Adsorption of phenol and methylene blue from aqueous solutions by pyrolytic tire char: Equilibrium and kinetic studies Vasiliki Makrigianni, Aris Giannakas, Yiannis Deligiannakis, Ioannis Konstantinou * Department of Environmental and Natural Resources Management, University of Patras, Agrinio 30100, Greece A R T I C L E I N F O Article history: Received 20 October 2014 Accepted 18 January 2015 Keywords: Pyrolytic tire char Adsorption Methylene blue dye Phenol Thermodynamics A B S T R A C T Acid-treated pyrolytic tire char (PTC) was studied as an adsorbent for the removal of phenol and methylene blue (MB) dye from aqueous solutions. PTC was derived via pyrolysis of used rubber tires at 450  C in oxygen-free atmosphere. Textural characteristics and surface chemical nature of the adsorbent were examined through a battery of techniques such as X-ray diffraction, porosimetry, Boehm titration, and point of zero charge determination. The adsorption characteristics of phenol and MB dye were evaluated on the basis of equilibrium and kinetic studies. The adsorption kinetic data fitted better to pseudo-second kinetic model. Equilibrium isotherms were analyzed by Langmuir, Freundlich, Dubinin– Radushkevich, Temkin and Frumkin models. Langmuir and Freundlich isotherm models were found to fit the adsorption data of phenol and MB, respectively. The maximum adsorption capacities of phenol and MB on PTC were found to be 51.92 and 65.81 mg g 1 , respectively. In the case of phenol, the adsorption mechanisms can be attributed to p–p electron donor/acceptor interaction, hydrogen bonding of phenol with surface groups of PTC. On the other hand, electrostatic attraction between the carboxylic groups of PTC surface and cationic dye is responsible for the adsorption of MB. The effect of temperature on the organics’ adsorption was studied and the thermodynamic parameters (DH, DS and DG) were determined for each of the studied systems. The adsorption for both pollutants was found to be thermodynamically spontaneous (DG < 0), although it was exothermic (DH < 0) for phenol vs endothermic (DH > 0) for MB. ã 2015 Elsevier Ltd. All rights reserved. Introduction The disposal of waste tires has become a serious source of environmental pollution [1]. More of 330 million of waste tires are discarded each year [2]. Tire rubbers consist mainly of synthetic and natural rubber, tire rubber additives i.e., carbon black, sulfur and zinc oxide [1,3]. A feasible solution for an environmentally friendly treatment of waste tires would be to recycle them to valuable products that can be used in various applications. Pyrolysis is an established process, which involves thermal decomposition of waste tires at high temperatures (450–900  C) under oxygen-free atmosphere, transforming them into useful products [5,13]. Tire rubber pyrolysis results in the production of oil- and gas-fractions, plus the carbonized solid residue, the pyrolytic tire char (PTC) [3]. Pyrolytic oil and gas can be used as a source of chemical feedstock or a fuel with high calorific value for in-process, on-site or off-side applications [4]. PTC may be used as a carbon black filler for the tire and printing-ink industries or as a precursor to manufacture low-cost adsorbent materials because of its high carbon content [5]. Adsorption has long been used as a wastewater purification technique for the removal of organic molecules at industrial scale [6]. Several characteristics such as surface area, pore volume and surface chemistry, govern the adsorption capacity of an adsorbent [6,7]. Low-cost adsorbents are needed for large scale water decontamination process. Some studies have reported the production of chars and activated carbons by pyrolysis of tire rubber for wastewater treatment [1,8–11]. Activated carbons, bearing a mesoporous– microporous structure, are widely used as adsorbents in liquid phase separation process for the removal of organic pollutants in aqueous solutions [6,12]. From the economic point of view, however, expensive running costs for the production of activated carbons, consist a serious drawback for their use as commercial adsorbents [13] thus, low cost adsorbents from industrial and agricultural wastes and by-products emerge as attractive alternatives. Previously, PTC has been used as a low-cost adsorbent of organic compounds from wastewater. Its well-developed * Corresponding author. Tel.: +30 2641074186; fax: +30 2641074176. E-mail addresses: iokonst@cc.uoi.gr, iokonst@upatras.gr (I. Konstantinou). http://dx.doi.org/10.1016/j.jece.2015.01.006 2213-3437/ ã 2015 Elsevier Ltd. All rights reserved. Journal of Environmental Chemical Engineering 3 (2015) 574–582 Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/je ce