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

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 photolysis of liquid H2O by vacuum-ultraviolet (VUV) photolysis, and in particular by the radiation emitted by Xe-excimer sources (172 nm) is reviewed, and the impact of the primary radicals (hydroxyl (HO ), hydrogen (H ) and hydroperoxyl (HO2 )) on dissolved organic and inorganic substrates described. Mechanisms of oxidative and reductive reaction manifolds are reported, interpreted and developed, in particular for nitrogen containing substrates. At the present state of research, at least semi-quantitative predictions on primary reaction paths and rate effects can be made, and oversimplified qualitative interpretations belong definitely to the past. Nevertheless, the review demonstrates clearly the lack of data concerning the reactivity of radical species, generated by inorganic anion oxidation, and their interaction with organic compounds and with the radicals derived from them.

Vacuum-ultraviolet photolysis of aqueous reaction systems

Photo-degradation of chlorophenols in the aqueous solution

The hydroxyl radical (HO•) is a strong oxidant that reacts with electron-rich sites of organic compounds and initiates complex chain mechanisms. In order to help understand the reaction mechanisms, a rule-based model was previously developed to predict the reaction pathways. For a kinetic model, there is a need to develop a rate constant estimator that predicts the rate constants for a variety of organic compounds. In this study, a group contribution method (GCM) is developed to predict the aqueous phase HO• rate constants for the following reaction mechanisms: (1) H-atom abstraction, (2) HO• addition to alkenes, (3) HO• addition to aromatic compounds, and (4) HO• interaction with sulfur (S)-, nitrogen (N)-, or phosphorus (P)-atom-containing compounds. The GCM hypothesizes that an observed experimental rate constant for a given organic compound is the combined rate of all elementary reactions involving HO•, which can be estimated using the Arrhenius activation energy, Ea, and temperature. Each Ea for thos...

Development of a Group Contribution Method To Predict Aqueous Phase Hydroxyl Radical (HO•) Reaction Rate Constants

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?

Lorraine Ashton

Papers

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?