The rate constant for the reaction of NO with ·O2− was determined to be (6.7 ± 0.9) × 109 1 mol−1 s−1, considerably higher than previously reported. Rate measurements were made from pH 5.6 to 12.5 ...

The reaction of no with superoxide

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.

Generation of sulfate radical through heterogeneous catalysis for organic contaminants removal: Current development, challenges and prospects

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.

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Persulfate-Based Advanced Oxidation: Critical Assessment of Opportunities and Roadblocks.

Chemistry of persulfates in water and wastewater treatment: A review

A rapid spectrophotometric determination of persulfate anion in ISCO

Rate constants have been determined for the reactions of SO4− with a series of alcohols, including hydrated formaldehyde. The SO4− radical was produced by the laser-flash photolysis of persulfate, S2O82−. Rate constants for the reactions of SO4− with alcohols range from 1.0 × 107 for methanol to 3.4 × 108 M−1 s−1 for 1-octanol. Rate constants for the reactions of SO4− with deuterated methanol and ethanol are lower by about a factor of 2.5. For methanol, ethanol, and 2-propanol, the temperature dependence of the rate constant was determined over the range 10–45°C.

Rate constants for hydrogen abstraction reactions of the sulfate radical, SO4−. Alcohols

Electron transfer reaction rates and equilibria of the carbonate and sulfate radical anions

Rate constants have been measured as a function of temperature over the range 10-60{degree}C for the reactions of the sulfate radical, SO{sub 4}{sup {minus}}, with azide, chloride, cyanate, cyanide, acetate, and carbonate anions Room-temperature rate constants ranged from about 5 {times} 10{sup 6} to 5 {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1} The variation of the rate constant with substrate depended mainly on the Arrhenius preexponential factor and less on the activation energy For the reaction of SO{sub 4}{sup {minus}} with Cl{sup {minus}}, the dependence of the rate constant on ionic strength was determined to be in agreement with the theoretical prediction

Temperature dependence of the rate constants for reactions of the sulfate radical, SO4-, with anions

Rate constants have been determined for the reactions of SO4− with a series of alkanes and ethers. The SO4− radical was produced by the laser-flash photolysis of persulfate, S2O82−. For methane, only an upper limit of 1 × 106 M−1 s−1 could be determined. For ethane, propane, and 2-methylpropane, rate constants of 0.44, 4.0, and 10.5 × 107 M−1 s−1 were found. For ethyl and n-propyl ether, rate constants of 1.3 × 108 and 2.2 × 108 M−1 s−1 were found and for 1,4-dioxane and tetrahydrofuran, rate constants of 7.2 × 107 and 2.8 × 108 were obtained. The reaction of SO4− with allyl alcohol was also studied and found to have a rate constant of 1.4 × 109 M−1 s−1.

Rate constants for hydrogen abstraction reactions of the sulfate radical, SO4−. Alkanes and ethers

Rate constants have been determined for the reactions of NO{sub 2} with SO{sub 3}{sup 2{minus}} and HSO{sub 3}{sup {minus}} in aqueous solutions. A pulse radiolysis apparatus with signal averaging, which has allowed us to monitor the decay of NO{sub 2} directly and to measure rate constants for the reaction of NO{sub 2} with SO{sub 3}{sup 2{minus}} and HSO{sub 3}{sup {minus}} over the pH range 5.3-13. The rate constant increases from about 1.2 {times} 10{sup 7} M{sup {minus}1} s{sup {minus}1} near pH 5 to 2.9 {times} 10{sup 7} M{sup {minus}1} s{sup {minus}1} at pH 13. The reaction appears to involve the formation of an intermediate complex that may undergo subsequent reaction with NO{sub 2} to yield the ultimate products or may react with other substrates present. The formation of a long-lived intermediate would have implications on the chemistry of flue gas scrubbers and on luminol-based NO{sub 2} detectors.

Carol L. Clifton

Papers

Rate constants for hydrogen abstraction reactions of the sulfate radical, SO4−. Alcohols

Electron transfer reaction rates and equilibria of the carbonate and sulfate radical anions

Temperature dependence of the rate constants for reactions of the sulfate radical, SO4-, with anions

Rate constants for hydrogen abstraction reactions of the sulfate radical, SO4−. Alkanes and ethers

Rate constant for the reaction of nitrogen dioxide with sulfur(IV) over the pH range 5.3-13.