Second-order Rate Constants For Reactions Research Articles

Investigation of CO2Absorption Kinetics and Desorption Performance in Aqueous 1-(2-Aminoethyl)-3-methylimidazolium Bromine Solution

The absorption rate and absorption amount of CO2 were determined at different 1-(2-aminoethyl)-3-methylimidazolium bromine ([Aemim][Br]) concentrations, absorption temperatures, and CO2 content in order to optimize operational conditions in our study. Meanwhile, the effects of regeneration temperature, regeneration time, and regeneration circles on the regeneration efficiency of CO2 saturated [Aemim][Br] solution were discussed, respectively. On the basis of these experiments, the kinetic data (i.e., in terms of the pseudo-first-order rate constant (kov), the second-order reaction rate constant (k2) and heat of CO2 absorption was calculated at 303 K as [Aemim][Br] concentration varied from 0.1 to 2.0 mol·L–1. The result showed that the process of CO2 absorption was a fast pseudo-first-order reaction regime. According to the kinetic data at 293–323 K, the activation energy of reaction was calculated by the Arrhenius equation, and the second-order reaction rate constant can be expressed as follows: k2 = 1.9...

Comparison of the degradation of molecular and ionic ibuprofen in a UV/H2O2 system.

The advanced oxidation technologies based on •OH can effectively degrade the pharmaceutical and personal care products under operating conditions of normal temperature and pressure. In this study, direct photolysis of ibuprofen (IBU) is slow due to the relatively low molar extinction coefficient and quantum yield. Compared to direct photolysis, the degradation kinetics of IBU was significantly enhanced in the UV/H2O2 system, mainly by •OH radical mediated oxidation. In the UV/H2O2 system, the degradation rate of ionic IBU was slightly faster than that of the molecular form. Kinetic analysis showed that the second-order reaction rate constant of ionic IBU (5.51 × 109 M-1 s-1) was higher than that of the molecular form (3.43 × 109 M-1 s-1). The pseudo first-order rate constant for IBU degradation (kobs) increased with increasing H2O2 dosage. kobs can be significantly decreased in the presence of natural organic matter (NOM), which is due to (i) NOM radical scavenging effects (dominant role) and (ii) UV absorption. The degradation of IBU was inhibited by HCO3-, which was attributed to its scavenging effect. Interestingly, when NO3- was present in aqueous solution, a slight increase in the degradation rate was observed, which was due to NO3- absorbing photons to generate •OH at a low quantum yield. No obvious effects were observed when SO42 and Cl- were present.

Activation of persulfate by magnetite: Implications for the degradation of low concentration sulfamethoxazole

Abstract Sulfamethoxazole (SMX) was widely detected in the effluent of sewage treatment plants and water environment. In this study, SMX with low concentration was effectively degraded by magnetite-activated persulfate (PS) system. The process mainly involved the sulphate radical (SO4 −) and hydroxyl radicals (HO ), formed from PS activated by Fe(II) and Fe(III) in Fe3O4. The second-order reaction rate constants for the reaction between SMX and HO /SO4 − were estimated. The SMX degradation rate decreased as the solution pH increased from pH 3.5–10.5 and the pH dependency of SMX degradation rate was closely related to the speciation of Fe2+. HO , SO4 − and O2 − were involved during PS activation. The effects of chloride were investigated. SMX degradation efficiency was significantly decreased due to the trapping of SO4 − by Cl− at pH 4.0, while it had no effect by Cl− at pH 10.5 due to the formation of HO . The potential recyclability of Fe3O4 was measured and it could be reused at least 4 times. The findings may have promising implications in the application of Fe3O4 on a new technology for the treatment of micro-contaminated waters and soils, especially the application of natural magnetite.

Bench- and pilot-scale studies on the removal of pesticides from water by VUV/UV process

