Power Play

If you still read texts the old-fashioned way, hold one up in front of you and look at it. Really look at it—trace the smooth texture of the paper, feel the weight of it in your hands. What will happen to this collection of colorful paper when you are finished with it? Will it go into a recycling bin? Or will it just become another item in the trash can, unidentifiable from the more than 250 million tons of garbage generated annually in the United States? The average person doesn’t think much about it. But Sahadat Hossain, professor of civil engineering, does. Dr. Hossain thinks about trash a lot.

As the world’s population continues to grow and urbanize, more and more trash is being generated, and all that waste has to go somewhere. Dealing with it improperly can lead to diseases like malaria, dengue fever, and cancer; poor air and water quality; and even violence, as seen in Naples, Italy, in 2008 and 2010 when citizens rioted over the planned opening of a new landfill.

To help address these global issues, The University of Texas at Arlington established the Solid Waste Institute for Sustainability (SWIS) in January 2015 to provide cities around the world with better solutions for common operational issues. Current members include Denton, Irving, and, further afield, Addis Ababa, Ethiopia. Hossain anticipates agreements soon with three more Texas municipalities.

“This is a win-win industry,” says Hossain, who serves as director of SWIS. “This is the only business where people pay to give you the raw material. Any other industry, if you need raw material, you have to buy it. But people are throwing garbage into the landfill, and they’re paying $40 per ton.”

As environmental and ecological threats continue to increase globally, finding sustainable solutions to problems like energy generation and consumption is becoming ever more pressing. The SWIS is just one example of the ways UTA is tackling the problem of renewable energy. In addition to Hossain’s work with landfills, two groups of engineers are making breakthroughs in converting a greenhouse gas into solar hydrocarbon fuels.

TRASH TROUBLES

Since 2009, Hossain has worked with the city of Denton on a series of projects to optimize its ECO-W.E.R.C.S. landfill. He initially converted it into a bioreactor with enhanced leachate recirculation (ELR). Adding regulated amounts of water can speed up the decomposition of refuse and the production of methane. In turn, that excess methane can be converted into energy, powering buildings and generating revenue for the city. Denton’s bioreactor/ELR landfill—the first in Texas—currently produces electricity for 1,600 homes; the city hopes to add two generators to triple that figure.

But landfills as they currently operate have limited lifespans, approximately 25-30 years. At its most basic, a landfill is a lined cell filled with a random assortment of trash until it’s full, at which point the cell is covered and closed before the process restarts with the next one. The landfill operation continues this way until all of the acreage has been used. But then what?

That’s where Hossain’s newest project with Denton comes in: landfill mining. Supported by a three-year grant from the city, he is examining landfill samples to see what remains of the previously buried trash and whether it can be removed and recycled in some way.

The bigger idea behind the project, however, is to eventually reclaim the physical space once a majority of the refuse has decomposed and gas production has declined—which, in an optimized bioreactor, takes only 10-15 years. So, for example, if the landfill were divided into four parts and it took 7-8 years to fill up and close one cell, then when you were finished with the third cell all of the waste in the first would have mostly decomposed. When you began filling the fourth and final cell, you would “mine” the first, readying it to take new trash when the final cell closed.

“If we start mining one while filling another, we’re recycling the landfill itself,” Hossain says. “It’s rotating again and again. You could run it for 150 years.” He expects to begin mining in Denton this fall or early next year.

SWIS is also working heavily with emerging nations. In the United States, food accounts for just 14.6 percent of the material thrown away, per the Environmental Protection Agency’s 2013 figures. (Paper is the most common material, at 27 percent.) In developing countries, it’s often much higher—in China, for example, it accounts for up to 70 percent. This is notable because high percentages of food waste create a natural bioreactor that already contains enough moisture for quick decomposition. In many of these nations, that waste either isn’t collected or it’s haphazardly deposited at open dumps, which ultimately adds methane and carbon dioxide to the air.

Hossain says it has been a challenge to convince officials in these countries that the initial investment in a waste management program will see a return. He believes that one way to overcome this hurdle is to educate young entrepreneurs about the social and economic benefits of proper waste management. To facilitate that goal, this past January UTA hosted the International Solid Waste Association (ISWA)’s Winter School on Solid Waste Management, a two-week training program that gathered industry professionals and graduate students from 27 different countries. It was such a success that the ISWA asked UTA to host its Winter School in 2017, too.

“The potential for these countries is tremendous,” Hossain says. “You’re not only generating electricity, you’re not only collecting the garbage, but you’re cleaning the city. If you’re cleaning the city, you’re reducing a lot of disease. So along with this environmental solution, you’re providing a public health service.”

