Competition Kinetics Method Research Articles

Mechanism of β-MnO2 particle for the peroxymonosulfate activation under electric field: the promoting effect of carbamazepine on Mn(III) and reactive oxygen species production

In this study, β-MnO2 was introduced into electro-peroxymonosulfate process as particle electrodes (E/PMS/β-MnO2) to reveal the pathway of reactive manganese species (RMnS) and its promoting effect on reactive oxygen species (ROS) production. A significant synergistic was observed in the E/PMS/β-MnO2 process, the reaction constant was 3 times higher than that in the E/PMS process. The mechanism experiments revealed the generation pathway of Mn(III)aq during the E/PMS/β-MnO2 process. The pyrophosphate complex experiments and electrochemical analysis demonstrated that Mn(III)aq was dominantly generated through the electron transfer between carbamazepine (CBZ) and Mn(IV) under electric field conditions, rather than the reduction process in the cathode area. Furthermore, quenching experiments and competition kinetics method demonstrated the production of Mn(III)aq has the potential to enhance the formation of reactive oxygen species (ROS), especially •OH and SO4•−. In addition, Mn(V)aq and Mn(VI)aq was proved unable to be produced during the E/PMS/β-MnO2 process. Different from other similar electrochemical studies, 1O2 was mainly generated by the reaction betweenO2•− and •OH in this research. It was observed that the inclusion of Cl— facilitated the removal of CBZ, whereas the addition of humic acid (HA) had negligible impact on CBZ removal during the E/PMS/β-MnO2 process. At last, the results of various water matrices effects and toxicology analysis showed that the E/PMS/β-MnO2 process has satisfactory performance in practical application. This study could provide new insights for electro-persulfate treatment processes.

Reactivity of Cyanobacteria Metabolites with Ozone: Multicompound Competition Kinetics.

Cyanobacterial blooms occur at increasing frequency and intensity, notably in freshwater. This leads to the introduction of complex mixtures of their products, i.e., cyano-metabolites, to drinking water treatment plants. To assess the fate of cyano-metabolite mixtures during ozonation, a novel multicompound ozone (O3) competition kinetics method was developed. Sixteen competitors with known second-order rate constants for their reaction with O3 ranging between 1 and 108 M-1 s-1 were applied to cover a wide range of the O3 reactivity. The apparent second-order rate constants (kapp,O3) at pH 7 were simultaneously determined for 31 cyano-metabolites. kapp,O3 for olefin- and phenol-containing cyano-metabolites were consistent with their expected reactivity (0.4-1.7 × 106 M-1 s-1) while kapp,O3 for tryptophan- and thioether-containing cyano-metabolites were significantly higher than expected (3.4-7.3 × 107 M-1 s-1). Cyano-metabolites containing these moieties are predicted to be well abated during ozonation. For cyano-metabolites containing heterocycles, kapp,O3 varied from <102 to 5.0 × 103 M-1 s-1, giving first insights into the O3 reactivity of this class of compounds. Due to lower O3 reactivities, heterocycle- and aliphatic amine-containing cyano-metabolites may be only partially degraded by a direct O3 reaction near circumneutral pH. Hydroxyl radicals, which are formed during ozonation, may be more important for their abatement. This novel multicompound kinetic method allows a high-throughput screening of ozonation kinetics.

Effect of sodium dodecyl sulfate on preferential degradation of naphthalene by thermally activated persulfate: Implications for soil flushing solution reuse