As an advanced oxidation process, vacuum ultraviolet/ultraviolet (VUV/UV) process has recently gained increasing attention in water treatment, but assessment of its technical and economic feasibility for pesticide removal has rarely been conducted. This study investigated the degradation of five typical pesticides, including aldicarb (ALD), alachlor (ALA), chloroneb (CHL), methiocarb (MET) and atrazine (ATR), by VUV/UV under simulated water treatment conditions at both bench- and pilot-scales. Bench-scale tests showed that pesticide degradation followed pseudo-first-order kinetics and all pesticides were degraded by >90% at a VUV fluence of 12 mJ cm−2. The second-order reaction rate constants of ALD, ALA, CHL, MET and ATR with HO were 7.57 × 109, 4.89 × 109, 2.57 × 109, 3.81 × 109 and 2.38 × 109 M−1 s−1, respectively. Solution pH had varying influences on the degradation of different pesticides by VUV/UV. Inorganic anions (Cl−, NO3−, HCO3−) and natural organic matter (NOM) inhibited pesticide degradation by scavenging HO, but SO42− slightly promoted pesticide degradation. The toxicity of pesticide solutions was reduced by 33.7% to 74.8% after the VUV/UV treatment. In a long-term (80 d) operation of the pilot reaction system using a sand-filtered water, the stable performance with acceptable energy consumption (0.27–1.52 kWh m−3 Order−1) suggested a good application potential of the VUV/UV process for pesticide removal in small-scale water treatment.

Elucidating the Stimulatory and Inhibitory Effects of Dissolved Organic Matter from Poultry Litter on Photodegradation of Antibiotics.

This study examined the photolytic fate of the chlortetracycline (CTC), ciprofloxacin (CIP), roxarsone (ROX), and sulfamethoxazole (SMX) antibiotics in agriculturally relevant matrices. The observed photodegradation kinetics for antibiotics in solutions containing dissolved organic matter (DOM) from three poultry litter extracts was modeled to identify contributions from direct and indirect photolysis. Suwannee River natural organic matter (SRN) was used as a surrogate DOM standard. Poultry litter-derived DOM generated lower concentrations of reactive species compared to SRN. Direct photolysis was the dominant transformation mechanism for CIP, whereas CTC, ROX, and SMX were sensitized by 3DOM* and 1O2. The impacts of agricultural DOM on photodegradation of antibiotics were identified in terms of pseudo-first-order rate constants for formation of reactive species and second-order rate constants for reaction of reactive species with DOM. Solutions containing poultry litter-derived DOM generated similar levels of 3DOM* and 1O2, enhancing degradation of CTC, ROX, and SMX. The reactivity of SMX was markedly different in solutions containing poultry litter DOM compared to solutions with SRN, indicating that the photolytic fate of select antibiotics varies for agricultural and surface water matrices. As the majority of antibiotics are consumed by animals, these findings provide new insight into agriculturally relevant transformation mechanisms and kinetics.

Priority pesticide dichlorvos removal from water by ozonation process: Reactivity, transformation products and associated toxicity

Abstract The treatability of waters contaminated with priority pesticide dichlorvos (DDVP) by means of ozonation has been assessed for the first time. In order to do so, reaction kinetics, transformation mechanisms and associated toxicity have been inspected in detail. Second-order rate constants of DDVP reactions with O 3 and OH were determined to be 590 and 2.2·10 9 M −1 s −1 , respectively. These values partly explained the degradation profiles obtained during experiments with and without the presence of an OH scavenger, in which the significant contribution of the indirect degradative route in the removal of DDVP was revealed. LC-MS analyses for ozonated samples allowed the elucidation of desmethyl dichlorvos (d-DDVP), dichloroacetic acid (DCA) and dimethyl phosphate (DMP) as main transformation products (TPs). DMP was found to be present in both O 3 and OH -mediated oxidation pathways. Three possible degradation routes were proposed for OH degradation, whereas the direct oxidation by O 3 was only well-explained by the addition of this oxidant to the double bond of DDVP dichlorovinyl moiety. Microtox® bioassays revealed the inability of molecular ozone to reduce the toxicity of the medium and pointed out the importance of OH contribution in the degradation process. In general, ozonation could be a suitable treatment alternative for DDVP, formed TPs and associated toxicity abatement.

Kinetic analysis of and platinum(II) migration in the reactions of tetrazolato-bridged dinuclear platinum(II) complexes with nucleotides

