Titanium dioxide (TiO2) photocatalysis is a well-established advanced oxidation process for degrading organic pollutants in water. The process relies on the generation of highly reactive species under light irradiation, which oxidize and mineralize contaminants into harmless byproducts such as CO2, water, and inorganic ions. The effectiveness of TiO2 photocatalysis depends on multiple factors, including the mechanisms of pollutant degradation, reaction kinetics, reactor configurations, and operational parameters.
The photocatalytic mechanism begins when TiO2 absorbs photons with energy equal to or greater than its bandgap (3.2 eV for anatase), exciting electrons from the valence band to the conduction band. This creates electron-hole pairs (e⁻/h⁺), which migrate to the catalyst surface and participate in redox reactions. The holes oxidize water or hydroxide ions to produce hydroxyl radicals (•OH), while electrons reduce oxygen to form superoxide radicals (•O₂⁻). These reactive oxygen species (ROS) attack organic pollutants, leading to their decomposition.
Dye degradation is a widely studied application of TiO2 photocatalysis. Azo dyes, for instance, undergo cleavage of the –N=N– bond, followed by hydroxylation and ring-opening reactions, ultimately forming small organic acids before complete mineralization. The degradation of pesticides such as atrazine involves dechlorination, hydroxylation, and the breakdown of triazine rings. The reaction pathways depend on the pollutant structure, but the dominant ROS (•OH) ensures non-selective oxidation, making TiO2 effective for diverse contaminants.
Kinetic models describe the rate of photocatalytic degradation, with the Langmuir-Hinshelwood (L-H) model being the most common. The L-H equation is expressed as:
r = (kₓ K C) / (1 + K C)
where r is the reaction rate, kₓ is the intrinsic rate constant, K is the adsorption equilibrium constant, and C is the pollutant concentration. At low concentrations, the equation simplifies to pseudo-first-order kinetics:
ln(C₀/C) = kₐₚₚ t
where kₐₐₚ is the apparent rate constant. Factors influencing kinetics include light intensity (I), where the rate often follows a power-law dependence (r ∝ Iⁿ, with n typically between 0.5 and 1.0 at moderate intensities). At very high intensities, electron-hole recombination dominates, reducing efficiency.
Reactor design plays a critical role in photocatalysis efficiency. Slurry reactors suspend TiO2 nanoparticles in the aqueous solution, maximizing catalyst-pollutant contact and light absorption. However, post-treatment separation of nanoparticles is required, often through filtration or centrifugation. Fixed-bed reactors immobilize TiO2 on supports like glass beads, fibers, or porous substrates, eliminating the need for catalyst recovery but potentially reducing active surface area.
Operational parameters significantly impact degradation efficiency. pH affects the surface charge of TiO2 (isoelectric point ~6.0) and pollutant speciation. Anionic pollutants adsorb better at low pH, while cationic species favor high pH. Optimal pH varies with the pollutant; for example, methyl orange degrades faster under acidic conditions, whereas rhodamine B shows better removal at neutral pH.
Light intensity and wavelength are crucial. UV light (λ < 387 nm) is typically used for anatase TiO2, but visible-light-active doped or modified TiO2 can utilize solar radiation. Higher light intensity increases ROS generation but may also accelerate electron-hole recombination. Dissolved oxygen is essential as an electron scavenger, preventing recombination and sustaining ROS production.
Temperature influences reaction rates, though TiO2 photocatalysis is often conducted near ambient conditions. Elevated temperatures can enhance mass transfer but may also reduce pollutant adsorption. The presence of inorganic ions (e.g., Cl⁻, SO₄²⁻, HCO₃⁻) can scavenge ROS, reducing efficiency, while some metal ions (e.g., Fe³⁺) may participate in Fenton-like reactions, enhancing degradation.
Practical applications require scaling up laboratory systems. Challenges include maintaining uniform light distribution in large reactors and minimizing energy consumption. Immobilized catalyst systems are preferred for continuous flow operations, while slurry reactors are more suitable for batch treatments. Pilot-scale studies have demonstrated TiO2 photocatalysis for textile wastewater treatment, achieving over 80% dye removal under optimized conditions.
Long-term stability and catalyst fouling are concerns. Organic intermediates or inorganic precipitates can block active sites, reducing efficiency. Periodic catalyst regeneration via UV irradiation or thermal treatment may be necessary. Despite these challenges, TiO2 photocatalysis remains a promising solution for water purification due to its robustness, non-toxicity, and ability to degrade a wide range of pollutants.
In summary, TiO2 photocatalysis effectively degrades organic pollutants through ROS-mediated oxidation, with kinetics following L-H models. Reactor designs balance catalyst activity and practicality, while operational parameters like pH, light intensity, and dissolved oxygen must be optimized for specific contaminants. Continued research focuses on improving visible-light activity and reactor scalability to enhance real-world applicability.