Various flushing solutions that contain hydrophobic organic compounds and surfactants are generated during surfactant-enhanced soil flushing processes. How to effectively remove the target pollutants while minimizing the degradation of surfactants for reuse of the flushing solutions has become a major problem. A series of batch experiments were conducted to investigate the preferential degradation mechanisms of naphthalene by sodium dodecyl sulfate (SDS) as well as the effect of SDS concentration on the degradation efficiency using a competitive kinetic method. In addition, the effects of persulfate (PS) dosage, temperature, and pH on the degradation efficiency of naphthalene were also studied, and a prediction model based on response surface methodology (RSM) was built to optimize the preferential degradation of naphthalene. One-dimensional column flushing experiments were performed to evaluate the feasibility of reusing soil flushing solutions at the predicted PS dosage. Results show that: (i) SDS inhibited naphthalene degradation owing to competitive oxidation and micellar protection, with a decreasing degradation rate constant as SDS concentration increased. However, naphthalene was still degraded by reactive oxygen species (ROS) at a higher rate than SDS, indicating the preferential degradation of naphthalene. When the concentration of SDS increased from 5 to 10 times of critical micelle concentration (CMC; 8.2 mM), the ratio for rate constants of naphthalene to SDS (kNAP/kSDS) decreased significantly from 31.61 to 3.38, illustrating the obvious effect of SDS concentration on preferential degradation of naphthalene; (ii) The formation of single-chambered vesicles with a higher thermodynamic stability was observed at 10 CMC, compared to the spherical micelles formed at 5 CMC. The accessibility of naphthalene to ROS was greatly reduced at 10 CMC, resulting in a significant reduction in kNAP/kSDS value. Moreover, the method in which ROS attacked naphthalene in surfactant micelles varied with SDS concentration, which also impacted the preferential degradation of naphthalene; (iii) Under the optimal PS dosage predicted by the RSM model, a good reusability of SDS was observed in the soil flushing solutions after thermally activated PS oxidation, with a significant reduction on the toxic risk of soil after flushing.

Mechanistic and kinetic aspects of florfenicol degradation by [rad]OH: Chloride moiety resistance

Antibiotic residues in aquatic environments can be effectively degraded by hydroxyl radical (OH)-based advanced oxidation processes (AOPs). However, the reaction kinetics and mechanisms have not been comprehensive determined due to the limitations of conventional competition kinetic methods and mass spectrometry-based product identification, as these methods fail to exclude interference from secondary radicals and are unable to capture unstable intermediates or transient species. In this study, these limitations were overcome using laser flash photolysis (LFP) and density functional theory (DFT) calculations, allowing accurate determination of the OH-initiated reaction kinetics and degradation mechanisms of florfenicol (FF), a model veterinary antibiotic compound that is frequently detected in the environment. Based on the LFP experiment results, the second order rate constant (k) between OH and FF was determined to be 1.96 × 109 M−1 s−1 by tracking the typical signal of (SCN)2−. Furthermore, DFT calculations revealed two distinct mechanisms for OH addition to the benzene ring and identified hydrogen atom abstraction (HAA) reaction from chiral carbons as being the most favorable initial reaction for OH. In addition, the destruction of chlorine moiety occurred via hydroxylation-chlorine abstraction, rather than direct chlorine abstraction or substitution reactions. The resistance of chlorine moiety to degradation during the initial oxidation process, resulted in the formation of chlorine-containing by-products. This study provided a novel approach to comprehensively investigate the mechanisms of emerging contaminants elimination during OH-based AOPs.

Experimental determination and QSAR analysis of the rate constants for [formula omitted] reactions with aromatic micropollutants in water

