Yadollah Abdollahi 1,2 Azmi Zakaria 1 Nor Asrina Sairi 2 1 Advanced Materials and Nanotechnology Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, Serdang, Selangor, Malaysia 2 Chemistry Department, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia Research Article Degradation of High Level m-Cresol by Zinc Oxide as Photocatalyst In this study, the high concentration of m-cresol as a sample of organic pollutants was degraded in the presence of zinc oxide and UV irradiation during 6 h at laboratory scales. The amount of photocatalyst, pH and m-cresol concentration were considered as effective factors on the photodegradation. The demineralization of m-cresol was measured by UV–Vis spectrophotometry while the total organic carbon-analyzer was used to determine the mineralization. The ultrahigh performance LC was used to identify probable intermediates. The results showed optimum condition at pH 7–9, which is the natural pH of industrial wastewater. Moreover, 100% of m-cresol was removed after 5 h of irradiation time, which is quite significant. The detected intermediates were 3,5-hydroxytoluene, 2,5-hydroxy-benzaldehyde, and 3-hydroxy- benzaldehyde after 3 h of reaction time. Reusability of the photocatalyst showed insignificant reduction in the photo-catalytic performance. In conclusion, this investigation indicated high potential of zinc oxide suspension to remove high level concentration of m-cresol under UV irradiation. Keywords: Advanced oxidation process; Demineralization; Photochemistry; Photodegradation; Wastewater treatment Received: June 8, 2013; revised: August 9, 2013; accepted: August 10, 2013 DOI: 10.1002/clen.201300451 1 Introduction According to the United Nations World Water Development Report 2012, up to 90% of untreated wastewater is flowing into rivers, lakes and highly productive coastal zones. The hazardous wastewater contains high concentration of phenolic compounds, which must be prevented of entering into the environment [1]. The global attention has been focused on the removal of the compounds from the wastewater using several methods, including biological oxidation systems, electrochemical and adsorption methods [2–6]. The methods are limited by a few drawbacks; the drawback of the biological method is the longer retention time, usually measured in days, to oxidize the organic compounds, the adsorption method cannot mineralize the pollutants and the electrochemical method generates the new toxic intermediates [7–10]. On the other hand, advanced oxidation processes (AOPs) mineralize the organics to harmless final products using stable and non-toxic photocatalysts, suitable reaction time, appropriate irradiation wavelength at ordinary temperature and atmospheric pressure [11–13]. The mineralization is carried out on the photocatalyst surface by hydroxyl radicals ( • OH) that are powerful and non-selective to oxide pollutants and probable intermediates [14, 15]. The process of the radicals’ generation starts when the valence band electrons are excited to the empty conduction band of the photocatalyst by an appropriate irradiation as the source of the energy. The excited electrons are trapped by adsorbed oxygen molecules over the suspension photocatalyst to produce O 2 • species. The species are converted to hydroxyl radicals by further oxida- tion [16, 17]. The AOPs usually use TiO 2 and ZnO as photocatalysts to degrade the organics to CO 2 and H 2 O that are environmental friendly products [15, 18–20]. However, the inexpensive ZnO photodegrades a broad range of organic compounds in acidic and basic medium as an excellent alternative for TiO 2 [21–24]. To the best of our knowledge, AOPs have never degraded m-cresol, one of the phenolic compounds, by ZnO under UV irradiation. In developing countries, m-cresol is extensively used in manufacturing products including cresol-based herbicides, pharmaceuticals, petrochemicals, and surfactants [25– 27]. The wastewater of the industries contains a huge amount of m-cresol which easily penetrates into groundwater because its water solubility is very high, 24.4 g L À1 at 25°C [28]. On the other hand, m-cresol was classified as persistent, priority, toxic chemical in 1970s because its quantitative structure–activity relationship shows chronic effects at 12 g L À1 [29]. Therefore, m-cresol with LC50 could be a significant threat to groundwater and must be controlled as near as possible where they are generated. Obviously, at those places, the concentration of the pollutant is very high therefore an effective method is necessary to remove the m-cresol. In our previous work, m-cresol was removed by ZnO suspension under visible-light irradiation [27]. Despite the fact that utilizing visible-light as a solar energy is quite convenient, there are two drawbacks which must be considered for high concentration contaminant solutions. The first one to mention here is that the removal of m-cresol amount was merely 25 mg L À1 in 360 min irradiation time, which is considered to Correspondence: Dr. Y. Abdollahi, Advanced Materials and Nanotechnology Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia E-mail: yadollahabdolla@upm.edu.my Abbreviations: AOP, advanced oxidation process; pH zpc , pH of zero point charge; TIC, total inorganic carbon; TOC, total organic carbon; UHPLC, ultrahigh performance LC 1 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com Clean – Soil, Air, Water 2014, 41 (9999), 1–6