Abandoned Mine Posts

By Bruce Golden, Regional Coordinator

Did you ever wonder why there’s such a wide range in the chemical make-up of coal mine drainage discharges? It’s because there’s a great diversity in the mining environments in which mine drainage is formed. This multipart series of AMP will explore, on an introductory level, the connection of mine drainage characteristics with the mining environments that created it. It’s not meant to rigorous, but to mainly get you thinking about the kinds of things that happen can underground that are influential in the making of polluted water.

Part 2. Chemical Beginnings.

This installment of AMD and Mining Environments focuses on the chemistry of AMD formation. For those of you that aren’t fully comfortable with chemistry concepts, I’ve made an extra effort to provide enough detail and background to give you a decent picture. For this reason this installment won’t be as brief as we would generally like. For those of you already comfortable with chemistry stuff, take the day off.

Some background first. On the earth’s surface two of the most common and important chemical actors, oxygen and water, are generally abundant. Underground in coal seams, quite the opposite is generally true. In fact, the last time the vegetation that eventually became coal had likely experienced free oxygen was a quarter of a billion years ago. Because of the abundance of oxygen in our atmosphere and oxygen’s propensity for entering into chemical reactions with any number of substances, we’re said to live in an oxidizing environment. Our very existence depends on our oxidizing environment. The notable example of breathing or respiration wouldn’t be possible without oxygen. But oxygen can also enter into chemical reactions that are not so beneficial from our perspective, as we’ll see shortly.

In the last installment, we said that the weathering of the mineral pyrite leads to the formation of AMD. Here are some points that may be helpful in better understanding this menacing mineralogical misfit:

• Pyrite is comprised of atoms of iron (Fe) and sulfur (S) in the ratio of two sulfur atoms for every iron atom. It’s molecular formula is FeS2 .
• Pyrite exists in coal as a small percentage by weight. The actual percentage may vary considerably from coal seam to coal seam and can even vary significantly within a given coal seam. For instance, the coal near the top (roof coal) of the famous Pittsburgh seam is quite high in pyrite compared to the middle of the seam (the good stuff). The pyrite percentage can be up to a few percent by weight. Much of the sulfur content in coal is actually pyrite. (When burned, low sulfur coal produces less sulfur dioxide air pollution.)
• Pyrite is not the sole mineralogical culprit in coal; close cousins of pyrite more or less behave the same way. Yet pyrite generally takes the rap for the entire family.
• Pyrite in coal has remained unchanged for eons in its oxygen deprived underground sanctuary/prison. Only when something unearths it, for instance mining, and exposes it to its incorrigible chemical cohorts, oxygen and water, does pyrite awake from its extended nap ready to get on with the chemical party.
• And perhaps it’s really the over-reactive oxygen that we should view as the real trouble-maker. Oxygen has a rap sheet a mile long for being the ring leader in any number of oxidation reactions, a lot of them seen as destructive. Poor pyrite hadn’t bothered anything for practically forever. If pyrite had a lawyer, this would undoubtedly be part of the defense.
• Now suppose some coal mining has occurred enabling pyrite to get a visit from its reactive cohorts oxygen and water. Let’s take a look at the chemical mayhem that ensues as this motley trio hooks up.

Act 1: Pyrite has a Transformational Experience

For the first time since its formation eons ago, pieces of pyrite embedded in coal are free of complete burial and now have some exposure to a new, less confining environment. Surfaces of coal may now be in the open. Pyrite will still be immobile as it’s locked as a solid within coal, but pieces of it may now be exposed to the atmosphere. Assuming the air is fresh, oxygen molecules will certainly be paying regular visits to the pyrite. Even though in direct contact, pyrite and oxygen by themselves don’t interact chemically. That changes when water is added to the mix. Only then, when all three players are present at the same time does the first of several chemical reactions take place. Our actors, pyrite (a solid), oxygen (a gas), and water (a liquid), collectively known as reactants, are symbolized as

Each reactant substance will undergo a transformation resulting in products with totally new identities. Atoms of the reactant substances will rearrange into new product substances. The individual atoms of original reactant substances break up, then recombine with different partner atoms to form products unlike the original reactants. Talk about a makeover! This process of atomic rearrangement is a chemical reaction. With the exception of the electron configuration surrounding atoms, the various atoms themselves remain unchanged throughout the process. An iron atom remains an iron atom, an oxygen atom continues to be an oxygen atom, etc. From an atom’s standpoint, the only thing different following a chemical reaction is its electrons have a new hair-do. Well, not really, because atoms don’t actually have hair, it would be more of an electron-do, and… I’ll just shut up.