S(IV)-based systems used for advanced oxidation processes (AOPs) have been constructed for the degradation of organic contaminants via oxysulfur radicals, including SO3•−, SO4•−, and SO5•−. Although SO5•− is proposed as an active species in AOPs processes, research on the reactivity of SO5•− has remained unclear. In this work, 53 target aromatic micropollutants (AMPs), including 13 phenols, 27 amines, and 13 PPCPs were selected to determine the second-order reaction rate constants for SO5•− using the competitive kinetics method, in which the kSO5•− values, observed at pH 4 ranged from (2.44 ± 0.00) × 105 M−1 s−1 to (4.41 ± 0.28) × 107 M−1 s−1. Quantitative structure-activity relationship (QSAR) models for the oxidation of AMPs by SO5•− were developed based on 40 kSO5•− values of amines and phenols, and their molecular descriptors, using the stepwise multiple linear regression method. This comprehensive model exhibited the excellent goodness-of-fit (Radj2 = 0.802), robustness (QLOO2 = 0.749), and predictability (Qext2 = 0.656), and the one-electron oxidation potential (Eox), energy of the highest occupied molecular orbital energy (EHOMO), and most positive net atomic charge on the carbon atoms (qC+) were considered the most influential descriptors for the comprehensive model, indicating that SO5•− oxidizes pollutants via single electron transfer reaction and exhibits a strong oxidation capacity, especially for pollutants containing electron-donating groups. Moreover, the kSO5•− values of 13 PPCPs were predicted using this comprehensive model, which suggested the practical application significance of the QSAR model. This study emphasizes the direct oxidation capacity of SO5•−, which is important to evaluate and simulate AOPs based on S(IV).

Kinetics and mechanism of dissolved organic phosphorus (DOP) digestion by the UV/O3 process

AbstractIn this study, the kinetics and mechanism of UV/O3 synergistic oxidative digestion of dissolved organic phosphorus (DOP) were investigated, focusing on the ozone direct oxidation and hydroxyl radical oxidation parts of glufosinate and triphenyl phosphate (TPhP). The p‐chlorobenzoic acid (p‐CBA) was selected as the probe compound, and two kinds of reaction kinetic models were proposed by competitive kinetic method with Rct according to the different scale of rate constants of hydroxyl radical oxidation. Under the condition of weakly alkaline (pH = 9.0) and weakly acidic environment (pH = 5.0), the second‐order rate constants of glufosinate and TPhP was determined indirectly to be ko3/glufosinate = (2.903 ± 0.247)M−1s−1 and ko3/TPhP = (3.307 ± 0.204) M−1s−1 by ozone direct oxidation, and k·OH/glufosinate = (1.257 ± 1.031) × 109 M−1s−1 and k·OH/TPhP = (7.120 × 108 ± 0.963) M−1s−1 by hydroxyl radical oxidation, respectively. The comparison of the contribution levels of the two parts to the digestion process showed that the contribution levels in the digestion of glufosinate and TPhP processes both the contribution of ·OH were higher than those of ozone, 86.3% and 72.6%, respectively.

Fast and Complete Destruction of the Anti-Cancer Drug Cytarabine from Water by Electrocatalytic Oxidation Using Electro-Fenton Process

The fast and complete removal of the anti-cancer drug cytarabine (CYT) from water was studied, for the first time, by the electro-Fenton process using a BDD anode and carbon felt cathode. A catalytic amount (10−4 M) of ferrous iron was initially added to the solution as catalyst and it was electrochemically regenerated in the process. Complete degradation of 0.1 mM (24.3 mg L−1) CYT was achieved quickly in 15 min at 300 mA constant current electrolysis by hydroxyl radicals (●OH) electrocatalytically generated in the system. Almost complete mineralization (91.14% TOC removal) of the solution was obtained after 4 h of treatment. The mineralization current efficiency (MCE) and energy consumption (EC) during the mineralization process were evaluated. The absolute (second order) rate constant for the hydroxylation reaction of CYT by hydroxyl radicals was assessed by applying the competition kinetics method and found to be 5.35 × 109 M−1 s−1. The formation and evolution of oxidation reaction intermediates, short-chain carboxylic acids and inorganic ions were identified by gas chromatography-mass spectrometry, high performance liquid chromatography and ion chromatography analyses, respectively. Based on the identified intermediate and end-products, a plausible mineralization pathway for the oxidation of CYT by hydroxyl radicals is proposed.

