Date of Award

2023

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Environmental Engineering (PhD)

Administrative Home Department

Department of Civil, Environmental, and Geospatial Engineering

Advisor 1

Daisuke Minakata

Committee Member 1

Jiehong Guo

Committee Member 2

Loredana Valenzano-Slough

Committee Member 3

Marina Tanasova

Committee Member 4

Paul Doskey

Abstract

Photochemically produced reactive intermediates (PPRIs) such as the hydroxyl radical, carbonate radical (CO3•-) singlet oxygen (1O2) and triplet state of chromophoric dissolved organic matter (3CDOM*) are formed in sunlit natural waters upon photoexcitation of chromophoric dissolved organic matter (CDOM). PPRIs react with the organic compounds involved in key environmental processes, resulting in transformation products of smaller molecular weight than their parent compounds. Photochemical transformation of these key water constituents due to their reactions with PPRIs may pose potential effects on human and aquatic ecosystems. Consequently, there is a need to improve our understanding of the reactivity of PPRIs with key organic compounds of environmental relevance.

Due to the large number of organic compounds present in natural waters and the complexity of their reactions with PPRIs, only a limited number of compounds have been experimentally studied. Moreover, the short lifetime and high reactivity of PPRIs makes it difficult to elucidate the reaction pathways embedded in the photochemical transformation of organic compounds. Furthermore, the structural complexity of CDOM hinders the elucidation of the impact of individual functional groups on the photochemical transformation of organic compounds. Quantum-mechanical density functional theory (DFT)-based calculations represent an attractive alternative to study the kinetics and reaction mechanisms of PPRIs with organic compounds.

This dissertation aims to advance the current understanding of the reactivity of PPRIs with organic compounds by combining experiments with DFT computations. I studied the kinetics and reaction mechanisms of 1O2, CO3•-, and 3CDOM* with organic compounds of environmental relevance. By developing linear free-energy relationships, I obtained mechanistic insights into the rate-limiting steps controlling the photochemical transformation of organic compounds. Subsequently, I theoretically studied the photochemical fate of methionine in the presence of three model CDOM (i.e., 1,4-naphthoquinone, 2-naphthaldehyde, and umbelliferone) under simulated sunlight irradiation by combining benchtop-scale results from our laboratory and DFT xxvii computations. I developed an elementary reaction-based kinetic model that was consistent with the experiments, validating the theoretical results. This dissertation highlights a roadmap of environmental research using an integrated approach of advanced experimental and theoretical techniques to discover unknown mechanisms that have not been elucidated by conventional approaches.

Download

Share

COinS