Alumina-titania particles for the heterogeneous photocatalytic oxidation of ciprofloxacin
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The global presence of xenobiotic organic compounds, such as pharmaceuticals and personal care products (PPCPs), in drinking water supplies and wastewater effluents has raised concerns regarding their potential effects on aquatic life, human health, and antibacterial resistance. Traditional treatment systems, not specifically designed to target removal of these compounds, vary with respect to their ability to remove PPCPs. Therefore, research focused on the development and optimization of treatment processes that can remove PPCPs is warranted. Advanced oxidation processes (AOP) have been shown to be effective at degrading a number of pharmaceuticals. These processes are defined by the production of radicals, such as superoxide and hydroxyl radicals, which are highly reactive and can, therefore, oxidize target contaminants more effectively than common oxidants such as ozone and ultraviolet (UV) light. One such AOP is photocatalysis, in which the radicals are created by the illumination of a photocatalyst. Photocatalysis has been previously demonstrated for degradation of the antibiotic ciprofloxacin, the selected pharmaceutical in this research. In this work, the impact of sorption on the photocatalysis of ciprofloxacin was studied by developing a mixed phase photocatalyst that optimized sorption. Titanium dioxide (TiO2) is a commonly used photocatalyst. However, TiO2 particles have limited surface area and varying affinity for ciprofloxacin as a function of pH. Aluminum oxide (Al2O3) was chosen as the sorbent to mix with the TiO2 due to its ability to sorb ciprofloxacin over the pH range of natural waters and its ability to prevent the recombination of photo-generated electron-hole pairs, which are vital for the production of hydroxyl radicals. The particles were synthesized using pure TiO2 and other particles that ranged in aluminum oxide content (68% - 100% by mass) using a sol-gel process to achieve high surface areas. The 100% TiO2 particle was pure anatase, while the mixtures and the 100% Al2O3 particle consisted gamma-alumina and anatase when the titania was present. Sorption was found to be pH dependent for both Al2O3 and TiO2 with maximum adsorption at pH 6 and 7, respectively. The synthesized particles resulted in linear equilibrium sorption constants at fixed pH that varied with alumina content, with the 90% alumina material adsorbing the most at pH 8 due to its high surface area. Photocatalysis using the synthesized particles under UV irradiation at 365 nm effectively oxidized ciprofloxacin and the rates were faster than photolysis, but slower than the standard photocatalyst Degussa P25 TiO2 and synthesized pure TiO2. Sorption followed the Langmuir isotherm and oxidation followed first order kinetics if the assumption of low concentration (relative to the Langmuir sorption coefficient) was applied, typical of environmental conditions. Experiments revealed that rapid exchange of ciprofloxacin with pure TiO2 occurred as rate constants determined from aqueous phase concentrations were comparable to rate constants determined by monitoring the concentration of ciprofloxacin within the total reactor (surface and aqueous phases). In contrast, differences between rate constants measured by these two methods suggest that at high aluminum contents a fraction of the sorbed ciprofloxacin was not rapidly accessible to the oxidant. Nevertheless, the ability to temporally separate adsorption and photocatalysis was demonstrated which lays the foundation for optimizing this treatment scheme for ciprofloxacin removal from water.