Photocatalytic degradation of diclofenac amide in a fixed-bed reactor using TiO₂/β-Bi₂O₃: Process optimization and stability analysis

In this study, we employed a recirculating photocatalytic fixed-bed reactor featuring a TiO₂/β-Bi₂O₃ (TB) composite as a photocatalyst immobilized onto borosilicate glass beads for the degradation of the water-dissolved pharmaceutical compound diclofenac amide (DCFA) under UV-LED illumination. Our findings from the DCFA degradation runs revealed that the initial concentration (Co) and the volumetric recirculation flow rate (Q) exert minimal influence on the DCFA degradation rate, indicating the absence of mass transfer limitations at the chosen Q. A kinetic model, developed in alignment with the experimental data, facilitated the calculation of kinetic adsorption (k₁ = 0.15 (ppm h)^-1, K_eq = 15 ppm⁻¹, S_t = 4.3 ppm) and reaction rate constants (k_r = 0.015 h⁻¹). These constants were pivotal in determining the photocatalytic space yield (PSTY—0.45 × (10^-6 m³ water m⁻³ reactor day⁻¹ kW⁻¹ lamp)) for the employed reactor system. The results from three consecutive cycles of DCFA degradation, conducted with the same TB-coated glass beads, demonstrated that the adsorption of DCFA in the dark primarily occurs through the glass material of the beads. Upon exposure to light, the DCFA degradation rate remained constant across all three oxidation cycles. Analysis of the kobs values for cycle 1 (0.0041 ± 0.00067 h⁻¹, R² = 0.91), cycle 2 (0.0083 ± 0.0013 h⁻¹, R² = 0.93), and cycle 3 (0.0073 ± 0.001 h⁻¹, R² = 0.94) indicated an initial excessive leaching of weakly bound TB powder during the first DCFA degradation cycle, potentially resulting in light scattering and reduced photon absorption. Nevertheless, the coatings exhibited reusability over three DCFA degradation cycles, highlighting their stability and reproducible activity. Furthermore, the long-term stability of the coated TB beads was demonstrated by comparing the photocatalytic activity between the present day and two years ago. Remarkably, comparable activity levels were achieved both today (k_obs = 0.0085 ± 0.0035 h⁻¹, R² = 0.71) and two years ago (k_obs = 0.0071 ± 0.0014 h⁻¹, R² = 0.90). These obtained results offer valuable insights for the refinement and optimization of fixed-bed photoreactors in the context of wastewater treatment.