The U.S. Army Engineer Research and Development Center’s Environmental Laboratory’s Research Environmental Engineer, Dr. Jonathon Brame, recently provided nanotechnology expertise to the United Nations for a policy brief, “Nanotechnology in Water Treatment,” published on Feb. 27, 2017. The material will also be used in a U.N. Global Sustainable Development report to be published in 2018. The brief was commissioned by the U.N. Policy Analysis Branch, Division for Sustainable Development, and was written by Chelsea Blaser, Pim ten Haaf, Juliana Kessler, and Fenna Wielenga.
“The brief was written for U.N. policymakers to provide them with a better understanding of the state of ‘nano’ in water treatment,” Brame said. “‘Nano’ refers to nanotechnology — the science and engineering of systems on a scale less than 100 nanometers; this is typically technology on the atomic and molecular scale,” he said. As a means of comparison, a piece of paper is about 100,000 nanometers thick. “The U.N. is interested in learning whether nanotechnology can be used to address seemingly unresolvable water problems, especially those that arise under the unique circumstances of developing nations,” he said. “I was one of a handful of researchers with applied research experience using nano-enabled technology to address a water-treatment challenge in the developing world,” he said. While Brame was at Rice University pursuing his doctorate in environmental engineering, the university was commissioned by the U.N. Food and Agriculture Organization to develop a low-cost water treatment system for contaminated surface water used for irrigation in the Kingdom of Swaziland. Brame was lead student on the team comprised of five Rice University staff and two members from the FAO; the team successfully developed a photocatalytic system in 2013 for treating Swaziland’s contaminated reservoir waters. The nano-enabled treatment system they created destroyed 99.9 percent of bacteria and viruses and reduced the concentration of a model pesticide by more than half in just 2 minutes and 30 seconds of treatment time.
Brame’s water treatment system used a nano-sized photocatalyst: material that converts light energy into chemical energy when the material is exposed to light. As the photocatalyst absorbs photons, it generates reactive oxygen species, such as hydroxyl radicals or super oxide. The reactive oxygen species react with and then destroy contaminants in the water — including bacteria, pesticides, E. coli, and pharmaceuticals. Brame’s team selected titanium dioxide as the photocatalyst for the Swaziland system; the TiO2 produced hydroxyl radicals when excited by ultraviolet light. TiO2 was selected as the photocatalyst over other alternatives, such as amino-fullerenes, because TiO2 was the cheapest material possible to implement, yet it was found by the team to be effective at removing organic contaminants, such as pesticides.
Brame’s team designed, built, and tested a fluidized bed photoreactor treatment system specifically for this application. Brame said, “The FBPR was built with readily available materials and the users would only need to change out the TiO2 every three to six months as a routine maintenance procedure.” The FBPR was a 12-centimeter-in-diameter, 26-centimeter-high cylinder with a low-power (16W) bactericidal, short-wavelength ultraviolet light (UV-C) encased down the center of the reactor. The water was passed up through the cylinder, through silica that became suspended in the flow. “The silica was coated with the TiO2, which was attached to the silica by a sol-gel precipitation process — the silica worked like an anchoring system for the photocatalyst,” Brame said. As the water flowed up through the cylinder and through the silica, the UV-C light both disinfected the water and simultaneously excited the photocatalyst, enhancing the effectiveness of both the UV-C and the photocatalyst treatments of the water.
Brame discovered that there was an optimal level of photocatalyst loading — or how much photocatalyst was put in the cylinder (0.2-1.5g L-1). “Too small a quantity of photocatalyst reduced the effectiveness of the material; too much photocatalyst obstructed the effectiveness of the UV-C disinfection treatment,” he said. At a total treatment time of 2 minutes 30 seconds, the nano-enabled FBPR destroyed 99.9 percent of bacteria and viruses and eliminated more than 50 percent of Carbaryl, a model pesticide. Brame said, “The photocatalytic system augmented the efficiency of the UV-C’s already-high E. coli removal rate by 5-10percent, and the rate of Carbaryl destruction was three times higher than that of commercially available UV treatment systems.” Brame said that the latter point is significant, because Carbaryl is a representative persistent organic pollutant. “These are compounds that don’t degrade easily in the environment and can accumulate in fatty tissues, producing adverse effects on humans,” he said.
A filter was located at the point of outflow at the top of the cylinder to keep the silica and the attached photocatalyst in the reactor. Brame’s team measured the amount of photocatalyst that escaped over the course of the 70-day experiment at nine different points during the testing process. In seven of the measurements, they measured zero micrograms of photocatalyst; on the two remaining days, they measured 2 micrograms per liter. “The EPA maximum allowable level for drinking water — a strict standard unnecessary for irrigation — is 15 micrograms per liter,” Brame said.
This was a year-long project. First, Brame and his team visited Swaziland to get a sense of the physical parameters, then the team took six months to design, build, and test the treatment system. They then took three months to complete laboratory-scale testing; finally, they presented the treatment system to Swaziland’s Minister of Agriculture. “The reaction and feedback was very positive,” Brame said. “The Swaziland government was very excited and requested that the U.N. pay for the treatment system for the reservoirs; however, as far as I am aware, the Kingdom of Swaziland was never able to commission the technology for use in reservoirs around the country due to a lack of funds,” he said.
ERDC presently has basic and applied research projects investigating nano-enabled photocatalytic water treatment. Brame is leading a five-person 6.1 basic research project focused on overcoming the inefficiency of having to shine UV light through the water to obtain the same treatment results. “Since UV energy is diminished when it bounces around inside the system and travels through water, we are investigating utilizing fiber optic illumination to access evanescent energy to streamline the method of exciting the photocatalyst,” Brame said. EL, the Construction Engineering Research Laboratory, Cold Regions Research and Engineering Laboratory, and the Geospatial Research Laboratory are also currently collaborating on 6.2 and 6.3 research projects: Advanced Level Logistics H2O. Brame is leading a seven-member team in the photocatalysis task of these projects. “We are looking at the most advanced oxidation processes to find how to most efficiently recycle water on military bases—we’re not looking solely at photocatalysis, but it is the main thrust of the research,” Brame said. “The Army is very interested in learning how to best recycle blackwater into greywater, and greywater into potable water,” he said.