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Antibiotics are the most widely prescribed medicine nowadays, with many finding their way into aquatic systems and causing problems due to their non-biodegradable nature and threat to water supplies. A team of researchers from Shoolini University of Biotechnology and Management Sciences in India have developed a method using a graphene bentonite/sand supported ZnFe2O4 superparamagnetic recoverable photocatalysts to effectively breakdown selected antibiotics in water streams.
As the most widely prescribed medicine, it comes of no surprise to many of us that antibiotics end up in water systems. However, despite our blasé approach of knowing it is there, more efforts are required to remove common antibiotics, before they start to cause problems with our water systems.
Many methods have been attempted to remove antibiotics from aqueous systems, but these methods have either failed to fully remove (or almost fully remove) the antibiotics, or have created a secondary pollutant.
In the last two decades, semiconductor photcatalysis has seen the fastest growth in wastewater technology. Photocatalysts can possess a high activity, recyclability and stability in various aquatic mediums. Iron-based ferrites, particularly zinc ferrite (ZnFe2O4) have lended themselves as promising photocatalysts in recent years due to possessing a high visible-light-responsivephotocatalytic nature. They also have the potential to be fabricated with other morphologies, particularly carbon-based structures.
Recent attempts at using magnetic nanoferrites has yielded agglomerated particles and reduced the large-scale applicability in waste water treatment applications.
The effort to produce materials suitable for efficient and scalable waste water treatment has yielded iron ferrites in conjunction with both bentonite clay and graphene. Bentonite clay is a useful material for nanoparticle support and is environmentally friendly. Graphene, with its delocalised and high charge carrier mobilised structure, is a very efficient material for the movement of electrons and can act as great electron facilitator in photocatalytic applications. Bentonite also possesses high absorption and ion exchange affinities for metal and organic pollutants.
The Photocatalysts
The researchers have combined these beneficial properties into working materials. The researchers have developed a superparamagnetic ZnFe2O4 supported on graphene and bentonitephotocatalyst. They also produced a similar material, but used sand instead of bentonite clay. Both bentonite and graphene possess a sufficient adsorption efficiency towards antibiotics and provide an efficient photocatalytic depredation reaction. The antibiotics the materials were tested against are ampicillin and oxytetracycline.
The presence of graphene and bentonite not only support ZnFe2O4, but also retarded the agglomeration of ZnFe2O4 molecules. The sand molecules were found to reduce the agglomeration of the graphene sheets.
The materials were produced by simple wet synthetic methods and were characterised by FESEM (Nava Nano SEM-45), HR-TEM and energy dispersive X-ray analysis (FP/5022 Tecnai G2 20 S-TWIN), FTIR (Perkin– Elmer Spectrum RX-I), XRD (PanalyticalX’PertProdiffractrometer), diffuse reflectance spectrometry (UV 3600, Shimadzu), amagnetometer (VSM), Raman spectroscopy (BRUKEF RFS 27) and BET analysis (Quantachrome Corp, Autosorb I).
The size of the sand-based catalyst was found to be twice the size of the bentonite catalyst, but both catalysts exhibited the same band gap of 1.95 eV. The superparamagnetic properties of the photocatalyst allow for a quick separation of the reaction media to occur. The photcatalytic ability of the catalysts were evaluated for photo-mineralisation purposes. The mineralisation process was found to have significant effect on specific antibiotics, with more than 90% mineralisation obtained within 10 hours.
The removal of the antibiotics was calculated using a power law model. The sand-based catalyst removed 92 and 97% of ampicillin and oxytetracycline, respectively within 2 hours, leading to removal efficiencies of 86 and 88% respectively; Whereas the bentonite catalyst only removed 72 and 74% of ampicillin and oxytetracycline, respectively within 2 hours, leading to removal efficiencies of 79 and 78%, respectively.
The two catalysts have shown great efficiencies for photocatalytic degradation application in water systems. They have the potential to be used for the breakdown of these two specific antibiotics, highlighted previously, in real-world aquatic systems.
Unlike previous materials, the magnetic nature of the nanoparticles allows them to be recovered through magnetic separation, presenting a very green and recyclable method. This major property will allow these nanoparticles to be implemented as an eco-friendly alternative to current methods to treat waste water containing high antibioticconcentrations under solar light.
Source:
Singh P., Gautam S., Shandilya P., Priya B., Singh V. P., Raizada P., Graphene bentonite supported ZnFe2O4 assuperparamagnetic photocatalyst for antibiotic degradation, Advanced Material Letters, 2017, 8(3), 229-238
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