Steady-state Radical Concentration Research Articles

Degradation mechanisms of antibiotics in UV222/H2O2 and UV222/persulfate systems: Dual roles of inorganic anions

Advanced oxidation processes based on KrCl* excimer lamps (UV222-AOPs) have recently received widespread attention in water treatment due to their high radical yields. In this investigation, the degradation of two typical antibiotics, florfenicol (FLO) and ciprofloxacin (CIP), by UV222 and its combination with hydrogen peroxide (H2O2) or persulfate (PDS) were studied, respectively. The dual effects of inorganic anions on the degradation performance of UV222/UV222-AOPs were determined. It was found that NO3– significantly enhanced the photolysis of CIP, and the degradation rate constants increased from 1.03 × 10-4 mJ−1 cm2 (without NO3–) to 2.56 × 10-4 mJ−1 cm2 (with 1 mM NO3–). However, NO3– markedly inhibited the photolysis of FLO, leading to a decrease in the degradation rate constants from 1.18 × 10-3 mJ−1 cm2 (without NO3–) to 5.27 × 10-4 mJ−1 cm2 (with 1 mM NO3–). HCO3– was found to have a certain promoting effect on the photolysis of FLO and CIP, and SO42- has a negligible effect on the photolysis of both antibiotics. The above three inorganic ions decelerated the degradation of CIP and FLO in UV222-AOP, where NO3– exerted the most inhibition. Through LC-MS analysis and ECOSAR model, the main transformation products were individually determined and evaluated for toxicity. In addition, the performance of UV222-AOPs and UV254-AOPs in reclaimed wastewater (RWW) treatment were compared, and the steady-state concentration of radicals was measured. This study provided novel insight into the application of UV222-AOPs in RWW treatment.

The effects of inorganic anions on degradation kinetics and isotope fractionation during the transformation of tris(2-chloroethyl) phosphate (TCEP) by UV/persulfate

Tris(2-chloroethyl) phosphate (TCEP), as a typical chlorinated flame retardant, is attracting more attention as a carcinogen. Although persulfate-based oxidation exhibits good performance in removing refractory organic pollutants, the kinetics of persulfate-based remediation are affected by inorganic anions, which causes inaccurate remediation efficiency. This study combines steady-state radical concentration modelling with isotope fractionation to investigate the effects of inorganic anions on TCEP degradation by UV/persulfate (UV/PS). In the absence of anions during UV/PS system, the observed degradation rate was (9.7 ± 0.1) × 10−5 s−1, which was approximately 93 % attributed to sulfate radical (SO4−•) oxidation based on radical modelling. Carbon isotope fractionation, coupled with the identification of transformation products by mass spectrometry, suggests a carbon bond split during TCEP degradation with a carbon isotopic fractionation value (ε) of −1.6 ± 0.2 ‰ (± 95 % confidence intervals). With respect to co-existing anions in UV/PS system, the addition of chloride (Cl−) had a negligible effect on degradation rates, while the addition of hydrogencarbonate (HCO3−) caused them to decrease, and the addition of hydrogenphosphate (HPO42−) caused them to increase. Radical modelling suggested that SO4−• was transformed to chlorine radicals (Cl•/Cl2−•), phosphate radicals (HPO4−•), and carbonate radicals (CO3−•). Furthermore, the overlapping 95 % confidence intervals (C.I.) and the statistical tests (p > 0.05) both agree that Cl− and HPO42− gain identical ε values. Nevertheless, when HCO3− coexisted in the UV/PS system, the ε values were distinct. The addition of HCO3− would result in ε variation of TCEP in the UV activated PS process, which should receive more attention when applying remediation.

Radicals in ‘biologically relevant’ concentrations behave differently: Uncovering new radical reactions following the reaction of hydroxyl radicals with DMSO

Methyl radicals play key roles in various chemical and biological processes. Mechanistic studies of methyl radicals with their precursor, Dimethyl Sulfoxide (DMSO), were extensively studied. Though the involved mechanisms seemed to be clarified, essentially none of the studies have been performed at conditions relevant to both biological and catalytic systems, i.e. low steady state radical concentrations. A chain-like reaction, as an inverse function of the radicals concentrations ([•CH3]ss), increases the methyl radical yields. The nature of the additional products obtained differs from those commonly observed. Furthermore it is shown that methyl radicals abstract a methyl group from DMSO to yield ethane. Herein we report a novel mechanism relevant mainly at low steady state radical concentrations, which may change the understanding of certain reaction routes present in both biological systems and catalytic chemical systems. Thus the results point out that mechanistic studies have to be carried out at dose rates forming radicals at analogous concentrations to those present in the process of interest.