The series of tetrazolato-bridged complexes with the formula [{cis‑Pt(NH3)2}2(μ-OH)(μ-5-H-tetrazolato-N1,N2)]2+ (5-H-X) or [{cis‑Pt(NH3)2}2(μ-OH)(μ-5-R-tetrazolato-N2,N3)]n+ (R=H (5-H-Y), CH3 (1), CH2COOCH2CH3 (2), CH2COO– (3), n=2 (5-H-Y, 1, 2) or 1 (3)) are promising candidate complexes for formulation as next-generation platinum-based anticancer drugs that form multimodal bindings with DNA molecules. These multimodal bindings involve both non-covalent and covalent interactions, the latter of which are acknowledged to be essential for platinum-based drugs to exert their anticancer activity. In the present study, the tetrazolato-bridged complexes reacted with two molar equivalents of guanosine-5′-monophosphate (GMP) to yield the 1:2 reaction products [{cis‑Pt(NH3)2(GMP-N7)}2(μ-5-R-tetrazolato-N1,N3)]2− or 1−. This reaction was accompanied by an intramolecular Pt(II) migration that contributed to the formation of diverse DNA crosslinking, such as interhelical crosslinks. The second-order reaction rate constants for the reactions performed in phosphate-buffered D2O solution showed that the reactivity of the complexes decreased in the order 5-H-X≳5-H-Y>2≳1>3 and that reactivity was correlated with the cytotoxicity of the complexes. A similar result was obtained for the reaction of the complexes with calf thymus DNA in which the formation of covalent DNA adducts was quantified by means of inductively coupled plasma mass spectrometry. These results suggest that overall charge affects the kinetics of the reactions of platinum complexes with GMP and calf thymus DNA. Thus, the positive charge of the complexes affects not only the non-covalent but also the covalent interactions between the complexes and nucleotides and DNA, which are negatively charged molecules.

Exploring ozonation as treatment alternative for methiocarb and formed transformation products abatement

Despite the high toxicity and resistance to conventional water treatments exhibited by methiocarb (MC), there are no reports regarding the degradation of this priority pesticide by means of alternative purification technologies. In this work, the removal of MC by means of ozonation was studied for the first time, employing a multi-reactor methodology and neutral pH conditions. The second-order rate constants of MC reaction with molecular ozone (O3) and formed hydroxyl radicals (OH·) were determined to be 1.7·106 and 8.2·109 M−1 s−1, respectively. During degradation experiments, direct ozone reaction was observed to effectively remove MC, but not its formed intermediates, whereas OH· could oxidize all species. The major identified TPs were methiocarb sulfoxide (MCX), methiocarb sulfoxide phenol (MCXP) and methiocarb sulfone phenol (MCNP), all of them formed through MC oxidation by O3 or OH· in combination with hydrolysis. A toxicity assessment evidenced a strong dependence on MCX concentration, even at very low values. Despite the OH· capability to degrade MC and its main metabolites, the relative resistance of TPs towards ozone attack enlarged the oxidant dosage (2.5 mg O3/mg DOC) necessary to achieve a relatively low toxicity of the medium. Even though ozonation could be a suitable technique for MC removal from water compartments, strategies aimed to further promote the indirect contribution of hydroxyl radicals during this process should be explored.

Sulfate radical induced degradation of β2-adrenoceptor agonists salbutamol and terbutaline: Phenoxyl radical dependent mechanisms

The present study investigated the reactivity and oxidation mechanisms of salbutamol (SAL) and terbutaline (TBL), two typical β2-adrenoceptor agonists, towards sulfate radical (SO4−) by using photo-activated persulfate (PS). The reaction pathways and mechanisms were proposed based on products identification using high resolution HPLC-ESI-MS, laser flash photolysis (LFP) and molecular orbital calculations. The results indicated that SO4− was the dominant reactive species in the UV/PS process. The second-order rate constants of sulfate radical reaction with SAL and TBL were measured as (3.7 ± 0.3) × 109 and (4.2 ± 0.3) × 109 M−1 s−1 by LFP, respectively. For both SAL and TBL, phenoxyl radicals were found to play key roles in the orientation of the primary pathways. For SAL, a benzophenone derivative was generated by oxidation of the phenoxyl radical. However, in the case of TBL, the transformation of the phenoxyl radical into benzoquinone was impossible. Instead, the addition of OSO3H on the aromatic ring was the major pathway. The same reactivity pattern was observed in the case of TBL structural analogs resorcinol and 3,5-dihydroxybenzyl alcohol. Our results revealed that basic conditions inhibited the decomposition of SAL and TBL, while, increasing PS dose enhanced the degradation. The present work could help for a better understanding of the difference in oxidation reactivity of substituted phenols widely present in natural waters.