High-performance reductive decomposition of trichloroacetamide by the vacuum-ultraviolet/sulfite process: Kinetics, mechanism and combined toxicity risk

Trichloroacetamide (TCAcAm) is among of the nitrogenous disinfection by-products (N-DBPs) with high cytotoxicity and genotoxicity, which is usually detected at low concentration (μg/L) in drinking water. In this study, advanced reduction process (ARP) based on vacuum ultraviolet (VUV) was employed to eliminate TCAcAm. Compared with VUV, VUV/sulfide, and VUV/ferrous iron processes, VUV/sulfite process demonstrated excellent performance for TCAcAm decomposition, the higher removal of TCAcAm could be achieved by VUV/sulfite process (85.6 %) than VUV direct photolysis (13.5 %) due to the production of a great number of reactive species. The degradation of TCAcAm followed the pseudo-first-order kinetics well in VUV/sulfite process, and the pseudo-first-order rate constant (kobs) increased with increasing sulfite concentration. Reactive species quenching experiments demonstrated that eaq−, SO3·− and H· were involved in the degradation of TCAcAm. The in situ generated eaq−, SO3·− and HO· via VUV/sulfite process were identified by electron paramagnetic resonance (EPR), and the eaq− was proved to be the dominated species (relative contribution: 83.5 %) for TCAcAm decomposition. The second-order rate constant of TCAcAm with eaq− was determined to be 2.41 × 1010 M−1 s−1 for the first time based on competitive kinetic method. The complete TCAcAm degradation could be achieved at pH > 8.3, while TCAcAm degradation efficiency decreased to 11.9 % at pH 5.8. TCAcAm decay could be divided into two stages: rapid growth (sulfite dosage: 0.25-1.0 mM) and slow growth (sulfite dosage: 1.0-4.0 mM). The yield of eaq− was controlled by sulfite dosage, and the predict yield of eaq− increased from 3.69 × 10−14 to 2.58 × 10−12 M with increasing the sulfite dosage from 0.25 to 4.0 mM by Kintecus 6.80, which resulted in an increase in TCAcAm removal. Meanwhile, the presence of dissolved oxygen (DO), chloride (Cl−), bicarbonate (HCO3−) and humic acid (HA) posed negative influence on TCAcAm decomposition to various degrees. Dichloroacetamide (DCAcAm), trichloroacetic acid (TCAA), dichloroacetic acid (DCAA) and Cl− were identified as intermediate products, indicated that reductive dechlorination and hydrolysis coexisted during the degradation of TCAcAm in VUV/sulfite process.

Oxidative digestion of dissolved N‐compounds by the UV/O3 process: Kinetic models and rate constants

AbstractIn this study, the reaction mechanisms and kinetic of UV/O3 digestion processes were explored with ammonium chloride and urea as the target pollutants, focused on the ozone direct oxidation and hydroxyl radical oxidation parts. Under the conditions of dual environmental digestion method, two kinds of kinetic models were proposed by competitive kinetic method with Rct based on the different scale of rate constants of hydroxyl radical oxidation. Subsequently, the second‐order rate constants of ammonia (pH 11) and urea (pH 2) was determined to be 4.613 ± 0.094 and 0.378 ± 0.006 M–1 s–1 by ozone direct oxidation, and (6.154 ± 0.971) × 107 and (2.257 ± 0.513) × 106 M–1 s–1 by hydroxyl radical oxidation, respectively. Compared with contribution level of two parts to the digestion process, it was shown that in the digestion of ammonia, ozone and •OH radical contribute equally (58.86%, 41.14%). In contrast, in the case of urea, ozone (86.10%) contributes more than •OH radical.

