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Environmental Factor, March 2012

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SRP researchers test less expensive method for tar remediation

By Nancy Lamontagne

Seven column of tar-contaminated sand, with decreasing amounts of sand

A solution of NaOH and xanthan gum is flowed downward through a column of tar-contaminated sand and imaged every 90 minutes. From left to right, the tar saturation is 32 percent in the first photo and 3.7 percent in the last, corresponding to a removal of 88 percent of the tar. The ruler is marked in centimeters. (Photo courtesy of Cass Miller)

Cass Miller, Ph.D.

Miller tested a new way to remove tars from sites of former manufactured gas plants. He says that these tars contain virtually every complication that one could conceive of in a subsurface and contaminant. (Photo courtesy of UNC)

Superfund Research Program (SRP) researchers from the University of North Carolina (UNC) at Chapel Hill have published an NIEHS-funded study showing that alkaline and alkaline-polymer solutions might offer an effective and less expensive way to remove tars from former manufactured gas plant sites. Manufactured gas plants made flammable gas for heating and lighting in the United States and Europe until the 1950s.

According to the study, there are an estimated 36,000 to 55,000 former manufactured gas plants and related facilities in the U.S. Most of these plants produced waste tars that contain many known or suspected carcinogens and are challenging to remove. Cleanup for one site typically costs millions of dollars, according to the U.S. Environmental Protection Agency.

“Our work demonstrates a physicochemical method that is relatively inexpensive to apply, and that effectively and efficiently mobilized tars that were trapped under capillary forces,” said Cass Miller, Ph.D., the Daniel A. Okun Distinguished Professor of environmental sciences and engineering at the UNC Gillings School of Global Public Health. “Because of the large number of tar-contaminated sites that exist and the economical nature of this proposed method, abundant opportunities exist for field-scale applications of the method.”

Challenges of remediation

As the researchers explain, remediation methods must take into account the complex characteristics of manufactured gas plant tars, which contain thousands of compounds and vary in composition from site to site. The tars don’t mix with water and, because they are also denser than water, they can move down through the water table until reaching a confining layer such as silt or clay. The tars can also become trapped in porous materials via capillary forces and can be non-Newtonian fluids, meaning they don’t behave like water or other similar fluids. “In short, tars contain virtually every complication that one could conceive of in a subsurface and contaminant,” Miller said.

The researchers investigated mobilizing pore-trapped tars with alkaline agents, because these chemicals are readily available, inexpensive, and exhibit properties beneficial for contaminant mobilization. To test this method, they injected tar into sand-filled glass columns, which replicated natural mineralogy. Flushing the columns with alkaline solutions of sodium hydroxide (NaOH) removed up to 44 percent of the tar, and adding the polymer xanthan gum to the NaOH solution boosted the tar removal up to 93 percent. Other experiments showed that solutions of NaOH significantly reduced the forces that trap the tar in porous material.

The researchers say that the tar removal likely occurs because of reduced interfacial tension between the tar and water, due to stable displacement of the tar, and because the trapped tar formed a more connected phase. The team is continuing to evaluate the chemical mechanisms involved in the mobilization technique and is performing additional experiments in heterogeneous three-dimensional systems typically found in subsurface environments. They are also investigating ways to remove the small amount of residual contamination that remains after mobilization.

Citation: Hauswirth SC, Birak PS, Rylander SC, Miller CT. 2012. Mobilization of manufactured gas plant tar with alkaline flushing solutions. Environ Sci Technol 46(1):426-433.

(Nancy Lamontagne is a science writer with MDB, Inc., a contractor for the NIEHS Division of Extramural Research and Training, Superfund Research Program, and Worker Education and Training Program.)

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