The Effect of Modifying TiO2 with Lanthanides on the Photocatalytic Degradation of Ciprofloxacin, a Hydrophobic Compound.

Abstract

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 at miniscule amounts. This issue is currently not resolved by conventional waste treatment plants, as their adequacy for handling these compounds, many of which non-polar, is quite limited. 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 towards degrading non-polar compounds. Ciprofloxacin, a widely used antibiotic, was chosen a model hydrophobic compound. Its degradation rate was compared with that of caffeine, used as a model hydrophilic compound. Fused silica plates were coated with REO-containing films comprising of 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 with fluorescence spectroscopy (ciprofloxacin). Further information was obtained by measuring the kinetics in the presence of charge scavengers, as well as by SEM-EDS mapping of the surface following photodeposition of platinum. Most of the REOs-modified TiO2 coatings showed an increase in the activity and selectivity towards ciprofloxacin, in comparison with coatings that did not contain REOs. A study on the role of the silica binder suggests that the hydrophobicity of the binder 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 to be responsible for the contribution of the REOs to the enhanced degradation of ciprofloxacin. The hydrophobicity of lanthanide oxides, while having some effect on the adsorption of the non-polar ciprofloxacin, cannot explain by-itself the observed effects. 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.