A step forward in heterogeneous photocatalysis: Process intensification by using a static mixer as catalyst support

Despite the proven efficiency of heterogeneous photocatalysis using immobilized catalysts, this process lacks integration with the industry due to limitations on mass and photon transfer. The current study aimed to go up against this scenario by applying for the first time a Kenics (R) static mixer as catalyst support for heterogeneous photocatalysis in tubular reactors. Stainless steel Kenics (R) static mixers coated with TiO2 or Fe2O3 photocatalyst were assembled in a borosilicate tube coupled to a compound parabolic collector (CPC) and applied to the degradation of the antibiotic oxytetracycline (OTC) in aqueous solution. The use of the Kenics (R) static mixer provided intense mixing even under laminar flow, and, beyond that, the use of sunlight and a CPC ensured that the entire photocatalyst surface area was receiving front side illumination. Fe2O3 catalyst showed superiority over TiO2 for OTC degradation, exhibiting a pseudo-first-order kinetic constant 79% higher. However, while no TiO2 leaching occurred, up to around 0.7 mg L-1 of total dissolved iron were leachated. Thus, both homogeneous and heterogeneous photo-Fenton reactions may have contributed to the higher OTC degradation by Fe2O3 photocatalysis. The effect of various parameters on the photocatalytic efficiency was assessed: (i) photocatalyst deposition technique, (ii) photocatalyst dosage, (iii) H2O2 concentration, and (iv) number of static mixing elements. Spray coating technique showed superiority over dip coating. OTC degradation was improved by using 50 mL of TiO2 suspension together with 77 mg L-1 of H2O2, and 75 mL of Fe2O3 suspension together with 154 mg L-1 of H2O2. The increase of the number of Kenics (R) static mixing elements from two to six did not improve the OTC degradation rate. Furthermore, the reusability of both catalysts was proved.