Abstract

Background: Recent years have seen the increased use of antibiotics and hormones in domestic, agricultural and healthcare applications. As a result, waste streams contain more and more of these compounds, which eventually end up in the environment, where they might cause serious damage to flora and fauna, even in miniscule amounts. This issue is currently not resolved by conventional waste treatment plants, as their adequacy for handling these compounds, many of which are non-polar, is quite limited.

Objective: This work studies the effect of modifying the hydrophilic photocatalyst TiO₂ with various rare earth oxides (REOs), of the lanthanide family (Er, La, Gd, Ce), on the photocatalytic activity toward degrading non-polar compounds. Ciprofloxacin, a widely used antibiotic, was chosen as a model hydrophobic compound. Its degradation rate was compared with that of caffeine, used as a model hydrophilic compound.

Methods: Fused silica plates were coated with REO-containing films comprising TiO₂ and silica. The latter was used as a binder to assure high integrity and strong adherence of the films to their substrates. The plates were characterized by SEM, EDS, XPS, and scratch-resistance measurements. The photocatalytic kinetics were determined with UV-Vis spectroscopy (caffeine) or fluorescence spectroscopy (ciprofloxacin). Further information was obtained by measuring the kinetics in the presence of charge scavengers and by SEM-EDS mapping of the surface following photodeposition of platinum.

Results: Most REOs-modified TiO₂ coatings showed increased activity and selectivity towards ciprofloxacin compared to coatings that did not contain REOs. A study on the silica binder's role suggests that the binder's hydrophobicity plays an important role in promoting ciprofloxacin degradation. With respect to REOs contribution, SEM-EDS mapping of REOs-containing films indicated that the REOs act as electron sinks, despite the position of their conduction bands. This charge accumulation is likely responsible for the contribution of the REOs to the enhanced degradation of ciprofloxacin. The hydrophobicity of lanthanide oxides, while affecting the adsorption of the non-polar ciprofloxacin, cannot explain the observed effects.

Conclusion: Oxides of erbium, gadolinium and lanthanum may be used to increase photocatalytic rates via electron accumulation, despite the location of their conduction bands. This is in parallel to their effect as adsorption promotors of hydrophobic compounds.

Keywords: TiO2, photocatalysis, lanthanides, coating, antibiotics, ciprofloxacin.

Graphical Abstract