Radical Probe Research Articles

Fluorine-free MXene activate peroxymonosulfate to remove tetracyclic antibiotics

The most typical synthesis method for MXene is to etch the Al layer from its MAX phase using an acidic solution containing fluorine. However, etching with fluorine-containing reagents can lead to environmental contamination and experimental safety issues. Herein, fluorine-free Ti3C2Tx (Ff-Ti3C2Tx) was successfully prepared by using a safer and more environmentally friendly tetramethylammonium hydroxide solution (TMAOH) at a controlled temperature of 40 °C and reacting continuously for one week. Based on the characterization results, we demonstrated that the prepared Ff-Ti3C2Tx exhibits outstanding hydrophilicity and exposes more active sites on the surface, which is beneficial towards activating peroxymonosulfate (PMS). The catalyst exhibited superior degradation of tetracycline hydrochloride (TC) than Fluorine-Ti3C2Tx (F-Ti3C2Tx). The effects of catalyst dosage, optimum concentration of PMS, initial pH, common coexisting ions (Cl−, SO42−, HCO3− and humic acid) and different real water environments on TC degradation were systematically investigated in the Ff-Ti3C2Tx/PMS system. Meanwhile, the degradation effects of both Doxycycline (DC) and Oxytetracycline (OTC) reached more than 89.7%. In addition, Ff-Ti3C2Tx exhibited excellent reusability with a mineralization efficiency of 49.7%. Based on the electron paramagnetic resonance (EPR) test, quenching experience, radical probe experiments, electrochemical measurements and XPS characterization, a possible activation mechanism was proposed. The contribution of reactive oxygen species (ROS) was calculated, 1O2 (45.3%) and ·O2− (30.2%) play a major role in the degradation of TC. Furthermore, a possible degradation pathway was proposed by liquid chromatography-mass spectrometry (LC-MS) with a decreasing trend in the toxicity of TC intermediates. This work not only demonstrates a novel strategy for preparing fluorine-free MXene via simple TMAOH etching, but it also provides new insight into the use of MXene as a catalyst directly in the activation of PMS.

Dark transformation from 17β-estradiol to estrone initiated by hydroxyl radical in dissolved organic matter

The occurrence and fate of 17β-estradiol (E2) in natural water have gained extensive attention owing to its high ecotoxic risk to wildlife. Dissolved organic matter (DOM) is a ubiquitous water constituent and contributes significantly to E2 removal, although the reaction mechanism is rarely clarified. The present study aims to investigate E2 transformation in water containing fresh or aged DOM surrogates at environmentally relevant concentrations in the dark. Experiments along with radical probes of benzene and furfuryl alcohol reveal that reactive radicals, particularly hydroxyl radical (·OH), formed non-photochemically at higher concentrations in aged DOM than in fresh DOM. The contribution of ·OH in E2 removal is indicated by the decreases in the removal of radical probes in the presence of E2; moreover, E2 removal is inhibited in the presence of radical scavengers. The dose-dependent inhibitive effect of substrate concentrations, including E2 and coexistent propylparaben, shows that the radical concentration is a limiting factor for E2 removal, which could be enhanced by increasing DOM concentration, dissolved oxygen, and light supply. As the main byproduct, estrone (E1) is persistent in the current DOM water in the dark, but it can be easily photodegraded when exposed to light. Theoretical analysis reveals that the initial step is ·OH-initiated H- abstraction on the hydroxyl group in the cyclopentane ring of E2. The formed singlet excited state of E2 undergoes further intramolecular rearrangement and oxidative dehydrogenation to generate E1 and the hydroperoxy radical (·HO2). Considering the universal occurrence of E2 in DOM-rich aquatic matrices, the present findings have special implications for the biogeochemical cycle and risk assessment of this pollutant in natural aquatic environments, particularly those beyond the photic zone.

