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Stephen P. Mezyk

Bio: Stephen P. Mezyk is an academic researcher from California State University, Long Beach. The author has contributed to research in topics: Radiolysis & Hydroxyl radical. The author has an hindex of 34, co-authored 163 publications receiving 3876 citations. Previous affiliations of Stephen P. Mezyk include Idaho National Laboratory & University of California, Irvine.


Papers
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Journal ArticleDOI
TL;DR: These values represent the first direct measurements of k*(OH, DOM,) and they compare well with literature values obtained via competition kinetic techniques during ozone or ultraviolet irradiation experiments, and more polar, lower-molecular-weight DOM isolates from wastewater have higher k(OH), DOM values.
Abstract: Pulse radiolysis experiments were conducted on dissolved organic matter (DOM) samples isolated as hydrophobic and hydrophilic acids and neutrals from different sources (i.e., stream, lake, wastewater treatment plant). Absolute bimolecular reaction rate constants for the reaction of hydroxyl radicals (•OH) with DOM (k•OH, DOM) were determined. k•OH, DOM values are expressed as moles of carbon. Based on direct measurement of transient DOM radicals (DOM•) and competition kinetic techniques, both using pulse radiolysis, the k•OH, DOM value for a standard fulvic acid from the Suwannee River purchased from the International Humic Substances Society was (1.60 ± 0.24) × 108 M-1 s-1. Both pulse radiolysis methods yielded comparable k•OH, DOM values. The k•OH, DOM values for the seven DOM isolates from different sources ranged from 1.39 × 108 M-1 s-1 to 4.53 × 108 M-1 s-1, and averaged 2.23 × 108 M-1 s-1 (equivalent to 1.9 × 104 (mgC/L)-1 s-1). These values represent the first direct measurements of k•OH, DOM, and ...

330 citations

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TL;DR: It is suggested that UV/H2O2 and UV/S2O8(2-) advanced oxidation processes (AOPs) are capable of degrading β-lactam antibiotics decreasing consequently the antibiotic activity of treated waters.
Abstract: The extensive production and usage of antibiotics have led to an increasing occurrence of antibiotic residuals in various aquatic compartments, presenting a significant threat to both ecosystem and human health. This study investigated the degradation of selected β-lactam antibiotics (penicillins: ampicillin, penicillin V, and piperacillin; cephalosporin: cephalothin) by UV-254 nm activated H 2 O 2 and S 2 O 8 2− photochemical processes. The UV irradiation alone resulted in various degrees of direct photolysis of the antibiotics; while the addition of the oxidants improved significantly the removal efficiency. The steady-state radical concentrations were estimated, revealing a non-negligible contribution of hydroxyl radicals in the UV/S 2 O 8 2− system. Mineralization of the β-lactams could be achieved at high UV fluence, with a slow formation of SO 4 2− and a much lower elimination of total organic carbon (TOC). The transformation mechanisms were also investigated showing the main reaction pathways of hydroxylation (+16 Da) at the aromatic ring and/or the sulfur atom, hydrolysis (+18 Da) at the β-lactam ring and decarboxylation (–44 Da) for the three penicillins. Oxidation of amine group was also observed for ampicillin. This study suggests that UV/H 2 O 2 and UV/S 2 O 8 2− advanced oxidation processes (AOPs) are capable of degrading β-lactam antibiotics decreasing consequently the antibiotic activity of treated waters.

226 citations

Journal ArticleDOI
TL;DR: In this article, the absolute rate constants for reaction of three β-blockers (atenolol, metoprolol, and propranolol) with the two major AO/RP radicals; the hydroxyl radical (•OH) and hydrated electron (e−aq).
Abstract: Many pharmaceutical compounds and metabolites are currently found in surface and ground waters which indicates their ineffective removal by conventional water treatment technologies. Advanced oxidation/reduction processes (AO/RPs) are alternatives to traditional water treatment, which utilize free radical reactions to directly degrade chemical contaminants. This study reports the absolute rate constants for reaction of three β-blockers (atenolol, metoprolol, and propranolol) with the two major AO/RP radicals; the hydroxyl radical (•OH) and hydrated electron (e−aq). The bimolecular reaction rate constants for •OH are (7.05 ± 0.27) × 109, (8.39 ± 0.06) × 109, and (1.07 ± 0.02) × 1010, and for e−aq they are (5.91 ± 0.21) × 108, (1.73 ± 0.03) × 108, and (1.26 ± 0.02) × 1010, respectively. Transient spectra were observed for the intermediate radicals produced by hydroxyl radical reactions. In addition, preliminary degradation mechanisms and major products were elucidated using 60Co γ-irradiation and LC-MS. The...

173 citations

Journal ArticleDOI
TL;DR: Fundamental mechanistic parameters, hydroxyl radical and hydrated electron rate constants, and degradation efficiencies that are critical for the evaluation and implementation of advanced oxidation processes (AOPs) are provided.
Abstract: radical oxidation were determined as (8.3 ( 0.8) 10 9 , (7.9 ( 0.4) 10 9 , (8.5 ( 0.3) 10 9 , and (7.8 ( 0.3) 10 9 M -1 s -1 , respectively, with corresponding degradation efficiencies of 36% ( 6%, 46% ( 8%, 53% ( 8%, and 35% ( 5%. The reduction of these four compounds by their reaction with the hydrated electron occurred with rate constants of (2.4 ( 0.1) 10 10 , (2.0 ( 0.1) 10 10 , (1.0 ( 0.03) 10 10 , and (2.0 ( 0.1) 10 10 M -1 s -1 , respectively, with efficiencies of 0.5% ( 4%, 61% ( 9%, 71% ( 10%, and 19% ( 5%. We propose that hydroxyl radical adds predominantly to the sulfanilic acid ring of the different sulfa drugs based on similar hydroxyl radical rate constants and transient absorption spectra. In contrast, the variation in the rate constants for hydrated electrons with the sulfa drugs suggests the reaction occurs at different reaction sites, likely the different heterocyclic rings. The results of this study provide fundamental mechanistic parameters, hydroxyl radical and hydrated electron rate constants, and degradation efficiencies that are critical for the evaluation and implementation of advanced oxidation processes (AOPs).

