Oct 25, 2012

Pick-Up-Truck Chemistry

Here you go Anna, I made these videos and wrote this up just for you. I posted it because I figure there might be more environmental educators or simply curious citizens who wanted to know some more about the chemistry of Acid Rock Drainage and a little about the science goes on at the West Virginia Department of Environmental Protection.  Last week I had the privilege of tagging along with a WVDEP chemist and we did a few pickup-truck-bed-experiments to check on the progress of some mine sites that we monitor. It's a little fast and my phone didn’t give me enough footage to really edit into something Bill Nye quality but I have some pretty dramatic videos of what happens in the field on a regular basis. For privacy reasons, the location of the sample and the identity of the speaker should remain anonymous.
Warning! Gratuitous Chemistry!

Coal is just carbon. Plain old, harmless carbon of which most of your body is made. The rocks that are typically found deposited near coal, on the other hand, can do some pretty serious environmental damage when disturbed. Underground coal mines and the exposed piles of crushed waste rock left over from mining (which you can read more about in my previous article) are often loaded with pyrite, or fool’s gold which is named for its metallic yellow luster. Chemically, it is ferrous sulfate (FeSO4) which has the nasty tendency to dissolve into sulfuric acid when it contacts water and oxygen from the atmosphere. When mining activity exposes rock to the air and the rainwater that percolates underground, the water becomes highly acidic- sometimes with pH readings lower than 1. A big problem with acids is that they are exceptional solvents, which means that the acidic water is now very good at dissolving minerals from the rock and carrying them downhill, turning mine waste piles into giant wet tea bags that leach out water loaded down with iron, aluminum, manganese, selenium and other trace metals. The acidic, metal-laden effluent flows underground until it is forced to the surface on hillsides or streams. This is called Acid Rock Drainage though you may see the older term Acid Mine Drainage which fell out of vogue since you don’t need a mine to mix sulfur-bearing minerals with oxygen and water and acid drainage can occur even during highway projects if the geology is right like it is in the Anthracite Coal region of Pennsylvania.
The deep vertical cuts in the rock (left) are holes drilled to insert explosives and expose the coal seam underneath it. The exposed rock will often produce ARD which can collect and form orange-brown streams (right)
No matter how it is formed, the water that emerges usually looks normal. The water is clear because the metals are dissolved and invisible just like the common and important minerals that you can find in clean drinking water. As the ARD is diluted with fresh water or runs over minerals like limestone that neutralizes the acidity, the water loses its ability to hold on to all that dissolved stuff.
Those minerals then start to precipitate, or fall out of solution and they rain down on the bottom of streams. Iron is the most common and conspicuous precipitate and when it comes out of solution it either turns blue in the absence of oxygen or it turns red in its presence. That’s because there are two types of iron that we're concerned with here: the blue, reduced form called feric iron (Fe II) and the more familiar red oxidized ferrous iron (Fe III) that we more commonly call rust. Oxygen has a tendency to rip electrons away from other elements so if there is an abundance of oxygen, many electrons are ripped away which oxidizes the iron and makes it more positively charged. Less oxygen means the iron keeps more electrons and is less positively charged (but still positive!). The roman numerals just indicate how positive the ion is, or how many electrons the iron needs to pick up before it is neutrally charged and has the same number of protons as electrons. The charge of the iron ions affect not just its color but also a number of important other properties that relate to how well it separates from the water column or binds to stones to form an “armor.” Suffice to say, water treatment demands we let the stuff settle out and form ferrous iron (the rusty kind). In mine water reclamation there may be a pond filling up with ARD and in the deeper, lower oxygen environment of the pond, the bottom is covered with bluish black ferric iron, but in the shallows or near a babbling stream, the bottom has a red, fuzzy-looking coating of the ferrous iron. This explains the pretty colors and why almost all ARD in streams is orange. Now, iron isn't toxic but if there is enough of it, it can coat the bottom of streams with rust that smothers every living thing on the stream bottom where benthic insects, the foundation of the food web, call their home. It's a great way to ruin a nice trout stream very, very quickly. It isn’t uncommon to find streams in the backwoods of Appalachia that have a bright orange blanket of goo on top of the sediment marking another section of stream that won’t support a valuable trout fishery or help provide people with clean drinking water. 

