Mono-component versus binary isotherm models for Cu(II) and Pb(II) sorption from binary metal solution by the green alga Pithophora oedogonia Dhananjay Kumar * , Alpana Singh, J.P. Gaur Laboratory of Algal Biology, Department of Botany, Banaras Hindu University, Varanasi 221 005, India article info Article history: Received 4 November 2007 Received in revised form 2 March 2008 Accepted 4 March 2008 Available online 14 April 2008 Keywords: Pithophora Cu(II) Pb(II) Mono-component isotherm Binary isotherm abstract The sorption of Cu(II) and Pb(II) by Pithophora markedly decreased as the concentration of the secondary metal ion, Cu(II) or Pb(II), increased in the binary metal solution. However, the test alga showed a greater affinity to sorb Cu(II) than Pb(II) from the binary metal solution. Mono-component Freundlich, Langmuir, Redlich–Peterson and Sips isotherms successfully predicted the sorption of Cu(II) and Pb(II) from both single and binary metal solutions. None of the tested binary sorption isotherms could realistically predict Cu(II) and Pb(II) sorption capacity and affinity of the test alga for the binary metal solutions of varying composition, which mono-component isotherms could very well accomplish. Hence, mono-component isotherm modeling at different concentrations of the secondary metal ion seems to be a better option than binary isotherms for metal sorption from binary metal solution. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Diverse types of biomass have shown remarkable ability to sorb metal ions thus opening up the possibility of their use in metal re- moval from wastewater (Mehta and Gaur, 2005; Romera et al., 2006). Metal biosorption studies have hitherto remained focused on single metal systems despite the fact that industrial effluents often contain several metal ions concomitantly (Kratochvil and Volesky, 1998; Ma and Tobin, 2003; Aksu and Dönmez, 2006). The available reports suggest that the biosorption of the metal ion of interest is greatly affected by the presence of other metal ions (Mehta and Gaur, 2005). Hence, it becomes necessary to gen- erate data on metal biosorption using multi-metal systems for their meaningful application in metal removal from industrial wastewaters. Biosorption isotherms describe metal sorption as a function of metal concentration in the solution at equilibrium and at a con- stant temperature. Freundlich, Langmuir, Redlich–Peterson and Sips are well-known isotherm models that can effectively define the sorption of metal ions on the biomass from the single metal solution. Insofar as metal sorption from binary metal solution is concerned, earlier researchers have used both mono-component and binary isotherms. The mono-component models are easy to use and interpret due to their two-dimensional nature. But, they are unable to predict the sorption behaviour of a metal ion of inter- est as a function of the concentration of both the metal ions in the binary metal solution. Several empirical and mechanistic binary sorption isotherms have recently been developed for some biomass types (see Chong and Volesky, 1995, 1996; Pagnanelli et al., 2002; Hammaini et al., 2003; Ma and Tobin, 2003; Apiratikul and Pavasant, 2006; Fagun- des-Klen et al., 2007). They provide information on sorption of the metal ion of interest at varying concentrations of both the metal ions in the form of three-dimensional graphs. However, suitability of these isotherms in describing the binary metal sorption by di- verse biosorbent types requires critical examination. Most of the studies on binary metal system predict isotherm parameters taking into account only the metal ion of interest, and thus ignore the estimation of parameters for the secondary metal ion and total metal sorption. Hence, there is a need to interpret the derived iso- therm parameters considering sorption data of primary and sec- ondary metal, and total metal for the same binary system. As discussed above, mono-component and binary isotherm models have their own strengths. However, nobody seems to have made an effort to find out which one is more appropriate for mod- eling metal sorption from binary metal solution. This major gap in knowledge prompted the present study. The present work was 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.03.008 * Corresponding author. Tel.: +91 542 2307147x342. E-mail address: dhananjaybhu@gmail.com (D. Kumar). Bioresource Technology 99 (2008) 8280–8287 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech