Structural Features Promoting Photocatalytic Degradation of Contaminants of Emerging Concern: Insights into Degradation Mechanism Employing QSA/PR Modeling

Although heterogeneous photocatalysis has shown promising results in degradation of contaminants of emerging concern (CECs), the mechanistic implications related to structural diversity of chemicals, affecting oxidative (by HO-) or reductive (by O-2(-)) degradation pathways are still scarce. In this study, the degradation extents and rates of selected organics in the absence and presence of common scavengers for reactive oxygen species (ROS) generated during photocatalytic treatment were determined. The obtained values were then brought into correlation as K coefficients ( MHO- /MO2-), denoting the ratio of organics degraded by two occurring mechanisms: oxidation and reduction via HO and O-2. The compounds possessing K >> 1 favor oxidative degradation over HO-, and vice versa for reductive degradation (i.e., if K << 1 compounds undergo reductive reactions driven by O-2(-)). Such empirical values were brought into correlation with structural features of CECs, represented by molecular descriptors, employing a quantitative structure activity/property relationship (QSA/PR) modeling. The functional stability and predictive power of the resulting QSA/PR model was confirmed by internal and external cross-validation. The most influential descriptors were found to be the size of the molecule and presence/absence of particular molecular fragments such as C O and C Cl bonds; the latter favors HO-driven reaction, while the former the reductive pathway. The developed QSA/PR models can be considered robust predictive tools for evaluating distribution between degradation mechanisms occurring in photocatalytic treatment.

Automated summary

Heterogeneous photocatalysis has shown potential in degrading contaminants of emerging concern (CECs), but there is still limited understanding of the mechanistic implications related to the structural diversity of chemicals. In this study, the degradation extents and rates of selected organics in the presence of reactive oxygen species (ROS) scavengers were determined, and K coefficients were used to correlate the oxidative and reductive degradation pathways. A quantitative structure activity/property relationship (QSA/PR) modeling was then developed to relate the empirical values with the structural features of CECs. The resulting QSA/PR models are robust predictive tools for assessing the distribution of degradation mechanisms in photocatalytic treatment.