Degradation of 1,2,3-trichloropropane by pyrite activating sodium percarbonate and the implications for groundwater remediation

1,2,3-trichloropropane (TCP) is a newly regulated groundwater contaminant, however, its remediation technology has not been fully investigated. In this research, pyrite (FeS2) was used as a novel activator of sodium percarbonate (SPC) for TCP degradation. TCP could be effectively removed by FeS2/SPC system with a much higher removal efficiency than the classic Fenton system. Free radical quenching test, hydroxyl radical probe reaction, EPR, XPS, XRD and SEM were used to elucidate the reaction mechanism. SPC was decomposed to produce H2O2, which was activated by Fe2+ dissolved from pyrite to produce HO· and O2-·. Both HO· and O2-· contributed to TCP degradation, but HO· played a more important role. The S elements of pyrite promoted the regeneration of Fe2+, thus enhancing the TCP degradation. The TCP removal efficiency significantly increased with increases in concentrations of pyrite and SPC and decreases in pyrite particle size and pH. Although HCO3-, Cl- and NOM inhibited TCP degradation by FeS2/SPC, the reaction system could still remove TCP in real groundwater. Overall, pyrite-activate SPC is a promising in-situ groundwater remediation technology.

Roles of radical species in vacuum-UV/UV/peroxydisulfate advanced oxidation processes and contributions of the species to contaminant degradation at different water depths

Vacuum-UV (VUV) (wavelength 185 nm)/ UV (wavelength 254 nm) are applied to improve performances of UV-based advanced oxidation processes. However, the improvements were strongly affected by water depth because of poor VUV transmittance in water. In this study, VUV/UV and peroxydisulfate (PDS) were used to degrade carbamazepine. More SO4•− oxidation occurred in VUV/UV/PDS than VUV/UV with similar •OH oxidation occurring. The additional SO4•− oxidation could be caused by VUV/PDS in superficial water or UV/PDS in deeper water. The synergistic factor for VUV/UV/PDS processes relative to VUV/UV and UV/PDS processes was 1.32. VUV/UV/PDS performances were affected by competition for photon absorption by dissolved organic matter (32–58 % inhibition), radical quenching by CO32−/HCO3− and NO3−, and conversion of •OH and SO4•− into reactive chlorine species by Cl−. Radical probe experiments and steady-state kinetic modeling simulations indicated that 34 %, 25 %, and 40 % of carbamazepine degradation occurring in 2-cm-deep bulk solution was due to •OH oxidation through VUV/H2O, SO4•− oxidation through VUV/PDS, and SO4•− oxidation through UV/PDS, respectively. Contribution of VUV-driven processes decreased with increasing water depth and became equivalent to contribution of 3.5-cm-deep UV-driven processes, which indicated the importance of optimizing water depth in VUV/UV-advanced oxidation process reactors.

Catechol‑iron-clay surface complexation enriches radical formation and efficiency in heterogeneous Fenton reactions

Heterogeneous Fenton reactions are complex and to date, the role of surface confinement (of the pollutant and oxidant) on the reaction rates, the parameters that control the type and concentration of radical species produced, and the properties that impact the extent of degradation are still not fully understood. Here, the degradation behavior of several phenolic and carboxylic compounds was monitored in the presence of H2O2 and an iron-oxide-coated clay catalyst. The role of compound structure on the extent of surface complexation, iron-oxide reduction, H2O2 activation, and phenol degradation was examined. The two compounds carrying a catechol group, catechin (CC) and 3,4-dihydroxybenzaldehyde (DHB), had the highest adsorption affinity towards the catalyst (KL = 34 and 29 L∙g−1, respectively), induced iron-reduction on the surface, and were rapidly degraded within 5 min. In contrast, the compounds with single phenol or carboxylic groups exhibited low surface complexation affinity, did not reduce the surface-bound iron(III), and needed 24 h to completely degrade. Radical probe experiments showed the formation of hydroxyl, superoxide, and singlet oxygen reactive species for CC and DHB. In contrast, mainly hydroxyl radicals were produced in the presence of the latter compounds. Furthermore, when the catechol-carrying compounds were added alongside the less reactive compounds, they helped induced rapid degradation, reducing the less reactive compounds' half-lifetime by an order of magnitude. These results suggest that wastewaters that are rich in catechol groups may be highly suited for this type of heterogeneous Fenton remediation process. In addition, the basic insight gained regarding surface complexation with the iron(III)-oxide-coated clay and the structure-activity relationship controlling radical formation can aid in the design of improved, efficient heterogeneous Fenton systems for recalcitrant pollutants in soils and water.