In light of these problems, the state of West Virginia and the US Federal Government have a set of standards for how acidic the water can be and how much iron and aluminum it is allowed to carry as it flows out of mining properties. The WVDEP works to enforce these standards and even takes complete control of cleanup efforts of centuries-old abandoned mines as well as more recent cases where a company has gone bankrupt or run out of money to clean up after itself. The DEP has a to-do list a mile long.

So how does DEP go about rehabilitating an acidic, orange mess into a clean, clear river? The key is to neutralize the water enough that it dumps all of its metals into a pond before the water can enter the environment. Digging a pond acts as a reservoir to collect all of the ARD so it has plenty of time to settle out, oxygenate and let wild iron-eating bacteria help to adjust the chemistry back to normal. With enough time, this is often plenty to bring the water back into compliance. Because all it takes is a strategically located hole with some engineered drainages, we call this 
Passive Treatment, but each site is different and some need more help.
Adding baffles to the pond (like at right) help to keep the water still and promotes settling out of suspended particles. In this pond, you can see how newly contaminated water in the front starts to improve as it has longer to settle out.
Active Treatment might entail forcing the water through a channel filled with limestone cobbles or adding powdered calcium carbonate (CaCO3) to neutralize the acid in one go. DEP sometimes has to install the aptly named AquaFix units which are giant hoppers full of calcium carbonate or sodium hydroxide which automatically add chemicals to the water to the stream flow. By adding a base (high OH-) to an acid (H+) you end up creating a lot of water (H2O) and if you do it right by adding the perfect amount, acid and base annihilate each other leaving only a harmless sludge of iron oxide, calcium and some residual salts. 

Sodium hydroxide drips into the ARD effluent.
When the sludge gets too thick in the pond, it gets shoveled out and dumped into a sludge pond (a hole in the ground) where it is stored or sometimes collected for use as a safe soil amendment for farmers who want to naturally adjust the acidity of their soil. Some enterprising individuals are trying to sell it to the pigment industry where iron oxide is used in ceramics and crazy educational activities.

The AquaFix uses a water wheel (left) to measure water volume and dispenses the right amount of CaCO3.
All this time I’ve focused on iron because it is more visible and is more common but taking care of aluminum is just as important. It’s also more complicated. Unlike iron, aluminum is actually toxic when it is dissolved in the water and unlike iron, it doesn’t just fall out of the water when the acidity is neutralized. To get aluminum out of acidic water we have to use sodium hydroxide (also called lye or NaOH) which is a much stronger base. If you've ever seen Fight Club, you know it is a VERY strong base that can cause severe, lip-print shaped burns and is used in the production of soap. The sodium hydroxide is used to neutralize the acid but then even more is added to bring the pH up to around 9. At that point, the aluminum can settle out but the water is now too basic to let down stream so it gets treated again until it ends up back down to a nice, neutral 7 for discharge.

Suspended aluminum reflects sky, creating a weird blue/green
Getting the metals out on the lab bench would be a simple matter of measurement in a few beakers, a few quick calculations and an easy titration with a pipette. Getting the chemistry right in the real world is much trickier with stream flows changing with the weather, erosion, and complicated geologic and hydrologic processes. Treatment takes frequent monitoring and often a bit of tweaking.
If we add too little base, we don’t do our job and the acid flows into the stream to dump its iron and aluminum in the environment. If we overdo it we risk wasting expensive treatment chemicals and even making the treatment pond too basic which has its own set of environmental problems. Here are some videos of a DEP officer and me conducting a test to figure out how acidic an ARD site is and how much metal is in it- figures we can use to determine how much of which chemicals we’ll have to add to the water to bring it to acceptable levels.

Can you read the display? Holy Crap!

In the photo on the far right you can see that the excess calcium carbonate has precipitated on the right while iron oxide from the ARD precipitates on the left. This is a classic example of overtreatment and the water here is incredibly basic, giving the water some interesting properties. For one thing, the iron becomes almost gelatinous and you can even see the trails of frogs and stream flows etched into the water like a jello mold.

A Quick Recap on pH

Are you following me with this pH business? If you've already got it figured out, you won't miss anything by skipping ahead, this is just here if you need a quick chemistry reboot.
pH is simply the ratio of hydrogen ions (H+) to hydroxide ions (OH-) in a solution. As the ratio tips in favor of H+, the solution becomes more acidic and the solution gets better at dissolving other substances. It's why a chunk of bread doesn't last very long in your stomach before it gets broken down and dissolved. If the solution has more OH-, the solution is called basic, or alkaline and it tends to be corrosive or caustic.

We measure acidity by measuring the ratio of H+ to OH- and we call it pH (think "proportion of H+"). The pH scale goes from 0, most acidic to 14, most basic with a neutral 7 in the middle being neither neutral nor basic. To be technical, the pH scale is a negative logarithm of hydrogen ions, or in other words, 0 has a very high concentration of positively charged hydrogen ions (H+) and very few hydroxide (OH-) ions while 14 has a very high concentration of OH- and a very low concentration of H+. A pH of 7 means that there is the same number of H+ and OH- in solution as is the case with pure water. The logarithmic part means that the scale slides by powers of ten; a pH of 3 is actually TEN TIMES more acidic than a 4 and a 9 is TEN TIMES more basic than an 8. You got it all? Good, now try and follow along with this cool field test.

Doing science like this in a controlled environment would be relatively straight forward, but our lab bench looks like this:

videoThis is science at the quick and dirty end of the spectrum. We start with a sample of highly acidic mine waste water. This sample started off with a pH around 3.1, which is solidly in stomach acid territory. We then add sodium hydroxide which, as you recall, is a very strong base. Here, we actually want to overpower the acid and make the whole sample so basic that the aluminum and iron can't stay dissolved in the water anymore. They precipitate out of the water and become a suspension, which is a bunch of tiny free-floating particles that float around for a while until they sink. Once they sink, you have clear water on the top with no dissolved minerals in it. Because we’ve added so much sodium hydroxide, it's still very basic.

The blue stuff on the bottom is suspended reduced ferric iron that has fallen out of solution and is now sinking to the bottom. The clear layer on top is just basic water. If given enough time, the blue iron would oxidize and turn orange.

If we’ve added enough base, we can suck up some water in a syringe and filter out all of the solid particles that have fallen out of solution and leave no dissolved metals remaining. We can test to see if any metals remain in solution by putting the water in a machine called a colorimeter which shines a light on the sample and measures what color the light is when it comes out the other side. Pure water should be clear but if any dissolved metals remain, the water will have a tint that the colorimeter can detect. If everything has been done right, the colorimeter should read zero, showing pure, clean water.

videoSo now we have water on the top without any dissolved minerals in it. None. Some of the minerals, like calcium and magnesium, are actually super important to buffer against changes in acidity. Buffering is the ability to soak up extra H+ or OH- ions, that is, the more calcium and magnesium is in the water, the more acid or base you need to add to the water to change its pH. By getting rid of all of the minerals, we have eliminated the iron and aluminum just like we wanted but we also made the water vulnerable to changes in pH that might occur from smaller ARD sources downstream. If we have done our chemistry right, however, we should have a very good idea how much sodium hydroxide we’ll need to treat the water without stripping it of its ability to resist chemical changes in the future. DEP has to treat the water but not treat it too much. Through decades of experience, the DEP now takes pride in treating some of the worst drainage runoff and discharging it later even more pure than the streams it flows into.
Keeping West Virginia 'Wild and Wonderful' is just the name of the game.


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