HYDROCARBON HYPOTHESIS

Last year in Paris, more than 190 countries pledged to help prevent global temperatures from rising more than 1.5-2 degrees Celsius by reducing their greenhouse-gas emissions. But the international agreement doesn’t go into effect until 2020; in the meantime, carbon dioxide levels continue to rise to dangerous heights. One prominent researcher has put the “safe” level of carbon dioxide at 350 parts per million, a number the Earth passed in the late 1980s.

Investing in more environmentally friendly technologies and employing more energy-efficient practices will likely not be enough to reach the Paris Agreement goals, especially because we don’t have a viable alternative to the fossil fuels that drive our economies, transport our goods, and grow our food.

“We can’t turn off the oil pumps tomorrow without starving a huge fraction of the world’s population,” says Frederick MacDonnell, chair of the Department of Chemistry and Biochemistry. “So no matter how green we get, we’re still going to be burning fossil fuels for many years to come. But the need to develop technologies to replace them is urgent.”

Dr. MacDonnell is one of a growing number of researchers pursuing technologies that won’t add more carbon dioxide to the atmosphere. He and two other UTA professors are working on what they see as a feasible process: using the sun and carbon dioxide pulled from the atmosphere to create hydrocarbon fuels.

“What do we do with all the accumulated carbon dioxide?” asks Krishnan Rajeshwar, Distinguished Professor of Chemistry and Biochemistry. “One of the ideas is to do a value-added conversion by turning a greenhouse gas into something that can be used as fuel. That’s an exciting area of research.”

He and Brian Dennis, professor of mechanical and aerospace engineering, are collaborating on a project to develop hybrid photoelectrode materials that would improve the efficiency and cost-effectiveness of solar fuel generation through carbon dioxide splitting.

Liquid hydrocarbon fuels would offer one very significant benefit over hydrogen and fuel cell technologies: The infrastructure to distribute and use them is already in place. Today’s cars and planes are primed to handle them like they do gasoline—but without the harmful environmental impact.

Dr. Rajeshwar, Dr. Dennis, and MacDonnell are the current core members of UTA’s Center for Renewable Energy and Science Technology (CREST). The center—founded in 2004 by Rajeshwar and Richard Billo, former associate dean for engineering research—has allowed the trio to apply their diverse expertise to energy research in a more comprehensive way.

“Many problems we have in energy require multiple points of view and disciplines working together to achieve solutions,” says Dennis, co-director of the center with Rajeshwar. “What’s unique about our approach is we’re actually looking at not just doing fundamental research, but also research that’s going to eventually lead to something that can be commercialized. And it takes people with different backgrounds in order to get to that point.”

COPPER CALCULATIONS

A major obstacle to any alternative energy technology is cost. Researchers around the globe are still trying to understand the mechanisms of carbon dioxide conversion as a pathway to boosting fuel output and making solar fuels economically viable.

“It all boils down to dollars and cents,” Rajeshwar says. “The challenge is to match what the fossil fuel prices are because, ultimately, the consumer really only cares about what he or she is going to have to pay.”

To that end, he is attempting to develop more efficient photocathodes under the umbrella of a separate four-year, $360,000 National Science Foundation (NSF) grant.

One promising avenue he has pursued with colleagues at UTA and in Hungary is a copper oxide/copper interface, where a copper oxide film is grown on copper foil. Although copper is in demand, it is both earth-abundant and non-toxic, and it can perform in both photoelectrochemical and electrochemical processes—i.e., with or without the use of light.

“Copper works amazingly well with the oxide being present, converting carbon dioxide into as many as 16 different products, including methanol, ethanol, and isopropanol,” Rajeshwar explains. “No other metal has shown this diversity.”

He and his colleagues have taken the copper oxide/copper and blended it with highly conductive carbon nanomaterials to create hybrid electrodes. They have experimented with a multi-step electrodeposition process that coats carbon nanotubes and graphene foam with copper oxide nanoparticles to analyze how the composition, crystal structure, and morphology affect properties and performance.

Their nanocarbon/inorganic semiconductor composites have shown five times higher electrical conductivity and also nearly tripled the photocurrent values used to split carbon dioxide when compared to copper oxide. The hybrids also are more stable than the copper oxide alone.

“You want to keep the ratio of the copper and copper oxide constant,” Rajeshwar says. “We are trying to implement a self-repair mechanism in the copper oxide because it is not completely chemically stable under light. The carbon phase is essentially regenerating the material.” This stabilizing component would help extend the life of the device.

Dennis has worked with the team to develop a reactor incorporating these materials that can demonstrate the production of alcohols. Gas chromatography mass spectrometry analysis has detected the formation of liquid fuels such as methanol and ethanol in their samples.

These hybrid photoelectrodes show promise, but there are myriad possible combinations out there to be explored, and Rajeshwar says they are not wedded to any particular material, copper or otherwise: “If tomorrow, one of my collaborators comes and says they’ve found something cheaper, better, we’ll switch in a heartbeat.”