Ozonation of the oxybenzone, octinoxate, and octocrylene UV-filters: Reaction kinetics, absorbance characteristics, and transformation products

UV-filters (UVFs) are active ingredients in personal care products that protect skin from exposure to UV light. Environmentally-relevant concentrations of UVFs have recently been linked to toxicity in aquatic organisms, necessitating research into improved UVF removal in water/wastewater treatment. Here, we investigated ozonation of the three most commonly employed UVFs: octinoxate (OMC), octocrylene (OC), and oxybenzone (OXY). Specific second-order rate constants for UVF reaction with ozone were identified as follows: OMC, 5.25×104M−1s−1; OC, 1.58M−1s−1; OXY (neutral), 3.80×102M−1s−1; and, OXY (anion), 1.51×106M‐1s‐1. These kinetic parameters indicated that OMC and OXY undergo significant (2-log or greater) transformation for typical ozone exposures in disinfection processes; however, minimal oxidation is expected for OC. UV absorbance mapping was employed to characterize the loss of UVF activity (i.e., absorbance across the UV-A, UV-B, and UV-C ranges) during ozonation. These 4-dimensional maps also confirmed ozone attack mechanisms, namely reaction at phenolate (OXY) and olefin (OMC, OC) groups. Primary transformation products from these reactions were identified for all three UVFs of concern. For OC and OXY, the benzophenone structure is conserved, suggesting that transformation products retain toxicity concerns.

Reactivity of Semivolatile Organic Compounds with Hydroxyl Radicals, Nitrate Radicals, and Ozone in Indoor Air

Indoor semivolatile organic compounds (SVOCs) originate from indoor and outdoor sources. These SVOCs partition among different phases and available surfaces, which increases their residence time indoors to several years. SVOCs may also react with indoor oxidants, such as hydroxyl radicals (OH), nitrate radicals (NO3), and ozone. In the present study, the second-order reaction rate constants of 72 SVOCs in indoor air (gas and particle phases) at room temperature and ambient air pressure were retrieved from the literature. The pseudo–first-order reaction rate constants of these SVOCs were calculated for the indoor concentration ranges of OH, NO3, and ozone. Then, the extent to which the chemical reaction had a meaningful impact on the removal of SVOCs from the indoor environment was quantitatively analyzed. The orders of magnitude of the second-order rate constant ranged between 10−15 and 10−10 cm3/(molecule·s) for OH/SVOC reactions, 10−17 and 10−12 cm3/(molecule·s) for NO3/SVOC reactions, and 10−20 and 10−16 cm3/(molecule·s) for ozone/SVOC reactions in indoor air. Assuming that the highest indoor reactant concentrations were 1.8 × 106 molecules/cm3 (7.3 × 10−5 ppb) for OH, 2.5 × 108 molecules/cm3 (10−2 ppb) for NO3, and 1.4 × 1012 molecules/cm3 (58 ppb) for ozone, the highest pseudo–first-order rate constants in the gas phase for the studied reactions of OH/SVOCs (n = 72), NO3/SVOCs (n = 3), and ozone/SVOCs (n = 14) reached 1.5 h−1 (OH/benzo[a]pyrene), 0.41 h−1 (NO3/acenaphthene), and 1.0 h−1 (ozone/aldrin and ozone/heptachlor), respectively. The pseudo–first-order rate constants in the particle phase for the studied reactions of OH/SVOCs (n = 13), NO3/SVOCs (n = 6), and ozone/SVOCs (n = 14) at the high indoor reactant concentrations reached 0.09 h−1 (OH/DEHP), 5.8 h−1 (NO3/pyrene), and 11 h−1 (ozone/benzo[a]pyrene), respectively. These results indicate that the chemical reactions of some SVOCs in indoor air have a meaningful impact compared to the air exchange rate, which should be considered in future studies on indoor air quality modeling.

Mathematical simulation of the hydrogenation of borodino coal

The kinetic and thermodynamic parameters of the hydrogenation of Borodino brown coal were calculated. With the use of equilibrium-kinetic analysis, the second-order reaction rate constants of forward and reverse reactions, the equilibrium constant, the activation energy, and the thermal effects of brown coal hydrogenation reactions were determined in a temperature range of 648–698 K at a hydrogen pressure of 10 MPa. In the course of the study, it was found that the experimental data are indicative of the adequacy of the equilibrium-kinetic analysis model for second-order reactions.

