ORNL laser technique tests for 'grassoline' use, weather conditions of the past

Which varieties of switchgrass have sufficient biomass and low enough levels of ash to be turned into high-quality ethanol, a biofuel for cars sometimes called “grassoline”? Can tree rings from trunk cores tell you which years had long droughts, excessive precipitation or normal amounts of rain to provide information on climate trends?

How are trees affected by forest fires and do trees damaged by controlled burns recover? Is the presence of certain elements important for the functioning of genes responsible for plant growth and a high yield of biomass suitable for production of renewable biofuels to replace non-renewable petroleum fuels?

These are some of the questions that have been addressed by Madhavi Martin, leader of the Biomaterials and Biomass Characterization Group in the Biosciences Division at Oak Ridge National Laboratory and her team in research described recently in her talk to Friends of ORNL. For 20 years she has helped shed light on these scientific issues through her use of laser-induced breakdown spectroscopy (LIBS).

It has enabled her to identify and measure the relative amounts of chemical elements and to detect changes in the composition of biological and environmental samples. Examples include insects, soils, plant roots and tissues, tree rings and wood.

In one application of LIBS, she detected metals present in polluted wastewater by analyzing the exoskeletons and endoskeletons of insects in, near and far away from the source of pollutants in wastewater. Her analysis of insects determined that the water was contaminated with potentially toxic copper and mercury.

How does laser-induced breakdown spectroscopy work?

Martin, a senior staff research scientist at ORNL, is a native of India who aspired to be a doctor as a teen. A teacher discouraged her from pursuing a biology degree after she fainted while dissecting a frog in high school. So, her physicist father encouraged her to study physics. She earned four degrees in the field, including two in India and a master’s degree and a doctorate in solid-state physics from the University of California at Los Angeles.

She explained how LIBS works. Pulses of a wide beam of laser light are focused onto a tiny area of a target material, ablating the sample surface and breaking the chemical bonds of the eroded material that forms a spark-like plasma. As the excited atoms and ions in the plasma decay to their original state, they emit wavelengths of light unique to the elements present, just as fingerprints are unique to each person.

The unique emission spectrum passes through an optical fiber to a spectrometer. Its grating separates the wavelengths and sends the optical information to a detector, which converts it to electrical signals that carry digital data to a computer. Its diagnostic software displays the full wavelength spectrum as a series of peaks with different heights and distances from each other. In this way, all the elements present in the sample are identified and quantified.

Martin and her team have used LIBS for determining the viability of varieties of switchgrass as biomass that can be used to produce high-quality biofuels. Switchgrass has been studied for years at ORNL as a candidate for making ethanol, the biofuel currently being made from corn for cars. Unlike corn stover, switchgrass can grow on millions of acres of unfertilized U.S. lands that cannot support crop or food production.

Unfortunately, switchgrass varieties have different qualities of biomass, making only some suitable enough for the production of the high-quality biofuels cars require. The problem is that some switchgrasses are high in ash, which can cause abrasive wear in biomass processing equipment and other problems.

Switchgrass, which is being converted to biofuel experimentally at the pilot plant stage (as in Vonore, Tennessee), is not yet profitable as a feedstock like corn because many varieties have lower biomass.

Another reason why, Martin explained, is that “because it’s a wild grass that grows close to the soil, it’s picking up more elements from the soil, making it harder for a biorefinery to manufacture a high-quality biofuel.” She added later that LIBS is a good technique for commercial biorefineries to have on hand to ensure that the biomass received has a high enough quality for production of saleable biofuels.

Martin’s team used LIBS to study 73 pellets, each formed at high pressure for 30 seconds from 100 milligrams of powdered switchgrass. The pellets were made of different switchgrass varieties. The team showed that the LIBS technique can not only scan the surface, but also “dig deeper into the heterogeneous material” to obtain a spectrum and detect the elements throughout the pellet since some elements can be forced to the bottom of each pellet in the process of palletization.

“We wanted to make sure we could get a depth profile of each pellet’s elemental content and see how the spectrum changes,” she added.

The ORNL team used LIBS to scan switchgrass leaves grown in a silver solution, showing that it entered the leaf veins, and then a palladium solution that was seen to spread throughout the leafy parts of the grass. By examining spectra changes, the team could discern which switchgrass varieties tend to have high ash content, medium ash content or low ash content. Analysis of the spectra showed differences in the concentrations of elements classified as ash and nutrients, including calcium, magnesium, sodium and zinc (high ash content) and aluminum, iron and manganese (medium ash content).

The LIBS analysis by the ORNL team revealed that the principal components of switchgrass are silicon from the soil, the nutrients potassium, calcium, and magnesium, as well as phosphorus and sulfur. The three-dimensional information that LIBS provides on the differences in the composition of the switchgrass pellets can guide the selection of parts of different switchgrass varieties that together can make a good feedstock amenable to being converted to a high-quality biofuel, Martin suggested.

Detecting weather conditions of the past with tree chemistry?

To determine if LIBS could detect weather conditions through analysis of tree chemistry, Martin’s team used it to analyze the elemental composition of each of 35 tree rings in slabs of wood cut from felled hardwood trees that had been part of the Walker Branch Throughfall Displacement Experiment (TDE), which was started in 2001 at ORNL. The samples came from chestnut oak, black gum, red maple and white oak trees grown in three treatment plots that subjected the trees present to ambient, wet or dry conditions. The dry plot’s troughs caught the ambient rain, keeping it from seeping into the soil and transferring it to the wet plot.

The LIBS spectra obtained enabled the identification of six critical nutrients in the four tree species: calcium, potassium, magnesium, sodium, nitrogen and phosphorus. The differences in the relative concentrations and distributions of the elements in the samples of the four different woods were compared over 13 years of treatment manipulations.

For example, the chestnut oak samples from the three different plots that exposed the trees to constant ambient, wet or dry climatic conditions were found by LIBS to contain the same elements. But their concentrations varied, and they were clustered differently, Martin said.

When asked if she used LIBS to determine whether some trees along mercury-contaminated East Fork Poplar Creek in Oak Ridge contain any mercury, she affirmed that LIBS scans of 58 samples from EFPC trees revealed the presence of mercury.

Her team used LIBS to analyze wood samples from trees before and after controlled burns of forests needed to make it easier to contain future wildfires. They found that the chemical content of the partially burned trees had changed because they picked up iron and sulfur from the soil and their calcium and sodium concentrations increased.

“Sometime after the fire event, we used LIBS on samples of burned trees that didn’t die and found that the chemical composition returned to their pre-fire state,” Martin said. “That’s good because our results told us that the damaged trees are growing again and will become healthy.”

Martin is collaborating with biologists to explore the roles of elements and genes in producing biomass in poplar trees, an application of the new field of ionomics, which connects elements to gene function. Using LIBS, her team found that phosphorus and copper are two of the important elements present in poplar trees.

Martin and her colleagues have also used LIBS to detect in plants the nutrient magnesium and tiny, sand-like silicon deposits called phytoliths. These deposits result from silicon accumulation found to be caused by a certain gene. The phytoliths form an enduring structure that traps carbon, thus increasing soil carbon storage.

Enhanced carbon sequestration, said Martin, will help slow climate change.

This article originally appeared on Oakridger: ORNL laser technique tests for 'grassoline' use, weather history