Environ. Sci. Technol. 2005, 39, 5203-5208
Multiphase Decomposition of Novel Oxygenated Organics in Aqueous and Organic Media TAMAR MOISE AND YINON RUDICH* Department of Environmental Sciences, Weizmann Institute, Rehovot 76100, Israel DAVY ROUSSE AND CHRISTIAN GEORGE Laboratoire d’Application de la Chimie a` l’Environnement (UCBL-CNRS), 43 boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France
Prior to the massive use of new oxygenated solvents, data on their multiphase reactivity must be obtained to assess their environmental fate and impact on water and air quality. For this, the kinetics and mechanisms of the photochemical and photocatalytic degradation of selected oxygenated solvents by common tropospheric oxidants (such as OH and ozone) must be characterized. We studied the oxidation kinetics of new oxygenated solvents as pure organic liquids and in an aqueous medium by ozone and by the OH radical, respectively. The studied chemicals are all unsaturated compounds, having none, one, or two ether groups. The results indicate that the OH reaction proceeds at the diffusion limit by addition to the double bond. The reactive uptake coefficients associated with the reaction initiated by ozone are of the order of 10-3. The reactions of compounds with two double bonds are very fast and probably occur at the surface. This kinetic information demonstrates that organic solvents in an organic medium or in an aqueous droplet will be oxidized rapidly by these oxidation reactions. These reactions, however, are not significant sinks for ozone and OH radicals.
Introduction It is now well accepted that the switch from traditional solvents (such as alkanes and aromatics) to oxygenated compounds might be beneficial both in terms of toxicity and in order to reduce the levels of oxidants formation in the troposphere. The solvent industry has targeted a limited range of relatively long chained ethers, ketones, esters, and glycols as replacements for the traditional solvents. These oxygenated organic compounds are to some extent soluble in water and are generally less volatile than the solvents they are envisaged to replace. However, to maintain their properties as organic solvents, most of them are also lipophilic. Because of their solubility in water they will be used as replacement both for organic solvents and in water-based technologies. They are expected to be released to the atmosphere either directly or through evaporation. Once in the atmosphere, these low vapor-pressure molecules can partition effectively into atmospheric aqueous and organic particles. Such particles are by nature of small size with a high organic content. They may be inhaled by humans leading to adverse effects on health. While the gas-phase reactions of certain oxygenated VOC are relatively well-known, their degradation in aqueous * Corresponding author e-mail:
[email protected]. 10.1021/es048488h CCC: $30.25 Published on Web 06/17/2005
2005 American Chemical Society
droplets and in organic particles is unexplored. It is known that aerosol particles with high organic content interact with radicals and oxidants such as ozone or OH, which transform them (1-12). These reactive interactions will determine the particles properties and hence total mass, deposition efficiency, and their ability to nucleate clouds. It is therefore crucial to understand the oxidation processes of these new oxygenated solvents in the aqueous phase and in an organic medium. The focus of this study is on a representative group of ethers which have been recently introduced by the industry and are expected to be used intensively in the near future as solvents in industrial applications. Some of them are currently in an industrial development stage. For such species, studies on the degradation pathways in the gas phase are being conducted. However, their degradation in or on particles is sparse and warrants further investigation. Therefore this paper reports on the multiphase oxidation of a series of saturated and unsaturated oxygenated compounds by the OH radical and by ozone. Such kinetic information may then be used to assess the environmental impact of these compounds.
Experimental Section OH Kinetics in Aqueous Medium. The Teflon waveguide photolysis system used is shown schematically in Figure 1 and is detailed in a previous study. (13) A “windowless” reaction cell can be created by using a liquid core waveguide (henceforth called LCW) made of Teflon AF 2400. The highly flexible Teflon AF 2400 tubing was loosely coiled (∼4 cm diameter) and placed in a thermostated reactor placed close to a photolytic source, an HPK lamp (Cathodeon, 150W) located within a Pyrex tube. The OH radicals were generated by photolysis of H2O2, which was dissolved in the water within the LCW. The generated OH radicals then reacted with the organic compound under study, which is also dissolved in the water. The concentration of OH radicals in the aqueous phase was not directly measured but derived from the “titration” reaction resulting from a competition reaction of the OH radical with the SCN- anion which produces (SCN)2radical anions. We adopted the rate coefficient reported by Chin and Wine (kref ) 1.29 × 1010 M-1 s-1 at 298 K) as the reference value for this competitive reaction. Note that a recent study (14) has questioned this reference value, while a second one has validated it and even extended its application to various ionic strengths (15). In this context, we decided to derive our rate constant by still using the rate coefficient reported by Chin and Wine as it has been widely used (16). To probe the (SCN)2- concentration in the LCW tube, the output of a 75 W xenon lamp was focused onto the entrance of a 200 µm diameter fused silica optical fiber held inside the LCW tube. The light escapes the solid optical fiber and is conducted to the other end of the liquid waveguide. There, another fused silica optical fiber (placed in the liquid) collects most of the transmitted light and conducts it to a spectrograph (Lot-Oriel, 127i) coupled to a CCD camera (Andor Technology). The CCD camera monitors the evolution of the UVvisible spectrum as a function of wavelength and time within the LCW. The (SCN)2- anion spectra were recorded in the wavelength range from 350 to 750 nm, but the kinetics were derived from absorbance changes at 465 nm. Aqueous solutions of the organic reactant and the radical precursors were continuously pumped into the LCW, which was used as a microflowtube. In all cases, the liquid passed through the LCW in less than 0.5 s. Under these experimental VOL. 39, NO. 14, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematics of the liquid core waveguide experimental setup. conditions, steady-state was obtained within the LCW and verified experimentally (see previous paper (13)) as well as theoretically using numerical simulations of the chemistry within the LCW. O3 Reactions in Organic Medium. The reactive uptake coefficients of ozone by the oxygenated organic compounds in their liquid and frozen states was studied using a cylindrical rotating flow reactor coupled to a mass spectrometer (8, 17). The experimental system has been described in detail previously (8, 11), and only specifics relevant to the current measurements are described here. Ozone was injected into the reactor through a moveable injector and interacted with the organic compounds coating the reactor’s inner walls. The reactor was etched with HF prior to each coating to ensure a smooth and continuous organic surface coating. The position of the injector dictates the reaction time of ozone with the organic coating, and the ozone signal, detected as O3- following its chemical ionization by SF6-, is monitored as the injectors’ position is changed. The reactive uptake probability was determined from the first-order loss rate of gas-phase ozone. The coating was formed with 1-2 cm3 of the pure organic liquid (∼10-2 mol). Ozone concentrations of approximately 1 × 1011 molecules cm-3 were used for the uptake measurements. Typical pressures and velocities in the reactor were 2-8 Torr and 50-800 cm s-1, respectively. Helium was used as the carrier gas. Laminar flow was established in the flow tube