Degradation of chlothianidin from Dantutso 50WG in electrochemical process: Kinetics and role of reactive species

The kinetic degradation of clothianidin (CLO) (in Dantuso 50 WG) during electrochemical oxidation (EO) process using sulfate- and chloride-supporting electrolytes was comprehensively investigated. The degradation of CLO was not due to direct electron oxidation, but was mainly due to •OH and other radicals generated from supporting electrolytes. The degradation of CLO was significantly inhibited when increasing the concentration of nitrobenzene (NB), methanol (MeOH) and benzoic acid (BA). The second-order rate constant of •OH toward CLO was determined to be 3.23×109 M-1 s-1 using competition kinetics method. When SO42- and Cl- were used as supporting electrolytes, the degradation of CLO by •OH was the same with kCLO = 0.0084 min-1. Meanwhile, the higher removal of CLO in SO42--supporting electrolyte was due to the contribution of (SO4•-/S2O82-) more than that of (Cl•/HClO/ClO-).

Comparison of UV and UV-LED activated sodium percarbonate for the degradation of O-desmethylvenlafaxine

As an active metabolite of venlafaxine and emerging antidepressant, O-desmethylvenlafaxine (ODVEN) was widely detected in different water bodies, which caused potential harm to human health and environmental safety. In this study, the comparative work on the ODVEN degradation by UV (254 nm) and UV-LED (275 nm) activated sodium percarbonate (SPC) systems was systematically performed. The higher removal rate of ODVEN can be achieved under UV-LED direct photolysis (14.99%) than UV direct photolysis (4.57%) due to the higher values of photolysis coefficient at the wavelength 275 nm. Significant synergistic effects were observed in the UV/SPC (80.38%) and UV-LED/SPC (53.57%) systems and the former exhibited better performance for the elimination of ODVEN. The degradation of ODVEN all followed the pseudo-first-order kinetics well in these processes, and the pseudo-first-order rate constant (kobs) increased with increasing SPC concentration. Radicals quenching experiments demonstrated that both ·OH and CO3·− were involved in the degradation of ODVEN and the second-order rate constant of ODVEN with CO3·− (1.58 × 108 (mol/L)−1 sec−1) was reported for the first time based on competitive kinetic method. The introduction of HA, Cl−, NO3− and HCO3− inhibited the ODVEN degradation to varying degrees in the both processes. According to quantum chemical calculation, radical addition at the ortho-position of the phenolic hydroxyl group was confirmed to be the main reaction pathways for the oxidation of ODVEN by ·OH. In addition, the oxidation of ODVEN may involve the demethylation, H-abstraction, OH-addition and C-N bond cleavage. Eventually, the UV-LED/SPC process was considered to be more cost-effective compared to the UV/SPC process, although the UV/SPC process possessed a higher removal rate of ODVEN.

Transformation of phenol and nitrobenzene by superoxide radicals: Kinetics and mechanisms

In this study, the kinetics and mechanisms of superoxide radical (O2·-)-mediated transformation of two monosubstituted aromatic contaminants with diametrically opposed electronic effect, namely phenol (PhOH) and nitrobenzene (NB), were investigated through combined experimental and theoretical approaches. The reaction rate constant between O2·- and PhOH was determined to be 269 M−1 s−1via competition kinetic method. Such weak role of O2·- was confirmed by density functional theory (DFT) calculations at SMD/CAM-B3LYP/6-311++G(2d,2p)//CAM-B3LYP/6-31+G(d,p) level of theory. However, NB exhibited a higher reactivity towards O2·- than PhOH, demonstrating the selectivity of O2·- to electron withdrawing group. Despite the difference in reactivity, O2·- shows very limited removal capacity for PhOH and NB. These results were also in reasonably good agreement with an indication from a quantitative structure–property relationship model. With these quantitative and mechanistic results, we provided a more comprehensive understanding on the O2·--mediated transformation of organic pollutants bearing different electron donating/withdrawing groups.

