Catalytic ozonation of sulfonated aromatic compounds in the presence of activated carbon P.C.C. Faria, J.J.M. O ´ rfa ˜o, M.F.R. Pereira * Laborato ´rio de Cata ´lise e Materiais, Departamento de Engenharia Quı ´mica, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal Received 30 November 2007; received in revised form 1 February 2008; accepted 4 February 2008 Available online 20 February 2008 Abstract The ozonation of two model compounds (benzenesulfonic acid and sulfanilic acid) was carried out in the presence of activated carbon. With the aim of evaluating the role of the activated carbon surface chemistry during the ozonation, two activated carbon samples were assessed. Activated carbon promoted ozonation increased the rate of removal of the selected aromatic compounds and, most of all, enhanced the removal of organic matter. Selected experiments were carried out in the presence of a radical scavenger, which evidenced the participation of HO  radicals in the oxidation mechanism in the degradation of benzenesulfonic acid and in the mineralization of oxidation by-products. No significant effect was observed in the degradation of sulfanilic acid. The removal of organic carbon content from solution is due to a complex mechanism involving direct ozone reactions, adsorption and free radicals mechanisms occurring both in the liquid phase and on the activated carbon surface. Regardless of its nature, activated carbon acts both as an adsorbent of the solutes and as a catalyst in the ozonation reaction, though best results are achieved with the most basic activated carbon sample. # 2008 Elsevier B.V. All rights reserved. Keywords: Ozonation; Activated carbon; Surface chemistry; Sulfanilic acid; Benzenesulfonic acid 1. Introduction Sulfonated aromatic compounds as well as aromatic amines are released into the environment in large amounts, mainly through the discharge of industrial effluents. Aromatic sulfonates are used in the production of azo dyes, drugs, detergents, etc. [1]. Production and biodegradation of dyestuffs, represent one of the main sources of these recalcitrant pollutants, especially azo dyes, which is one of the largest class of dyes applied in textile processing [2]. Generally, textile effluents are treated through conventional biological processes. The biodegradation of azo dyes may proceed in two stages. The first one involves the reductive cleavage of –N N-moieties in anaerobic conditions, resulting in the formation of hazardous aromatic amines, such as aminobenzenesulfonic acids. The second path involves the aerobic degradation of the latter compounds. Sulfonated aromatic amines constitute a group of aromatic compounds particularly difficult to degrade. Many of those formed during the reduction of sulfonated azo dyes are not susceptible to anaerobic or aerobic biodegradation, and conventional biological treatments fail to mineralize most of these compounds [1,2]. The organosulfonate group plays an important role in altering the solubility and dispersion properties of the molecule and increases its refractory character to biodegradation due to the thermodynamic stability of the carbon-sulfur bond [3]. Consequently, effluents from dye- houses and textile industries are often contaminated with sulfonated aromatic compounds. Chemical oxidation processes may be used as an alternative and/or as a complement to biological treatments, in order to accomplish a faster and higher mineralization of these compounds. Ozonation and catalytic ozonation processes have been widely studied in the scope of wastewater treatments. Heterogeneous catalytic ozonation aims to enhance removal of more refractory compounds through the transformation of ozone into more reactive species and/or through adsorption and reaction of the pollutants on the surface of the catalyst. Different supported and unsupported catalysts have been tested in the ozonation of several organic compounds [4]. Among the www.elsevier.com/locate/apcatb Available online at www.sciencedirect.com Applied Catalysis B: Environmental 83 (2008) 150–159 * Corresponding author. E-mail addresses: pfaria@fe.up.pt (P.C.C. Faria), jjmo@fe.up.pt (J.J.M. O ´ rfa ˜o), fpereira@fe.up.pt (M.F.R. Pereira). 0926-3373/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2008.02.010