Ratio Of Rate Constants Research Articles

Solvent effect on electron transfer through alkanethiols

The data on the rate constants for ferrocene/ferrocenium (Fc/Fc+) electron transfer in ionic liquid 1-butyl-3-methyl imidazolium tetrafluoroborate ([bmim][BF4]) are reported for a series of -S-(CH2)n-Fc adlayers (n=8…16) on polycrystalline gold. The values and trends are compared to the previously published data for similar series in aqueous medium, as well as to the results for ‘free’ Fc at -S-(CH2)n−1-CH3 modified gold in the same ionic liquid. For the latter case, the trend is opposite as compared to that in the available aqueous series (attached and ‘free’ ruthenium ammine complex): the ratio of rate constants for attached and ‘free’ reactants decreases with electrode-reactant distance in the former case, and increases in the latter. The presented comparison discovers the role of reactant and product work terms, which are essential for attached reactants as well because of conformational diversity. Corresponding contributions are estimated from molecular modeling and considered in terms of reaction volume.

A New Mechanism of Free-Radical Nonbranched-Chain Oxidation: Oxygen as an Antioxidant

New reaction scheme is suggested for the initiated nonbranched-chain addition of free radicals to the multiple bond of the molecular oxygen. The scheme includes the addition reaction of the peroxyl free radical to the oxygen molecule to form the tetraoxyl free radical. This reaction competes with chain propagation reactions through a reactive free radical. The chain evolution stage in this scheme involves a few of free radicals, one of which (tetraoxyl) is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. Based on the proposed scheme rate equations (containing one to three parameters to be determined directly) are deduced using quasi-steady-state treatment. The kinetic description with use the obtained rate equations is applied to the γ-induced nonbranched-chain processes of the free-radical oxidation of liquid o-xylene at 373 K and hydrogen dissolved in water containing different amounts of oxygen at 296 K. The ratios of rate constants of competing reactions and rate constants of addition reactions to the molecular oxygen are defined. In these processes the oxygen with the increase of its concentration begins to act as an oxidation autoinhibitor (or an antioxidant), and the rate of peroxide formation as a function of the dissolved oxygen concentration has a maximum. From the energetic standpoint possible nonchain pathways of the free-radical oxidation of hydrogen and the routes of ozone decay via the reaction with the hydroxyl free radical in the upper atmosphere.

Modeling and Experimentation of RAFT Solution Copolymerization of Styrene and Butyl Acrylate, Effect of Chain Transfer Reactions on Polymer Molecular Weight Distribution

AbstractChain transfer reactions widely exist in the free radical polymerization and controlled radical polymerization, which can significantly influence polymer molecular weight and molecular weight distribution. In this work, the chain transfer reactions in modeling the reversible addition–fragmentation transfer (RAFT) solution copolymerization are included and the effects of chain transfer rate constant, monomer concentration, and comonomer ratio on the polymerization kinetics and polymer molecular weight development are investigated. The model is verified with the experimental RAFT solution copolymerization of styrene and butyl acrylate, with good agreements achieved. This work has demonstrated that the chain transfer reactions to monomer and solvent can have significant impacts on the number‐average molecular weight (Mn) and dispersity (Ð).

Application of kinetic modeling to predict the fate of two indoxacarb metabolites and their bound residues in soil

Insecticide indoxacarb metabolites JT333 and MP819 were used as model compounds to assess the utilization of kinetic modeling to elucidate metabolic pathways, determine degradation kinetics of non-extractable residues (NER) and predict the accumulation potential of the released NER in soil. Soil adsorption coefficients and degradation product formation were determined in different soils in laboratory. Inverse kinetic modeling was applied to explore the dynamics of dissipation of parent (P), formation of extractable metabolites (MET), NER and CO2, and to identify their routes of degradation in soil. These two compounds share similar structural characteristics, have high affinity to soil (Koc>5000L/kg), short half-life (DT50 of 4–9days), and significant CO2 formation in soil. However, kinetic modeling showed that they degraded via different pathways. The P-MET-CO2 conversion route was the major degradation pathway for JT333 in aerobic soil. Multiple pathways were involved in MP819 degradation, while the formation of NER was predominant. The time-exposure area under the curves (AUC) for the MET or NER in soils were derived from the time-%concentration plots for the evaluation of rate limiting steps in their degradation pathways. In P-MET-CO2 pathway the MET-CO2 conversion is the rate limiting step for both compounds. Higher P-MET conversion/MET-CO2 conversion rate constant ratio resulted in larger MET AUC. The rate of NER degradation appeared much slower compared to the rates of P-MET and MET-CO2 conversions, likely due to the rate-limiting step of NER release from the bound-state, indicating that in this situation the free-state NER would be unlikely to accumulate in soil. The study reported here demonstrates the utility of kinetic modeling to quantify the dynamics of NER formation/dissipation vs. P-MET-CO2 conversion, and the application of kinetic modeling to predict the possibility of free-sate NER accumulation in soil, therefore, reveals the potential for the quantitative NER environmental risk assessment.

