Titanium dioxide (TiO2) has emerged as a leading photocatalyst for the degradation of pharmaceutical contaminants, including antibiotics and non-steroidal anti-inflammatory drugs (NSAIDs), due to its strong oxidative capability, chemical stability, and cost-effectiveness. Under ultraviolet (UV) or visible light irradiation, TiO2 generates electron-hole pairs that initiate redox reactions, breaking down complex pharmaceutical molecules into simpler intermediates and ultimately mineralizing them into CO2, H2O, and inorganic ions. The process is governed by reactive oxygen species (ROS) such as hydroxyl radicals (•OH), superoxide radicals (•O2−), and holes (h+), which attack organic pollutants through oxidation, hydroxylation, and cleavage reactions.
The degradation pathways of pharmaceuticals depend on their molecular structure. For antibiotics like ciprofloxacin, the primary attack sites are the piperazine ring and quinolone moiety, leading to decarboxylation, defluorination, and ring-opening reactions. NSAIDs such as ibuprofen undergo hydroxylation at the aromatic ring and side-chain cleavage, forming smaller carboxylic acids and aldehydes before complete mineralization. The toxicity of intermediates is a critical consideration, as some byproducts may exhibit higher toxicity than the parent compounds. For instance, during the degradation of diclofenac, chlorinated intermediates like 2,6-dichloraniline can form, which are more persistent and toxic. However, prolonged photocatalytic treatment typically reduces toxicity as these intermediates are further broken down.
Mineralization efficiency, measured as the percentage of total organic carbon (TOC) removed, varies depending on the pharmaceutical compound and reaction conditions. Studies report TOC removal rates ranging from 40% to 90% for common pharmaceuticals after several hours of treatment. Complete mineralization is often hindered by the formation of recalcitrant intermediates, such as short-chain carboxylic acids (e.g., oxalic acid), which require extended treatment times or optimized conditions for further degradation. The presence of inorganic ions like chloride or sulfate can also influence mineralization by competing for ROS or forming secondary radicals.
Matrix effects significantly impact TiO2-mediated degradation. pH plays a crucial role in determining the surface charge of TiO2 and the ionization state of pharmaceutical molecules. At acidic pH (below the point of zero charge of TiO2, ~6.3), the catalyst surface is positively charged, favoring the adsorption of anionic pollutants like diclofenac. Conversely, at alkaline pH, the negatively charged surface enhances the adsorption of cationic species like certain antibiotics. pH also affects ROS generation, with neutral to slightly acidic conditions generally favoring •OH production. However, extreme pH values can destabilize the catalyst or reduce photocatalytic activity.
The presence of natural organic matter (NOM) in water matrices complicates the degradation process. NOM competes with pharmaceuticals for adsorption sites on TiO2 and scavenges ROS, reducing degradation efficiency. Humic acids, for example, can decrease the degradation rate of sulfamethoxazole by up to 50% due to competitive inhibition. Inorganic ions such as bicarbonate (HCO3−) and chloride (Cl−) also act as radical scavengers, though chloride can sometimes enhance degradation by forming secondary reactive species like chlorine radicals (Cl•). Dissolved oxygen is essential for sustaining the photocatalytic cycle, as it acts as an electron acceptor, preventing electron-hole recombination and promoting •O2− formation.
The crystallographic phase of TiO2 (anatase, rutile, or mixed-phase) influences its photocatalytic performance. Anatase generally exhibits higher activity due to its wider bandgap and slower electron-hole recombination rates. Doping TiO2 with non-metals (e.g., nitrogen) or metals (e.g., iron) can extend its light absorption into the visible spectrum, improving solar efficiency. However, excessive doping may introduce recombination centers, reducing overall activity.
Practical challenges include catalyst recovery and reuse, as nanoparticle suspensions are difficult to separate from treated water. Immobilization of TiO2 on supports like glass beads or alumina can mitigate this issue but may reduce active surface area. Light penetration depth is another limitation, particularly in turbid waters where scattering reduces photon absorption. Optimizing reactor design, such as using thin-film or annular reactors, can improve light utilization.
Despite these challenges, TiO2 photocatalysis remains a promising solution for pharmaceutical contamination, offering a green alternative to conventional treatments. Future research should focus on enhancing visible-light activity, minimizing toxic intermediates, and scaling up systems for real-world applications. Understanding the complex interplay between reaction pathways, matrix effects, and mineralization kinetics is essential for optimizing TiO2-based water treatment technologies.