Spectroscopy-Based Tool Detects, Measures Contaminants in Landfills

A method known as laser-induced breakdown spectroscopy (LIBS) could offer a cleaner, faster and simpler approach than existing technologies for detecting contaminants in the fluids coming from landfills. Although conventional LIBS presents some limitations when used in the single-pulse configuration, the use of LIBS in the double-pulse (DP) configuration demonstrated rapid detection of mercury (Hg) and could potentially be applied to other contaminants.

Targeting a leachate sample with a high-intensity double-pulse laser generates an extremely hot plasma. The light emitted by the plasma is then analyzed to assess the quantity of mercury present in the sample. Courtesy of Gustavo Nicolodelli, Embrapa Instrumentation.

In LIBS, a sample is targeted with an intense laser pulse, which generates a very hot plasma. The light emitted from the plasma is then captured and measured by a spectrometer, which can be calibrated to detect the chemical signatures of specific contaminants.

Researchers at the Brazilian company Embrapa Instrumentation applied LIBS to the qualitative and quantitative analysis of Hg in landfill leachates. They used a DP setup in which a series of two laser pulses targeted the sample, generating a more intense plasma than single-pulse and thus increasing the sensitivity of this approach.

The DP LIBS approach also made it possible for researchers to use a different emission line to detect Hg. The emission line near 253 nm is often used to detect Hg, but when iron is also present, the iron emission line can cause interference at 253 nm, necessitating complex data analysis to separate the Hg fingerprint from that of iron. Use of the DP laser made it possible to observe a different Hg emission line near 194 nm, thus avoiding interference with the iron emission line.

The team tested their system experimentally using leachate that had been laced with Hg. The lowest mercury concentration detectable in their tests was 76 ppm. The researchers said further refinements should allow detection of lower levels of Hg, ultimately to 5 ppm or below, in order for the system to be useful for ensuring compliance with legal standards. In validation experiments, the system showed an average error of about 20 percent, which the researchers said should be satisfactory for quantifying Hg in landfill leachate.

The Embrapa study is the first to apply LIBS to the detection of Hg in landfill leachate.

“This was the first time that the double pulse LIBS was applied to measure mercury in a solid sample,” said researcher Carlos Menegatti. “It is well-established in the literature that double-pulse LIBS has more sensitivity than the single-pulse LIBS, so we have achieved better detection limits in solid samples than previous work.”

The researchers plan to further refine the LIBS instrumentation to improve the ability to detect Hg at lower concentrations and to more accurately quantify the amount of Hg present. Although Hg was the focus for this proof-of-concept demonstration, the system could be calibrated to measure the chemical signatures of contaminants other than Hg.

“This concept can be applied to other chemical elements,” said Menegatti. “Depending on the type of sample, you can choose more appropriate lines to avoid interference in the spectrum caused by the emission lines of other elements.

“LIBS is an environmentally clean technique that is free of chemical residues, compared to standard reference techniques currently used for the same type of analysis,” he said. “Moreover, LIBS is a much faster technique and does not require pre-preparation of the samples.”

The research was published in Applied Optics, a publication of OSA, The Optical Society of America (doi: 10.1364/AO.56.003730).