Heterogeneous photocatalysis for pharmaceutical wastewater treatment

Environmentally sustainable solutions for wastewater management including the improvement of water quality and water recycling are considered key priority areas globally. The challenges facing our water resources are unprecedented due to the presence of organic and inorganic pollutants derived from numerous anthropogenic activities. This situation has been further complicated by emerging persistent contaminants such as pharmaceuticals which possess low biodegradability and resistance to chemical and biological treatments. Excretion of pharmaceuticals and their metabolites via human waste, improper disposal and veterinary applications is recognized as the principal sources of pharmaceuticals ending up in various compartments of the environment. Heterogeneous photocatalysis using semiconductor titanium dioxide (TiO₂) has proven to be a promising treatment method for the degradation of pharmaceuticals. Here, we review recent research concerning the application of TiO₂ photocatalysis for the removal of selected pharmaceuticals belonging to different therapeutic drug classes. These classes of pharmaceuticals were chosen based on their environmental prevalence and potential adverse effects. The highlighted conclusions from this review are that (1) TiO₂ photocatalysis may play a major role in the degradation of pharmaceuticals; (2) various factors such as catalyst loading, initial concentrations, and water matrices significantly influence both the degradation rate and kinetics of degradation; (3) mineralization remains incomplete, despite complete abatement of the parent pharmaceutical due to the formation of stable byproducts; (4) structures or number of intermediates formed differ due to the variation of photocatalytic experimental parameters, and (5) laboratory-scale experiments with artificial pharmaceutical solutions, in particular single compounds, are more common than pilot-scale or real wastewater samples. The main conclusion is that the use of heterogeneous photocatalysis can be considered a state-of-the-art pharmaceutical wastewater treatment. Further studies are needed to optimize the operating conditions for maximum degradation of wastewater containing multiple pharmaceuticals under realistic conditions and on industrial scales