Computational Investigation On Aquatic Environmental Photochemical Behavior Of2-phenylbenzimidazole-5-Sulfonic Acid And P-Aminobenzoic Acid Based On Density Functional Theory

Photochemical transformation can influence the environmental persistence and ecological risk of toxic organic pollutants. As the number of organic pollutants is huge, and effects of water constituents on photodegradation are complex, experimental approaches for studying the photolytic pathways cannot meet the great need of ecological risk assessment of toxic chemicals. It is necessary to develop computational prediction methods on the environmental photochemical behavior of organic pollutants. In this study, computational methods based on density functional theory (DFT) were employed to investigate the photochemical behavior of two model compounds,2-phenylbenzimidazole-5-sulfonic acid (PBSA) and p-aminobenzoic acid (PABA). Effects of water constituents on the photodegradation of PBSA were computationally predicted and experimentally verified. Reaction mechanisms of PBSA and PABA with singlet oxygen (1O2) were computationally clarified, and thermodynamic parameters were calculated. This work lays a foundation for the development of computational methods on the environmental photochemical behavior of organic pollutants.Effects of dissolved oxygen on the photodegradation of PBSA were evaluated by computing the photosensitization reactions based on DFT, and verified by experiments. The computational results indicate that the lowest singlet and triplet excited states of PBSA can sensitize dissolved oxygen via energy and electron transfer to generate reactive oxygen species; the electron donating ability of PBSA, and thus the photogeneration of superoxide anion radicals (O2·-), is dependent on protonation states. It can be inferred that the photolysis of PBSA is dependent on dissolved oxygen, and possibly includes self-sensitized photooxidation, which is dependent on the protonated states of PBSA in the condition that the photooxidation is induced by O2·-. Experimental results confirm that PBSA can undergo self-sensitized photooxidation induced by O2·-, and photolytic rates vary with protonated states;1O2is not involved in the photolysis. Photolytic products provide further evidence on confirming the photooxidation of PBSA in aerated solutions.Effects of dissolved organic matters (DOM), halide ions (X-), HCO3-and CO32-on the photodegradation of PBSA were computed, and verified by experiments. The computational results indicate that DOM of4sources, including Suwannee River fulvic acid (SRFA), cannot photosensitize PBSA through energy or electron transfer, but the excited states of PBSA can be quenched by DOM; the excited states and cation radicals of PBSA can be quenched by I-and CO32-via spontaneous electron transfer. Therefore, the photodegradation of PBSA is possibly inhibited by DOM, I-and CO32-. Experimental results confirm that SRFA can inhibit the direct and self-sensitized photodegradation of PBSA; halide ions and HCO37CO32-can inhibit the photodegradation of PBSA to different extents, with I-showing significant inhibition effects.In order to reveal the mechanisms that1O2cannot react with PBSA, DFT computations were employed. Results indicate that PBSA can undergo thermodynamically favorable1,2-cycloaddtion with1O2, however, activation energies of the reactions are high. With the above method, reactions of PABA with1O2were computed, and the effects of protonation states were evaluated. The results show that PABA anion (PABA-) can undergo concerted1,3-addition,1,4-and1,2-cycloaddition, and stepwise1,2-cycloaddition with102. The1,3-addition is exothermic, spontaneous and lower in activation energies, and can generate hydroperoxide products, which is structurally similar to a photolytic product of PABA reported in a previous study. Other three protonated states of PABA, i.e. cation, neutral molecule and zwitterion, can undergo concerted1,3-addition with102. However, the activation energies vary with protonation states. The barrier is low for PABA with carboxyl group ionized, and increased if the amino group is protonated. Negative charge transfer from PABA to1O2was observed for all the reaction pathways, implying that transition states show zwitterionic characteristics. Dissociation enthalpies for O-O bond and vertical excitation energies of hydroperoxides calculated based on DFT imply that the hydroperoxides can potentially undergo O-O bond photodissociation.In summary, the study indicates that DFT computations can be employed for predicting the effects of water constituents on the photochemical behavior of organic pollutants; unveiling the mechanisms of the photooxidation pathways of organic pollutants with1O2, and predicting potential products.