Reaction kinetics of dissolved black carbon with hydroxyl radical, sulfate radical and reactive chlorine radicals

As an important component of dissolved organic matter (DOM), dissolved black carbon (DBC) which is characterized of abundant aromatic and oxygen-containing functional groups, is widely distributed in aquatic environments. Its presence may hinder the oxidation of organic micro-pollutants during advanced oxidation processes (AOPs) via free radicals scavenging effect. However, the second-order reaction rate constants of DBC with different free radicals including hydroxyl radical (OH•), sulfate radical (SO4•-), reactive chlorine radicals (RCR) are unknown and the relationship between the chemical composition of DBC and the second-order reaction rate constants during different AOPs (UV/H2O2, UV/PDS, UV/Chlorine) is also unclear. In this study, a plant-derived DBC was extracted from wheat biochar and fractionated according to molecular weight (i.e., <10 k, <3 k, and < 1 k Da). The second order rate constants of DBC reaction with different free radicals were determined by competitive kinetic method. Besides, the chemical composition of DBC was revealed by ultraviolet-visible (UV–Vis) spectroscopy, fluorescence excitation-emission-matrix (EEM) spectroscopy Fourier Transform Infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) combined with statistical analysis. The results showed that the second-order rate constants decreased as the molecular weight increased. For the <1 k Da DBC, the kDBC-OH•, kDBC-SO4•-−, kDBC-RCR were (1.83 ± 0.06) × 104, (7.60 ± 0.21) × 103, and (1.71 ± 0.13) × 104 L·mgC−1·s−1, which were 1.98, 2.19, 1.43 times of that for the <10 k Da fraction and 1.38, 1.36, 1.24 times of that for the <3 k Da fraction in UV/H2O2, UV/PDS and UV/Chlorine processes. In addition, the results of chemical composition analysis showed that DBC mainly contained humic substances and was rich in O-containing functional groups such as CO. The second order reaction rate constants of DBC with different free radicals decreased with increasing the molecular weight of DBC due to the more aggregated structure of the small molecules that the inner carbon of DBC was not easily exposed to free radicals.

Kinetics and Mechanism of Degradation of Reactive Radical-Mediated Probe Compounds by the UV/Chlorine Process: Theoretical Calculation and Experimental Verification.

The UV/chlorine process, by combining chlorination with UV irradiation, has been recently considered as a highly efficient advanced oxidation process (AOP) technology in water treatment. Nitrobenzene (NB), benzoic acid (BA), and p-chlorobenzoic acid (pCBA) are widely used as model probe compounds in the UV/chlorine system to calculate the second-order rate constants of the specific radical reaction with target contaminates by a competitive kinetics method. A comprehensive understanding of probe compounds’ reaction mechanism with reactive radicals is critical for investigation of the UV/chlorine reaction system. Here, we evaluated the radical-mediated reaction kinetics and mechanism of NB, BA, and pCBA in the UV/chlorine process using theoretical calculations and experimental studies. The main reactive radicals •OH, •ClO, and •Cl in the UV/chlorine process for the initial reaction with NB, BA, and pCBA can be explained by H-abstraction and addition pathways. The ΔE0,≠ values for the •OH reaction with NB, BA, and pCBA were in the range of 5.0–8.0, 3.7–8.2, and 3.4–8.2 kcal mol–1, respectively. The ΔE0,≠ values for •ClO and •Cl reactions with these three probe compounds were higher than those of •OH, indicating slower •ClO- and •Cl-initiated reactions than that of the •OH-initiated reaction. The theoretically calculated radical-mediated reaction kinetic rate constants (kCPC) for NB, BA, and pCBA were 4.58 × 10–3, 1.28 × 10–2, and 1.6 × 10–2 s–1, respectively, which was consistent with the experimentally determined pseudo-first-order rate constant (kCPRR) in the UV/chlorine process. Interestingly, theoretical calculations showed that •ClO and •Cl played an important role in subsequent reactions of NB-OH radicals, converting to hydroxylated and chlorinated products, which were further confirmed by experimental products’ identification. The findings from this study indicated that quantum chemistry calculations provide an effective means to investigate the reaction kinetics and mechanism of chemicals in the UV/chlorine process.