Tuning nanostructured CuCo2O4 on 3D macro-support for enhanced degradation of carbofuran via catalytic activation monopersulfate: Key roles of morphology and active species

While a few OH-based oxidation technologies have been used for eliminating the highly toxic pesticide Carbofuran (CFR), very few studies have ever assessed SO4−-based oxidation technologies for degrading CFR. Even though monopersulfate (MPS) is increasingly used for producing SO4− and cobalt (Co) is the most useful element for activating MPS, almost no study has ever been implemented to evaluate cobaltic catalysts for activating MPS to degrade CFR. Thus, the goal of this study is to develop Co-based catalysts for activating MPS to degrade CFR. In particular, a hierarchic composite which is fabricated based on macroscale Cu foam (CF) with the decoration of nanoscale Co-based oxide is fabricated to combine the macroscale practicability, and nanoscale functionality of Co-based oxide. The modification of CF with Co would grow CuCo2O4 directly on CF, leading to the formation of CuCo2O4@CF (CCO@CF). Moreover, different cobalt salts and dosages of urea might alter the formation of CuCo2O4, leading to various morphologies and nanostructures, such as CCO@CF with filament (CCF), CCOF@CF with thorn (CCT), and CCOF@CF with sheet (CCS). Interestingly, CCS exhibited the faster interfacial reaction rate, lower charge transfer resistance, and more abundant Co2+ sites, and oxygen vacancy, enabling CCS to show the highest catalytic activity for MPS activation to degrade CFR among these three CCO@CFs. CCS also exhibits a much lower Ea of CFR degradation than CCF, CCT, and CuOF, revealing its advantage over other catalysts. Moreover, the degradation mechanism of CFR by CCO@CF-activated MPS has been also elucidated using the tests of radical probes, and electron paramagnetic resonance for identifying contributions of OH, SO4−, and 1O2 to CFR degradation. These results validate that CCO@CFs are useful heterogeneous catalysts for MPS activation, and especially, CCS appears as the most favorable CCO@CF to eliminate the toxic CFR.

Structural Anomaly in Glasses: Molecular Dynamics Study of Organic Radical in Dibutylphthalate at Different Temperatures

Understanding the heterogeneous nano/microscopic structures of various organic glasses is fundamental and necessary for many applications. Recently, unusual structural phenomena have been observed experimentally in various organic glasses near their glass transition temperatures (Tg), including dibutyl phthalate (DBP). In particular, the librational motion of radical probe in the glass is progressively suppressed upon temperature increase. In this work, we report in-depth molecular dynamics studies of structural anomalies in DBP glass, that revealed insights into the general mechanism of these phenomena. In particular, we have evidenced that the two types of solvation within alkyl chains coexist, allowing only small-angle wobbling of the solute molecule (TEMPO radical), and another favouring large-angle rotations. The former solvation assumes constrained location of the solute near carboxyl groups of DBP, while the latter is coupled to the concerted movement of butyl chains. Remarkably, excellent qualitative and quantitative agreement with previous experimental results were obtained. As such, we are certain that the above-mentioned dynamic phenomena explain the intriguing structural anomalies observed in DBP and some other glasses in the vicinity of Tg.

Tin removal by an annular surface wave plasma antenna in an extreme ultraviolet lithography source

Tin contamination of the collector mirror surface remains one of the crucial issues of EUV (Extreme Ultraviolet) sources, directly impacting the availability of the tool. Hydrogen plasma-based tin removal processes employ hydrogen radicals and ions to interact with tin deposits to form gaseous tin hydride (SnH4), which can be removed through pumping. An annular surface wave plasma (SWP) source developed at the University of Illinois—Urbana Champaign is integrated into the cone and perimeter of the collection mirror for in situ tin removal. The SWP is characterized by high ion and radical densities, low electron temperature, and local generation where etching is needed. This method has the potential to significantly reduce downtime and increase mirror lifetime. Radical probe measurements show hydrogen radical densities in the order of 1019 m−3, while Langmuir probe measurements show electron temperatures of up to 6 eV and plasma densities on the order of 1017–18 m−3. The generated ions are essential to the tin cleaning and have sufficiently low energy to cause no damage to the collector capping layer. Tin etch rates of up to 270 nm/min were observed in a variety of experimental conditions, including various powers, pressures, flowrates, and temperatures. The high etch rates demonstrated in this study exceed the expected contamination rate of the EUV source.