Experimental and simulation investigations of UV/persulfate treatment in presence of bromide: Effects on degradation kinetics, formation of brominated disinfection byproducts and bromate

Bromide (Br−), which is omnipresent in water bodies, can not only affect the degradation kinetics of target pollutants but may also form undesired byproducts (such as bromate (BrO3-) and brominated disinfection byproducts (Br-DBPs)) through sulfate radical (SO4∙-)-based advanced oxidation processes (AOPs). This study found that Br− significantly suppressed the degradation of diethyl phthalate (DEP) based on the ultraviolet (UV)/persulfate (PS) treatment and was efficiently converted into reactive species (radicals and free bromine) by the SO4∙- or hydroxyl radical (HO) generated in the UV/PS system. These reactive bromine species could, in turn, brominate the phenolic degradation intermediates of DEP as well as natural organic matter (NOM), yielding Br-DBPs, including tribromomethane (TBM), which was observed when Br−, DEP and/or NOM coexisted in this UV/PS system. However, the Br-DBPs were a short-lived form of bromine during the transformation of Br– and were degraded by the excessive oxidants (e.g., SO4∙- and HO). Instead, Br− was eventually transformed into BrO3-, with free bromine acting as the requisite intermediate. The BrO3- formation initially showed a delay before increasing monotonically. In general, raising the PS dosage and the initial Br− concentrations enhanced both the maximum concentration of TBM and the formation of BrO3-, while increasing the amounts of DEP and NOM facilitated the former and inhibited the latter. The maximum concentration of TBM increased while the formation of BrO3- was suppressed with increasing pH from 5.0 to 8.0. In addition, through simulating the steady-state concentration of radicals, it was found that the contribution of bromine atom radical (Br) towards oxidizing free bromine to BrO3- was far greater than those of SO4∙- and HO. The findings demonstrate the potential negative effects of Br− on SO4∙--based AOPs, which need to be considered when this technology is applied in practice.

Highly efficient removal of trimethoprim based on peroxymonosulfate activation by carbonized resin with Co doping: Performance, mechanism and degradation pathway

We first report the syntheses of carbon-supported Co (CS-Co) composite via a carbonization process from a saturated resin. CS-Co exhibited favorable catalysis activities in activation of peroxymonosulfate (PMS) to generate SO− 4 and OH for trimethoprim (TMP) degradation. The performance of composites were studied with respect to diverse pHs (3.0–9.0), catalyst dosages (0.05–0.5 g/L), PMS dosages (0.05–1.0 mM), TMP concentration (5–20 mg/L) and temperature (15–30 °C). Water matrix concerning various levels of humic acid (HA) showed negative effect for TMP removal due to the intrinsic competition between HA. Scavenging test indicated that SO− 4 and OH were the dominant reactive radicals, and an uncomplicated method to calculate the normalized steady-state concentration of radicals in CS-Co/PMS process was established. The electron transfer concerning PMS, carbon surface and Co2+ was attributed as the main activation mechanism. The main intermediate products of TMP were identified by LC/MS/MS technology with four degradation pathways proposed, including hydroxylation, electron transfer mechanism, demethylation and ring-cleavage.

Influence of mediators on laccase catalyzed radical formation in lignin

Laccases (EC 1.10.3.2) catalyze oxidation of phenolic groups in lignin to phenoxyl radicals during reduction of O2 to H2O. Here, we examine the influence on this radical formation of mediators which are presumed to act by shuttling electrons between the laccase and the subunits in lignin that the enzyme cannot approach directly. Treatments of three different lignins with laccase-mediator-systems (LMS) including laccases derived from Trametes versicolor and Myceliophthora thermophila, respectively, and four individual mediators, 1-hydroxybenzotriazole (HBT), N-hydroxyphthalimide (HPI), 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO), and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) were assessed by real time electron paramagnetic resonance measurements. Radical steady state concentrations and radical formation rates were quantified. LMS treatments with 500 μM N-OH type mediators (HPI or HBT) did not affect the lignin radical formation, but increased doses of those mediators (5 mM) surprisingly led to significantly decreased radical formation rates and lowered steady state radical concentrations. Laccase-TEMPO treatment at a 5 mM mediator dose was the only system that significantly increased steady state radical concentration and rate of radical formation in beech organosolv lignin. The data suggest that electron shuttling by mediators is not a significant general mechanism for enhancing laccase catalyzed oxidation of biorefinery lignin substrates, and the results thus provide a new view on laccase catalyzed lignin modification.