Catalytic diversity conferred by confinement of protons within porous aluminosilicates in Prins condensation reactions

The effects of confinement of transition states and bound intermediates are described here based on kinetic and theoretical studies for Prins condensation and oligomerization reactions of isobutanal-isobutene reactant mixtures catalyzed by Brønsted acid sites of similar strength but confined within diverse microporous (TON, BEA, MFI, FAU) and mesoporous (Al-MCM-41) aluminosilicate structures. Kinetic data and density functional theory (DFT) show that smaller voids lead to more stable bound monomers derived from isobutene and isobutanal and to protons nearly saturated with H-bonded isobutanal. Prins condensation forms 2,5-dimethyl-hexadiene regioisomers (2,5-DMH) with rate constants that reflect the different stability of the kinetically-relevant CC bond formation transition states and their H-bonded isobutanal precursors. Their differences in size lead to reactivities that depend sensitively on the size and shape of the confining voids in each aluminosilicate and even on the location of the Al-atoms (and the protons) within a given framework. Specifically, FAU leads to the highest turnover rates, because larger voids in Al-MCM-41 form less effective van der Waals contacts with the transition state, while the smaller voids within TON and BEA lead to enthalpic and entropic penalties brought forth by host-guest distortions. MFI cage-like intersections provide more effective stabilization than BEA cylindrical channels of similar size and lead to higher turnover rates and smaller DFT-derived activation free energies; these subtle effects of void shape on reactivity are neglected in descriptions of the confining frameworks based on spherical constructs. Reactivity descriptors require instead theoretical treatments that rigorously account for van der Waals interactions and for the entropy losses and host-guest distortions required for confinement, as reported here from confinement free energies ensemble-averaged over all crystallographic locations in each aluminosilicate structure. The ratios of rate constants for these parallel isobutene dimerization and Prins condensation reactions depend only weakly on the nature of the aluminosilicate framework, because of the similar size and shape of their respective transition states and bound precursors. Transition states for secondary skeletal isomerization and cyclization of 2,5-DMH condensation products, however, are stabilized more effectively (relative to condensation transition states) within voids of intermediate size (BEA, MFI), leading to higher selectivity to secondary products. MFI cage-like structures specifically catalyze the conversion of 2,5-DMH to cyclic 1,4-dimethyl-cyclohexene isomers, direct precursors to p-xylene, with unique specificity.

Kinetics of thorium and particle cycling along the U.S. GEOTRACES North Atlantic Transect

The high particle reactivity of thorium has resulted in its widespread use in tracing processes impacting marine particles and their chemical constituents. The use of thorium isotopes as tracers of particle dynamics, however, largely relies on our understanding of how the element scavenges onto particles. Here, we estimate apparent rate constants of Th adsorption (k1), Th desorption (k−1), bulk particle degradation (β-1), and bulk particle sinking speed (w) along the water column at 11 open-ocean stations occupied during the GEOTRACES North Atlantic Section (GA03). First, we provide evidence that the budgets of Th isotopes and particles at these stations appear to be generally dominated by radioactive production and decay sorption reactions, particle degradation, and particle sinking. Rate parameters are then estimated by fitting a Th and particle cycling model to data of dissolved and particulate 228,230,234Th, 228Ra, particle concentrations, and 234,238U estimates based on salinity, using a nonlinear programming technique.We find that the adsorption rate constant (k1) generally decreases with depth across the section: broadly, the time scale 1/k1 averages 1.0 yr in the upper 1000m and (1.4–1.5) yr below. A positive relationship between k1 and particle concentration (P) is found, i.e., k1∝Pb, where b≥1, consistent with the notion that k1 increases with the number of surface sites available for adsorption. The rate constant ratio, K=k1/(k−1+β−1), which measures the collective influence of rate parameters on Th scavenging, averages 0.2 for most stations and most depths. We clarify the conditions under which K/P is equivalent to the distribution coefficient, KD, test that the conditions are met at the stations, and find that K/P decreases with P, in line with a particle concentration effect (dKD/dP<0). In contrast to the influence of colloids as envisioned by the Brownian pumping hypothesis, we provide evidence that the particle concentration effect arises from the joint effect of P on the rate constants for thorium attachment to, and detachment from, particles.

