Ozonation of drinking water: part I. Oxidation kinetics and product formation.

Many, if not most, redox reactions are coupled to proton transfers. This includes most common sources of chemical potential energy, from the bioenergetic processes that power cells to the fossil fuel combustion that powers cars. These proton-coupled electron transfer or PCET processes may involve multiple electrons and multiple protons, as in the 4 e–, 4 H+ reduction of dioxygen (O2) to water (eq 1), or can involve one electron and one proton such as the formation of tyrosyl radicals from tyrosine residues (TyrOH) in enzymatic catalytic cycles (eq 2). In addition, many multi-electron, multi-proton processes proceed in one-electron and one-proton steps. Organic reactions that proceed in one-electron steps involve radical intermediates, which play critical roles in a wide range of chemical, biological, and industrial processes. This broad and diverse class of PCET reactions are central to a great many chemical and biochemical processes, from biological catalysis and energy transduction, to bulk industrial chemical processes, to new approaches to solar energy conversion. PCET is therefore of broad and increasing interest, as illustrated by this issue and a number of other recent reviews.

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Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

In vitro studies suggested that sub-millisecond pulses of radiation elicit less genomic instability than continuous, protracted irradiation at the same total dose. To determine the potential of ultrahigh dose-rate irradiation in radiotherapy, we investigated lung fibrogenesis in C57BL/6J mice exposed either to short pulses (≤500 ms) of radiation delivered at ultrahigh dose rate (≥40 Gy/s, FLASH) or to conventional dose-rate irradiation (≤0.03 Gy/s, CONV) in single doses. The growth of human HBCx-12A and HEp-2 tumor xenografts in nude mice and syngeneic TC-1 Luc + orthotopic lung tumors in C57BL/6J mice was monitored under similar radiation conditions. CONV (15 Gy) triggered lung fibrosis associated with activation of the TGF-b (transforming growth factor–b) cascade, whereas no complications developed after doses of FLASH below 20 Gy for more than 36 weeks after irradiation. FLASH irradiation also spared normal smooth muscle and epithelial cells from acute radiation-induced apoptosis, which could be reinduced by administrationofsystemicTNF-a(tumornecrosisfactor–a)beforeirradiation.Incontrast,FLASHwasasefficientasCONVinthe repression of tumor growth. Together, these results suggest that FLASH radiotherapy might allow complete eradication of lung tumors and reduce the occurrence and severity of early and late complications affecting normal tissue.

http://www.fondaction.ch/wp-content/uploads/2014/10/245ra93.full_.pdf

Ultrahigh dose-rate FLASH irradiation increases the differential response between normal and tumor tissue in mice

Fluoroquinolones and their metabolites are found in surface and ground waters, indicating their ineffective removal by conventional water treatment technologies. Advanced oxidation processes (AOPs) are alternatives to traditional water treatments. They utilize free radical reactions to directly degrade fluoroquinolones. This work reports absolute rate constants for the reaction of ciprofloxacin with several free radicals, OH, N 3 and SO 4 − as well as hydrated electrons. Pulsed radiolysis experiments showed that OH, N 3 and e aq − reacted quickly with ciprofloxacin, with bimolecular reaction rate constants of (2.15 ± 0.10) × 10 10 , (2.90 ± 0.12) × 10 10 and (2.65 ± 0.15) × 10 10  M −1  s −1 , respectively, while the SO 4 − radical appeared not to react with ciprofloxacin. Transient spectra were observed for the intermediate radicals produced by hydroxyl and azide radical reactions. Moreover, ciprofloxacin can be degraded rapidly using a typical advanced oxidation process, TiO 2 photocatalysis, with half-lives of 1.9–10.9 min depending upon pH values. Seven degradation products were elucidated by LC/MS/MS analysis, and the degradation mechanism of ciprofloxacin was also tentatively proposed by combining the experimental evidence with theoretical calculations of frontier electron densities. The calculations suggest that the addition of a hydroxyl radical to ciprofloxacin and photo-hole direct attack is two predominant reaction pathways.