Degradation kinetics and mechanism of β-lactam antibiotics by the activation of H2O2 and Na2S2O8 under UV-254 nm irradiation

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-254nm activated H2O2 and S2O82− 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/S2O82− system. Mineralization of the β-lactams could be achieved at high UV fluence, with a slow formation of SO42− and a much lower elimination of total organic carbon (TOC). The transformation mechanisms were also investigated showing the main reaction pathways of hydroxylation (+16Da) at the aromatic ring and/or the sulfur atom, hydrolysis (+18Da) at the β-lactam ring and decarboxylation (–44Da) for the three penicillins. Oxidation of amine group was also observed for ampicillin. This study suggests that UV/H2O2 and UV/S2O82− advanced oxidation processes (AOPs) are capable of degrading β-lactam antibiotics decreasing consequently the antibiotic activity of treated waters.

Petroleum films exposed to sunlight produce hydroxyl radical

Sunlight exposed oil films on seawater or pure water produced substantial amounts of hydroxyl radical as a result of irradiation. Oil was collected from the surface of the Gulf of Mexico following the Deepwater Horizon spill and exposed to simulated sunlight in thin films over water. Photochemical production of hydroxyl radical was measured with benzoic acid as a selective chemical probe in the aqueous layer. Total hydroxyl radical formation was studied using high benzoic acid concentrations and varying exposure time. The total amount of hydroxyl radical produced in 24h irradiations of thin oil films over Gulf of Mexico water and pure water were 3.7×10−7 and 4.2×10−7moles respectively. Steady state concentrations of hydroxyl radical were measured using a competition kinetics approach. Hydroxyl radical concentrations of 1.2×10−16 to 2.4×10−16M were observed for seawater and pure water under oil films. Titanium dioxide (TiO2) nanomaterials were added to the system in an effort to determine if the photocatalyst would enhance oil photodegradation. The addition of TiO2 nanoparticles dramatically changed the observed formation rate of hydroxyl radical in the systems with NP water at pH 3, showing increased formation rate in many cases. With photocatalyst, the steady state concentration of radical decreased, predominantly due to an increase in the hydroxyl radical scavenging rate with oxide present. This study illustrates that oil is a strong and important source of hydroxyl radical when exposed to sunlight. The fate of oil and other dissolved species following oil spills will be heavily dependent on the formation and fate of hydroxyl radical.

Phototransformation of Wastewater-Derived Trace Organic Contaminants in Open-Water Unit Process Treatment Wetlands

Open-water cells in unit process treatment wetlands can be used to exploit sunlight photolysis to remove trace organic contaminants from municipal wastewater effluent. To assess the performance of these novel systems, a photochemical model was calibrated using measured photolysis rates for atenolol, carbamazepine, propranolol, and sulfamethoxazole in wetland water under representative conditions. Contaminant transformation by hydroxyl radical ((•)OH) and carbonate radical ((•)CO3(-)) were predicted from steady-state radical concentrations measured at pH values between 8 and 10. Direct photolysis rates and the effects of light screening by dissolved organic matter on photolysis rates were estimated using solar irradiance data, contaminant quantum yields, and light screening factors. The model was applied to predict the land area required for 90% removal of a suite of wastewater-derived organic contaminants by sunlight-induced reactions under a variety of conditions. Results suggest that during summer, open-water cells that receive a million gallons of water per day (i.e., about 4.4 × 10(-2) m(3) s(-1)) of nitrified wastewater effluent can achieve 90% removal of most compounds in an area of about 15 ha. Transformation rates were strongly affected by pH, with some compounds exhibiting faster transformation rates under the high pH conditions associated with photosynthetic algae at the sediment-water interface and other contaminants exhibiting faster transformation rates at the circumneutral pH values characteristic of algae-free cells. Lower dissolved organic carbon concentrations typically resulted in increased transformation rates.

Kinetics of polymerization reactions with reversible-chain termination

General kinetic features of radical and ionic polymerization processes accompanied by reversible chain termination reactions are considered. Special attention is paid to the conditions of applicability of the steady-state approximation usually employed to analyze the kinetics of radical polymerization. It is shown that the steady-state concentration of radicals is attained at practically the very beginning of the reaction, while the steady-state concentration of a reversible termination agent is reached with a certain delay. A kinetic explanation of the reversible termination reaction effect on the pattern of the molecular-mass distribution is suggested. Conditions providing the obtainment of a polymer with a narrow molecular-mass distribution in processes with reversible termination are formulated.