Elimination of bisphenol A with visible light-enhanced peroxydisulfate activation process mediated by Fe3+-nitrilotriacetic acid complex

In recent years, visible light assisted advanced oxidation processes (AOPs) are appealing in the elimination of pollutants. Herein, an innovative and eco-friendly visible light enhanced Fe3+-nitrilotriacetic acid system for the activation of peroxydisulfate (Vis/Fe3+-NTA/PDS) was proposed for the removal of bisphenol A (BPA). Fe3+-NTA could be dissociated through ligand-to-metal charge transfer (LMCT) to realize the generation of Fe2+ for the continuous activation of PDS to remove BPA. The use of 0.10 mM Fe3+, 0.10 mM NTA and 1.00 mM PDS led to 97.5% decay of 0.05 mM BPA and 66.3% of TOC removal in 30 min with the illumination of visible light at initial pH 3.0. The sulfate and hydroxyl radicals were proved to be the dominant species leading to BPA removal by means of radical scavenging experiments, radical probes and electron paramagnetic resonance (EPR) technique. The effects of various operating parameters, natural water constituents as well as different water matrices on BPA abatement were explored. The intermediate products of BPA degradation were identified and a possible transformation pathway was proposed. Briefly, this research provides an attractive strategy for the remediation of refractory wastewater using NTA assisted with visible light in the homogeneous Fe3+/PDS system.

Oxo-Carotenoids as Efficient Superoxide Radical Scavengers.

Oxo-carotenoids containing conjugated carbonyl groups in their chains were designed to be more efficient superoxide radical scavengers than natural carotenoids, β-carotene and canthaxanthin. A practical chain-extension method for polyene dials (e.g., crocetin dial) was also proposed based on Horner–Wadsworth–Emmons olefination. Double aldol condensation between polyene dials and acetophenones with ring substituents produced oxo-carotenoids with substituted benzene rings. The antioxidant activity of oxo-carotenoids was measured using DPPH (radical) and ABTS (cationic radical) scavenging assays and compared with the analysis with the superoxide (anionic radical) probe. An effective conjugation length by carbon–carbon double bonds is important to provide superior antioxidant activity for oxo-carotenoids, regardless of the type of radical probe used in the assay. Increasing electron density is favorable to strong antioxidant activity in DPPH, and the phenol group is favored in ABTS, whereas electron deficient oxo-carotenoids are very potent in the superoxide radical assay. All oxo-carotenoids exhibited 105~151% better superoxide radical scavenging activity compared to beta-carotene (100%), whereas 38~155% in DPPH and 16~96% in ABTS radical scavenging activities were observed.

Application of a dual-thermopile radical probe to expanding hydrogen plasmas

We compare the performance of a hydrogen radical probe to historic data determined via two-photon absorption laser induced fluorescence using a comparable cascaded arc source under similar operating conditions. This probe has dual heat flux sensors (DHFS) each coated with materials with different catalytic properties for hydrogen atoms. In the ideal situation, the hydrogen radical flux can be deduced based on the difference between the heat loads measured by these two sensors. The influence of DHFS temperature on the performance was also assessed. The experimental results showed measurement errors of <10% could be obtained regardless of the probe temperature during plasma exposures. To convert heat fluxes into atomic fluxes, we calibrated the difference of the recombination coefficients using a vacuum ultraviolet absorption technique, which is more reliable than modeled values based on assumptions or scattered values reported in literature. As a result, we measured the hydrogen plasma and radical parameters at various settings using both a double Langmuir probe and the DHFS. The typical atom flux in the 1022 m−2s−1 range was in good agreement with those obtained using optical techniques. We also observed that the ion and atom fluxes are both sensitive to the background gas pressure. These findings validate application of the DHFS to the cascaded arc source, and could pave the way for optimization of the source performance in the plasma material processing experiments.

Three-Component Ru-Catalyzed Regioselective Alkylarylation of Vinylarenes via Meta-Selective C(sp2)-H Bond Functionalization.

