AMD and Mining Environments: Part 4
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 be rigorous, but to mainly get you thinking about the kinds of things that can happen underground that are influential in the making of polluted water.
Part 4. That Chemical Ball Keeps on Rollin’… then Stops.
In Part 2 of AMD and Mining Environments, I talked in some detail about the first act of several in the formation of AMD. To review, the chemical reaction that starts the whole AMD pollution ball rolling is the oxidation of pyrite in the presence of oxygen and water
which results in a complete chemical makeover with the chemical species of ferrous ions, sulfate ions, and hydrogen ions being dissolved in water.
In the last installment of AMD and Mining Environments, I told you about the next act, the ferrous to ferric oxidation reaction,
in which ferrous ions (from the first reaction) are converted to ferric ions with a corresponding reduction in the numbers of hydrogen ions.
So let’s move on. Fair warning though: get yourself a cup of coffee / Jolt cola to get you through. Who knew there could be so much to say about one stinking reaction?
Act 3.
Even though the newly formed ferric ions (Fe3+) at least symbolically may look like (and indeed have been transformed from) ferrous ions (Fe2+), don’t be fooled into thinking that things haven’t changed a lot. The loss of that single little electron means we’re talking about one very changed ion, one with a whole new attitude. Ferric is all grown up and it’s time for this ion to settle down.
You see, ferric isn’t all that enthralled with the whole aqueous scene. The freedom with being dissolved amidst all that water, as was the case in its former ferrous life, is no longer so appealing.
The problem is a single ferric ion isn’t capable of leaving the aqueous life by itself… all because of that blasted +3 charge. Ferric desperately needs help if it’s to escape its wild aqueous surroundings… help in the form of an offsetting negative charge. Only a complete neutralization of that charge will do.
As fate would have it, help is all around. The very thing ferric is trying to be free of will ultimately be its salvation… water. Water molecules find ferric quite attractive, but realize that without a significant commitment, ferric will not be interested. Remember that nothing short of complete neutralization of’ the +3 charge will satisfy ferric. Water by itself has no charge and nothing of interest to ferric. However, water has a trick… it’s able to split itself into two parts: a negatively charged hydroxide ion (OH-) and a positively charged hydrogen ion (H+).
Hydroxide’s negative charge definitely has spurred ferric’s interest, yet ferric knows hydroxide’s single negative charge will never be enough to satisfy its appetite for negative charge completely. Similarly hydroxide knows that ferric is far too much ion for it to handle by itself. A little out of the box ionic thinking is all it takes to come up with the solution. The wedding of ferric with not one, but three hydroxides is the ideal way to provide neutralization bliss as a compound, joined together by the attraction of their opposite charges. Not only do the happy ions enjoy complete neutralization with their union, the miracle of phase change is bestowed on them as they make the transition from the aqueous to the solid state as the newly formed ferric hydroxide (Fe(OH)3(s)). Music please!
The ceremony (reaction) is known as the hydrolysis of ferric ion.
Before you raise your eyebrows of ferric taking three partners, know that this sort of nuptial is quite acceptable, indeed expected, in the world of atoms and molecules where neutralization of charge is a very proper thing.
The ceremony, unfortunately, saw some unsavory guests appear. You may remember the Acidz, a.k.a. hydrogen ions, from the previous installment. As a consequence of water making itself attractive to ferric by splitting into hydroxide ion, it also created its counterpart, hydrogen ion. Three new Acidz came into existence as a result of the ferric – hydroxide marriage. Those chemical gangstas are far too often implicated with all sorts of trouble, if not now, then almost assuredly later.
Following the ceremony, the happy compound, now a respected member of the solid and stable Rust family, must say goodbye to the aqueous environment. It’s no longer able to mix well with all the water surrounding it. Ferric hydroxide must immediately embark on the new journey of precipitation which, aided by gravity, will take it to the bottom where it will join other ferric hydroxides that have preceded it as it settles into its new neighborhood. Should the compound remain underwater, as many do, it will join its neighbors known as the Yellowboy clan. However, should ferric hydroxide find itself high and dry, things become unbearable for the hydroxides which still yearn to be near their former water siblings. Inevitably, something’s got to give. Ferric hydroxide experiences a breakup where two hydrogens and an oxygen go their own way as a water molecule, leaving a somewhat lighter, but wiser and more stable compound formally known as ferric oxyhydroxide, but informally called iron oxide among its friends.
Iron oxide can look forward to a rather uneventful existence and a marriage characterized by solid bonds that are likely to last a very, very long time.
Ferric, once a part of another long lasting union with sulfur in its marriage as the mineral pyrite, and now a part of a new union in the Rust family, is now safe because the oxidizing environment that broke up the pyrite marriage is no longer able to sing its siren song.
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Okay, let’s review the hydrolysis of ferric ions, this time without the dramatization. Here’s the balanced reaction.
Here’s one way to “read” the equation. “An aqueous ferric ion combines with 3 water molecules to form 1 molecule of solid ferric hydroxide and 3 aqueous hydrogen ions.”
Here are some salient points:
- This is called a hydrolysis reaction because of the reaction with water. This general form of reaction is common with other metal ions. I’ll again use aluminum as our example.
Hydrolysis reactions of this sort produce a number of hydrogen ions equal to the charge on the metal… 3 in this case. - This is an acid forming reaction. A lowering of pH is common.
- This reaction generally happens very quickly following the ferrous to ferric oxidation reaction.
- In water, the solid ferric hydroxide particle that forms will join up with many other ferric hydroxide particles in forming a larger particle. How big this particle grows will determine how fast it settles. In still water, bigger particles settle out faster.
- Ferric hydroxide also goes by common name yellowboy. Its color can vary from a yellow to a deep orange. In a stream, as yellowboy settles and coats the bottom of a stream, it fills in the innumerable nooks and crannies created by pebbles, rocks and other features on the stream bottom. In doing so, the habitats of benthic (bottom dwelling) macroinvertebrates (small creatures without backbones, a.k.a. bugs) are destroyed, creating serious food-web implications for fish.
- Given the chance to dry out, yellowboy undergoes a reaction where a water molecule is lost, forming ferric oxyhydroxide.
Similar reactions can also occur which result in various forms of rust, or iron oxides. These compounds are commonly used as pigments. Iron oxides will stain your skin, but don’t represent a safety hazard. - In an oxidizing environment (as we live in with our abundance of atmospheric oxygen) rust-like compounds are pretty much the end of the line chemically. They are very stable and not much is ikely to come along and react with them.
We’ll I’m going to stop now before I think of anything else. Thanks for hanging in there. Join me again for the next installment of AMD and Mining Environments where we’ll shift our attention to the neighborhoods where these chemical reactions take place.