Photo-Transformation Mechanisms of Trace Organic Compounds by Reactive Oxygen Species

Abstract

The increasing demand for potable water is forcing municipalities around the world to utilize alternative water sources. In the most water-stressed regions the use of these alternatives has or will become a necessity to meet future water demand. One of the most promising alternatives is the reuse of municipal wastewater for potable purposes. However, the well-documented presence of persistent Trace Organic Compounds (TOrCs) at concentrations that are physiologically active in this water resource provides environmental and human health challenges that must be addressed. Despite the existence of proven technologies for the removal of pathogens and organic pollutants in wastewater, the presence of TOrCs that survive conventional wastewater treatment may cause potential effects to ecosystems and human health. While it has been proven that some TOrCs are naturally attenuated via biodegradation, photo-degradation and/or adsorption, many others persist in the environment as contaminants in surface and ground waters. Many different treatments have been tested for the removal of TOrCs. However, variability of: (i) chemical and physical properties of TOrCs, (ii) chemical and physical characteristics of the wastewater effluent, and (iii) environmental exposure, all play a significant role in TOrCs fate, which highlights the need to understand their transformation pathways and mechanisms. This work presents research focused on understanding the degradation of TOrCs by indirect photolysis, which comprises reactions generated by different sources of light and Reactive Oxygen Species (ROS). The photo-transformation of different TOrCs (furfuryl alcohol, p-cresol, caffeine, carbamazepine and Bisphenol A) under conditions relevant to both engineered treatment systems and natural attenuation was investigated. A comprehensive mechanism and complementary kinetic model were proposed to predict the degradation of TOrCs by reaction with singlet oxygen, hydroxyl radicals, and triplet excited states that are created by effluent organic matter (EfOM). The effect of light wavelength was also explored. Mathematical modeling based on quantum efficiencies and reaction rates accounted for light shading and competitive effects. The resulting modelling comprises a base for developing predictive software for the degradation of TOrCs in wastewater by oxidation processes.