Electrostatic roles in electron transfer from [NiFe] hydrogenase to cytochrome c3 from Desulfovibrio vulgaris Miyazaki F

Electrostatic interactions between proteins are key factors that govern the association and reaction rate. We spectroscopically determine the second-order reaction rate constant (k) of electron transfer from [NiFe] hydrogenase (H2ase) to cytochrome (cyt) c3 at various ionic strengths (I). The k value decreases with I. To analyze the results, we develop a semi-analytical formula for I dependence of k based on the assumptions that molecules are spherical and the reaction proceeds via a transition state. Fitting of the formula to the experimental data reveals that the interaction occurs in limited regions with opposite charges and with radii much smaller than those estimated from crystal structures. This suggests that local charges in H2ase and cyt c3 play important roles in the reaction. Although the crystallographic data indicate a positive electrostatic potential over almost the entire surface of the proteins, there exists a small region with negative potential on H2ase at which the electron transfer from H2ase to cyt c3 may occur. This local negative potential region is identical to the hypothetical interaction sphere predicted by the analysis. Furthermore, I dependence of k is predicted by the Adaptive Poisson–Boltzmann Solver considering all charges of the amino acids in the proteins and the configuration of H2ase/cyt c3 complex. The calculation reproduces the experimental results except at extremely low I. These results indicate that the stabilization derived from the local electrostatic interaction in the H2ase/cyt c3 complex overcomes the destabilization derived from the electrostatic repulsion of the overall positive charge of both proteins.

Analytical model for the design of in situ horizontal permeable reactive barriers (HPRBs) for the mitigation of chlorinated solvent vapors in the unsaturated zone

In this work we introduce a 1-D analytical solution that can be used for the design of horizontal permeable reactive barriers (HPRBs) as a vapor mitigation system at sites contaminated by chlorinated solvents. The developed model incorporates a transient diffusion-dominated transport with a second-order reaction rate constant. Furthermore, the model accounts for the HPRB lifetime as a function of the oxidant consumption by reaction with upward vapors and its progressive dissolution and leaching by infiltrating water. Simulation results by this new model closely replicate previous lab-scale tests carried out on trichloroethylene (TCE) using a HPRB containing a mixture of potassium permanganate, water and sand. In view of field applications, design criteria, in terms of the minimum HPRB thickness required to attenuate vapors at acceptable risk-based levels and the expected HPRB lifetime, are determined from site-specific conditions such as vapor source concentration, water infiltration rate and HPRB mixture. The results clearly show the field-scale feasibility of this alternative vapor mitigation system for the treatment of chlorinated solvents. Depending on the oxidation kinetic of the target contaminant, a 1m thick HPRB can ensure an attenuation of vapor concentrations of orders of magnitude up to 20years, even for vapor source concentrations up to 10g/m3. A demonstrative application for representative contaminated site conditions also shows the feasibility of this mitigation system from an economical point of view with capital costs potentially somewhat lower than those of other remediation options, such as soil vapor extraction systems. Overall, based on the experimental and theoretical evaluation thus far, field-scale tests are warranted to verify the potential and cost-effectiveness of HPRBs for vapor mitigation control under various conditions of application.

Kinetics of Oxidation of Iodide (I–) and Hypoiodous Acid (HOI) by Peroxymonosulfate (PMS) and Formation of Iodinated Products in the PMS/I–/NOM System

In this work, the kinetics of transformation of iodide (I–) and hypoiodous acid (HOI) by peroxymonosulfate (PMS) and potential formation of worrisome iodinated products in the presence of natural organic matter (NOM) were investigated. As the pH increased from 5 to 10, the apparent second-order rate constants of reaction of PMS with I– gradually decreased from 1.01 × 103 to 3.86 × 102 M–1 s–1, while those for HOI increased dramatically from 1.08 × 102 to 7.90 × 104 M–1 s–1. The obtained pH-dependent rate profiles were explained well by the effects of pH-affected speciation of PMS and/or HOI. Considerable amounts of total organic iodine (TOI) could be formed in the PMS/I–/NOM system over a wide pH range. Under similar conditions, the TOI levels formed in the PMS/I–/NOM system were generally higher than those formed in the case of HOCl but much lower than those formed in the case of NH2Cl. Also, specific iodoform (IF) and monoiodoacetic acid (MIAA) were detected in both simulated and authentic waters during...