Degradation and Mineralization Study of Promecarb by Electro Fenton Process

Organic substances as pesticides, especially aromatic compounds are a major environmental concern. In the present work, solutions of Promecarb or 3-isopropyl-5-methylphenyl-N-methylcarbamate of pH = 3 have been degraded by electro Fenton process, using a volumic electrochemical reactor filled with carbon graphite. Effects of nature of material of cathode, initial concentration of insecticide and applied current on the kinetics of oxidative degradation and mineralization efficiency have been investigated. Kinetic analysis showed that the oxidation of Promecarb by hydroxyl radicals follows a reaction kinetic of pseudo first order. The absolute rate constant for Promecarb oxidation by hydroxyl radicals was determined as 10.88 × 10⁹ L mol^-1 s^-1 by competitive kinetics method and benzoic acid was used as reference compound. Mineralization of this pesticide by hydroxyl radicals consists in its transformation to mineral products. The evolution of the mineralization during Promecarb treatment by electro Fenton process was followed by analysis of Total Organic Carbon TOC. Thus, after 3 hours of electrolysis and at I = 800 mA, more than 50% of the organic carbon present in the solution is mineralized. Several degradation products were formed during its electro Fenton treatment. These intermediates were identified using High Performance Liquid Chromatography HPLC, Ionic Chromatography IC and Liquid Chromatography - Mass Spectrometry LC-MS. Based on identification of aromatic intermediates and carboxylic acids, a plausible Promecarb mineralization pathway is proposed. Also, we realized the measurement of the Biochemical Oxygen Demand BOD₅ of insecticide solution after treatment by electro Fenton process, to evaluate its biodegradability.

Rapid removal of fungicide thiram in aqueous medium by electro-Fenton process with Pt and BDD anodes

• Advanced oxidation processes (AOP) are effective for the wastewater treatment. • These processes are based to the production of power oxidizing hydroxyl radicals ( • OH). • Electro-Fenton (EF) process is one of the AOP widely used to treat water polluted by pesticides. • Constant rate for oxidation of thiram by • OH was found to be 5.54 × 10 9 M −1 s −1 . • Almost complete mineralization of thiram was reached by electro-Fenton with BDD anode. The electro-Fenton (EF) process was used to assess the electrochemical degradation of the fungicide thiram and its complete removal from water using an undivided electrolytic cell equipped with Pt or BDD anode and carbon felt cathode. Hydroxyl radicals, produced homogeneously in bulk solution from electrochemically generated Fenton's reagent ( • OH) and heterogeneously on the anode surface (M( • OH)) from oxidation of water, reacted with thiram leading to its fast oxidation. Oxidative degradation and mineralization kinetics were monitored by chromatographic analysis (HPLC) and total organic carbon (TOC) measurements. The electrochemical degradation of thiram by hydroxyl radicals followed a pseudo-first-order reaction kinetics with an absolute rate constant k abs(Thir) of 5.54 (±0.03) × 10 9 M −1 s −1 , determined by competition kinetics method. The TOC removal rate values were found significantly higher with BDD anode than Pt anode. Thus, almost complete mineralization (92%) of thiram solution was obtained when using BDD anode. These results highlight the major role of heterogeneous BDD( • OH) formed in the mineralization of thiram. The contribution of homogeneous • OH in mineralization of thiram was found relatively low due to its specific aliphatic structure. The efficiency of the EF process was evaluated by determining mineralization current efficiency and energy consumption per gram of TOC removed. Degradation by-products and inorganic ions, such as nitrate (NO 3 − ), nitrite (NO 2 − ), ammonium (NH 4 + ) and sulfate (SO 4 2 − ) formed during mineralization process, were identified by GC–MS and ionic chromatography analyses and a plausible mineralization pathway was proposed.

Kinetic Study on the Reactivity of Azanone (HNO) toward Cyclic C-Nucleophiles.

