chemistry

Simultaneously extracted metals and acid-volatile sulfide

Simultaneously extracted metals and acid-volatile sulfide is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Simultaneously extracted metals and acid-volatile sulfide rather than just read about it. In short: 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 in a sediment sample are measured in a laboratory; at the same time, the amount of acid-volatile sulfide (sulfide which can be…

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Reference excerpt

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 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. 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 (S2−) is present in some anoxic sediments as a result of bacterial activity. In environments containing little or no oxygen gas (O2) but large amounts of sulfate ion (SO42−), 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 (Ksp) 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 M2+ 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). Iron (II) is abundant in anoxic sediment, and the Ksp 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 Ksp values substantially lower than those of the sulfides of iron and manganese - for example, cadmium (II) sulfide (CdS) has a Ksp 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 Ksp 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. 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 (H2S). 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.

Sulfide Determination When HCl is mixed with metal sulfides in the sediment, a reaction occurs that generates H2S 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 H2S produced by the reaction can be calculated.

A few important things should be noted about this reaction:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Simultaneously extracted metals and acid-volatile sulfide

Start with the simplest possible case. Write down what Simultaneously extracted metals and acid-volatile sulfide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Simultaneously extracted metals and acid-volatile sulfide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Simultaneously extracted metals and acid-volatile sulfide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Simultaneously extracted metals and acid-volatile sulfide

In research: Simultaneously extracted metals and acid-volatile sulfide appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Simultaneously extracted metals and acid-volatile sulfide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Simultaneously extracted metals and acid-volatile sulfide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aquatic ecology, Environmental toxicology, so understanding it makes those chapters shorter.
In everyday life: Look for Simultaneously extracted metals and acid-volatile sulfide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is Simultaneously extracted metals and acid-volatile sulfide in simple terms?

Why does Simultaneously extracted metals and acid-volatile sulfide matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Simultaneously extracted metals and acid-volatile sulfide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

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It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Simultaneously extracted metals and acid-volatile sulfide.