By: Christina Dierkes, Ohio Sea Grant
Removal of polluted sediment from lake and river bottoms can be costly and time consuming. Ohio Sea Grant researchers are developing a new method using ultrasound and chemical agents that bind to contaminants and render them inactive on the river bottom. The new approach means larger quantities of sediment can be scrubbed more thoroughly with each round of treatment, potentially making pollutant clean up faster and less costly. The overall goal is to treat contaminated sediments right where they are instead of having to dredge them up for treatment or disposal.
The work of Dr. Linda Weavers and her team at The Ohio State University has recently progressed from experiments where the contaminated sediment samples are mixed into water to one where the sediment has settled into the bottom of a glass column, closer to how they would find contamination in an actual river or lake. Dr. Weaver’s team is using a basic model of a river cross-section in their lab to test the new methods of contamination removal.
“When we looked at our bromide tracer in the water, we saw it spread through the sediment more with ultrasound than without,” said Weavers, professor in the Department of Civil, Environmental and Geodetic Engineering at Ohio State. “So if you think about the bigger picture, if we look at our contaminants, we have a better ability to reach more spaces within the contaminated sediment and are able to affect a larger area.”
The ultrasound treatment creates cavitation bubbles in the water around sediment particles that carry contaminants. As those bubbles form and collapse, the tiny shockwaves produced in the process act a little like a pressure washer, pushing heated water against the sediment particles and removing stuck-on contaminants. In addition, the pressure waves created by the ultrasound also spread apart the sediment particles so water can reach deeper into the sediment.
Currently, Weavers and her students are working on sediment samples that were provided by the Ohio Environmental Protection Agency (OEPA). Those samples contain polycyclic aromatic hydrocarbons (PAHs, for short), which are a major problem for Lake Erie and its tributaries.
“We have these contaminated sediments from a former creosote facility, which is one of the sources of PAH contamination,” Weavers said. “If you think of wood telephone poles, they don’t decay even though they’re made of wood, and that’s because they’re treated with creosote, which is heavy in PAHs.” Another example of creosote compounds is the black tarry sealant used on many driveways.
More information, including details on a potential way to inactivate contaminants once they’re removed from the sediment particles, is available in the Winter/Spring 2017 issue of Ohio Sea Grant’s Twine Line magazine, available directly at ohioseagrant.osu.edu/p/l0ewl/view#page=10 (PDF link) or at ohioseagrant.osu.edu/products/twineline.