Rates of reactions of OH and HO/sub 2/ with organic and inorganic molecules, ions and transients in aqueous solution have been tabulated, as well as the rates for the corresponding radical ions in aqueous solution (O/sup -/ and O/sub 2//sup -/). Most of the rates have been obtained by radiation chemistry methods, both pulsed and steady-state; data from photochemistry and thermal methods are also included. Rates for over one thousand reactions are listed.

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Selected specific rates of reactions of transients from water in aqueous solution. III. Hydroxyl radical and perhydroxyl radical and their radical ions

The oxidation of hazardous organic compounds leads to the formation of several by-products, being oxalic acid and oxamic acid final oxidation products refractory to ozonation. The present work aimed to study the ozonation of those carboxylic acids in the presence of activated carbon at different solution pH. For comparative purposes, experiments of adsorption on activated carbon, ozonation, and ozonation in the presence of activated carbon were carried out. In order to clarify the reaction mechanism, some experiments were done in the presence of a radical scavenger. With the aim of evaluating the role of the activated carbon surface chemistry during the ozonation, two activated carbon samples were assessed. A significant synergistic effect between ozone and activated carbon was evidenced in the oxidation of oxalic acid. Oxamic acid was found to be refractory to oxidation at pH 7. On the other hand, at pH 3, the mineralization of oxamic acid was significantly enhanced by the presence of activated carbon. Generally, the presence of activated carbon during ozonation increased the rate of degradation of both carboxylic acids leading to mineralization. Best results were achieved with the most basic activated carbon. In both cases, the efficiency of activated carbon promoted ozonation decreases with the increase of solution pH.

Activated carbon catalytic ozonation of oxamic and oxalic acids

The OH radical is the most important oxidant in both the tropospheric gas and aqueous phase. Its main sink processes in clouds appear to be reactions with organics but due to the lack of appropriate kinetic data current cloud chemistry models consider only reactions with C1 and C2 compounds. Therefore, in this study temperature dependent rate constants for the reactions of the OH radical with organic compounds (≥C2) were determined. These investigations were performed by competition kinetics (reference substance: SCN−). Initially the experimental system was checked reinvestigating kinetic data for OH reactions with formate (R-1) and tert-butanol (R-2) available from literature. For the reactions (R-1) and (R-2) the following results were obtained: k1(298 K)=(2.4±0.4)×109 M−1 s−1; A1=(7.9±0.7)×1010 M−1 s−1; EA,1=(9±5) kJ mol−1 and k2(298 K)=(5.0±0.6)×108 M−1 s−1; A2=(3.3±0.1)×1010 M−1 s−1; EA,2=(10±3) kJ mol−1 for formate and tert-butanol, respectively. Temperature dependent rate constants for the reactions of OH with ethanol (k3(298 K)=(2.1±0.1)×109 M−1 s−1; A3=(1.0±0.1)×1011 M−1 s−1; EA,3=(10±5) kJ mol−1), 1-propanol (k4(298 K)=(3.2±0.2)×109 M−1 s−1; A4=(5.6±0.6)×1010 M−1 s−1; EA,4=(8±6) kJ mol−1), acetone (k5(298 K)=(2.1±0.6)×108 M−1 s−1; A5=(3.4±0.4)×1011 M−1 s−1; EA,5=(18±11) kJ mol−1) and methylglyoxal (k6(298 K)=(1.1±0.1)×109 M−1 s−1; A6=(2.9±0.3)×1011 M−1 s−1; EA,6=(13±6) kJ mol−1), Propionic acid: k7(298 K)=(3.2±0.5)×108 M−1 s−1; A7=(7.6±0.9)×1011 M−1 s−1; EA,7=(19±8) kJ mol−1; propionate: k8(298 K)=(7.2±0.4)×108 M−1 s−1; A8=(3.2±0.2)×1011 M−1 s−1; EA,8=(15±4) kJ mol−1; glyoxylic acid: k9(298 K)=(3.6±0.2)×108 M−1 s−1, A9=(8.1±0.4)×109 M−1 s−1, EA,9=(8±3) kJ mol−1, glyoxylate: k10(298 K)=(2.6±0.9)×109 M−1 s−1, A10=(6.0±0.4)×1015 M−1 s−1; EA,10=(36±8) kJ mol−1; pyruvic acid k11(298 K)=(1.2±0.4)×108 M−1 s−1; A11=(1.0±0.1)×1012 M−1 s−1; EA,11=(23±4) kJ mol−1; pyruvate: k12(298 K)=(7±2)×108 M−1 s−1; A12=(1.3±0.1)×1012 M−1 s−1; EA,12=(19±4) kJ mol−1; oxalate (monoanion): k13(298 K)=(1.9±0.6)×108 M−1 s−1; A13=(2.5±0.1)×1012 M−1 s−1; EA,13=(23±4) kJ mol−1; oxalate (dianion): k14(298 K)=(1.6±0.6)×108 M−1 s−1, A14=(4.6±0.5)×1014 M−1 s−1; EA,14=(36±10) kJ mol−1; malonate (monoanion): k15(298 K)=(6±1)×107 M−1 s−1, A15=(3.2±0.4)×109 M−1 s−1; EA,15=(11±5) kJ mol−1; succinic acid k18(298 K)=(1.1±0.1)×108 M−1 s−1, A18=(8±1)×109 M−1 s−1; EA,18=(11±6) kJ mol−1 and succinate (dianion): k19(298 K)=(5.0±0.5)×108 M−1 s−1, A19=(5.0±0.4)×1010 M−1 s−1; EA,19=(11±5) kJ mol−1 were determined.

Temperature-dependent rate constants for hydroxyl radical reactions with organic compounds in aqueous solutions

The effect of chelating agents on the Fenton heterogeneous oxidation rate of pentachlorophenol (PCP) in the presence of magnetite (Fe3O4) was investigated in opened batch reactor at neutral pH. Six kinds of chelating agents (CA) were selected including EDTA, CMCD, oxalate, tartrate, citrate and succinate. The PCP oxidation rate in the Fenton-like system was significantly improved by using chelating agents at neutral pH. This observation was supported by the increased concentration of chloride produced by the degradation of PCP. The enhancement factor of heterogeneous oxidation rate was found to be not correlated with that of dissolved iron amount. However, the propagation of homogeneous reaction by the dissolved iron contributed to the improvement of the whole oxidation rate. In homogeneous Fenton system (dissolved Fe2+ or Fe3+), EDTA-driven Fenton reaction showed the highest oxidation rate, while oxalate seems to be more efficient in heterogeneous Fenton system (Fe3O4). In fact, EDTA can bind more strongly than oxalate to magnetite surface and compete more actively than H2O2 or PCP for the sorption on the surface active sites. Decrease in H2O2 decomposition rate value was observed when EDTA is previously adsorbed on the catalyst surface. The surface mechanism of oxidant with iron sites on the catalyst surface appears to be the rate-determining step in heterogeneous Fenton system.

Effect of chelating agent on the oxidation rate of PCP in the magnetite/H2O2 system at neutral pH

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...

F. Schwoerer

Papers

Pulse radiolysis of oxalic acid and oxalates