Carbon materials and catalytic wet air oxidation of organic pollutants in wastewater F. Stu¨ber a,* , J. Font a , A. Fortuny b , C. Bengoa a , A. Eftaxias a , and A. Fabregat a a Departament d’Enginyeria Quı´mica, ETSEQ, Universitat Rovira i Virgili, Paisos Catalans 26, 43007 Tarragona, Catalunya, Spain b Departament d’Enginyeria Quı´mica, EUPVG, Universitat Polite`cnica de Catalunya, Av. Victor Balaguer, s/n, 08800 Vilanova i la Geltru ´, Catalunya, Spain The use of carbon materials as catalytic support or direct catalyst in catalytic wet air oxidation (WAO) of organic pollutants is reviewed. The discussion covers important engineering aspects including the characterisation, activity and stability of carbon catalysts, process performance, reaction kinetics and reactor modelling. Recommendations for further research in catalytic WAO are outlined. KEY WORDS: activated carbon; characterisation; CWAO; kinetics; modelling; organic pollutants; review; trickle bed reactor 1. Introduction Currently half of the available freshwater is appro- priated for human uses [1] indicating a high level of exploitation and contamination of the existing water resources. Large quantities of aqueous effluents arise from diverse industrial (petrochemical, chemical, phar- maceutical and agro-alimentary) or domestic activities and have to be treated before returning to the aquatic environment. The presence of highly bio-toxic and refractory organic pollutants in these effluents is chal- lenging the traditional treatments including physical separation, incineration or biological abatement. At the same time, new regulations for the disposal and treat- ment of multicomponent toxic and potentially hazard- ous wastes are imposing lower discharge limits that are difficult to meet with the current technologies. Significantly less contamination of water resources can be achieved in situ by the prevention of end-stream pollution in the specific production processes. Concepts of green chemistry and process sustainability have become issues of global importance for industrial companies. Nevertheless, the implantation of new pro- duction strategies is slowly developing in the competi- tively orientated industrial environment. Meanwhile, more efficient and economic solutions for end-stream treatment, avoiding high-energy input technologies [2], are imperative to produce reusable process water and environmentally friendly effluents. A first step towards improved remediation of waste- water was the development and successful application of the Wet Air Oxidation technique (WAO) to treat sewage and other industrial effluents [3,4]. Investment and operation costs of WAO plants, however, are not favourable as WAO employs excessive temperatures between 150 and 300 °C and air pressures up to 200 bar. The effective treatment of effluents containing new highly refractory organic pollutants would make this method even more expensive. WAO performance can be improved by the addition of homogeneous [5,6] or heterogeneous [4] catalysts at subcritical conditions or at severe ‘supercritical’ operat- ing conditions [7,8]. Alternatively, low temperature oxidation processes replace oxygen by stronger oxidants such as O 3 [9,10] or H 2 O 2 [9,11,12] in combination with an energy input to form reactive OH-radicals [13,14]. Several reviews on catalytic and non-catalytic WAO [4,5, 15–19], supercritical water oxidation (SCWO) [7,8, 20–22] and advanced oxidation processes (AOP) [23–25] are now available in literature. Moreover, the intensification or integration of waste- water treatments provides superior cleaning effective- ness compared to single step oxidation processes. Examples of such new concepts are the simultaneous adsorption and oxidation [26–29] or sequential adsorp- tion–oxidation [30] of pollutants as well as adsorption- regeneration cycles in the same reactor unit [31–34]. Better performance of a continuous trickle bed reactor (TBR) for the phenol CWAO was also achieved by applying forced periodic operation of liquid flow [35]. The integration of a chemical oxidation/reduction pre- treatment step takes advantage of the partial contam- inant mineralisation reducing the effluent toxicity to levels acceptable for biological end-treatment plants [36–38]. Finally, abatement techniques that appear more appropriate than CWAO should be considered for specific pollutant groups including catalytic hydrotreat- ment for polyphenols [39], chlorophenols [40–44] and nitrites or nitrates [42,45–48]. Over the last 10–15 years, continuous SCWO tech- nology proved to be an extremely powerful treatment for all kinds and concentration range of toxic organic wastewater [7,8, 20–22]. Related problems with *To whom correspondence should be addressed. E-mail: fstuber@etseq.urv.es Topics in Catalysis Vol. 33, Nos. 1–4, April 2005 (Ó 2005) 3 DOI: 10.1007/s11244-005-2497-1 1022-5528/05/0400–0003/0 Ó 2005 Springer ScienceþBusiness Media Inc.