This work reports the improvement in the photon absorption and degradation of acetaminophen (ACF) and diclofenac (DFC) by photosensitizing TiO₂ with two types of dyes Eosin Y (Ey) and Rhodamine B (RhB). Experimental tests were carried out in a solar simulator for three hours for different systems and both pollutants. The influences of the TiO₂ concentration (100, 200 and 800 mg L⁻¹) and the catalyst–dye ratio (2%, 5% and 10%) were investigated. The degradation of the compounds was higher in the presence of TiO₂-Ey compared to the TiO₂-RhB and TiO₂ for both pharmaceutical compounds, which was attributed to the anionic nature of Ey. DFC total degradation was achieved using 100 mg L⁻¹ of catalyst loading and 10% of catalyst–dye ratio and the highest ACF degradation (71%) was obtained at 800 mg L⁻¹ of catalyst loading and 5% of catalyst–dye ratio. The photon absorption was studied for both dyes using the six-flux absorption scattering model (SFM) for estimating the LVRPA (local volumetric rate of photon absorption). This was done by modifying the apparent optical thickness equation. It was found that the presence of dye in the photocatalytic systems considerably increases the LVRPA. The rate coefficients for the degradation of pharmaceutical compounds in the presence of the organic dyes were also obtained.

This paper was accepted for publication in the journal Chemical Engineering Journal and the definitive published version is available at http://dx.doi.org/10.1016/j.cej.2016.04.127© 2016 Elsevier B.V.Robust and practical models describing the radiation field in heterogeneous photocatalytic systems, used in emerging environmental, photochemical and renewable energy applications, are fundamental for the further development of these technologies. The six-flux radiation absorption-scattering model (SFM) has shown to be particularly suitable for the modeling of the radiation field in solar pilot-plant photoreactors. In this study, the SFM was coupled to the Henyey-Greenstein (HG) scattering phase function in order to assemble the model with a more accurate description of the scattering phenomenon provided by this phase function. This new version of SFM, named as SFM-HG, was developed through fitting the Local Volumetric Rate of Photon Absorption (LVRPA) determined in a flat photoreactor to the "pseudo-experimental" LVRPA calculated by a Monte Carlo (MC) approach, which included the HG expression. As a result, simple mathematical correlations describing the SFM-HG scattering probabilities as function of the HG scattering parameter were determined. The SFM-HG was validated through a comparison with the MC model predictions of the Total Rate of Photon Absorption (TRPA) in the slab photoreactor. A RMSE% of approximately 5% demonstrated satisfactory agreement between the models. The SFM-HG was further applied to evaluate the impact of selected scattering phase functions on the absorption of radiation in solar photoreactors, operated with commercial TiO2 photocatalyst. The results have established that, the apparent optical thickness, τapp (or τapp,max for tubes) a parameter derived from the SFM approach, is the most appropriate for the design and optimization of photocatalytic reactors. This parameter is insensitive to scattering albedos and phase functions. CPC, tubular and flat-plate photoreactors should be designed with τapp,max = 12, τapp,max = 7 and τapp = 4.5 respectively