UNIVERSITY PARK, Pa. — A study led by Penn State researchers suggests that recycled concrete aggregate and an iron-rich byproduct of steel production could help treat acid mine drainage, the toxic runoff that pollutes thousands of miles of streams in Pennsylvania and other coal-mining regions.
The research, published in Mine Water and the Environment, evaluated whether recycled concrete aggregate (RCA) and an iron-rich material (IRM) could replace or supplement limestone, the standard material used in many systems that treat acid mine drainage. The findings, according to the researchers, suggest RCA could be a viable alternative in some locations, while IRM was less effective.
Acid mine drainage forms when mining exposes sulfur-containing rocks to air and water. The resulting chemical reaction creates sulfuric acid, which dissolves metals such as iron, aluminum and manganese. When contaminated water enters streams, it lowers the water's pH, deposits orange-colored iron and harms aquatic life, vegetation and drinking water supplies. This is typically treated by implementing a limestone treatment system, which neutralizes acidity and helps remove dissolved metals. Researchers, however, are exploring whether less expensive waste materials could serve a similar purpose, potentially turning materials that would otherwise be discarded into useful commodities for treating acid mine drainage.
More than 5,000 miles of streams in Pennsylvania are affected by acid mine drainage, underscoring the scale of the problem in a state with a long history of coal mining.
The study was led by Allison Fenske, a Penn State alumna and assistant professor of environmental engineering at Juniata College, and Nathaniel Warner, associate professor of civil and environmental engineering at Penn State. The researchers explored whether industrial waste materials already available in large quantities could provide a lower-cost option for treating polluted water.
“We selected RCA and IRM because both materials are readily available, inexpensive and are known to raise the pH of water, making them potential alternatives to limestone for treating acid mine drainage,” Fenske said. “We thought there could be a chance to use a waste product to treat AMD.”
To test the idea, the team conducted a series of laboratory experiments. They first measured whether the materials released harmful metals into acidic water. They then assessed how well the materials reduced acidity in short-term experiments before simulating how they would perform over time in a passive treatment system.
The tests showed that RCA performed best, producing the greatest reduction in acidity. In some batch tests, RCA outperformed limestone, although its performance gradually declined in continuous-flow tests. IRM also improved water chemistry but was less effective, failing rapidly in continuous flow and increasing manganese concentrations in the treated water. Both materials released small amounts of metals, although generally at levels comparable to limestone.
However, the researchers found that, over time, iron and other minerals coated the surface of both treatment materials, preventing water from reaching fresh surfaces that could continue neutralizing acidity. As the coating built up, the materials became less effective.
Fenske said this buildup is a common challenge for passive treatment systems that use limestone.
RCA maintained its performance longer than IRM, but neither material proved to be a simple replacement for limestone. IRM lost effectiveness more quickly and increased manganese concentrations in treated water, limiting its usefulness in many passive treatment systems.
The researchers also evaluated the cost of RCA, finding it could be cost-effective in parts of Pennsylvania where concrete waste is readily available and transportation distances are short. Because both the chemistry of acid mine drainage and the composition of recycled concrete vary from site to site, each location would need to be evaluated before using the material. IRM was not included in the regional cost comparison because the researchers identified only one supplier in Pennsylvania. Their modeling nevertheless indicated that IRM would require more material than limestone to provide the same treatment life, potentially increasing transportation and construction costs.
Rather than replacing limestone everywhere, the researchers said RCA could expand treatment options at some abandoned mine sites where traditional systems are too expensive.
"Ideally, this could enable more acid mine drainage to be treated and help restore miles of Pennsylvania streams so they can once again support fishing, swimming and drinking water," Warner said.
Warner said the findings also highlight a broader opportunity to find new uses for industrial waste.
“For communities affected by abandoned mines, this research suggests that locally available waste materials could become part of a more affordable strategy for restoring polluted streams when matched to the right site conditions,” Warner said.
The research was supported by the U.S. National Science Foundation under award number 1942601, as well as Befesa Inc. under award number 170440. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funders.