic fauna">Benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
organisms were then exposed to the sediments, and their mortality was measured and compared to metal-free controls. Ninety-two different trials were conducted, using several test species exposed to cadmium, copper, nickel, lead, and zinc.
In sediments where
was less than or equal to 1, only 1.1% of trials showed greater toxicity than in controls. Where the ratio was greater than 1, 73.5% of trials showed greater toxicity than controls. These results held for both fresh and salt water, for different metal types and test species, and across a range of SEM and AVS concentrations. A study that deployed spiked sediments in a pond found a similar threshold at
= 1 for effects to local benthic fauna.
Spike studies that measured heavy metals concentrations in the pore water - that is, before extracting with HCl - found that, when
pore water heavy metal concentrations were undetectable or nearly so.
These results strongly support the basic theoretical framework of the SEM-AVS approach.
The results of applying the SEM-AVS approach to contaminated sites in the field were mixed. One study found that
was a good predictor of toxic effects in laboratory exposures to field-contaminated sites.
was neither a more sensitive predictor of toxic effects nor a more efficient one than simply measuring metals concentration per sediment dry weight.
In addition, in Flemish rivers polluted with metals, AVS concentration had little to no effect on the accumulation of metals in benthic organisms (though no measurements of toxicity were done).
The potential for SEM-AVS to act as a screening tool in evaluating sediment toxicity due to metals has led several regulatory bodies to use it in the establishment of regulatory assessments of sediment quality.
Under the Water Framework Directive, implemented by the European Union in 2003, Environmental Quality Standards (EQS) for metals-contaminated sediments incorporate
In sediment quality assessments in Australia, SEM-AVS is employed in second-tier assessment. That is, when initial screening indicates metals concentrations in excess of guidelines,
is calculated to determine if sufficient sulfide is present to mitigate metal bioavailability.
In a 2002 guidance manual for assessment of contaminated freshwater sediment, the US Environmental Protection Agency (EPA) listed
(a variant of the approach in which data are normalized by subtraction rather than division) as a "Moderate Priority" metric of ecosystem health. Since 2005, EPA Equilibrium Partitioning Sediment Benchmarks (ESBs) for metal mixtures have been derived from a combination of Water Quality Criteria (WQC) and a variant of
corrected for the organic carbon content of the sediment.
In conducting its National Sediment Quality Survey, the EPA used
concentration greater than 5 μg/g dry weight was classified as "Tier 1: Associated Adverse Effects on Aquatic Life or Human Health Are Probable." Values between 0 and 5 μg/g dry weight were classified as "Tier 2: Associated Adverse Effects on Aquatic Life or Human Health Are Possible."