Low-magnification SEM image of the hierarchical nanostructure consisting of radially aligned ZnO nanowires grown on electrospun poly-l-lactide nanofibers. Inset: schematic representation of the hierarchical nanostructure. Credit: JACerS.
In a story just published in one of ACerS’ technical journals, a group of researchers from Sweden’s Royal Institute of Technology (KTH) report they have discovered what appears to be a relatively easy and inexpensive way to make a mat of photocatalytic water purification fibers that are coated with radial arrays of ZnO nanowires. The KTH group says that a prototype of a continuous-flow water purification system teamed with a UV light source worked well at decomposing three organic pollutants: methylene blue, monocrotophos and diphenylamine. Their paper is available in the “Early View” (online only) section of the Journal of the American Ceramic Society.
There is much interest in any purification system that combines UV and ZnO. UV irradiation is a great sterilization technique. Moreover, ZnO in the presence of UV light provides an enhanced antibacterial effect.
The ability of metal oxides, such as TiO2 and ZnO, to act as a photocatalytic agent isn’t new, and several other groups have previously demonstrated this also using nanoparticles of the oxides. These efforts have mainly been fairly simple demonstrations where a colloidal suspension of nanoparticles are mixed with water and an organic contaminant. Nanoparticles are an obvious choice because they offer the potential of a high collective surface area. While proving that the contaminants are decomposed, these demonstrations, however, haven’t addressed a practical way of removing the nanoparticles from the water other than batch centrufugation.
In effect, the KTH group is trying to tackle this problem by coming up with a fast, cost-efficient way to achieve the same water purification effects through other methods. There solution is to grow metal oxide nanowire arrays (NWA) on an anchored substrate. ZnO NWAs can be grown on a substrate by various techniques like gas phase deposition or chemical bath deposition. The KTH group also knew that researchers already had shown that ZnO NWAs could be grown on nonwoven polyethylene nanofibers using the above-mentioned chemical bath deposition technique. The nanofibers, themselves, could be formed via standard electrospinning techniques.
Looking for durable fibers that could withstand substantial water flow and turbulence, the group opted for nanofibers of poly-l-lactide (PLLA), a highly flexible material.
The KTH group’s procedure is to electrospin and aggregate an initial mat of PLLA fibers. The PLLA fibers then go through a two-step process that, first, “seeds” the fibers with ZnO nanoparticles to provide the initial nucleation sites for the NWAs. In the second step, the NWAs are grown by putting the seeded fibers in a mixed aqueous solution of of Zn(NO3)2 and hexamine, and heated. After several hours in the solution, the fibers are removed and finally dried.
To create a functioning proof-of-concept water purification system, the researchers filled a glass column with the NWA-coated fibers. The column was connected to a pump-and-reservoir continuous-flow loop system (see column sample and schematic diagram). To provide a UV light source, the glass column was illuminated by a mercury vapor lamp. Part of the system passed through a UV-vis spectrophotometer to monitor the changes in absorption spectra in real time.
The first sign of success for the system came when the group tested its ability to handle a 5 ppm solution of methylene blue. After 80 minutes, about 90% of the methylene blue had decomposed. Repeated tests using the same PLLA–ZnO fibers gave identical results, indicating the structural and chemical stability of the nanostructure.
The researchers then tested the systems ability to handle two widely used and toxic organic compounds, monocrotophos (MCP), an organophosphate insecticide, and diphenylamine (DPA) an antioxidant for fruit storage. For both MCP and DPA, the purification system work, but not nearly as well as with methylene blue. After 125 minutes, the system reached what appeared to be its maximum decomposition ceiling of 50%. They weren’t surprised by the lower decomposition levels of MCP and DPA and noted that other researchers found similar results in the absence of additives to enhance the photocatalytic decomposition. They say they are currently investigating methods to improve the decomposition percentage by testing the effects of various additives.
They are also testing a system that uses only solar radiation instead of a dedicated UV source.
Regardless, it looks like their work could be an important step in developing an efficient, low-cost system to remove agricultural and industrial pollutants. Scaling up the production of the PLLA-ZnO NWAs is fairly straightforward, so the main hurdles appear to be finding the right additives to surpass the decomposition ceiling.