We report a novel Ru-catalyzed regioselective alkylarylation of vinylarenes with alkyl halides and arenes via meta-C(sp2)-H bond functionalization to construct 1,1-diarylalkanes that generally show bioactivity. In this transformation, a wide spectrum of primary, secondary, and tertiary alkyl halides and electronically varied arenes was well-tolerated. This reaction is characterized by its exquisite regioselectivity of vinylarenes, unique meta-C(sp2)-H selectivity, and redox-neutral conditions. The mechanism presented was supported by radical probes and kinetic isotope effect studies.

Simulating micropollutant abatement during cobalt mediated peroxymonosulfate process by probe-based kinetic models

Prediction of micropollutant abatement during advanced oxidation processes by modelling can provide valuable information to optimize the process for efficient water treatment. In this study, a probe-based kinetic model was developed to simulate micropollutant abatement during water treatment by a cobalt-mediated peroxymonosulfate (Co(II)/PMS) process. Eight micropollutants (atrazine, primidone, sulfisoxazole, sulfamethoxypyridazine, gemfibrozil, chloramphenicol, p-chlorobenzoic acid, and perfluorooctanoic acid) were spiked in various water matrices (synthetic water and real surface water), which were then treated by the Co(II)/PMS process. By following the decay of PMS, atrazine, and primidone (as sulfate and hydroxyl radical (SO4·- and ·OH) probes) during the process, the exposures of PMS, SO4·- and ·OH were calculated using the model. Then, based on the determined exposures, the abatement efficiencies of the other micropollutants could be reasonably predicted with the model. The results indicate that by using a couple of probes to measure the SO4·- and ·OH exposures, the model can be used for the generalized prediction of the abatement of various micropollutants during water treatment by the Co(II)/PMS process. In addition, modelling results show that for a given water matrix, the ratios of SO4·- and ·OH concentrations to PMS concentrations (RSP and RHP) are almost constant during the Co(II)/PMS process. This finding allows the transient concentrations of SO4·- and ·OH to be quantified based on the constant RSP and RHP ratios and the PMS concentrations observed during the process. The results of this study suggest that the probe-based kinetic models can provide a useful tool to characterize the persulfate-based process and to predict the abatement efficiencies of various micropollutants during water treatment.

Efficient naphthalene degradation in FeS2-activated nano calcium peroxide system: Performance and mechanisms

Naphthalene (NAP) has received increasing concern due to frequent detection in groundwater and harm to humans. In this study, FeS2 was selected as a novel catalyst to activate nano calcium peroxide (nCP) for NAP degradation. Batch experiments were conducted in a 250 mL glass reactor containing 0.1 mM NAP solution to investigate the effect of reagents dosage, pH, air conditions (with or without N2 purge), and different solution matrixes on NAP degradation. Scavenging tests, electron paramagnetic resonance (EPR) spectrum, and radical probe tests were conducted to identify the main radicals. Results indicated that over 96% NAP was removed in a wide pH range (3.0–9.0) within 180 min at optimal dosage of nCP = 1.0 mM and FeS2 = 5.0 g L−1 in nCP/FeS2 system. Aerobic condition was more beneficial to NAP degradation and the system could tolerate complex solution conditions. Moreover, HO• was determined to be responsible for NAP degradation. NAP degradation intermediates were detected by gas chromatography-mass spectrometry (GC-MS) and the possible degradation pathways were revealed. Finally, the efficient degradation of other organic pollutants confirmed the broad-spectrum reactivity of the nCP/FeS2 system. Overall, these findings strongly demonstrated the potential applicability of nCP/FeS2 system in remediating organic contaminated groundwater.

Hydrogen Peroxide Activation with Sulfidated Zero-Valent Iron for Synchronous Removal of Cr(VI) and BPA

