Research Interests

Dr. Stephen Mezyk's Research

Environmental Remediation of Contaminated Waters

Background: Water shortages currently affect about 600 million people in over 30 countries around the world. Even when there is sufficient quantity, water quality still remains a major concern. The removal of unwanted chemicals and microbial or pathogen based diseases from water is a major, continuous, effort. To purify drinking water strong oxidants such as chlorine and ozone are typically used, however, these chemicals also react with dissolved natural organic matter to form unwanted products. These disinfection by-products (DBPs) include chlorinated/brominated and oxygenated species of methane's, acetic acids, acetonitriles, ketones and acetaldehydes. Many of these DBPs are suspected to have adverse health effects!

No universally acceptable solution exists for removing chemical contaminants or DBPs from water. Therefore, there is new interest in alternative approaches that offer in-situ chemical destruction. One new group of destructive technologies that has emerged for water clean-up (remediation) are those that generate the hydroxyl radical, ·OH, commonly referred to as Advanced Oxidation Processes (AOPs). These include addition of ozone, ozone in combination with ultraviolet (u.v.) light, or hydrogen peroxide and u.v. light. Several AOP's also produce both oxidizing and reducing species, such as heterogeneous catalysis with u.v./TiO2, sonolysis, and the electron beam irradiation process.

Our Research: We are interested in understanding the kinetics and mechanisms of reaction behind these AOP processes. Recent work has demonstrated that high-energy electron beam irradiation is effective in water disinfection, and that it can simultaneously destroy a wide range of organic compounds. Therefore, this method could potentially be used to selectively remove DBPs from disinfected water once the chemistry involved is sufficiently understood. Our focus therefore is on the electron beam AOP, as it generates all of the three radicals (·H, ·OH, and eaq-) that are used in all AOPs. The electron beam uses radiation chemistry as the basis for environmental remediation.

Presently, we are investigating the free-radical-induced chemistry of many different classes of compounds. We perform kinetic experiments to determine rate constants for their oxidation (by ·OH radicals) or reduction (by eaq-) in water. In addition, steady-state irradations (60Co) on aqueous solutions also allow us to determine the stable degradation products produced, which gives further insight into the mechanisms of these radicals' reactions. These data are all combined into computer kinetic models, which confirms our experimental findings, and allows optimization of these AOP applications on a commercial engineering scale. 

The projects involved in this area include:
  • Measuring absolute rate constants for the reactions of ·OH, eaq- and ·H with many different classes of contaminant chemicals/Disinfection by products (DBPs) (including nitrosamines (R1R2N-NO), nitramines (R1R2N-NO2), hydrazines (R1R2N-NH2), sulfa-based antibiotics, and sunscreen components)
  • Determining total degradation reaction mechanisms for the destruction of these contaminant chemicals, using transient and final product (HPLC, GC/MS) analyses
  • Measuring large-scale removal efficiencies of contaminants from water under natural conditions
  • Evaluating the impact of natural interferences such as carbonate, dissolved organic matter, pH, temperature, etc.
  • Creating kinetic computer programs to model the effectiveness of contaminant remediation by AOTs.

Kinetics and Mechanisms of Small Radicals in Solution

One of the fundamental interests in both pure and applied chemistry is understanding how solvent structures itself around different solutes, and how these structures change depending on external environment. This behaviour is exhibited in all phases, from gases to supercritical fluids. To better understand solvating processes and its effect on chemistry I am studying small radicals in condensed phase (water, organic solvents) to help characterise diffusion-, entropic- and enthalpic-controlled reactions. Specific projects include:
  • Studying radicals in different solvents over wide temperature ranges.
  • Investigating temperature effects on the reaction kinetics, thermodynamics and mechanisms on large, sterically-hindered organic radicals in non-aqueous solvents.
  • Studying radical reactions in mixed solvent systems.
Chemistry of Cancer

Nitrosoamines are ubiquitous within a number of environments and are of major concern as they have been shown to be carcinogenic, mutagenic, and teratogenic. They have been found in many food and beverage products,especially after fermentation or cooking, in cigarette smoke, and even in highly purified wastewater and drinking waters. The largest non-occupational exposure to nitrosamines is experienced by tobacco users due to tobacco-specific nitrosamines that are formed from nicotine during the processing of tobacco or during cigarette smoking.

Nitrosamine carcinogenesis has been studied for many years. Early investigations in rodents revealed a high degree of organ and cell specificity and that these carcinogens are activated at the target sites. This specificity has been attributed to different contributions from three different events; the organ-specific metabolic activation of these chemicals by different cytochrome P450 isoenzymes to yield alkylating agents and the extent of their reaction with DNA, the relative proportion of alkylation of DNA at oxygen, as opposed to nitrogen, atoms of purine and pyrimidine bases, and the DNA repair of O-alkylated bases.32

The cytochrome P450 activation of nitrosamines into mutagenic or carcinogenic species is understood to be initiated by the formation of an a-nitrosamine radical. This radical either combines with a hydroxyl radical within the catalytic site to form the alpha-hydroxynitrosamine, decomposes into free nitric oxide and N-methylformaldimine, or reacts with phosphates to produce mutagenic a-phosphonooxy derivatives.

It has also been demonstrated that nitrosamine activation can occur which is not P450-dependent. This pathway also requires the initial formation of the alpha-carbon radical, for example by the reaction of a reactive oxygen species (ROS) such as the hydroxyl radical, or ultraviolet light.

To establish this free-radical chemistry, we are investigating the formation of nitrosamine radicals under various conditions. This involves determination of the kinetics, and thermodynamics, of oxidation (hydroxyl radical reactions) and reduction (hydrated electron reactions) of many different nitrosamines in order to establish structure-activity relationships that can then be extrapolated to the more complex nitrosamines found in tobacco products. Following this formation, we are presently determining the quantitative yields of free nitric oxide formation, compared to the other alkylating pathways. In addition to these studies, we are also investigating the analogous classes of oxidized (nitramines) and reduced (hydroxylamines and hydrazines) nitrosamines.

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