Sulfamethazine degradation in water by the VUV/UV process: Kinetics, mechanism and antibacterial activity determination based on a mini-fluidic VUV/UV photoreaction system

A mini-fluidic VUV/UV photoreaction system (MVPS) was developed in our previous study, and it was demonstrated as a powerful tool for studies on pollutant degradation by the VUV/UV process. In this study, we investigated the VUV/UV photodegradation of sulfamethazine (SMN), one of the most frequently detected antibiotics in the environment. The determination methods of photochemical kinetic parameters (e.g., photon fluence-based rate constant and quantum yield) were developed based on the MVPS. The photon fluence-based reaction rate constants for SMN degradation by UV alone and VUV/UV processes were determined as 0.07 × 103 and 4.11 × 103 m2 einstein−1, respectively, while their quantum yields were calculated as 0.019 and 0.369, respectively. The second-order reaction rate constant between hydroxyl radical (HO•) and SMN was determined to be 8.9 × 109 M−1 s−1 in VUV/UV irradiation experiments, which were conducted without addition of any other chemical. The pH effect on the SMN degradation by the VUV/UV process arose principally from SMN and HO speciation. In addition, six byproducts were identified and the potential degradation pathways of SMN including hydroxylation and SO2 elimination were proposed. The antibacterial activity of the SMN solution, assessed by the growth inhibition tests of Escherichia coli, decreased by about 80% after VUV/UV treatment up to a photon fluence of 3.58 × 10−3 einstein m−2. This study has developed methods for the determination of photochemical kinetic parameters using the newly developed MVPS and has demonstrated that the VUV/UV process is an effective technology to remove sulfonamide antibiotics in water.

34 - The Kinetic Study on the Reactivity of HNO (Azanone) Towards Nitrite Ions, Hydroxylamine, Selected Nitrosocompounds and Selected Phosphines

Nitroxyl (HNO, IUPAC name azanone), being formally the protonated product of one-electron reduction of nitric oxide (NO), is an elusive reactive nitrogen species possessing interesting biological chemistry and high pharmacological importance. 1,2 Contrary to nitric oxide, HNO is a strong electrophile highly reactive towards nucleophiles. Here, we present the study on the kinetics of HNO reaction with nitrate ions, hydroxylamine, selected nitrosocompounds and selected phosphines. In this study we applied previously described 4 competition kinetics method based on two parallel, competing HNO reactions – with thiol and with molecular oxygen. The latter, relatively fast reaction (k = (1.8 ± 0.3) ∙ 10 4 M -1 s -1 ), 3 results in the formation of peroxynitrite, 3 that can be easily detected fluorometrically with the use of fluorogenic boronate derivative of resorufin, PC1. PC1 reacts rapidly and directly with ONOO – (k = 1∙10 6 M -1 s -1 ) with the formation of fluorescent resorufin as the major product. In this study the formation of resorufin was monitored by Applied Photophysics SX20 stopped-flow spectrophotometer equipped with a fluorescence detector. HNO released from Angeli's salt (3 μM) reacts either with the azanone scavenger, or with molecular oxygen (225 μM) forming peroxynitrite detected by PC1 (25 μM). Table 1. The determined values of the second-order rate constant of HNO reaction. HNO scavenger k (M -1 s -1 ) Nitrite ions (4,8 ± 0,2) × 10 3 Hydroxyloamine (2,0 ± 0,1) × 10 4 2-Nitroso-1-naphtol (9,8 ± 0,2) × 10 5 Nitrosobenzene >1,1× 10 5 S-Nitrosoglutathione 2,9 × 10 5 Triphenylophosphine > 6,1 × 10 6 Triphenylphosphine-3,3′,3′′-trisulfonic acid trisodium salt (1,5 ± 0,1) × 10 7 References 1: J. M. Fukuto, M. D. Bartberger et al., Chem. Res. Toxicol., 2005, 18 , 790. 2: N. Paolocci, M. I. Jackson et al. , Pharmacol. Ther. , 2007, 113 , 442. 3: R. Smulik, D. Debski et al. , J. Biol. Chem., 2014, 289 , 35570.

A study of the reaction of ferrate with pentachlorophenol - kinetics and degradation products.