145 citations

Journal ArticleDOI
TL;DR: Hydroxyl radical and chlorine radical steady-state concentrations are greatest under acidic conditions for all tested wavelengths and are highest using 254 and 311 nm irradiation.
Abstract: Chlorine photolysis is an advanced oxidation process which relies on photolytic cleavage of free available chlorine (i.e., hypochlorous acid and hypochlorite) to generate hydroxyl radical, along with ozone and a suite of halogen radicals. Little is known about the impact of wavelength on reactive oxidant generation even though chlorine absorbs light within the solar spectrum. This study investigates the formation of reactive oxidants during chlorine photolysis as a function of pH (6-10) and irradiation wavelength (254, 311, and 365 nm) using a combination of reactive oxidant quantification with validated probe compounds and kinetic modeling. Observed chlorine loss rate constants increase with pH during irradiation at high wavelengths due to the higher molar absorptivity of hypochlorite (p Ka = 7.5), while there is no change at 254 nm. Hydroxyl radical and chlorine radical steady-state concentrations are greatest under acidic conditions for all tested wavelengths and are highest using 254 and 311 nm irradiation. Ozone generation is observed under all conditions, with maximum cumulative concentrations at pH 8 for 311 and 365 nm. A comprehensive kinetic model generally predicts the trends in chlorine loss and oxidant concentrations, but a comparison of previously published kinetic models reveals the challenges of modeling this complex system.

128 citations


Cited by
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TL;DR: In this paper, the authors provide a state-of-the-art review on the development in heterogeneous catalysts including single metal, mixed metal, and nonmetal carbon catalysts for organic contaminants removal, with particular focus on peroxymonosulfate (PMS) activation.
Abstract: Sulfate radical-based advanced oxidation processes (SR-AOPs) employing heterogeneous catalysts to generate sulfate radical (SO4 −) from peroxymonosulfate (PMS) and persulfate (PS) have been extensively employed for organic contaminant removal in water. This article aims to provide a state–of–the–art review on the recent development in heterogeneous catalysts including single metal, mixed metal, and nonmetal carbon catalysts for organic contaminants removal, with particular focus on PMS activation. The hybrid heterogeneous catalyst/PMS systems integrated with other advanced oxidation technologies is also discussed. Several strategies for the identification of principal reactive radicals in SO4 −–oxidation systems are evaluated, namely (i) use of chemical probe or spin trapping agent coupled with analytical tools, and (ii) competitive kinetic approach using selective radical scavengers. The main challenges and mitigation strategies pertinent to the SR-AOPs are identified, which include (i) possible formation of oxyanions and disinfection byproducts, and (ii) dealing with sulfate produced and residual PMS. Potential future applications and research direction of SR-AOPs are proposed. These include (i) novel reactor design for heterogeneous catalytic system based on batch or continuous flow (e.g. completely mixed or plug flow) reactor configuration with catalyst recovery, and (ii) catalytic ceramic membrane incorporating SR-AOPs.

1,802 citations

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1,682 citations

Journal ArticleDOI
TL;DR: This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfates and the formation pathways of oxidizing species and the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry.
Abstract: Reports that promote persulfate-based advanced oxidation process (AOP) as a viable alternative to hydrogen peroxide-based processes have been rapidly accumulating in recent water treatment literature. Various strategies to activate peroxide bonds in persulfate precursors have been proposed and the capacity to degrade a wide range of organic pollutants has been demonstrated. Compared to traditional AOPs in which hydroxyl radical serves as the main oxidant, persulfate-based AOPs have been claimed to involve different in situ generated oxidants such as sulfate radical and singlet oxygen as well as nonradical oxidation pathways. However, there exist controversial observations and interpretations around some of these claims, challenging robust scientific progress of this technology toward practical use. This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfate and the formation pathways of oxidizing species. Properties of the main oxidizing species are scrutinized and the role of singlet oxygen is debated. In addition, the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry are discussed. The opportunity for niche applications is also presented, emphasizing the need for parallel efforts to remove currently prevalent knowledge roadblocks.

1,412 citations

Journal ArticleDOI
TL;DR: In this article, a review describes recent advances in the fundamental understanding of CO2 photoreduction on the surface of heterogeneous catalysts and particularly provides an overview of enhancing the adsorption/activation of CO 2 molecules.
Abstract: Large amounts of anthropogenic CO2 emissions associated with increased fossil fuel consumption have led to global warming and an energy crisis. The photocatalytic reduction of CO2 into solar fuels such as methane or methanol is believed to be one of the best methods to address these two problems. In addition to light harvesting and charge separation, the adsorption/activation and reduction of CO2 on the surface of heterogeneous catalysts remain a scientifically critical challenge, which greatly limits the overall photoconversion efficiency and selectivity of CO2 reduction. This review describes recent advances in the fundamental understanding of CO2 photoreduction on the surface of heterogeneous catalysts and particularly provides an overview of enhancing the adsorption/activation of CO2 molecules. The reaction mechanism and pathways of CO2 reduction as well as their dependent factors are also analyzed and discussed, which is expected to enable an increase in the overall efficiency of CO2 reduction through minimizing the reaction barriers and controlling the selectivity towards the desired products. The challenges and perspectives of CO2 photoreduction over heterogeneous catalysts are presented as well.

1,315 citations