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Modelling of photocatalytic degradation of pharmaceuticals in water by multi-faceted approach

Pharmaceuticals, as one of the largest groups of contaminants of emerging concern (CECs), pose a threat to the environment and human health. Advanced Oxidation Processes (AOPs) have proven to be effective in the degradation of persistent, toxic, and non-biodegradable pollutants in water. Heterogeneous photocatalytic processes are multiparameter systems pertaining to AOPs based on in-situ generated radical, with overall effectiveness influenced by the different process parameters. Predicting the behaviour of photocatalytic AOPs by means of mathematical simulations is crucial not only for scale-up and process optimization, but also for controlling undesirable environmental effects such as the formation of by products with a higher toxicity than the parent compound. In this work, a multi-faceted approach was applied to develop a simulation model for the photocatalytic process for the degradation and mineralisation of pharmaceutical ibuprofen by UV- A/TiO2 P25. The mathematical – mechanistic model of heterogeneous photocatalysis includes a set of differential equations and takes into account the configuration of the photocatalytic reactor, the irradiation emission, the scattering of irradiation, the process parameters, the reaction kinetics and the degradation mechanism. The developed model was verified by experimental results obtained at different photocatalyst loadings. Coumarin and 1, 4-benzoquinone were used as chemical probes to confirm the generation of hydroxyl and superoxide radicals and to fine-tune the chemical reactions in the model. To increase the robustness of the developed model, a variety of organic compounds whose structural features influence important mechanisms in photocatalytic treatment, such as adsorption on the TiO2 P25 surface and the prevalence of degradation by hydroxyl and superoxide radicals, were investigated using quantitative-structure activity/property relationship (QSA/PR) modelling. Adsorption was first investigated by a combined experimental/statistical approach using response surface method (RSM), which yielded a quadratic polynomial equation (QPE) describing adsorption for each organic compound studied. The coefficients of the QPE were related to structural features employing QSA/PR. The degradation of organic compounds by the UV-A/TiO2 P25 process in the presence of coumarin and 1, 4-benzoquinone was studied, common radical scavengers for hydroxyl and superoxide radicals were investigated and coefficients were determined based on the kinetics obtained, that diversified the prevalence of oxidation or reduction mechanisms in the degradation of organic compounds. The determined coefficients were correlated with structural features of the investigated organic compounds by QSA/PR modelling. The simulation model developed by the proposed multi-faceted approach achieved good agreement between the predicted and experimental data. The QSA/PR modelling combined with RSM methodology accurately predicted the adsorption of organic compounds with complex molecular structure. The QSA/PR technique also successfully captured relevant structural features that determine degradation kinetics, so it can be used to increase the robustness of mathematical – mechanistic models which ensures their simulation capability for a wide range of organic structures.

adsorption, advanced oxidation processes, heterogenous photocatalysis, mathematical – mechanistic modelling, pharmaceuticals, RSM, TiO2 P25, QSA/PR

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