Kinetics and mechanism of advanced oxidation processes (AOPs) in degradation of ciprofloxacin in water

Dissolved organic matter (DOM) has recently been shown to reduce the transformation rate of various aqueous organic contaminants submitted to oxidation by excited triplet states, apparently by inhibiting the transformation of oxidation intermediates. The main goals of the present study were to evaluate in more detail the effect of concentration and type of DOM on the triplet-induced transformation rate of four selected organic compounds and to check for an analogous inhibition effect in the case of oxidation induced by hydroxyl radical. A marked inhibition by DOM of triplet-induced oxidation was observed for N,N-dimethylaniline (DMA) and the two antibiotics sulfamethoxazole (SMX) and trimethoprim (TRI), with DOM of terrestrial origin being a more effective inhibitor than DOM of aquatic origin. The results are important to understand the role of DOM both as a photosensitizer and as an inhibitor for the triplet-induced transformation of aquatic contaminants. In contrast, no DOM-induced reduction in second-order rate constant could be observed in competition kinetics experiments for the reaction of hydroxyl radical with a series of 15 organic compounds, covering several classes of aromatic contaminants, indicating that Suwannee River fulvic acid (SRFA) used as reference DOM does not affect this reaction mechanism.

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Effect of Dissolved Organic Matter on the Transformation of Contaminants Induced by Excited Triplet States and the Hydroxyl Radical

The thiocyanate dosimeter (10–2 mol dm–3 SCN– in O2-saturated water) has been standardised against the super-Fricke dosimeter (10–2 mol dm–3 FeII in O2-saturated 0.4 mol dm–3 H2SO4) using the hexacyanoferrate(II) dosimeter [5 × 10–3 mol dm–3 Fe(CN)64– in O2-saturated water] as a secondary standard. On the basis that G(FeIII)= 1.67 × 10–6 mol J–1 and IµFeIII= 220.4 m2 mol–1 at 304 nm and 25 °C in the super-Fricke dosimeter, we obtain GIµ[Fe(CN)63–]=(3.47 ± 0.06)× 10–5 m2 J–1 at 420 nm and GIµ(SCN)2˙–=(2.59 ± 0.05)× 10–4 m2 J–1 at 475 nm. These values remain unchanged when the solutions are saturated with air instead of O2 and are doubled in N2O-saturated solution.

Re-evaluation of the thiocyanate dosimeter for pulse radiolysis

The rates of reaction of OH with benzene, chlorobenzene, nitrobenzene, ion and benzoic acid have been measured in aqueous solution up to 200 °C using pulse radiolysis to generate OH. The temperature dependence of the observed rate constant, kobs, is essentially the same for each compound and kobs changes by less than three-fold between 20 °C and 200 °C. The kinetic data are consistent with a mechanism whereby OH reversibly forms a π-complex with the aromatic compound, irrespective of the substituent on the ring, which then transforms to a σ-bonded hydroxycyclohexadienyl radical. The values of kobs were determined from the rate of formation of this radical. There is no evidence for dissociation of the σ-bonded radical nor for H atom abstraction from the ring which have been reported for the gas phase. The apparent mechanistic differences between the two phases may be due to the different timescales over which the kinetics measurements were made.

Temperature dependence of the rate of reaction of OH with some aromatic compounds in aqueous solution. Evidence for the formation of a π-complex intermediate?

Rate constants for the reactions of eaq– and ˙OH with N2H4 and N2H5+ in liquid water up to 200 °C have been measured by pulse radiolysis. Linear Arrhenius plots for the reactions of eaq– gave k(20 °C)= 106 and 1.6 × 108 dm3 mol–1 s–1, and Ea= 13.5 and 18.2 kJ mol–1, respectively. H is the product of the reaction with N2H5+. Non-linear Arrhenius behaviour was observed for the reactions of ˙OH with k(20 °C)= 4.5 × 109 and 8.2 × 107 dm3 mol–1 s–1, respectively. The pKa of N2H5+ decreases linearly with temperature from 8.1 at 20 °C to 4.2 at 200 °C. The products of the ˙OH reactions are ˙N2H3 and ˙N2H4+, respectively, and the pKa of ˙N2H4+ also decreases with increasing temperature. The self-reaction of ˙N2H3 shows the same temperature dependence as that of ˙OH with k(20 °C)= 2 × 109 dm3 mol–1 s–1. The product of this reaction is tetrazene. Up to 200 °C the data are consistent with successive eliminations of NH3 to form triazene and then N2. The pH-dependent kinetics of these processes indicate that the decomposition of N3H3 is acid- and base-catalysed over the whole temperature range.

