Dr Dustin McIntyre | Using Laser-Induced Breakdown Spectroscopy to Detect Rare Earth Elements and Monitor Carbon Sequestration

Whether identifying contaminants in our water courses or searching for the rare minerals our modern technology depends on, being able to monitor the natural world in real-time has never been more critical. Dr Dustin McIntyre and his colleagues from the US Department of Energy’s National Energy Technology Laboratory have been working to understand better the impact of some of our most vital industries using novel approaches in laser-induced breakdown spectroscopy.

Delicate Technology in a Harsh Environment

Environmental monitoring is essential for understanding and protecting the natural world. Unfortunately, many current monitoring technologies are delicate, complex, and unsuited to being taken into the field. For instance, sensors and spectrometers typically require careful handling and calibration to ensure accurate readings. Traditional analysis methods often require samples to be returned to a lab and prepared to measure their chemical makeup, making them inappropriate for real-time monitoring.

Dr Dustin McIntyre and his team at the National Energy Technology Laboratory (NETL) saw an opportunity to deploy laser-induced breakdown spectroscopy (LIBS) outside of the lab to monitor groundwater contamination and detect rare earth elements – both high priorities in the development of low carbon and negative emissions technologies. This interdisciplinary team set out to use their expertise across spectroscopy, chemistry, and environmental science to develop a portable sensor that could meet the challenges researchers and environmental monitoring programmes face.

The Benefits of Laser-induced Breakdown Spectroscopy

Dr McIntyre identified LIBS as a strong candidate for improving in situ analysis as the technology requires very little sample preparation and provides results instantaneously. As the name suggests, LIBS works by sending a high-energy laser pulse at a sample to create a tiny, intense flash of plasma. The plasma emits light that can be analysed to determine the composition of the material. This methodology lends itself to in situ analysis across a wide range of applications, as LIBS works on solids, liquids and gases.

While previous research had demonstrated that LIBS can work well underwater, no one had developed specialised sensors for environmental monitoring and detection. Dr McIntyre suspected that LIBS could provide the level of accuracy required for environmental monitoring under field conditions with the right design features.

A New Sensor Design

Dr McIntyre and his team designed a prototype LIBS sensor that could handle being used in the field. They used a special type of laser called a passively Q-switched microchip laser. This laser was chosen for its simplicity and reliability. The design included a fibre optic cable that separates the sensor from more delicate equipment, keeping expensive parts safe on the surface. In contrast, only the robust sensor head is exposed to harsh conditions where it interacts with the sample, such as underground or deep underwater.

Once the design was finalised, the team assembled a prototype using off-the-shelf components. The first step was to test if the new sensor was sensitive enough for two key applications: detecting rare earth elements, and detecting contaminants in groundwater associated with CO₂ leakage from geological carbon storage (GCS).

Detecting Rare Earth Elements

Rare Earth Elements (REEs) are essential for various high-tech applications, including electronics, lasers, and batteries. The increasing demand for these materials has raised concerns about their ongoing availability. Additionally, current methods of extracting REEs have a high environmental and economic cost. Dr McIntyre saw the opportunity to use their LIBS sensor to improve the detection of REEs, supporting the industry in finding new sources and optimising extraction from existing sources.

To test the prototype sensor’s ability to detect REEs, Dr McIntyre ran a series of lab experiments on liquid and solid samples from acid mine drainage hydraulic fracturing produced water sources. The team used Europium (Eu) and Ytterbium (Yb) in varying liquid concentrations and solid pellets. The team were excited to find that their sensor was incredibly effective at accurately identifying concentrations of Eu and Yb and could detect these REEs in concentrations as low as 1 part per million. These results demonstrate that Dr McIntyre’s LIBS sensor was comparable to or better than those achieved with traditional laboratory systems and showed great potential for field deployment.

Estimating CO₂ Leakage from Groundwater Contamination

Alongside REE detection, Dr McIntyre’s team wanted to test how well their sensor could detect groundwater contaminants. In particular, those associated with geological carbon storage. GCS involves injecting CO₂ into deep rock formations, oil fields or mining sites to remove it from the atmosphere. A potential barrier to the widespread use of this relatively new technique is the risk of CO₂ leaks. If CO₂ leaks occur, they can contaminate groundwater supplies with metals and acids found in the injection site. These contaminants can pollute aquifers, vital for drinking water and agriculture.

Dr McIntyre predicted that if his new LIBS sensor could detect dissolved metals in groundwater, such as potassium (K), calcium (Ca) and strontium (Sr), it could be used to indirectly detect CO₂ leaks and monitor the impact of injection projects on groundwater.

Dr McIntyre and his team tested their prototype sensor’s ability to detect and quantify K, Ca and Sr in aqueous solution, simulating the underwater conditions found in the field. Like the REE experiment, the team was ecstatic to see that, after some optimisation, the sensor would accurately measure the concentration of the tested dissolved metals. They also showed that a LIBS approach was comparable to, or better than, traditional lab-based methods.

Assessing CO₂ Injection Sites

When CO₂ is injected into a storage site it produces carbonic acid when it contacts water in the subsurface, it can react with the surrounding rock and affect its ability to store the gas, risking a leak and further site degradation. At high CO₂ and low pH conditions, metal carbonates found in the rock, such as CaCO₃, dissolve into groundwater, changing the structure of the rock and potentially causing contamination. The dissolution rate of carbonates is a key metric in understanding how an injection site reacts to the CO₂.

After the successes of their initial experiments on dissolved metals, Dr McIntyre wanted to demonstrate that LIBS had real practical application potential. He saw that if he could demonstrate that his LIBS sensor could be used to measure the dissolution rate of carbonates in real-time, it could provide an easy and convenient tool for assessing and monitoring CO₂ leak risks during GCS projects.

The team set up another series of lab experiments to study the dissolution of CaCO₃, followed by further experiments on three other common mineral carbonates (MgCO₃, MnCO₃, SrCO₃) under varying CO₂ pressures (ambient to 250 bar). Again, the LIBS sensor was used to great effect. These experiments showed that the methodology for underwater LIBS could track the changing metal concentrations as the carbonates dissolve under a range of practically relevant conditions and to a high degree of accuracy.

The NETL team’s work to develop and optimise LIBS across these projects provides a foundation for the application of LIBS in environmental monitoring, carbon management, and resource exploration. The researchers remain committed to bringing this novel approach out of the lab and into real-world application.