Environ. Sci. Technol. 2010, 44, 8248–8253
Degradation of Ozone-Refractory Organic Phosphates in Wastewater by Ozone and Ozone/Hydrogen Peroxide (Peroxone): The Role of Ozone Consumption by Dissolved Organic Matter J . P A B L O P O C O S T A L E S , * ,†,‡ MYINT M. SEIN,† WOLFGANG KNOLLE,§ C L E M E N S V O N S O N N T A G , * ,†,|,⊥ A N D TORSTEN C. SCHMIDT† Instrumental Analytical Chemistry, University Duisburg-Essen, Universitätsstr. 5, 45141 Essen, Germany, Departamento de Ingenierı´a Quı´mica y Quı´mica-Fı´sica, Universidad de Extremadura, Avenida de Elvas S/N, 06071 Badajoz, Spain, Leibniz-Institut fu ¨ r Oberfla¨chenmodifizierung (IOM), Permoserstrasse 15, 04303 Leipzig, Germany, Max-Planck-Institut fu ¨ r Bioanoganische Chemie, Stiftstrasse 34-36, 45413 Mu ¨ lheim an der Ruhr, Germany, and Universita¨t Dortmund, Fachbereich Bio- und Ingenieurwesen, Lehrstuhl Umwelttechnik, Emil-Figgestrasse 70, 44227 Dortmund, Germany
Received May 29, 2010. Revised manuscript received September 6, 2010. Accepted September 14, 2010.
Ozonation is very effective in eliminating micropollutants that react fast with ozone (k > 103 M-1 s-1), but there are also ozonerefractory (k < 10 M-1 s-1) micropollutants such as X-ray contrast media, organic phosphates, and others. Yet, they are degraded upon ozonation to some extent, and this is due to • OH radicals generated in the reaction of ozone with organic matter in wastewater (DOM, determined as DOC). The elimination of tri-n-butyl phosphate (TnBP) and tris-2-chloroisopropyl phosphate (TCPP), added to wastewater in trace amounts, was studied as a function of the ozone dose and found to follow first-order kinetics. TnBP and TCPP concentrations are halved at ozone to DOC ratios of ∼0.25 and ∼1.0, respectively. The •OH rate constant of TCPP was estimated at (7 ( 2) × 108 M-1 s-1 by pulse radiolysis. Addition of 1 mg H2O2/L for increasing the •OH yield had very little effect. This is due to the low rate of reaction of H2O2 with ozone at wastewater conditions (pH 8) that competes unfavorably with the reaction of ozone with wastewater DOC. Simulations based on the reported (No¨the et al., ES&T 2009, 43, 5990-5995) •OH yield (13%) and •OH scavenger capacity of wastewater (3.2 × 104 (mgC/ L)-1 s-1) confirm the experimental data. Based on a typically applied molar ratio of ozone and H2O2 of 2, the contribution of H2O2 addition on the •OH yield is shown to become important only at high ozone doses. * Corresponding authors: e-mail:
[email protected] (J.P.P.) and
[email protected] (C.v.S.). † University Duisburg-Essen. ‡ Universidad de Extremadura. § Leibniz-Institut fu ¨ r Oberfla¨chenmodifizierung. | Max-Planck-Institut fu ¨ r Bioanoganische Chemie. ⊥ Universita¨t Dortmund. 8248
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 21, 2010
Introduction After biological treatment, wastewater still contains a large number of unwanted micropollutants, and means are developed to further reduce their levels. Adsorption on activated carbon (1) and oxidation by ozone (2, 3) are the most promising approaches. In both cases, the water matrix competes with the desired removal of micropollutants. As a consequence, only those micropollutants are eliminated effectively by ozone that have high ozone rate constants, and at moderate ozone doses (∼5 mg/L) only those micropollutants that react with ozone at k > 103 M-1 s-1 stand a chance of being eliminated by ozone in a direct reaction. There is, however, a second route in ozone reactions, the formation of •OH radicals. These radicals are much more reactive than ozone and thus also react readily with ozonerefractory compounds. In wastewater, the •OH yield is ∼13 mol % with respect to the applied ozone dose (4). Yet, there is again a competition with the water matrix, and those micropollutants that are more reactive toward •OH are more readily eliminated (5, 6). The •OH scavenging capacity of wastewater has been measured at 3.2 × 104 (mgC/L)-1 s-1 (4). This value is similar to those found for DOMs of other waters (7). The inorganic constituents of wastewater, notably bicarbonate, contribute to the •OH scavenging capacity only little in comparison (∼10%), and this contribution has been included in the above value. In the present paper, we have studied the elimination of two ozone-refractory micropollutants of the organic phosphate family, tri-n-butyl phosphate (TnBP) and tris-2chloroisopropyl phosphate (TCPP) (for formulas see Supporting Information Figure S1). TnBP is mainly used as a plasticizer and as an additive in the clothing industry. TCPP is a flame retardant. The pollution of rivers by such compounds has decreased over the years (8, 9), but they are still detected in wastewater as they are only poorly degraded biologically in municipal wastewater treatment plants (WWTPs). They are on the list of hazardous compounds whose levels should be reduced before the WWTP effluents are released into the environment (10). Their occurrence in WWTPs, surface, and drinking waters has been studied in detail (11-13). Being saturated aliphatic compounds, the ozone rate constants of TnBP and TCPP are that low (