Radiation chemistry of aqueous solutions of hydrazine at elevated temperatures. Part 1.—Oxygen-free solutions

The rate of reaction for has been
measured by pulse radiolysis up to 110°C in aqueous solutions of
hydrazine having a pH of 10.3 at room temperature. The values of the
kinetic parameters are
k
 1
=(3.8±0.1)×10
 8

dm3 mol
 -1
 s
 -1
 at 20°C and
E
 act
=5.6±2.1 kJ mol
 -1
. This
small activation energy is interpreted in terms of the formation of an
intermediate adduct 
N
 2
H
 3
O
 2

which is in a pre-equilibrium with the reactants and whose stability
with respect to dissociation back to the reactants decreases with
increasing temperature. The rate of the thermal reaction
has also been determined up to 110°C
by measuring the decrease in k
 1
 with increasing
age of the oxygenated hydrazine solution in order to monitor the
depletion of [O
 2
]. The kinetic parameters are
k
 12
=(1.2±0.8)×10
 -3

dm3 mol
 -1
 s
 -1
 at 20°C and
E
 act
=70±5 kJ mol
 -1
.
γ-Radiolysis studies show that N
 2
H
 4
 is
destroyed in a short chain reaction in oxygenated hydrazine solutions
with G(-N
 2
H
 4
)∝(dose
rate)
 -0.5
, indicating chain termination by
radical–radical reactions. Added EDTA shortens the chain but
does not alter the form of the rate law, indicating that metal ions
present as impurities accelerate the chain-propagating reactions more
than the termination steps. In each case the chain length increases as
ca. [N
 2
H
 4
]
 0.5
.

Radiation chemistry of aqueous solutions of hydrazine at elevated temperatures . part 2. solutions containing oxygen

The yields of eaq-, eaq-+OD and eaq-+D+OD have been measured in D2O at ambient temperature as a function of scavenger concentration. At scavenging powers ⩽107 s-1G(eaq-) is 10% larger than its value in H2O, but for scavenging powers 3×108 s-1 the yields of eaq-, eaq-+OD and eaq-+D+OD are the same in both solvents, indicating that scavenging in the spurs suppresses the reactions leading to the different primary yields. The absorption spectrum of OD is found to be very similar to that of OH, comprising a weak band rising into the UV between 320 and 220 nm with e250=50±2 mol-1 at 20°C. The shape of the band is the same at 200°C. Rate constants for the reactions OD+OD and D+OD have been measured up to 200°C, on the assumption that e250 for OD is independent of temperature. As with the corresponding reactions in H2O, they show non-linear Arrhenius behaviour, being close to the diffusion-controlled value at ambient temperature, but an order of magnitude less than this value at 200°C. The values of the rate constants are very similar in both solvents.

Radiation chemistry of D2O Time dependence of the yields of the radicals at ambient temperature and rate constants for the reactions OD+OD and D+OD up to 200°C

Craig R. Stuart

Papers

Re-evaluation of the thiocyanate dosimeter for pulse radiolysis

Temperature dependence of the rate of reaction of OH with some aromatic compounds in aqueous solution. Evidence for the formation of a π-complex intermediate?

Radiation chemistry of aqueous solutions of hydrazine at elevated temperatures. Part 1.—Oxygen-free solutions

Radiation chemistry of aqueous solutions of hydrazine at elevated temperatures . part 2. solutions containing oxygen

Radiation chemistry of D2O Time dependence of the yields of the radicals at ambient temperature and rate constants for the reactions OD+OD and D+OD up to 200°C