Pentachlorophenol (PCP) is a persistent pollutant which has been widely used as a pesticide and a wood preservative. As PCP is toxic and is present in significant quantities in the environment, there is considerable interest in elimination of PCP from waters. One of the promising methods is the application of ferrate. Ferrate is an oxidant and coagulant. It can be applied as a multi-purpose chemical for water and wastewater treatment as it degrades a wide range of environmental pollutants. Moreover, ferrate is considered a green oxidant and disinfectant. This study focuses on the kinetics of PCP degradation by ferrate under different pH conditions. The formation of degradation products is also considered. The second-order rate constants of the PCP reaction with ferrate increased from 23 M-1 s-1 to 4,948 M-1 s-1 with a decrease in pH from 9 to 6. At neutral pH the degradation was fast, indicating that ferrate could be used for rapid removal of PCP. The total degradation of PCP was confirmed by comparing the initial PCP molarity with the molarity of chloride ions released. We conclude no harmful products are formed during ferrate treatment as all PCP chlorine was released as chloride. Specifically, no polychlorinated dibenzo-p-dioxins and dibenzofurans were detected.

Electrochemical Transformation of Trace Organic Contaminants in Latrine Wastewater.

Solar-powered electrochemical systems have shown promise for onsite wastewater treatment in regions where basic infrastructure for conventional wastewater treatment is not available. To assess the applicability of these systems for trace organic contaminant treatment, test compound electrolysis rate constants were measured in authentic latrine wastewater using mixed-metal oxide anodes coupled with stainless steel cathodes. Complete removal of ranitidine and cimetidine was achieved within 30 min of electrolysis at an applied potential of 3.5 V (0.7 A L(-1)). Removal of acetaminophen, ciprofloxacin, trimethoprim, propranolol, and carbamazepine (>80%) was achieved within 3 h of electrolysis. Oxidation of ranitidine, cimetidine, and ciprofloxacin was primarily attributed to reaction with NH2Cl. Transformation of trimethoprim, propranolol, and carbamazepine was attributed to direct electron transfer and to reactions with surface-bound reactive chlorine species. Relative contributions of aqueous phase ·OH, ·Cl, ·Cl2(-), HOCl/OCl(-), and Cl2 were determined to be negligible based on measured second-order reaction rate constants, probe compound reaction rates, and experiments in buffered Cl(-) solutions. Electrical energy per order of removal (EEO) increased with increasing applied potentials and current densities. Test compound removal was most efficient at elevated Cl(-) concentrations present when treated wastewater is recycled for use as flushing water (i.e., ∼ 75 mM Cl(-); EEO = 0.2-6.9 kWh log(-1) m(-3)). Identified halogenated and oxygenated electrolysis products typically underwent further transformations to unidentifiable products within the 3 h treatment cycle. Identifiable halogenated byproduct formation and accumulation was minimized during electrolysis of wastewater containing 75 mM Cl(-).

Mechanistic Insights into C-H Oxidations by Ruthenium(III)-Pterin Complexes: Impact of Basicity of the Pterin Ligand and Electron Acceptability of the Metal Center on the Transition States.

A ruthenium(II) complex, [Ru(dmdmp)Cl(MeBPA)] (2) (Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, MeBPA = N-methyl-N,N-bis(pyridylmethyl)amine), having a pterin derivative as a proton-accepting ligand, was synthesized and characterized. Complex 2 shows higher basicity than that of a previously reported Ru(II)-pterin complex, [Ru(dmdmp) (TPA)](+) (1) (TPA = tris(2-pyridylmethyl)amine). On the other hand, 1e(-)-oxidized species of 1 (1OX) exhibits higher electron-acceptability than that of 1e(-)-oxidized 2 (2OX). Bond dissociation enthalpies (BDE) of the two Ru(II) complexes having Hdmdmp as a ligand in proton-coupled electron transfer (PCET) to generate 1OX and 2OX were calculated to be 85 kcal mol(-1) for 1OX and 78 kcal mol(-1) for 2OX. The BDE values are large enough to perform H atom transfer from C-H bonds of organic molecules to the 1e(-)-oxidized complexes through PCET. The second-order rate constants (k) of PCET oxidation reactions were determined for 1OX and 2OX. The logarithms of normalized k values were proportional to the BDE values of C-H bonds of the substrates with slopes of -0.27 for 1OX and -0.44 for 2OX. The difference between 1OX and 2OX in the slopes suggests that the transition states in PCET oxidations of substrates by the two complexes bear different polarization, as reflection of difference in the electron acceptability and basicity of 1OX and 2OX. The more basic 2OX attracts a proton from a C-H bond via a more polarized transition state than that of 1OX; on the contrary, the more electron-deficient 1OX forms less polarized transition states in PCET oxidation reactions of C-H bonds.