Kinetics and Mechanism of Ultrasonic Activation of Persulfate: An in Situ EPR Spin Trapping Study

Ultrasound (US) was shown to activate persulfate (PS) providing an alternative activation method to base or heat as an in situ chemical oxidation (ISCO) method. The kinetics and mechanism of ultrasonic activation of PS were examined in aqueous solution using an in situ electron paramagnetic resonance (EPR) spin trapping technique and radical trapping with probe compounds. Using the spin trap, 5,5-dimethyl-1-pyrroline-<i>N</i>-oxide (DMPO), hydroxyl radical (^•OH) and sulfate radical anion (SO₄^•–) were measured from ultrasonic activation of persulfate (US-PS). The yield of ^•OH was up to 1 order of magnitude greater than that of SO₄^•–. The comparatively high ^•OH yield was attributed to the hydrolysis of SO₄^•– in the warm interfacial region of cavitation bubbles formed from US. Using steady-state approximations, the dissociation rate of PS in cavitating bubble systems was determined to be 3 orders of magnitude greater than control experiments without sonication at ambient temperature. From calculations of the interfacial volume surrounding cavitation bubbles and using the Arrhenius equation, an effective mean temperature of 340 K at the bubble–water interface was estimated. Comparative studies using the probe compounds <i>tert</i>-butyl alcohol and nitrobenzene verified the bubble–water interface as the location for PS activation by high temperature with ^•OH contributing a minor role in activating PS to SO₄^•–. The mechanisms unveiled in this study provide a basis for optimizing US-PS as an ISCO technology

Kinetics and Mechanism of the Oxidation of Cyclic Methylsiloxanes by Hydroxyl Radical in the Gas Phase: An Experimental and Theoretical Study

The ubiquitous presence of cyclic volatile methylsiloxanes (cVMS) in the global atmosphere has recently raised environmental concern. In order to assess the persistence and long-range transport potential of cVMS, their second-order rate constants (<i>k</i>) for reactions with hydroxyl radical (^•OH) in the gas phase are needed. We experimentally and theoretically investigated the kinetics and mechanism of ^•OH oxidation of a series of cVMS, hexamethylcyclotrisiloxane (D₃), octamethycyclotetrasiloxane (D₄), and decamethycyclopentasiloxane (D₅). Experimentally, we measured <i>k</i> values for D₃, D₄, and D₅ with ^•OH in a gas-phase reaction chamber. The Arrhenius activation energies for these reactions in the temperature range from 313 to 353 K were small (−2.92 to 0.79 kcal·mol^–1), indicating a weak temperature dependence. We also calculated the thermodynamic and kinetic behaviors for reactions at the M06-2X/6-311++G**//M06-2X/6-31+G** level of theory over a wider temperature range of 238–358 K that encompasses temperatures in the troposphere. The calculated Arrhenius activation energies range from −2.71 to −1.64 kcal·mol^–1, also exhibiting weak temperature dependence. The measured <i>k</i> values were approximately an order of magnitude higher than the theoretical values but have the same trend with increasing size of the siloxane ring. The calculated energy barriers for H-atom abstraction at different positions were similar, which provides theoretical support for extrapolating <i>k</i> for other cyclic siloxanes from the number of abstractable hydrogens

Mechanistic study on the role of soluble microbial products in sulfate radical-mediated degradation of pharmaceuticals

The role of soluble microbial products (SMP), the most important component of effluent organic matter from municipal wastewater treatment plants, in sulfate radical (SO4•–)-based advanced oxidation technologies (AOTs) remains substantially unclear. In this study, we first utilized a suite of macro- and microanalytical techniques to characterize the SMP from a membrane bioreactor for its fundamental molecular, spectroscopic, and reactivity properties. The degradation kinetics of three representative pharmaceuticals (i.e., naproxen, gemfibrozil, and sulfadiazine) in the presence of SMP was significantly reduced as compared to in its absence. Possible mechanisms for the interference by SMP in degrading these target compounds (TCs) were investigated. The low percentage of bound TCs to SMP ruled out the cage effect. The measurement of steady-state 1O2 concentration indicated that formation of 1O2 upon UV irradiation on SMP was not primarily responsible for the degradation of TCs. However, the comparative and quenching results reveal that SMP absorbs UV light acting as an inner filter toward the TCs, and meanwhile scavenges SO4•– with a high second-order rate constant of 2.48 × 108 MC–1 s–1

Determination and Environmental Implications of Aqueous-Phase Rate Constants in Radical Reactions

Interests in the kinetics of radical–induced reactions in aqueous solution have grown remarkably due to their water engineering significance (e.g., advanced oxidation processes). Although compilations of the rate constants (k) for various radicals have been documented, surprisingly a systematic review has yet to be reported on the development of reliable methods for determining k values. A knowledge gap exists to critically evaluate and screen the various methods to measure them. In this review, we summarize the direct and indirect methods under steady–state and non–steady–state conditions, followed by critical evaluations on their advantages and disadvantages. The radicals of ·OH, SO4·−, O2·−, and Cl· were chosen based on their significant aquatic environmental relevance. MS excel spreadsheets that demonstrate the determination processes were provided allowing one to reproduce the data and/or to analyze the unprocessed raw data as a “template”. We formulated a standard operation procedure for the k determination, although there is simply no “versatile” method fitting for all radical reactions. Finally, existing challenges and future research focus are discussed. This is the first review covering methodological approaches and considerations, aiming to provide a holistic and fundamental basis to choose an appropriate method for determining the k values for bimolecular reactions between target compounds and radicals in the aqueous phase

Mechanistic insight into superoxide radical-mediated degradation of carbon tetrachloride in aqueous solution: An in situ spectroscopic and computational study

Contemporary studies emphasize that superoxide radical (O2∙-) exhibits the potential to degrade organic contaminants, but practical application of this radical in engineered waters require an in-depth understanding of its kinetic profiles in a quantitative way. Here, we developed, for the first time, a convenient and reliable approach to generate micromolar level O2∙- in aqueous solution by photolysis of formate and H2O2. The presence of O2∙- was confirmed by comparing the UV spectra under pulse radiolysis and chromogenic reaction. We then constructed an in situ long-path spectroscopy to investigate the kinetics and mechanisms of O2∙--mediated degradation of carbon tetrachloride (CCl4), a halogenated model contaminant. The rate constant for the reaction of O2∙- and CCl4 was determined to be 478 M−1 s−1. In addition, we employed the transition state theory to model the reaction rate constants. Both results show that O2∙- exhibited low reactivity towards CCl4 with bimolecular rate constant lower by at least one order of magnitude than those radicals generated in typical advanced oxidation processes such as hydroxyl and sulfate radicals. Our results also indicate that nucleophilic substitution is the major pathway, and the solvation effect plays an important role in the reaction. The complementary experimental and theoretical approaches provide a mechanistic basis for better understanding aqueous–phase O2∙- chemistry and a holistic evaluation on the application of O2∙- for the degradation of organic contaminants of emerging concern