Heterogeneous Catalytic Oxidation of Aqueous Phenol on Red Mud- Supported Cobalt Catalysts Syaifullah Muhammad, †,‡ Edy Saputra, †,§ Hongqi Sun, † Ha-Ming Ang, † Moses O. Tade ́ , † and Shaobin Wang* ,† † Department of Chemical Engineering and CRC for Contamination Assessment and Remediation of the Environment (CRC-CARE), Curtin University, GPO Box U1987, Perth, WA 6845, Australia ‡ Department of Chemical Engineering, Syiah Kuala University, Banda Aceh, Indonesia § Department of Chemical Engineering, Riau University, Pekanbaru 28293, Indonesia ABSTRACT: Red mud (RM), an industrial waste from the alumina refinery industry, was used as a support for preparation of Co-oxide-based catalysts (Co/RM). The samples were characterized by N 2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and UV-vis diffusive reflectance spectroscopy. The heterogeneous catalytic activity was evaluated in aqueous phenol degradation using peroxymonosulphate (PMS) as an oxidizing agent. It was found that Co 3 O 4 particles were highly dispersed on the RM surface and that pretreatment of the red mud (RM-T) would significantly influence catalytic activity. Co/RM-T catalysts exhibited high effectiveness in heterogeneous activation of PMS to produce sulfate radicals for phenol degradation compared with Co/RM-NT. Phenol degradation followed first-order kinetics, and activation energies on Co/RM-T and Co/RM-NT are 46.2 and 47.0 kJ/mol, respectively. 1. INTRODUCTION Industrial activities produce large amounts of organics- containing wastewater, which is hazardous to the environment and has to be processed before discharge into natural water bodies. There are many types of organics in wastewater; however, one important class of water pollutants is phenol and its derivatives because of their strong toxicity to many living organisms even at low concentrations. 1 These pollutants have been found on the EPAs priority list since 1976. 2 Phenol and its derivatives can be found as byproducts from many industries such as petroleum refining, petrochemical, pharmaceutical, plastic, and pesticide chemical industries. 3,4 Among the possible technologies for wastewater treatment, advanced oxidation processes (AOPs) using chemicals as oxidants are the most suitable processes to degrade toxic organics in aqueous solution owing to low operation cost, no need for special equipment, less energy consumption, and high conversion of the organic pollutants. 5-7 Homogenous catalytic oxidation using Fenton’s reagent (H 2 O 2 and ferrous ion) has been identified as an effective process to degrade toxic organics in aqueous solutions. 8-11 However, it requires a longer process for separation of the catalysts. 12 Moreover, most of the dissolved metal catalysts are harmful to the environment. This disadvantage can be overcome by using heterogeneous catalysts which will be easily recoverable and reusable. 13,14 Furthermore, heterogeneous catalysis can completely convert organics to CO 2 and H 2 O or partially oxidize the organic compounds to less toxic ones. 6,7 Heterogeneous catalytic oxidation of organic compounds, such as dyes, phenol, and its derivatives, have been widely used as a technology for reducing these substances in wastewater from industries. 3,5,11,15,16 Currently, most AOPs are based on the generation of very reactive species, such as hydroxyl radicals (OH • ) that oxidize a broad range of pollutants rapidly and nonselectively. 11,17 Apart from OH • , sulfate radicals have also been recently suggested as an alternative due to their higher oxidation potential. 18 For sulfate radical production, the reaction between Co ions and peroxymonosulphate (PMS, HSO 5 - ) has been found to be an effective route. The radical generation and organic degradation processes can be described as shown below. 18 + → + + + − + −• − Co HSO Co SO OH 2 5 3 4 (1) + → + + + − + −• + Co HSO Co SO H 3 5 2 5 (2) + → + + −• − • + SO HO SO OH H 4 2 4 2 (3) + → → + + −• − SO organics several steps CO HO SO 4 2 2 4 2 (4) In the past few years, several heterogeneous Co catalysts have been reported for PMS activation. Most of them are supported Co 3 O 4 systems using oxides, 19-27 carbons, 5,28-30 and zeolites. 31 Red mud (RM) is a solid byproduct from the alumina refinery industry. For production of every ton of alumina, the Bayer process plant will generate between 1 to 2 tonnes of RM residues. 32-34 Every year, 90 million tonnes of RM are generated globally and 33% of them are created in Australia. 34 RM is mainly composed of fine particles containing alumina, hematite, goethite, boehmite, quartz, and gypsum. 34,35 Furthermore, RM cannot be easily disposed due to its high alkalinity with pH 10-13; this waste residue can cause serious impacts to the environment. 35,36 Thus, RM needs proper Received: June 20, 2012 Revised: October 3, 2012 Accepted: November 12, 2012 Published: November 12, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 15351 dx.doi.org/10.1021/ie301639t | Ind. Eng. Chem. Res. 2012, 51, 15351-15359