Batch experiments were conducted to assess the effect of metal concentration and initial pH of solution on the biosorption of UO22+ and Pb2+ by chemical modified Cystoseira indica. The sorption capacity of Pb2+ was enhanced by increasing initial pH from 3.0 to 5.5 and maximum sorption of UO22+ biosorption was occurred around initial pH 4.0. The single metal isotherm data, at 25 degrees C, were fitted on Langmuir equation and vacancy solution theory model using Flory-Huggins activity coefficients (VST-Flory-Huggins). The equilibrium adsorption experiments of the binary solution systems were carried out and the results were modeled by four types of extended Langmuir and multi-component VST-Flory-Huggins isotherm equations. Notwithstanding presence of any adjustable parameter, it was concluded that multi component VST-Flory-Huggins predicted the binary solution isotherms more accurate than extended Langmuir isotherms. (C) 2013 Elsevier B.V. All rights reserved.