Chemical Engineering Journal 133 (2007) 239–246 Surface characterization and dye adsorptive capacities of char obtained from pyrolysis/gasification of sewage sludge Charothon Jindarom a , Vissanu Meeyoo b,∗ , Boonyarach Kitiyanan a , Thirasak Rirksomboon a , Pramoch Rangsunvigit a a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand b Center for Advanced Materials and Environmental Research, Mahanakorn University of Technology, Bangkok 10530, Thailand Received 19 June 2006; received in revised form 25 October 2006; accepted 4 February 2007 Abstract Sewage sludge was used to develop a potential adsorbent for dye removal by pyrolysis under either N 2 or CO 2 atmospheres. The results showed that the surface area of the char increases as the pyrolysis temperature increase under the CO 2 atmosphere. The maximum surface area of the char is achieved with pyrolysis at 750 ◦ C under the CO 2 atmosphere, 60.7 m 2 g -1 with mainly mesopores. The FT-IR spectra of the char prepared under both N 2 and CO 2 atmospheres indicate a decrease in –OH, –NH and C O functionalities with increasing the pyrolysis temperature, corresponding to a decrease in the acidity of the char. The maximum adsorption capacities of acid and basic dyes were found to increase with an increase in the pyrolysis temperature while that of the reactive dye possessed no correlation. The adsorption mechanism is governed by the combination of the electrostatic interactions and dispersive interactions. The equilibrium data fit well with the Langmuir model of adsorption suggesting a monolayer coverage of dye molecules at the outer surface of sewage sludge derived chars. The maximum adsorption capacities of acid yellow 49, basic blue 41 and reactive red 198 dyes are reported at 116, 588 and 25 mg g -1 of char, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Azo dye adsorption; Adsorbent; CO 2 pyrolysis; Surface chemistry 1. Introduction Sewage sludge is a by-product from wastewater treatment plants, and contains significant amounts of heavy metals, organic toxins and pathogenic microorganisms, which are considered to be harmful to the environment and all living organisms [1]. Agricultural use, land filling and incineration are commonly used as disposal methods. It was, however, reported that sewage sludge applications in agriculture gives rise to an accumulation of harmful components (heavy metals and organic compounds) in soil [2,3]. The problems encountered with the land filling are the leachate and the landfill gas. The major gas released from the land filling is methane, which is a significant contributor to the climate change [4]. Since hazardous air pollutants (HAPs) are usually present during incineration, an expensive pollutant reduction unit has to be implemented in the incineration. ∗ Corresponding author. Tel.: +66 2988 3655x108; fax: +66 2988 4039. E-mail address: vissanu@mut.ac.th (V. Meeyoo). Pyrolysis can be considered as a promising technique to treat the sewage sludge including the production of fuels [5]. Gen- erally, pyrolysis products are divided into a volatile fraction consisting of gasses; vapours and tar components; and a carbon rich solid residue. The processing conditions can be optimized to maximize the production of these products [6–8]. The solid residue usually has a porous structure and surface area that is appropriate for use as an adsorbent. Typically, pyrolysis utilizes an inert gas, e.g. nitrogen or helium. In commercial-scale, a pyrolysis plant, however, has been operated under low oxygen atmosphere. Therefore, a recycle of gas product stream, CO 2 rich stream [9], seems to be the economical way. CO 2 is an unreactive molecule at ambient temperature. If the temperature is high enough, CO 2 is turned to be the reactive molecule and can be used as an activating agent in gasification. Under such a reactive atmosphere, it may significantly influence the yield and the quality of the products [10]. However, a few literatures have been found on the effect of CO 2 on pyrolysis. In general, activated carbon productions involve in high tem- peratures (900–1200 ◦ C) and also in the presence of an activating agent such as CO 2 . It was reported that CO 2 can enhance the 1385-8947/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2007.02.002