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Chemistry Unexpected

Simple chemistry brings a Pennsylvania creek back to life

by Stephen K. Ritter
August 27, 2007 | A version of this story appeared in Volume 85, Issue 35

Credit: Courtesy of Carl Kirby
Red iron hydroxide precipitates from groundwater flowing out of an abandoned coal mine.
Credit: Courtesy of Carl Kirby
Red iron hydroxide precipitates from groundwater flowing out of an abandoned coal mine.

Being a chemist, I go through life expecting to find chemistry at every turn. Even so, I was pleasantly surprised a few weekends ago to discover chemistry at work in an unexpected way and in an unexpected place: Simple limestone is being used to adjust the pH of streams affected by acidic coal-mine drainage in Pennsylvania forests.

Large-scale anthracite coal mining began in eastern Pennsylvania before the 1850s and reached its peak in the late 1910s, during World War I, when some 100 million tons of coal per year was being extracted. The industry has declined significantly since then, and most of the coal mines in the region were abandoned before 1960. Lower quality bituminous coal is more plentiful and still heavily mined elsewhere in the state and the rest of the country.

No one can doubt the benefits of using anthracite coal to produce electricity, make carbon black for water filtration, and, in the past, heat homes. But the consequence of coal-mine drainage in this area of Pennsylvania is that more than 2,400 miles of streams have been degraded by acidity or deposits of iron hydroxide, Fe(OH)3.

The chemistry of coal-mine drainage is fascinating. The acidity arises primarily from the oxidation of pyrite (FeS2), a mineral associated with coal deposits. Exposed to water and oxygen, pyrite is converted into ferrous iron (Fe2+) and what essentially is sulfuric acid (H+ and SO42-). Ferrous iron is then oxidized to Fe3+ in a process that consumes H+ and raises the pH of the water. If the pH of the stream rises above about 3.5, the aqueous ferric iron precipitates as red-orange Fe(OH)3, a material called "yellow boy." That's what one sees seeping from the ground or coating the rocks in stream beds near the outflows of abandoned mines. Heavy deposits of Fe(OH)3 choke out most aquatic life in the streams, and although the water is clear, the pH can remain quite low.

That leads me back to where I started. I was hiking on the Appalachian Trail east of Harrisburg when I came across a pair of "diversion wells" built alongside Rausch Creek. A sign conveniently explained that the wells were built to increase the pH of the clear stream, which flows through a bucolic wooded glen.

Here's how the wells work: Water is diverted from the creek into a pipe that leads to a concrete chamber containing limestone gravel (calcium carbonate, CaCO3). Water entering the chamber under pressure pulverizes the limestone, then drains out of the chamber along with fine limestone particles and reenters the stream bed. The particles dissolve, releasing Ca2+ and HCO3-, which neutralizes the H+ in the acidic water.

This treatment raises the pH of Rausch Creek from about 4.0 to above 6.0 (a pH of about 7.0 is ideal for a cold-water stream). The stream still isn't pristine, but the simple act of correcting the pH has made a difference. Upstream of the creek's wells, fish can't survive. But just downstream from them, healthy brook trout are swimming.

The Rausch Creek diversion well system, patterned after a Swedish design for treating acid rain runoff, was the first of its kind built in the U.S. One well was constructed in 1987 as a joint project of the Pennsylvania Cooperative Fish & Wildlife Research Unit and the Doc Fritchey Chapter of Trout Unlimited, a nonprofit organization whose mission is to protect cold-water watersheds for fishing. A second well was built in 2000 to handle the overflow from the first one during heavy rain. Weekly maintenance, carried out by volunteers from the Doc Fritchey Chapter, is required to shovel in fresh limestone.

It turns out that diversion wells are one of several types of treatments now used to remediate acidic mine drainage in eastern Pennsylvania, according to Carl S. Kirby, a geology professor at Bucknell University, Lewisburg, Pa., who studies mine drainage. The systems include simply dumping truckloads of powdered limestone into a stream, constructing retention ponds, and building full-scale water-treatment plants. The type of system used depends on the flow rate, land available, and unique chemistry of the site, Kirby says.

As a sidenote, Kirby hit upon the notion to use yellow boy as a paint pigment. To promote the idea, he has used Fe(OH)3 collected from streams to color T-shirts and even to have his 1982 Volkswagen van professionally painted burnt red. Separately, Hedin Environmental, a Pittsburgh-based consulting firm that specializes in remediating coal-mining sites, is providing ton quantities of Fe(OH)3 to a company that is making iron oxide paint pigments. Kirby is collaborating with Hedin Environmental to explore other possible uses for yellow boy. He hopes that producing commercial products from the deposits will spur efforts to clean up mining sites and streams affected by mine drainage.

In the past, people haven't always been able to foresee the environmental problems that might arise from extracting natural resources or from making new classes of chemicals, pharmaceuticals, and materials. Mother Nature has been rather forgiving, showing an amazing ability to correct for the past transgressions of humankind-consider the ongoing recovery of the Antarctic ozone hole, forests damaged by acid rain, and the bald eagle. Streams in Pennsylvania are another example.

But nature can't do it alone. As chemists, we have learned that we need to use our chemical knowledge to anticipate or correct such problems. It's refreshing to see that we are doing so in what might be considered unexpected ways.



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