Environmental engineering researchers in the US have developed a two-part approach for cleaning up toxic chlorinated solvents spilled into underground water supplies from former dry cleaning and industrial operations.
The patent-pending technique, which uses a macroemulsion composed of alcohol and food-grade surfactants, is said to simultaneously reduce the density of the pollutant, to keep it from sinking farther into the groundwater, and helps separate it from soil particles so it can be flushed out.
Known as ‘density modified displacement,’ the approach could cut the cost of environmental remediation by reducing both the time required for clean up and the amount of contaminated effluent that must be treated.
Researchers from the Georgia Institute of Technology, the University of Michigan and the University of Oklahoma participated in the research, which was sponsored by the US Environmental Protection Agency (EPA).
‘We’re trying to make remediation of contaminated groundwater more efficient, because it is now largely driven by economics,’ said Kurt Pennell, an associate professor in Georgia Tech’s School of Civil and Environmental Engineering. ‘The idea is to make this process so efficient that the cost of cleaning up a site is less expensive than traditional approaches which rely on groundwater extraction and long-term monitoring.’
The technique offers a new approach to removing dense non-aqueous phase liquids (DNAPLs), including tetrachloroethane (PCE), trichloroethene (TCE) and chlorobenzene (CB), which are heavier-than-water compounds. Relatively stable chemicals that don’t readily degrade, their concentrations in groundwater must be kept to a few parts-per-billion (ppb) to meet environmental standards in the US.
Established remediation techniques (pump and treat) often rely on pumping large amounts of contaminated water out of the ground, flushing the pollutants with it. However, these techniques require large volumes of water and may need to be operated for many years. The cost of treating the contaminated water and the time required make this approach very expensive and serves only to contain the contaminated groundwater.
The approach developed by the Georgia Tech team could allow remediation engineers to directly address pollution mass removal, with recovery rates exceeding 90 percent.
‘Our approach is to aggressively treat the source zone where the actual spill occurred and remove the compound, then separate and treat or recycle it above ground,’ Pennell explained. ‘We are trying to remove the long-term source of groundwater contamination in a manner that will produce the most results for the least cost. It’s generally not economically feasible to treat the entire aquifer.’
One issue that all such solvent remediation efforts must address is containing the problem. Because these chlorinated compounds are heavier than water, removal efforts can inadvertently drive them deeper into the ground if underground water supplies lack natural boundaries such as impermeable clay or bedrock.
To keep the dense DNAPLs from flowing deeper into the earth, engineers have injected alcohols such as n-butanol into the pollution mass to lower its density. They have also injected surfactant chemicals, which reduce the interfacial tension to separate the solvent from soil particles.
The patent-pending technique developed by Pennell and collaborators C. Andrew Ramsburg, Tohren C.G. Kibbey and Kim F. Hayes combines both approaches, making the compounds lighter and their removal from the soil easier.
In laboratory studies using a test cell to simulate underground conditions, the researchers first flowed their macroemulsion through a layer of trichloroethene, then flushed it out with water, removing 93% of the solvent. Most of the surfactants and alcohol are removed from the soil; the small amounts that may remain will encourage biological processes to break down remaining contaminants over a longer period of time, Pennell said.
In addition to the alcohol, the process uses a variety of surfactant chemicals, including Polysorbate-20, Tween or Span. Because they are produced in large volumes for other purposes, these materials can cost as little as $1.50 per pound.
Based on laboratory results and interest from the remediation industry, Pennell hopes to attract a partner to begin field testing the technique.