Abstract
Solar-driven heterogeneous photocatalysis using semiconductor thin-films immobilised on inert surfaces has restricted applicability, mainly due to the low catalyst surface per reactor volume and mass and photon transfer limitations. Furthermore, sunlight capture hardware presents some setbacks relating to high implementation and maintenance costs. Therefore, this work aims to overcome those limitations by using a stainless steel (SS) Kenics® static mixer (SM) coated with a thin-film of TiO2-P25 and assembled inside a borosilicate glass absorber tube (AT) placed above distinct solar collectors composed of different materials (uncoated [R85], coated [MS], and soiled anodised aluminium [R85s] and SS) and optics (traditional [DP] and [SP] simple double parabola, and flat [F]). The photon flux reaching/entering the AT (measured by 2-nitrobenzaldehyde actinometry) agreed with the theoretical light efficiency estimated for each reflective surface (based on material specular reflectance and geometry ray-wtrace analysis), increasing as follows: SS-F ≈ R85-F < R85s-DP ≈ SS-SP < SS-DP < MS-DP < R85-DP. The ability of each solar collector in combination with the Kenics® SM as TiO2-P25 support on promoting heterogeneous photocatalysis was compared using oxytetracycline (OTC) as the target pollutant. OTC degradation rates over time agreed with actinometric results, leading to similar apparent photonic efficiencies for all reflective surfaces, excepting R85-DP and MS-DP reflectors, which were slightly lower. Moreover, the semiconductor thin-film deposited on the Kenics® SM surface after calcination remained stable for at least seven consecutive cycles. Besides, the Kenics® SM geometry combined with flat reflectors allowed a more efficient absorption of solar UV photons by the catalyst when using two ATs, resulting in the highest volumetric treatment capacity per unit of time and collector surface. The enduring SS-F was found to be the most compact (37% fewer square meters) and cost-effective solution (50% less expensive).
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