CHARACTERIZATION AND TRANSFORMATION OF DISSOLVED ORGANIC MATTER (DOM) IN ENGINEERED ULTRAVIOLET (UV) PHOTOLYSIS AND UV-BASED ADVANCED OXIDATION PROCESSES

Dissolved organic matter (DOM) is a complex mixture of heterogeneous surrogate organic compounds and includes natural organic matter (NOM), dissolved organic nitrogen (DON), dissolved organic phosphorous (DOP) and soluble microbial products (SMP). DOM plays a key role in moderating the aquatic biogeochemical cycle in freshwater ecosystems. Shedding light into the composition of freshwater DOM will aid in understanding the biogeochemical dynamics of freshwater ecosystems and consequently the role of DOM as a local climate regulator. An increase in de facto wastewater reuse has made the removal of DOM present in wastewater imperative, as DOM can be potentially toxic to human health. Additionally, wastewater DOM can reduce the efficiency of the treatment performances of advanced treatment techniques such as UV-advanced oxidation processes (UV-AOPs), as DOM can screen UV light and scavenge reactive radical species, thereby reducing the concentrations of reactive radical species that are available to destroy the target trace organic contaminants. Therefore, understanding the molecular composition of DOM present in wastewater as well as in the effluent to surface water is crucial, as this knowledge will aid in upgrading treatment performances and subsequently contribute towards the effective removal of target contaminants. In this regard, ultrahigh resolution mass spectrometry was used to evaluate the molecular composition of DOM present in wastewater as well as in the effluent to surface water. DOM composition is characterized in terms of H/C and O/C elemental ratios and double bond equivalents (DBEs). Effluent DOM was found to comprise of a higher number of unsaturated aromatic species relative to wastewater DOM. Species with nitrogen and sulfur heteroatoms are less common among effluent DOM relative to wastewater DOM. In order to better understand the interference of DOM in the treatment performances of individual UV-AOPs, the transformation of a standard DOM isolate, i.e. Suwannee River Fulvic Acid (SRFA) as well as municipal effluent wastewater DOM (EfOM) were investigated in UV/H2O2, UV/free chlorine and UV/persulfate AOPs using ultrahigh resolution mass spectrometry. The changes in the H/C and O/C elemental ratios, DBEs, and the low-molecular-weight transformation product concentrations reveal that different DOM and EfOM transformation patterns are induced by each UV-AOP. DOM and EfOM transformation routes induced by hydroxyl, chlorine and sulfate radicals were mechanistically elucidated by comparing the known reactivities of each active radical species with specific organic compounds. In the UV/H2O2 system, distinct transformation of aliphatic components of DOM and transformation of olefinic as well as aliphatic components of EfOM was observed. In the UV/free chlorine system, transformation of aliphatic as well as olefinic species of DOM and transformation of aromatic species of EfOM was observed. Transformation of aromatic and olefinic moieties of both DOM as well as EfOM was observed in the UV/persulfate system. Transformation products of DOM such as chlorinated compounds are of concern due to their adverse health impacts. However, only a small portion of these compounds have been well-characterized, thus necessitating the characterization of the unknown compounds. Plausible structural information about chlorinated compounds produced from UV/free chlorine triggered transformation of DOM is provided through the means of tandem mass spectrometry (MS/MS) with ultrahigh resolution mass analysis. The two compounds of interest were hypothesized to be composed of alcohol, carboxylic acid and aldehyde/ketone groups