Tanzania water source. Credit: Bob Metcalf
I don’t have enough historical perspective to know if this is truly a eureka moment or not, but a group from Rice University reports that adding a small amount of silica (or another inexpensive silicon source) can amp up the anti-viral, anti-bacterial power of titanium dioxide for producing disinfected drinking water.
The researchers Huma R. Jafry, Michael V. Liga, Qilin Li and Andrew R. Barron report in Environmental Science & Technology that, at least in non in situ testing with model bacteriophage MS2, adding a dose of silica to a commercial TiO2 product, P25, triples the material’s ability to kill viruses and increases the adsorption of viruses onto P25 nanoparticles.
Liga’s website mentions that researchers are looking at several TiO2 dopants, and have also been testing the improved photocatalyst against a model organic pollutant, congo red dye, and discusses the possibility of even stronger effects:
“Our composite catalysts have been found to inactivate viruses over five times faster than the base titanium dioxide material. Future research will be focused on inactivating pathogenic adenovirus, which is of concern to the drinking water treatment industry. We will also attempt the construction of a lab scale treatment reactor employing the photocatalyst.””If you’re using titanium dioxide, just take it, treat it for a few minutes with silicone grease or silica or silicic acid, and you will increase its efficiency as a catalyst,” he said.
In a news release from Rice, Barron says they have supercharged the performance of the titania with little increase in cost. “Basically, we’re taking white paint pigment and functionalizing it with sand,” he says.
Barron and the others say they are setting the performance bar fairly high by looking at how the silica-titania mix would perform against Yangtze river contamination. “We chose the Yangtze River as our baseline for testing, because it’s considered the most polluted river in the world, with the highest viral content. Even at that level of viral contamination, we’re getting complete destruction of the viruses in water that matches the level of pollution in the Yangtze,” he says.
A couple of the researchers admit they accidentally stumbled into this revelation. Liga says he noticed a jump in the TiO2 performance when then-grad student Huma Jafry was heating a titania solution in a sealed flask. Jafry reported that she had done nothing knowingly different, but eventually she and Liga realized it might be the silicone grease used to lubricate the sealing stopper, a hypothesis they later confirmed.
Barron attributes the improved power of TiO2 to band bending that creates a path for electrons freed by the UV to react with water to create a surge of hydroxyl radicals. “Because the silicon-oxygen bond is very strong, you can think of it as a dielectric. If you put a dielectric next to a semiconductor, you bend the conduction and valence bands. And therefore, you shift the absorption of the ultraviolet (used to activate the catalyst) . . . If your conduction band bends to the degree that electrons find it easier to pop out and do something else, your process becomes more efficient,” he says.
“In places where they don’t have treatment plants or even electricity, the SODIS [solar disinfection – ed.] method is great, but it takes a very long time to make water safe to drink,” says coauthor Quilin Li. “Our goal is to incorporate this photocatalyst so that instead of taking six hours, it only takes 15 minutes.”