Reversible chain reaction between N,N′-diphenyl-1,4-benzoquinonediimine and 2,5-dichlorohydroquinone: Kinetics, mechanism, and the rate constants of elementary steps

The kinetics of the reversible chain reaction between N,N′-diphenyl-1,4-benzoquinonediimine and 2,5-dichlorohydroquinone was studied in chlorobenzene at 298 and 343 K. Experiments in the presence of an initiator proved that the reaction proceeds via a chain mechanism with a chain length of ∼104 to 105 units, depending on the reactant and initiator concentrations and temperature. The reaction rate first increases and then decreases with increasing concentrations of the reaction products (N,N′-diphenyl-1,4-phenylenediamine and 2,5-dichloroquinone) due to the pronounced reversibility of the chain termination and propagation steps involving the reaction products. The reaction orders with respect to the components were determined. The rate constants and activation energies of most of the elementary steps of the forward and backward chain reactions were determined or reliably estimated. Induction periods were observed for the first time in the reversible chain reactions in the quinoneimine + hydroquinone systems. The induction periods are due to the long time required for the establishment of the steady-state radical concentrations in the system.

Light-induced formation of free radicals in cream cheese

Abstract Radicals were found, by electron spin resonance (ESR) spectroscopy, to accumulate in cream cheese (26% fat, 7% protein) and more significantly in low fat cream cheese (17% fat, 11% protein) upon light exposure. The decay of radicals following illumination (875 lux, with a strong UV-component for up to 80 min) followed first-order kinetics with a half-life at room temperature of around 0.5 h both for cream cheese and low fat cream cheese. The surprisingly long-lived radicals had a broad structureless ESR-spectrum ( g -value of 2.006) which, for partly desiccated cream cheese, changed towards a nitrogen-centred ESR powder spectrum ( g -value of 2.0014) typical for immobilised protein-based radicals. The protein oxidation product, dimethyl disulphide, and the lipid oxidation products, hexanal and 2-butanone, were detected by GC-analysis in higher concentrations in the outer 1 mm layer than in the second layer (of 1 mm thickness) of the product, in agreement with absorption of 99% of the UV-light intensity in the outer 1 mm layer. The low fat cream cheese had higher levels of both lipid and protein oxidation products, in agreement with the higher steady state concentration of radicals and confirming the role of proteins in oxidative changes also for lipids in cheese. The low fat cream cheese was initially more oxidised than was the cream cheese, as seen from the peroxide values, and oxidation products in lipid droplets could be visualised in three dimensions by confocal laser scanning microscopy, using the fluorescent probe C 11 -Bodipy (581/591).

Reversibly cross-linked surface-grafted polymer brushes.

Reversibly cross-linked surface-grafted polymer brushes.

Role of catalyst deactivation and regeneration in the heck reaction involving unactivated aryl bromides

The kinetics of low-temperature autooxidation of methyl oleate for a bimolecular mechanism of the degenerate branched reaction were analyzed taking into account changes in the methyl oleate concentration for different amounts of peroxides formed. This analysis made it possible to explain the experimental data. The role of the initial peroxide concentration in the mechanism and kinetics of the chain degenerate branched reaction of methyl oleate autooxidation was studied in the steady-state approximation and in the course of establishing a steady-state concentration of radicals. A systematic approach to estimating the antioxidant properties of lipids on the basis of the methyl oleate model was proposed.

The influence of laccase on the chemo-enzymatic synthesis of lignin graft-copolymers

The mechanism unterlying the chemo-enzymatic graft copolymerization of lignin with acrylic compounds was studied. When lignin sulfonate (LS) was incubated with the phenoloxidase laccase in the presence of a peroxide species ( t-butylhydroperoxide, i.e. t-BHP), the decrease of the phenolic groups was significantly higher than when LS was incubated with laccase or t-BHP, separately. The enzymatic oxidation of the phenolic groups did lead to the formation of quinones; however, the number of quinone groups formed in LS in the presence of t-BHP was not significantly higher than in its absence. Still, a certain proportion of the quinones formed in the presence of t-BHP appeared to be further oxidized resulting in an opening of the aromatic rings. In addition, the rate of LS polymerization, i.e. its increase in M w instigated by laccase, was significantly reduced in the presence of t-BHP even though the activity of laccase was only slightly affected by the peroxide. Incubation of LS with laccase prior to the grafting reaction resulted in a significant increase of the spin concentration as detected in the EPR spectra. In spite of a high steady state concentration of radicals, the generation rate of these radicals in pre-incubated LS was lower than in unpretreated LS. Thus, the polymer yields obtained with pre-incubated LS were lower than those obtained with unpretreated LS.