Azanone (HNO) is an elusive electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present a comparative kinetic study of HNO reactivity toward selected cyclic C-nucleophiles under aqueous conditions at pH 7.4. We applied the competition kinetics method, which is based on the use of a fluorescein-derived boronate probe FlBA and two parallel HNO reactions: with the studied scavenger or with O2 (k = 1.8 × 104 M−1s−1). We determined the second-order rate constants of HNO reactions with 13 structurally diverse C-nucleophiles (k = 33–20,000 M−1s−1). The results show that the reactivity of HNO toward C-nucleophiles depends strongly on the structure of the scavenger. The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with C-nucleophiles is provided.

The optimal dose of oxidants in UV-based advanced oxidation processes with respect to primary radical concentrations

UV-based advanced oxidation processes (AOPs) via photolysis of precursor chemical oxidants have been of interest to numerous researchers over the past several decades due to their capacity to generate highly active radical species and interesting radical chemistry. However, applications of UV-based AOPs have been commonly optimized case by case, due to the lack of theoretical investigations on process optimization, especially on oxidant doses. In this study, a simple equation for UV/H2O2 (•OH as the sole primary reactive species (PRS)) to obtain the theoretical optimal concentration (Copt-theoretical) for H2O2 was derived (Copt-theoretical=Ab·Scε·k). The equation was then validated for its accuracy in the calculation of Copt-theoretical for H2O2 in the UV/H2O2 AOP using a well-established comprehensive kinetic model. A competition kinetics method for the measurement of scavenging capacity (Sc, the unknown parameter for the simple equation) was designed, for which nitrobenzene was employed as the probe compound and tert‑butyl alcohol was introduced as the standard compound. Based on this simple equation, we calculated the Copt-theoretical of 77 environmental water samples and introduced the concept of a practical optimal oxidants dose for the UV/H2O2 AOP, while minimizing the operation costs in engineering applications. Moreover, this study mathematically proved that the simple equation obtained from UV/H2O2 could be successfully extended to other UV-based AOPs, including UV/chlorine, UV/NH2Cl, UV/S2O82−, and UV/peracetic acid. The simple equation of Copt-theoretical derived in this study may not only help to provide instructions for engineering applications, but also point out the ultimate treatment capability of each UV-based AOPs.

Kinetics of Azanone (HNO) Reactions with Thiols: Effect of pH

HNO (nitroxyl, IUPAC name azanone) is an electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present data on the pH-dependent kinetics of azanone reactions with the low molecular thiols glutathione and N-acetylcysteine, as well as with important serum proteins: bovine serum albumin and human serum albumin. The competition kinetics method used is based on two parallel HNO reactions: with RSH/RS− or with O2. The results provide evidence that the reaction of azanone with the anionic form of thiols (RS−) is favored over reactions with the protonated form (RSH). The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with thiolates is provided.

Aqueous picloram degradation by hydroxyl radicals: Unveiling mechanism, kinetics, and ecotoxicity through experimental and theoretical approaches

Pesticides are chemical compounds widely used to combat pests in crops, and they thus play a key role in agricultural production. However, due to their persistence in aquatic environments, even at low concentrations, their use has been considered an environmental problem and caused concern regarding the adverse effects on human health. This paper reports, for the first time, the mechanisms, kinetics, and an evaluation of the toxicity of picloram degradation initiated by OH radicals in the aqueous environment using quantum chemistry and computational toxicology calculations. The rate constants are calculated using a combination of formulations derived from the Transition State Theory in a realistic temperature range (250–310 K). The results indicate that the two favorable pathways (R1 and R5) of OH -based reactions occur by addition to the pyridine ring. The calculated rate constant at 298 K is compared with the overall second-order reaction rate constant, quantified herein experimentally via the competition kinetics method and data available in the literature showing an excellent agreement. The toxicity assessment and a photolysis study provide important information: i) picloram and the majority of degradation products are estimated as harmful; however, ii) these compounds can suffer photolysis in sunlight. The results of the present study can help understand the mechanism of picloram, also providing important clues regarding risk assessment in aquatic environments as well as novel experimental information.