Light for clean water: Nano-enabled photonics technologies might ease the drought

Fresh Water is getting scarce. Population growth and industrialization have led to a global six-fold increase of fresh-water consumption in the last 100 years. Exacerbated by droughts, many areas of the world suffer serious water shortages, which increases food insecurity and health risks, drives migration, and fuels social tensions.

Half a billion people are considered to face severe water scarcity year-round. The 2030 Agenda for Sustainable Development, adopted in 2015 by all United Nations member states, acknowledges this by naming clean water as one of 17 goals. Water also needs to be clean when it is released back into the water cycle to achieve two other UN sustainability goals: protecting life below water and reaching zero hunger.

Photonics researchers like Stephanie Loeb, an assistant professor for environmental engineering at McGill University, have been working on both sides of the problem to alleviate water stress: purifying unsuitable water and keeping pollutants from entering the water cycle. The technology for purification before use depends on the water source, the underlying pollutants, and the available infrastructure. For example, brackish water needs to be cleaned of excess salt, while surface water from a lake might need stronger disinfection.

For her doctoral research during the late 2010s at Yale University, Loeb wanted to develop a water-disinfection device that employs nanoparticles to harvest the power of sunlight. Sunlight had been identified as a disinfectant by the end of the 19th Century. The first large-scale application of its antiseptic wavelengths—the ultraviolet (UV)—for water treatment opened in 1906 in Marseilles, France.

Since those early days, many communal water-treatment facilities—like the Groundwater Replenishment System in Orange County, California, one of the world’s largest water-purification systems for potable use—have relied on electric discharges in fragile glass tubes filled with toxic mercury vapor to produce the desired UV light for disinfection. In 2017, Loeb wanted to see if UV light-emitting diodes (UV-LEDs) could do a better job. They have several advantages over conventional lamps, for example, a less-fragile housing, no toxic mercury, and they do not need to warm up. But UV-LEDs were expensive and had too little power.

In the last ten years, this technology has seen what Loeb calls “incremental advances of efficiency,” a slow but steady process. As it turns out, the German gas and water industry trade association is currently preparing a testing standard for the certification of such equipment. They hope to get approval from Germany’s environmental protection agency by 2026, before the European Union phases out mercury in UV-lamps completely. Also, water bottles with integrated UV-LED lights have reached the consumer market. The Orange County Water District, however, is still waiting for cheaper LEDs.

For communities without centralized water treatment looking for low-cost and low-maintenance point-of-use solutions, solar disinfection is promoted as a practical method: The water is exposed to sunlight in transparent containers for a couple of hours. The method needs almost no investment, but outcomes are unpredictable. Boiling disinfects water consistently, but doing so with solar radiation directly is impractical, and burning fuel is not desirable. Can novel materials and technology help?

SolMem’s parabolic trough. Photo credit: Qilin Li

Loeb took a closer look at that question as another part of her doctoral studies. When light hits the surface of a conductor at the correct angle, electrons can become excited. This surface plasmon resonance, as the excitation is known, has been studied since the late 1970s. Nanoparticles suspended in water can be activated by the resonance to superheat the water to hundreds of degrees Celsius. In 2018, Loeb tested the concept successfully with contaminated water, killing different bacteria and viruses under solar radiation. The photothermal material, however, was gold nanocubes and nanorods, and those are not cheap materials. Commercial applications of this concept are thus unlikely. “But it was an interesting proof of concept,” Loeb recalls. And it can be used to clean water in a different way.

Loeb’s doctoral studies at Yale were part of the NSF-funded Nanotechnology-Enabled Water Treatment (NEWT) Research Center. Scientists at Rice University who participated in NEWT patented a technology known as nanophotonics-
enabled solar membrane distillation (NEMSD). They embedded carbon black nanoparticles in nanofibers and spun the fibers in a dense network onto a commercially available microfiltration membrane. Water flows over the fibrous layer, where incoming sunlight is scattered and absorbed. The locally generated heat produces vapor that passes through the membrane and condenses on the other side, clearing the water of pathogens, salts, and other contaminants.

But “the temperature of the overall system is really not that high,” explains Qilin Li, a professor of civil and environmental engineering at Rice. Even with concentrated light, her system works below 70 degrees C.

With a patent for the process pending, Li co-founded SolMem in 2017. The company, with three full-time and two part-time employees, is working to commercialize the technology. For example, the early process of making the photothermal distillation membrane was complicated and not scalable. So now, the carbon black is no longer embedded within the fibers. We found some membrane material that has a support layer with this fiber structure on it already.” says Li. “So, we’re just doing a coating to form the core-shell structure.” The water comes into direct contact with the carbon black, leading to higher efficiency and allowing for sheet sizes needed in devices of commercial scale.

Li sees concentrated brines as the best applications of this technology, which is being tested this year. Desalination is often done by physically pushing water through a semipermeable membrane, a process called reverse osmosis. It leaves behind a brine that needs to be disposed of, often in open-air evaporation ponds. The membrane distillation works well beyond the salinity limit of reverse osmosis. “We can take that brine we can concentrate it much more and extract even more freshwater out of that brine and reduce the brine volume manifolds.”