FLUORESCENCE TECHNIQUE TO STUDY FREE-RADICAL POLYMERIZATION OF 2-VINYLNAPHTHALENE

A novel technique based on steady-state fluorescence measurements was introduced to study free-radical polymerization of 2-vinylnaphthalene (2VN). Both spectroscopic and kinetic studies of polymerization of 2VN in toluene solution have been carried out over the temperature range between 50 and 70°C. The effective radius of interaction of 2VN required for excimer formation was found to be around 3.5 nm. The product of reaction rate constant kp and steady-state radical concentration [M•], kp[M•] was calculated for 2-vinylnaphthalene (2VN) polymerization carried out at the temperatures 50, 60, 65, and 70°C. A sudden change in the value of kp[M•] was observed above 60°C, indicating that polymerization proceeds effectively just at and above this temperature.

STEADY-STATE CONCENTRATIONS OF CARBONATE RADICALS IN FIELD WATERS

The carbonate radical [•CO−3] is a secondary radical that results from the scavenging of hydroxyl radical (•OH) by carbonate/bicarbonate. N,N-dimethylaniline was found to be a selective probe for measuring the steady-state concentration of this radical in a variety of natural waters. The pH, nitrate, total carbonate/bicarbonate, and dissolved organic carbon (DOC) in selected field waters were quantitatively analyzed. It was assumed that hydroxyl radicals were produced primarily from the slow photolysis of nitrate and DOC and then were scavenged by carbonate/bicarbonate that were competing with DOC. Steady-state carbonate radical concentrations [•CO−3]ss in natural waters were measured by irradiating solutions of natural waters containing N,N-dimethylaniline in a photoreactor with varying intensity as well as using natural sunlight. The measured steady-state concentration of carbonate radicals was found to be strongly dependent on light intensity and ranged from 5 × 10−15 to 10−13 M. Further kinetic analysis of the data showed that carbonate radical concentrations were related to the concentrations of nitrate and dissolved organic carbon in natural waters through a simple linear relationship.

Quantitative studies on the respiratory burst generated in peritoneal macrophages

The luminol-dependent chemiluminescence of macrophages during the zymosan-stimulated respiratory burst has been studied both in the absence and in the presence of the radical inhibitor 3,5 di- tert-butyl-4-hydroxyphenyl propionic acid. In addition, the consumption of luminol and of the inhibitor has been followed analytically. Based on the rates of the consumption of the inhibitor, an iteration procedure yields a value of 2.2 × 10 −7 M for the steady-state concentration of radicals generated by cells at the maximum of the chemiluminescence in the presence of inhibitor. Approximate calculations have indicated that under the experimental conditions applied, additional formation of superoxide anion radicals by the oxidation of luminol is negligible. By assuming that in an inhibitor-free system the disappearance of radicals takes place via their combination process as well as by their interaction with luminol and/or with luminol-derived species, numerical integration yields a calculated curve of radical concentration versus time in fair agreement with experimental data and a rate-constant value for the combination of radicals of ∼ 10 6 M −1 s −1, supporting literature findings according to which primarily superoxide anion radicals are formed.

In VivoExperimental Studies on the Role of Free Radicals in Photodynamic Therapy

The effect of excited Photofrin II molecules on the zymosan stimulated and luminol dependent chemiluminescence (CL) of macrophages, known as respiratory burst, has been studied. Excitation was carried out by light irradiation in the vicinity of the maximal CL by varying light doses. In separate experiments potassium salt of 3,5-di-tert-butyl-4-hydroxyphenyl propionic acid radical inhibitor was added to the system and its consumption measured spectroscopically. Based on the experimental results the decrease in the steady state radical concentrations under the influence of radical inhibitor and of triplet sensitizer generatedin situhas been calculated. It has been established that the corresponding photodynamic effect is an inhibition-like process proceeding by triplet–doublet interactions.

Peroxy radicals from photostationary state deviations and steady state calculations during the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, 1993

Concentrations of peroxy radicals and a number of other trace gases were measured during the Tropospheric OH Photochemistry Experiment in August and September, 1993. The trace gas concentrations were used to derive two estimates of peroxy radical levels: deviations from the photostationary state, and theoretical calculations with the assumption of steady state radical concentrations. As in many previous studies, the photostationary state method yielded midday peroxy radical levels about twice those measured. Calculations were performed to assess the expected uncertainty in the photostationary state derived peroxy radical concentrations from the uncertainty of the input parameters. The agreement between measurements and steady state determinations was better than the photostationary state estimates, with the measurements slightly higher on average. Radical formation and destruction rates for clear sky conditions with low and high NOx were derived from the steady state calculations and the results demonstrate the processes that control peroxy radical levels in clean and polluted continental atmospheres.