PhD Dissertation Defense: Mamatha Hopanna

Dr. Lee Blaney

Photolytic fate of organo-selenium and-tin compounds in natural and engineered water systems

Organometallic chemicals contain at least one metal atom covalently bound to at least one carbon atom. Due to their sorption, redox, and catalysis properties, organometallics are commonly used as antibiotics, antioxidants, chemotherapy agents, pesticides, and semiconductors in the biomedical, agricultural, and electrical fields. Unlike conventional organic compounds, organometallics have unique properties that introduce distinct environmental concerns and imply interesting environmental chemistry. While photochemical degradation is a significant abiotic process that governs the fate of organic pollutants in water, few studies have elucidated the photochemical transformation of organometallics in natural (i.e., 365 nm) and engineered (i.e., 254 nm) systems. 

This dissertation (i) assessed the suitability of conventional protocols, which are employed to study the photochemistry of organic contaminants, for organometallics, (ii) measured quantum yields for direct photolysis of organometallics, specifically organo-selenium (ebselen) and -tin (triphenyltin hydroxide), at 254 nm and 365 nm, (iii) determined the second-order rate constants for the reaction of organometallic chemicals with the singlet oxygen (1O2), hydroxyl radical (•OH), and triplet state dissolved organic matter (3DOM*) reactive species, (iv) predicted the half-lives of organometallics in diverse water sources using the Aqueous Photochemistry of Environmentally occurring Xenobiotics (APEX) modeling tool, and (v) identified the primary photoproducts and toxicity of organometallics in the UV-254 and UV-H2O2 treatment systems. 

Ebselen exhibited a higher photoreactivity compared to its carbon analog (C-ebselen). In particular, the apparent quantum yields at 254 nm for ebselen and C-ebselen were (13.6 ± 0.1) × 10-2 mol Ein-1 and (2.0 ± 0.7) × 10-2 mol Ein-1, respectively. For 365 nm, ebselen exhibited an apparent quantum yield of (1.4 ± 0.1) × 10-3 mol Ein-1, but the quantum yield of C-ebselen was much lower (i.e., < 2.8 × 10-5 mol Ein-1). Atypical phototransformation kinetics were observed for ebselen in the presence of reactive species sensitizers, scavengers, and quenching agents due to ebselen reaction with active intermediates (e.g., superoxide radical anion, triplet sensitizers, and secondary radicals) that are not kinetically relevant for most organic contaminants. These findings confirmed that the selenium atom leads to the high photoreactivity of ebselen and informed proper protocols for future study of organoselenium compounds. 

Triphenyltin hydroxide exhibited negligible direct photolysis at 365 nm, and indirect photolysis by 1O2, •OH, and 3DOM* were the dominant photodegradation mechanisms. The second-order rate constants for triphenyltin hydroxide reaction with 1O2, ⦁OH, and 3DOM* were (3.9 ± 0.5) × 106 M-1 s-1, (7.81 ± 0.37) × 108 M-1 s-1, and (1.41 ± 0.06) × 106 M-1 s-1, respectively. APEX model simulations indicated that the interquartile range (i.e., 25-75th percentiles) of triphenyltin hydroxide half-lives were as follows: 126-262 d in surface water; 77-178 d in wastewater effluent; 55-126 d in stormwater; 51-78 d in wetlands; and, 106-202 d in natural organic matter extracts. In contrast, triphenyltin hydroxide underwent rapid degradation at 254 nm with an apparent quantum yield of 0.18 ± 0.02 mol Ein-1. However, the triphenyltin hydroxide removal efficiency was only 2-10% for typical operating conditions of UV-254 systems. The UV-H2O2 advanced oxidation process not only improved the removal efficiency to 50-92%, but also resulted in less toxic photoproducts, identified by growth inhibition of Staphylococcus spp., compared to UV-254 treatment. 

In summary, this dissertation reports critical knowledge on the complex photochemical behavior of organo-selenium and -tin compounds for UV-based treatment processes, advanced oxidation systems, and the natural environment. The information reported in this dissertation will assist with (i) understanding the fate of organometallics in natural systems and current water/ wastewater treatment processes and (ii) selecting appropriate photochemical/photocatalytic treatment systems for legacy and emerging organometallic chemicals.

11:00 AM EST - Presentation followed by questions from the audience.