A place where Li can test her technologies in a real-world setting outside the lab is the Brackish Groundwater National Desalination Research Facility in New Mexico. It provides produced water—a byproduct from oil and gas extraction—and high- salinity wastewater like the leftovers from reverse osmosis. Since April, SolMem has set up its mobile pilot system there. The part that needs sunlight—the module itself and the parabolic trough—only measures about
2 × 1 m. All the tanks, valves, and pumps are in a 20-ft cargo container. Once the tests are completed, Li wants to start producing modules. She is currently talking to potential customers.

Because her technology is dependent on sunlight, Li often gets asked what to do at night: “It all depends on how much water you need and how much space you have.” When the Sun is not strong you can still collect light and distill water at a lower rate, she says. When the Sun goes down, water can be put into tanks and treated later. But this might require huge amounts of storage. SolMem’s solution: “The brine we generate during the day has high temperature and we store that in a thermal storage tank,” Li says. At night, the heat is used directly. Skipping the photothermal step is less effective, but the device keeps producing, although at a lower rate.

Another method that many photonic researchers think will change the way we clean our water is photocatalysis. At its most basic, photocatalytic water treatment can be described as using light to activate a catalyst, which generates charges that create reactive oxygen species. Those purify the water. The activating light range depends on the material. Semiconductors, like titanium dioxide, are optimal because they utilize light in visible and UV wavelengths. The charge carriers need to get to the surface of the catalyst to react before being neutralized. That’s why nanomaterials come into play.

Despite many studies over the last 20 years, commercialization of photocatalytic water treatment has not happened. In 2019, Li, Loeb, and other researchers from NEWT and beyond, published a paper analyzing the state and future of this method. They say expectations have been tempered by obstacles like chemical instability, high price, toxicity of the catalyst, and limited catalyst efficiency.

All of SolMem’s tanks, valves, and pumps fit in a 20-ft cargo container. Photo credit: Qilin Li.

Today, Loeb leads her own research group with a particular focus on water decontamination. “Sunlight-based reactions are very near and dear to me,” she says, however, “I don’t think there is anything since writing that paper that I would describe as extremely breakthrough.”

Regardless, researchers keep working on photocatalytic water treatment to deal with a vast array of pollutants from industrial sources and communal waste: organic dyes, pharmaceutical residues, pesticides, lignin, or per-/polyfluoroalkyl “forever chemicals.”

Microplastics are an exceptionally stubborn case. They degrade slowly, can be ingested, and accumulate in animals including humans. Microplastics contribute to coral bleaching and some plastics additives disrupt the hormone systems of aquatic animals. Also, microplastics have been linked to cardiovascular and inflammatory diseases. But the full ecological impact and health risks, especially long-term, are far from being understood.

One of the many sources for plastic in water is the fiber in our clothing. Classical water treatment plants retain large quantities of microplastics, but they’re not specific targets of water treatment.

Joydeep Dutta has been working on degrading microplastics in water with sunlight for a long time. In his opinion, photocatalysis will not have commercial success in this endeavor. “But I think the photo-Fenton process would be a possibility.”

In the standard Fenton reaction, iron cycles between the states Fe⁺² and  Fe⁺³, which converts added hydrogen peroxide into water and free radicals. Those can degrade organic pollutants. The photo-Fenton reaction uses a catalyst and sunlight to accelerate the process.

Early on, “nobody believed that I could degrade microplastics,” Dutta says. That was around 2005 when he was a professor at the Asian Institute of  Technology in Thailand. Now, his office and laboratory are at Sweden’s Royal Institute of Technology, where he devised a nano-enabled photo-Fenton reactor, but “We don’t have sun! That’s why I went to Greece,” he exclaims. He set up the prototype on the roof of a wastewater plant in Athens, filtering the plant’s outflow.

The heart of Dutta’s reactor is made of transparent tubes stuffed with glass wool overgrown with semiconductor nanorods. The nanorods in turn have been decorated with iron nanoparticles. “You can think about little tomatoes hanging out of these rods,” says Dutta.

The tubing is where the magic happens. Not only do the plastics degrade at least 10 times faster than in a conventional Fenton process, but the reactor also reduces the amount of hydrogen peroxide needed and does not produce iron sludge, Dutta says.

More specifically, the reactor works in two ways: It traps the plastic particles inside and while it continues to filter water, the catalytic reaction degrades the trapped plastic. In tests, after being exposed to sunlight inside the reactor for one week, plastic particles shrank by more than 90 percent in volume. And, using a standardized test for ecotoxicity with aquatic organisms, Dutta showed that the treated water had no harmful residues.

This approach to the microplastics flood is at an early stage of development. Water disinfection with UV-LEDs, on the other hand, is within grasp for large-scale systems because the technology has become cheaper and more effective. Pushing the latter development are environmental regulations that aim to end the use of mercury in UV lamps. Without that pressure, the economics still favor the old lamps.

The commercial potential of SolMem’s solar membrane distillation for a cleaner environment is about to be shown, Li says, and the laws governing brine disposal could have a huge impact on its future. Dutta’s noteworthy efforts aside, microplastics in water will probably be floating around for some time to come.

Niko Komin is a freelance science writer. His curiosity first brought him to study theoretical physics and then led him to tell scientific stories of all flavors.