In this work, the synchronous removal of Cr(VI) and bisphenol A (BPA) in a heterogeneous Fenton process with sulfidated nanoscale zero-valent iron (S-nZVI) as the reductant and catalyst was systematically evaluated. Compared to other systems including S-nZVI or H2O2 alone, a simultaneous BPA degradation and Cr(VI) removal could be achieved in the S-nZVI/H2O2 system at an optimum pH of 3. It was, interestingly, found that 7.8% of BPA and 98.2% of Cr(VI) were removed within 60 min in presence of S-nZVI alone, whereas, correspondingly, 98.2% of BPA and 96.9% of Cr(VI) were eliminated in the S-nZVI/H2O2 system. Specifically, humic acid (HA) and H2PO4− inhibited the deterioration of BPA but posed no significant effect on Cr(VI) removal. NO3− had a slight lifting effect on the removal of BPA and Cr(VI), while HCO3− showed a relatively weak prohibition. Experiments with EPR and radical probe tests also provide direct evidence that hydroxyl radicals was monitored in the S-nZVI/H2O2 system, which not only degraded BPA but also inhibited the reduction of Cr(VI). It could not be ignored that FeS accelerated Fe0 corrosion to release Fe2+. In, addition, Fe0, Fe2+ and S2+ could react with Cr(VI) while the most of produced Cr(III) was co-precipitated in the form of CrxFe1−xOOH film. The study confirmed that it was feasible for S-nZVI/H2O2 system to remove synchronously organic pollutants and heavy metal.

Phase control and optimization of photocatalytical properties of samarium doped TiO2 synthesized by coupled ultraviolet and microwave radiations

The scientific community has been looking for strategies to synthesize active catalysts under Ultraviolet_Visible (UV_Vis) light to atrazine (ATZ) degradation. In this study, we applied for the first time the microwave (MW) energy and a Microwave Discharge Electrodeless Lamp (MDEL) as an UV_Vis light source to synthesize photocatalysts. The synergistic use of MW_UV_Vis energies allowed obtaining Sm-doped TiO2 with photoactivity under UV-A light (365 nm) to degrade ATZ. Different characterization analyzes showed that Sm3+ was inserted into the TiO2 lattice and prevented the conversion of anatase into rutile, in addition to giving rise to brookite phase, which increase under MDEL assisted synthesis. Different techniques proved the doping, and especially photoluminescence (PL) showed that electrons are excited to f levels of Sm3+ and can be trapped in the structural defects of the photocatalyst to be consumed later. Moreover, the catalyst synthesized under MDEL irradiation (STiM) showed lower recombination rates and a performance to degrade ATZ of 45% and 15% higher than pristine TiO2 (Ti) and Sm-doped TiO2 (STi), respectively. Photocatalytic degradation mechanism was unveiled by monitoring Desethyl-atrazine (DEAT), Desisopropyl-atrazine (DIAT), and Hydroxy-atrazine (HAT), which are by-products of ATZ. The doped materials showed high efficiency to activate the ATZ hydrolysis reaction, while the water oxidation to generate •OH and converted ATZ to HAT was more pronounced in STiM. We proved through radical probe tests that STiM produces more hydroxyl radicals and that its photoactivity remains even after 4 application cycles.

Peroxidase-Mimetic Copper-Doped Carbon-Dots for Oxidative Stress-Mediated Broad-Spectrum and Efficient Antibacterial Activity.

Carbon dots (CDs) have recently emerged as antibacterial agents and have attracted considerable attention owing to their fascinating merits of small size, facile fabrication, and surface functionalization. Most of them are involved in external light activation or hybridization with other functional nanomaterials. Herein, we present peroxidase-like Cu-doped CDs (Cu-CDs) for in vitro antibacterial applications. The unique peroxidase-mimicking property of the Cu-CDs was demonstrated by tetramethylbenzidine chromogenic assay, electron paramagnetic resonance spectra, and hydroxy radical probe. Escherichia coli and Staphylococcus aureus were chosen as representative gram-negative/positive models against which Cu-CDs exhibited superior antimicrobial activity even at a dosage down to 5 μg/mL. A possible mechanism of action was that the Cu-CDs triggered a catalytic redox reaction of endogenous H2 O2 and glutathione depletion in the bacteria cells, with subsequent oxidative stress and membrane disruption. This work provides a new strategy for the design of microenvironment-responsive antimicrobial nano-agents.

