Separation and Purification Technology 50 (2006) 132–140 Sorption of Pb(II), Ni(II), Cu(II) and Cd(II) from aqueous solution by olive stone waste N´ uria Fiol a , Isabel Villaescusa a,∗ , Mar´ ıa Mart´ ınez b , N´ uria Miralles b , Jordi Poch c , Joan Serarols c a Departament d Enginyeria Qu´ ımica, Agr ` aria i Tecnologia Agroaliment` aria, Universitat de Girona, Avda Llu´ ıs Santal ´ o, 17003 Girona, Spain b Departament d Enginyeria Qu´ ımica, Universitat Polit` ecnica de Catalunya, E.T.S.E.I.B., Avda Diagonal, 647, 08028 Barcelona, Spain c Departament d’Inform ` atica i Matem ` atica Aplicada, Universitat de Girona, Avda Lluis Santal´ o, 17003 Girona, Spain Received 21 June 2005; received in revised form 11 November 2005; accepted 15 November 2005 Abstract Olive stone waste generated in the olive oil production process has been investigated as metal biosorbent for Pb(II), Ni(II), Cu(II) and Cd(II) from aqueous solutions for its wide availability as agricultural waste and also for its cellulosic matrix rich of potential metal binding active sites. The effect of contact time, solution pH, ionic medium and initial metal concentration were studied in batch experiments at room temperature. Maximum metal sorption was found to occur at initial pH around 5.5–6.0. Kinetic studies revealed that the initial uptake was rapid and equilibrium was established in 1 h for all the studied metals and that the data followed the pseudo-second order reaction. The equilibrium sorption data for single metal systems at initial pH 5.5 were described by the Langmuir and Freundlich isotherm models; however, the non-competitive Freundlich model has been found to provide the best correlation. The highest value of Langmuir maximum uptake, (q max ), was found for cadmium (6.88 × 10 -5 mol g -1 ) followed by lead (4.47 × 10 -5 mol g -1 ), nickel (3.63 × 10 -5 mol g -1 ) and copper (3.19 × 10 -5 mol g -1 ). Similar Freundlich empirical constants, k, were obtained for all metals (2.4 × 10 -5 to 2.8 × 10 -5 ). Adsorption-complexation, in addition to ion-exchange, must be involved in the sorption process of copper, lead and cadmium while ion-exchange is the most important mechanism for Ni sorption. An increase of ionic strength concentration caused a decrease in metal removal. Sorption experiments with equimolar concentration of each metal in binary mixtures were also performed and then the extended Langmuir isotherm model fits adequately the experimental data. Desorption experiments put into evidence that after three contacts neither HCl nor EDTA solutions were able to desorb metals from the olive stones completely. The results obtained show that olive stone waste, which has a very low economical value, may be used for the treatment of wastewaters contaminated with heavy metals. © 2005 Elsevier B.V. All rights reserved. Keywords: Metal removal; Low cost sorbent; Biosorption; Sorption isotherms; Binary mixtures 1. Introduction The removal of toxic metals from wastewater is a matter of great interest in the field of water pollution, which is a serious cause of environmental degradation. Besides the classical waste- water treatments, biosorption of heavy metals is an alternative technique, primarily because it utilizes inactive/dead biologi- cal materials as sorbents which are generally available at low cost, non hazardous and abound in nature [1,2]. In the last years, certain raw waste products from industrial or agricultural oper- ∗ Corresponding author. Tel.: +34 972418416; fax: +34 972418399. E-mail address: isabel.villaescusa@udg.es (I. Villaescusa). ations, i.e. pine bark [3], grape stalks [4], crop milling waste [5] have been tested for the decontamination of metal-containing effluents. Nowadays, the production of olive oil in Spain is obtained by a continuous extraction process, which uses two-phase decanters to separate the oil. This new technology leads to a new type of waste named “alperujo” [6,7]. Alperujo contains little amount of olive oil and it is normally sent to special processing plants to be treated with a second extraction to get the residual oil. After this process, the resulting waste is dried and the olive cake and olive stones can be separated. In order to valorize the wastes generated during olive oil production some applications, i.e. used as combustible, natu- ral fertilizer and additive in animal nutrition [8–10], have been 1383-5866/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2005.11.016