I am making the very important assumption that the amount of water available is far in excess of what is actually needed for the actual reaction to occur. In this sense, water will have a dual role both as a reactant and as the medium where the chemical transformation will physically take place. Having water as the medium for reactions to occur is so common that we lovingly refer to that what happens chemically in water as aqueous chemistry. The “aq” symbol you’ll see later simply means there’s plenty of water all around.

Let’s see what the makeover produces.

• The first is that the iron (Fe) atom’s bonds with the two sulfur (S) atoms in pyrite (FeS2) are broken.
• The iron atom then becomes dissolved in the excess water as a positively charged particle deficient in two electrons.It’s called a ferrous (Fe2+) ion.
• Each of the sulfur atoms teams up (bonds) with four oxygen atoms and a couple of electrons to form a negatively charged complex called sulfate ion (SO42-), which is also dissolved in water.
• The bonds between the atoms of water molecules that enter into the reaction are broken. The oxygen atom is used in the production of sulfate. The two hydrogen atoms become positively charged hydrogen ions (H+) dissolved in the excess water.
• The reaction products, all of which are dissolved in water, are symbolized as

By the way, all of the above reaction products are colorless and odorless when dissolved in water.

We can now put the reactants together with the products to symbolize the entire reaction as

The arrow symbolizes a chemical reaction proceeding from reactants to products. Shown this way, it’s kind of like a recipe, with the ingredients on the left and what you get on the right.

You may have noticed that something is missing… namely the amounts or proportions of both the individual reactants and products. Since a reaction is a rearrangement of atoms, there have to be equal numbers of oxygen atoms on both the reactant and product sides of the equation. This must also be true for iron, sulfur, and hydrogen atoms. In other words, the number and kinds of atoms must balance on both the reactant and product sides. They have to: they’re the same atoms, just rearranged. This clearly isn’t the case for the reaction symbolized above. For instance, there are 2 sulfurs on the left, and only one on the right. See if you can find other discrepancies. This is an unbalanced equation and only shows the substances involved and not their quantities.
Without going into the detail of how it’s done, I’ll now show a balanced form of the reaction in which the number preceding each substance indicates the proportion needed in balancing the reaction.

If you count up the numbers of each kind of atom of the left (reactant) side of a properly balanced equation, they now exactly equal the numbers of each kind of atom of the right (product) side. With a little practice, you can look at symbolism of this sort and spout off something like “Four molecules of solid pyrite react with 14 molecules of oxygen gas and 4 molecules of liquid water react to produce 4 dissolved ferrous ions, 8 dissolved sulfate ions, and 8 dissolved hydrogen ions”. Yes, it may sound impressive, but take my advice… do NOT use it as a pick-up line at a party… bad idea. Just be content the recipe is complete showing all the reactant and product substances with an accurate accounting of their relative amounts.

We’ll call this reaction the pyrite oxidation reaction, the one that gets the AMD pollution ball rolling. Repeated over and over and over, the amounts of ferrous, sulfate and hydrogen ions increase. Increases in hydrogen ion concentrations mean the water is getting more acidic. Sulfates aren’t generally considered to be a health concern, but at high concentrations can cause diarrhea with subsequent dehydration in extreme cases. And we’re concerned with what ferrous ions become as they are involved in subsequent chemical reactions.

Well, that about does it for our first chemical act where pyrite, awoken from a deep sleep, went from a glittery, kind of a classy looking mineral, to a totally changed life as water pollution at the urgings of two of the most common chemicals, oxygen and water.

Join us on the road to chemical ruin in our next installment of AMD and Mining Environments (Part 3).

This entry was posted on Friday, October 12th, 2007 at 4:31 pm and is filed under General, Industry, Water Monitoring, Helpful Tools, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.