Are microbubbles magic or just small? a direct comparison of hydroxyl radical generation between microbubble and conventional bubble ozonation under typical operational conditions

The application of microbubbles for water treatment is an emerging technology which has been shown to significantly enhance gas–liquid contacting processes. When applied to ozonation, microbubble technology has been shown to enhance mass transfer and the speed and extent of compound removal compared with conventional bubbling techniques. One explanation as to why microbubble systems outperform conventional systems is that microbubbles shrink, collapse and spontaneously generate hydroxyl radicals which is thought to enhance the speed of compound removal. This study compared microbubble (mean diameter 37 μm) and conventional bubble (mean diameter 5.4 mm) ozonation systems under identical conditions. The experiments were normalised for effective ozone dose to determine whether microbubble ozonation generated significantly more hydroxyl radicals than conventional bubble ozonation. 4-chlorobenzoic was used as the hydroxyl radical probe and the proportion of hydroxyl radicals generated for a given effective ozone dose was quantified. The •OH-exposure to O3-exposure (the Rct) was used to compare the systems. The ratio of the mean RctMicrobubble to mean RctConventional was 0.73, 0.84 and 1.12 at pH 6, 7 and 8 respectively. Statistical assessment of the Rct showed that there was no significant difference between the bubble systems. No evidence was found to support the hypothesis that microbubble systems generate more •OH. Instead, the level of •OH-exposure is linked to the effective dose and pH of the system and future designs should focus on those factors to deliver •OH based benefits.

Linear Relationship between Temperature and the Apparent Reaction Rate Constant of Hydroxyl Radical with 4-chlorobenzoic Acid

ABSTRACT 4-Chlorobenzoic acid (p-CBA) is frequently used as a hydroxyl radical (HO·) probe substance in studies of ozonation and advanced oxidation processes. However, the temperature dependence of the reaction between HO· and p-CBA remains unclear. In this context, we identified the relationship between temperature ( T , K) and the apparent second-order reaction rate constant of HO· with p- CBA ( k H O ⋅ , p − C B A T , M−1 s−1): 10 10 . They were measured by a novel competitive method using 2-methylpropan-2-ol (tert-butyl alcohol) as a reference substance in the range of 1.0–40.0℃. The linear regression equation was more appropriate than the exponential regression equation to express this relationship. More generally, our simulation shows that the linear regression equation can be more accurate than the exponential regression equation to express the relationship between temperature and apparent reaction rate constants of HO.

Ultrasound/chlorine sono-hybrid-advanced oxidation process: Impact of dissolved organic matter and mineral constituents

In this work, after exploring the first report on the synergism of combining ultrasound (US: 600 kHz) and chlorine toward the degradation of Allura Red AC (ARAC) textile dye, as a contaminant model, the impact of various mineral water constituents (Cl−, SO42−, NO3−, HCO3− and NO2−) and natural organic matter, i.e., humic acid (HA), on the performance of the US/chlorine sono-hybrid process was assessed for the first time. Additionally, the process effectiveness was evaluated in a real natural mineral water (NMW) of a known composition. Firstly, it was found that the combination of ultrasound and chlorine (0.25 mM) at pH 5.5 in cylindrical standing wave ultrasonic reactor (f = 600 kHz and Pe = 120 W, equivalent to PA ∼ 2.3 atm) enhanced in a drastic manner the degradation rate of ARAC; the removal rate being 320% much higher than the arithmetic sum of the two separated processes. The source of the synergistic effect was attributed to the effective implication of reactive chlorine species (RCS: Cl, ClO and Cl2−) in the degradation process. Radical probe technique using nitrobenzene (NB) as a specific quencher of the acoustically generated hydroxyl radical confirmed the dominant implication of RCS in the overall degradation rate of ARAC by US/chlorine system. Overall, the presence of humic acid and mineral anions decreased the efficiency of the sono-hybrid process; however, the inhibition degrees depend on the type and the concentration of the selected additives. The reaction of these additives with the generated RCS is presumably the reason for the finding results. The inhibiting effect of Cl−, SO42−, NO3− and NO2− was more pronounced in US/chlorine process as compared to US alone, whereas the inverse scenario was remarked for the effect of HA. These outcomes were associated to the difference in the reactivity of HA and mineral anions toward RCS and OH oxidizing species, in addition to the more selective character of RCS than hydroxyl radical. The displacement of the reaction zone with increasing the additive concentration may also be another influencing factor that favors competition reactions, which subsequently reduce the available reactive species in the reacting medium. The NMW exerted reductions of 43% and 10% in the process efficiency at pH 5.5 and 8, respectively, thereby confirming the RCS-quenching mechanism by the water matrix constituents. Hence, this work provided a precise understanding of the overall mechanism of chlorine activation by ultrasound to promote organic compounds degradation in water.