Phenol depletion by thermally activated peroxydisulfate at 70 °C Verónica C. Mora, Janina A. Rosso, Daniel O. Mártire, Mónica C. Gonzalez ⇑ Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, 1900 La Plata, Argentina article info Article history: Received 21 December 2010 Received in revised form 20 April 2011 Accepted 21 April 2011 Available online 23 May 2011 Keywords: Sulfate radicals Peroxydisulfate Phenol Thermal activation Phenol polymers Phenoxyl radical abstract The ability of thermal activated peroxydisulfate (PS) of mineralizing phenol at 70 °C from contaminated waters is investigated. Phenol in concentrations of 10 À4 to 5 Â 10 À4 M is quantitatively depleted by 5 Â 10 À3 to 10 À2 M activated PS in 15 min of reaction. However, mineralization of the organic carbon is not observed. Instead, an insoluble phenol polymer-type product is formed. A reaction mechanism including the formation of phenoxyl radicals and validated by computer simulations is proposed. High molecular weight phenolic products are formed by phenoxyl radical H-abstraction reactions. This is not the case for the room temperature degradation of phenol by sulfate radicals where sulfate addition to the aromatic ring mainly leads to the generation of hydroxycyclohexadienyl radicals leading to hydroxybenzenes and oxidized open chain products. Therefore, a change in the reaction mechanism is observed with increasing temperature, and thermal activation of PS at 70 °C does not lead to the miner- alization of phenol. Thus PS activation at 70 °C may be considered a potential method to reduce the load of phenol in polluted waters by polymerization. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Activated sodium or potassium peroxydisulfate (PS) has the po- tential to destroy in situ many organic contaminants commonly present in soil and groundwater, including chlorinated methanes, ethanes, ethenes and trichloroacetic acid (Killian and Bruell, 2003; Liang et al., 2003, 2004; Huang et al., 2005; Waldemer et al., 2007; Liang et al., 2008; Liang and Su, 2009; Mora et al., 2009). PS can be chemically, photochemically or thermally acti- vated to generate the stronger oxidant sulfate radicals, SO ÅÀ 4 , with a redox potential E o (SO ÅÀ 4 /SO 2À 4 ) = 2.6 V (Wardman, 1989). Genera- tion of these radicals can significantly accelerate the kinetics of substrate oxidation in a wide range of matrix conditions. These properties, combined with PS safe handling and high water solubil- ity make it an excellent additive for waste treatment. However, despite in situ chemical oxidation can rapidly degrade contaminants, a judicious use of this technology requires determi- nation of effectiveness and ruling out potential negative effects on waste water treatment in advance of field or pilot scale use (EPA, 1989). Phenol is a ‘priority’ contaminant in industrial waste from the coal–gas, coal-coking, petroleum, and pharmaceutical industries, as well as in wastes from processes involving the use of phenol as a raw material (Barron, 2002). Room temperature oxidation of phenolic substrates initiated by reaction with SO ÅÀ 4 radicals is well-documented in the literature (see, for example Gonzalez and Mártire, 1999; Anipsitakis et al., 2006; Caregnato et al., 2008; Antoniou et al., 2010; Zhao et al., 2010). Heat-activated PS has been demonstrated to deplete phenol in alkaline aqueous solutions at 70 °C(Liang and Su, 2009) and in pressurized hot water (Kronholm and Liisariekkola, 1999). How- ever, these studies do not investigate the mineralization efficiency of the organic carbon, nor the chemical nature of the reaction inter- mediates formed. The determination of the organic carbon after treatment with the oxidant is of importance to evaluate the realis- tic potentiality of a method for environmental applications. Deple- tion of a pollutant does not necessarily imply the elimination of the contamination. In the present study, a detailed investigation on the reaction mechanism by which phenol is removed, but not mineral- ized by thermally activated PS at pH 2 and 70 °C is reported. 2. Materials and methods Na 2 S 2 O 8 (Riedel-de Haen, AG), phenol, 1,4-benzoquinone, hydroquinone, methanol, chloroform, and acetone (Baker ACS), and dichloromethane (Anedra) were used without further purifica- tion. Distilled water (>18 MX cm, <20 ppb of organic carbon) was obtained from a Millipore system. The temperature was controlled to ±1 °C. UV–Vis absorption spectra were measured with a Heweltt Packard 8452A diode array spectrophotometer. 0045-6535/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2011.04.062 ⇑ Corresponding author. Address: Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Diag. 113 y 64, (1900) La Plata, Argentina. Tel.: +54 221 425 7291. E-mail address: gonzalez@inifta.unlp.edu.ar (M.C. Gonzalez). Chemosphere 84 (2011) 1270–1275 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere