Journal of Hazardous Materials 164 (2009) 448–458 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Vat dye sorption onto crude dehydrated sewage sludge H. Dhaouadi ∗ , F. M’Henni Faculté des Sciences, Département de Chimie, Laboratoire de Chimie Appliquée et Environnement, Bvd de l’Environnement 5000 Monastir, Tunisia article info Article history: Received 13 March 2008 Received in revised form 23 July 2008 Accepted 8 August 2008 Available online 19 August 2008 Keywords: Vat dye retention Sewage sludge Adsorbent characterization Kinetic and equilibrium modelization Isotherm sensitivity analysis abstract In this work, sewage sludge is used as a textile dye adsorbent. A sample of crude dehydrated sewage sludge issued from an urban wastewater treatment plant (high-rate aeration, activated sludge process, Sahline, Tunisia) is utilized for vat dye retention. The main objective of this work is to evaluate the “efficiency” of the crude material on vat dye sorption. However, no treatment to modify any of the adsorbent properties was considered. Although the relatively low specific surface area (about 3.2m 2 g -1 ) compared to more conventional adsorbents, the used material shows very interesting retention capacities when used with water pollutants. The pseudo, first and second order kinetic models have been used to investigate the retention mechanism. When linearized, the pseudo-second order fit, for the both used dyes, in a better way the obtained experimental results than the pseudo-first order kinetic model. For equilibrium dye uptake amount it is found that the used material has a capacity (Langmuir Freundlich monolayer) of 73.1mg/g to fix the VAT RED 10 and 58.7mg/g to fix the VAT ORANGE 11. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Sewage sludges are by-products of urban and industrial wastew- ater treatment plants activities. This “waste of the waste”, if it is heavy metals free and after several treatment steps (mainly sta- bilization and composting) may be disposed through landfills and land reclamation. For the moment, tons of sewage sludges are pro- duced in Tunisia every day and this production is expected to rise (70,000 ton in 2006 against 40,000 ton in 1996). They are now stocked in the municipal wastewater treatment plants, if possi- ble, or discharged in controlled municipal areas. However many legal standards limit the handling or the use of sewage sludge on farmland. Even if many management options for the sewage sludge exist, the utilization of solid waste from one industrial activity to treat the wastewater of another one is environmentally help- ful and attractive especially if the used material is cost free. On the other hand, dyes used in the textile industry represent a large and important group of chemicals among the different aqueous pollutants that get mixed in wastewater. The lack of our knowl- edge of the toxic effects of dyes on environment and human health draws our attention. Activated sludge inhibition process by acid black 1 azo dye discharge is, for example reported by Martin et al. [1]. Textile mill wastewater may contain a substantial amount of dyes which require the adoption of various techniques for their elimination. Liquid phase adsorption remains the most popular ∗ Corresponding author. Tel.: +216 98202932; fax: +216 73500278. E-mail address: hatem.dhaouadi@fsm.rnu.tn (H. Dhaouadi). method for dye-containing wastewater treatment. It is especially a promising option for non-biodegradable organic dyes removal. This technology can prove to be attractive in case the adsorbents do not cost much and are ready for use. Many methods show reliance on dye retention using non-conventional materials. Several stud- ies are conducted to evaluate the capacity of such materials to eliminate different kinds of dyes. A quick review of most recent published papers dealing with vat dye retention and sewage sludge based activated carbons used for dye removal is given in Table 1. In nearly all the mentioned studies and in many other papers, the used adsorbent has been treated to modify some of its physicochemical properties to ensure a higher sorption capacity. In the present work, the main objective is the evaluation of the crude dehydrated sewage sludge capacity to fix vat dyes used in the textile industries. 2. Adsorption isotherm and kinetic models Equilibrium data, commonly known as adsorption isotherms, are basic requirement for the design of adsorption systems used for the retention of organic pollutants. In the removal of dyes from wastewater, it is necessary to know the rate of retention for the design and the quantitative evaluation of adsorbent. Moreover, the kinetics describe the solute uptake rate which, in turn, control the residence time of dye uptake at the solid–solution interface. There- fore it is important to be able to predict the rate at which pollutant is removed from aqueous solutions in order to design appropriate adsorption treatment plant. Various models have been used to describe thermodynamic equilibrium in the present studied sys- tems. The Langmuir adsorption isotherm assumes that adsorption 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.08.029