Simultaneously extracted metals/Acid-volatile sulfide (SEM-AVS) is an approach used in the field of
aquatic toxicology
Aquatic toxicology is the study of the effects of manufactured chemicals and other anthropogenic and natural materials and activities on aquatic organisms at various levels of organization, from subcellular through individual organisms to communit ...
to assess the potential for metal ions found in
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
to cause toxic effects in organisms dwelling in the sediment. In this approach, the amounts of several
heavy metals
upright=1.2, Crystals of lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead
Heavy metals are generally defined as metals with relatively high density, densities, atomi ...
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
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dig ...
) is determined. Based on the chemical interactions between heavy metals (SEM) and acid-volatile sulfide (AVS), the
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'' ...
s 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
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
ic 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
Partition equilibrium is a special case of chemical equilibrium. The most common chemical equilibrium systems involve reactants and products in the same phase - either all gases or all solutions. However, it is also possible to get equilibria b ...
. 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
The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:
* Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
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
Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termin ...
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
In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...
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
In pharmacology, bioavailability is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation.
By definition, when a medication is administered intravenously, its bioavailability is 100%. ...
. For organisms residing in contaminated sediments, the contaminants are most bioavailable when dissolved in the
pore water
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidat ...
, 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 equ ...
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
Colorimetry is "the science and technology used to quantify and describe physically the human color perception".
It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color ...
, 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
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
methods, including
atomic emission spectrometry,
atomic absorption spectrometry
Atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES) is a spectroanalytical procedure for the quantitative determination of chemical elemlight) by free atoms in the gaseous state. Atomic absorption spectroscopy is based ...
(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 u ...
.
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."