On stratified variable thermal conductivity stretched flow of Walter-B material subject to non-Fourier flux theory

The objective here is to examine the characteristics of non-Fourier flux theory in flow induced by a nonlinear stretched surface. Constitutive expression for an incompressible Walter-B liquid is taken into account. Consideration of thermal stratification and variable thermal conductivity characterizes the heat transfer process. The concept of boundary layer is adopted for the formulation purpose. Modern methodology for the computational process is implemented. Surface drag force is computed and discussed. Salient features of significant variables on the physical quantities are reported graphically. It is explored that velocity is enhanced for a larger ratio of rate constants. The increasing values of thermal relaxation factor correspond to less temperature.

Oxidation of benzalkonium chloride by gamma irradiation: kinetics and decrease in toxicity

The gamma degradation of toxic non-oxidizing biocide dodecyl dimethyl benzyl ammonium chloride (DDBAC) was investigated. The degradation of DDBAC achieved 70–100% depending on the initial concentration and the absorbed dose, but only 10–33% dissolved organic carbon was removed. The presence of NO3−, HCO3−, 2-propanol and tert-butanol inhibited the degradation of DDBAC. The DDBAC degradation rate constant ratios of ·OH, ·H and eaq− was calculated as 7.4:1.4:1. The acute toxicity of 10 mg L−1 DDBAC was removed by 60% at absorbed doses of 0.5–3.0 kGy. The results showed that gamma irradiation was effective to remove DDBAC and its toxicity.

Mixed convective stagnation point flow of Carreau fluid with variable properties

Here, our aim is to address the mixed convective stagnation point flow of Carreau liquid over a moving permeable surface. Constitutive expression of an incompressible Carreau liquid is taken into account. The assumptions of boundary layer are implemented in the mathematical modeling of considered physical phenomenon. A well-known analytical technique homotopy analysis method is employed for the computations of governing equations. The numerical data of skin friction coefficient and local Nusselt number is obtained and explored. The velocity is enhanced for larger ratio of rate constants. The increasing values of suction parameter correspond to less velocity and temperature profiles. Further, a benchmark is presented to validate the solutions obtained here. It is noted that the computed analytical solutions have excellent match with previous published materials in a limiting manner.

Reaction rate of H atoms with N2O in hot water

The rate constant of H• atoms with N2O in water has been measured by a competition method up to 300°C. Radiolysis with 2.5MeV electrons generated H• atoms, and the HD product from their reaction with deuterated tetrahydrofuran (THF-d8) was measured with mass spectroscopy. The concentration of THF-d8 was changed by an order of magnitude in the presence of 25mM N2O to obtain the ratio of rate constants. To determine the rate constant of H• with THF-d8, a similar competition vs. 0.2mM OH– ion was also measured. The reaction rate of H• with OH– has been accurately determined vs. temperature in previous work, allowing the two unknown rate constants to be deduced. Rate constant of H• with THF-d8 follows the Arrhenius law ln(k/M−1s−1)=27.33 - (32.30kJ/mol)/RT. Rate constant of H• with N2O follows the Arrhenius law ln(k/M−1s−1)=24.50 - (30.42kJ/mol)/RT. In all likelihood, the N2O reaction proceeds via cis-HNNO• radical intermediate as in the gas phase, but with participation of a bridging water molecule in the 1,3 hydrogen shift to form N2 and •OH products.

Formation of secondary organic aerosols from the ozonolysis of dihydrofurans

Abstract. In this work we report the study of the ozonolysis of 2,5-dihydrofuran and 2,3-dihydrofuran and the reaction conditions leading to the formation of secondary organic aerosols. The reactions have been carried out in a Teflon chamber filled with synthetic air mixtures at atmospheric pressure and room temperature. The ozonolysis only produced particles in the presence of SO2. Rising relative humidity from 0 to 40 % had no effect on the production of secondary organic aerosol in the case of 2,5-dihydrofuran, while it reduced the particle number and particle mass concentrations from the 2,3-dihydrofuran ozonolysis. The water-to-SO2 rate constant ratio for the 2,3-dihydrofuran Criegee intermediate was derived from the secondary organic aerosol (SOA) yields in experiments with different relative humidity values, kH2O/kSO2 = (9.8 ± 3.7) × 10−5. The experimental results show that SO3 may not be the only intermediate involved in the formation or growth of new particles in contrast to the data reported for other Criegee intermediate–SO2 reactions. For the studied reactions, SO2 concentrations remained constant during the experiments, behaving as a catalyst in the production of condensable products. Computational calculations also show that the stabilised Criegee intermediates from the ozonolysis reaction of both 2,5-dihydrofuran and 2,3-dihydrofuran may react with SO2, resulting in the regeneration of SO2 and the formation of low-volatility organic acids.

Drug disposition model of radiolabeled etelcalcetide in patients with chronic kidney disease and secondary hyperparathyroidism on hemodialysis.

Etelcalcetide (AMG 416) is an allosteric activator of the calcium-sensing receptor for treatment of secondary hyperparathyroidism in patients with chronic kidney disease (CKD) on hemodialysis. To characterize the time course of etelcalcetide in different matrices (plasma, dialysate, urine, and feces), a drug disposition model was developed. Nonlinear mixed-effect modeling was used to describe data from six adults with CKD on hemodialysis who received a single intravenous dose of [14C]etelcalcetide (10mg; 710 nCi) after hemodialysis (study NCT02054572). A three-compartment model with the following attributes adequately described the observed concentration-time profiles of etelcalcetide in the different matrices: biotransformation in the central compartment; elimination in dialysate, urine, and feces; and a nonspecific elimination process. The terminal half-life of total C-14 in plasma was approximately 56days. The ratio of conjugation-deconjugation rate constants between etelcalcetide and biotransformed products was 11.3. Simulations showed that three hemodialysis sessions per week for 52weeks would contribute to 60.1% of the total clearance of etelcalcetide following single-dose intravenous etelcalcetide administration. Minimal amounts were eliminated in urine (2.5%) and feces (5.7%), whereas nonspecific elimination accounted for 31.2% of total elimination. In addition to removal of etelcalcetide,~10% of small-molecular weight biotransformed products was estimated to have been removed through hemodialysis and in urine. This model provided a quantitative approach to describe biotransformation, distribution, and elimination of etelcalcetide, a unique synthetic D-amino acid peptide, in the relevant patient population.

Homotopic solutions for stagnation point flow of third-grade nanoliquid subject to magnetohydrodynamics

The prime intention here in this investigation is to report the influences of Brownian diffusion and thermophoresis in stagnation point flow towards a nonlinear stretching surface. Magnetohydrodynamic (MHD) third grade fluid is considered. Consideration of non-uniform heat generation/absorption characterizes the heat transfer process. Homotopy algorithm is adopted for local similar solution of highly nonlinear problems. Characteristics of material parameters, Hartman number, ratio of rate constants, thermophoresis parameter, heat generation/absorption parameter, Prandtl number, Brownian diffusion parameter and Schmidt number on the velocity, temperature, nanoparticles concentration, surface drag force and Nusselt and Sherwood numbers are addressed via graphs and tables. The results established here disclose that the velocity distribution decays via larger Hartman number. Moreover an increment in Brownian diffusion and thermophoresis variables cause an enhancement in temperature distribution whereas opposite scenario is noticed for these aspects for nanoparticle concentration.

How side chains affect conformation and electrical properties of poly(acrylic acid) in solution?

To better understand the effect of side chains on the chain conformation and electrical properties of polyelectrolytes, dielectric measurements were carried out on solutions of poly(acrylic acid) (PAA), poly(acrylic acid)-graft-dodecyl (PAA-g-dodecyl), and poly(acrylic acid)-graft-poly(ethylene oxide) (PAA-g-PEO) over a wide concentration range. Double dielectric relaxations with counterion distribution were observed for these polymers and a refined double-layer polarization model was proposed to analyze these, by which valuable information about conformations and interfacial electrokinetic properties was obtained. The transitional concentrations for the overlapping and entanglement of chains were identified from results for the dielectric increment and relaxation time. The concentration dependences of the ratio of effective charges were estimated from conductivity data. It was shown that effective charges on PAA were greatly influenced by PEO or dodecyl side chains, which caused steric hindrance of counterion binding and further dissociation of carboxylic groups or bound counterions. Moreover, a mutual superposition and offsetting effect of PEO and dodecyl side chains was observed. An enhancement in the interpenetration of counterion atmospheres as a result of side chains was also found. In addition, the rate constant ratio and the distance of counterion fluctuations perpendicular to the chains were estimated. It was demonstrated that the effects of side chains on the effective charges or ionization properties of GCP play an important role in their conformation, counterion distribution, and fluctuation.

Competitive C-C and C-H bond scission in the ethanol oxidation reaction on Cu(100) and the effect of an alkaline environment.

Direct ethanol fuel cell technology is impeded by inefficient, yet expensive anode catalysts. As such, research on effective and cheap anode catalysts towards complete ethanol oxidation reaction (EOR) is greatly needed. Herein, we report the investigations of the competitive C-C and C-H bond scissions in the EOR involving CH3CO, CH2CO, and CHCO species on Cu(100) using density functional theory and transition state theory calculations. The easiest C-C bond cleavage was found in CH2CO while the most difficult C-H bond cleavage was also found in CH2CO, both with an activation energy of 1.02 eV. The feasible C-C bond scission may take place in CH2CO with a rate constant ratio of the C-C to the C-H bond scission at 100 °C of 0.32. Furthermore, in an alkaline environment, the C-H bond scission activation barrier is considerably lowered but the C-C bond cleavage activation barrier is slightly increased for both CH3CO and CH2CO species. The reaction of CH3CO species on Cu(100) under alkaline conditions produces mainly acetic acid with a barrier of 0.49 eV and a rate constant of 4.93 × 105 s-1 at 100 °C.

Phosphate Changes Effect of Humic Acids on TiO2 Photocatalysis: From Inhibition to Mitigation of Electron-Hole Recombination.

A major challenge for photocatalytic water purification with TiO2 is the strong inhibitory effect of natural organic matter (NOM), which can scavenge photogenerated holes and radicals and occlude ROS generation sites upon adsorption. This study shows that phosphate counteracts the inhibitory effect of humic acids (HA) by decreasing HA adsorption and mitigating electron-hole recombination. As a measure of the inhibitory effect of HA, the ratios of first-order reaction rate constants between photocatalytic phenol degradation in the absence versus presence of HA were calculated. This ratio was very high, up to 5.72 at 30 mg/L HA and pH 4.8 without phosphate, but was decreased to 0.76 (5 mg/L HA, pH 8.4) with 2 mM phosphate. The latter ratio indicates a surprising favorable effect of HA on TiO2 photocatalysis. FTIR analyses suggest that this favorable effect is likely due to a change in the conformation of adsorbed HA, from a multiligand exchange arrangement to a complexation predominantly between COOH groups in HA and the TiO2 surface in the presence of phosphate. This configuration can reduce hole consumption and facilitate electron transfer to O2 by the adsorbed HA (indicated by linear sweep voltammetry), which mitigates electron-hole recombination and enhances contaminant degradation. A decrease in HA surface adsorption and hole scavenging (the predominant inhibitory mechanisms of HA) by phosphate (2 mM) was indicated by a 50% decrease in the photocatalytic degradation rate of HA and 80% decrease in the decay rate coefficient of interfacial-related photooxidation in photocurrent transients. These results, which were validated with other compounds (FFA and cimetidine), indicate that anchoring phosphate - or anions that exert similar effects on the TiO2 surface - might be a feasible strategy to counteract the inhibitory effect of NOM during photocatalytic water treatment.

Numerical simulation for melting heat transfer and radiation effects in stagnation point flow of carbon–water nanofluid

This attempt addresses the simultaneous characteristics of thermal radiation and melting heat transfer effects in stagnation point flow of carbon nanotubes due to a stretching cylinder. Velocity slip phenomenon is also retained. Two types of carbon nanotubes (single and multi-walls) are homogeneously dispersed in the base fluid (water). The governing physical problem is modeled and converted into set of coupled nonlinear ODE’s utilizing transformations. Resulting problems are computed numerically by fifth order Range–Kutta Fehlberg scheme. The physical characteristics of various variables on the velocity and thermal fields are examined. Numerical data for skin friction and Nusselt number have been prepared and deliberated. It is explored that velocity is increased for larger ratio of rate constants. The increasing values of melting parameter correspond to higher velocity and less temperature. Besides this the accuracy of present results is also affirmed. It is noted that the computed numerical solutions have excellent match with previous published materials in a limiting sense.

Free Radical Polymerization Behavior of the Vinyl Monomers from Plant Oil Triglycerides

A one-step method of plant oil direct transesterification was used to synthesize new vinyl monomers from sunflower (SFM), linseed (LSM), soybean (SBM), and olive (OVM) oils. The degree of unsaturation in plant oil fatty acids was used as a criterion to compare the free radical polymerization behavior of new monomers. The number-average molecular weight of plant oil-based homopolymers synthesized in toluene in the presence of AIBN at 75 °C varies at 11 000–25 000 and decreases as follows: poly(OVM) > poly(SFM) > poly(SBM) > poly(LSM), corresponding to increasing degree of unsaturation in the monomers. Rate of polymerization depends noticeably on the degree of unsaturation in monomers. Due to the allylic termination, chain propagation coexists with effective chain transfer during polymerization. The obtained values of CM (ratio of chain transfer and propagation rate constants) depends on monomer structure as follows: CM(LSM) > CM(SBM) > CM(SFM) > CM(OVM). 1H NMR spectroscopy shows that the fraction of the r...

Acid strength and metal-acid proximity effects on methylcyclohexane ring contraction turnover rates and selectivities

Methylcyclohexane ring contraction is used here to assess the effects of acid strength and metal-acid site proximity on turnover rates and selectivities for bifunctional catalysts consisting of Keggin type polyoxometalates (POM) with different central atoms that act as Brønsted acids. Bifunctional catalysts with metal sites that fully equilibrate cycloalkanes and cycloalkenes give methylcyclohexene conversion rate constants that decreased exponentially with increasing deprotonation energy, a rigorous descriptor of acid strength, consistent with the ion-pair character of transition states that mediate the kinetically-relevant ring contraction of bound methylcyclohexoxide intermediates. The measured rates of formation of each alkylcyclopentane isomer, however, do not reflect their intrinsic formation kinetics, because of fast diffusion-enhanced interconversions within acid domains; thus, isomer selectivities cannot be used to infer, even indirectly, the strength of acid sites, as often proposed in previous studies. Selectivities reflect instead diffusional effects that become more severe as the number and strength of acid sites and the size and diffusive resistances increase within these acid domains, which shift, in turn, the products formed from relative abundances dictated by kinetics to those prescribed by thermodynamics. A rigorous accounting of these diffusional effects using a kinetic-transport model leads to ratios of intrinsic rate constants for the formation of the different alkylcyclopentane isomers that do not depend on acid strength, because all isomerization routes are mediated by bicyclo[3.1.0]hexyl cation transition states similar in the amount and location of charge and that therefore benefit to the same extent from the more stable conjugate anions characteristic of stronger acids.

Decomposition of dexamethasone by gamma irradiation: Kinetics, degradation mechanisms and impact on algae growth

Gamma-irradiation induced degradation of dexamethasone (DEX) in aqueous solution was investigated under different conditions. The results indicate that low-concentration DEX could be effectively decomposed during gamma irradiation. H2O2 and Fenton reagent were favorable for DEX radiolytic removal. However, DEX radiolysis was restrained in the presence of NO3-, CO32-, 2-propanol and tert-butanol. DEX radiolytic degradation was in accordance with the pseudo first-order reaction kinetics. The rate constant ratios of OH, eaq- and H towards DEX degradation could be calculated as kOH: keaq-: kH=10:1.5:1. Besides, it is observed that radiolytic degradation of DEX restrained algae growth. Finally, LC-MS analysis was employed to identify the intermediates and Gaussian 03 software was used to optimize DEX geometry structure. On the basis of LC-MS analysis and Gaussian calculations, we attributed DEX radiolytic removal to OH oxidation, OH radical substitution and direct cleavage of DEX molecules.