Excimer irradiation can clean up tainted soil

01 Sep 2017

High power beam can break down toxic waste, promising better soil remediation, says Northeastern University research group.

Instead of washing or solvent-based methods, laser irradiation offers a new and improved method of ridding contaminated soil of toxins and pollutants. A team at Northeastern University, Boston, MA says that by directly breaking down pollutants, a high power excimer laser can now be more efficient and cheaper than conventional decontamination techniques.

“Other methods are either costly, labor intensive, have low efficiency, or take a long time," said Ming Su, an associate professor of chemical engineer at Northeastern. With two of his graduate students, Wenjun Zheng and Sichao Hou, Prof. Su has shown how such a laser system could work, describing the proof-of-principle in the Journal of Applied Physics.

The biggest advantage of lasers, says Su, is that they can be used at the site of decontamination. Many conventional decontamination methods require digging up contaminated soil, hauling it somewhere else to be cleaned, and then returning it -- a process that is expensive and time-consuming.

Current methods also have shortcomings: washing doesn't eradicate contaminants and it leaves the problem of contaminated water; organic solvents can be harmful and the process can create byproducts that become secondary contaminants.

Soil vapor extraction, in which air is pumped into the ground to remove volatile organic compounds, only works on permeable or homogeneous soils. Biological approaches only work for low concentrations of certain contaminants.

Laser advantages

Lasers, however, can be used on-site to completely break down contaminants. "There is no other method that can do it at such high efficiency," Su said.

To demonstrate that the new method is feasible, the researchers tested it on a simulated soil made from porous silica. They contaminated their artificial soil with a carcinogenic chemical called DDE, which is a product of DDT, the carcinogenic pesticide that was banned in the U.S. in 1972. The DDE molecules fluoresce under ultraviolet light, making them easier to detect.

Almost immediately after directing a high-powered infrared laser on the contaminated artificial soil, the glowing ceased – indicating that the DDE was no longer present.

To remove the harmful substance, the laser light heats up the pollutant locally, reaching temperatures of thousands of degrees Celsius. This heat is sufficient to break the chemical bonds of the pollutant, fragmenting DDE into smaller, safer molecules such as carbon dioxide and water.

Prof Su says that, in principle, lasers should be able to work on all types of contaminants, from organic compounds to metal ions. But first, the researchers need to do more experiments with other contaminants. Future studies also need to involve more careful analysis to determine whether all of the contaminant is broken down sufficiently to meet environmental standards.

Eventually, Prof Su envisions a multi-laser system carried on the back of a truck: “The laser light, channeled through fiber-optic cables that penetrate the soil, could perhaps couple to a plow that loosens the dirt, better exposing it to the laser light,” he said.

Paper conclusions

The Journal of Applied Physics paper reports the feasibility study of using a high power excimer laser to remove organic contaminants from porous materials using a focused laser beam. It states: “Laser produced lithographic patterns are used to offer quantitative results regarding power, speed, frequency, and energy efficiency.

“A heat transfer model is created to simulate and predict the interactions between the laser and the organic material. The simulation and experimental results are in good agreement with each other. Given the high efficiency of laser energy conversion, this method may be useful for large scale in situ soil decontamination.”