The biosorption of Cr (VI) and Ni (II) ions, both singly and in a binary system, by dried biomasses of Rhizopusarrhizus and Mucor hiemalis was investigated in a batch system as a function of initial pH. The working initialpH values for Cr (VI) and Ni (II) in a single biosorption system were determined as 2.0 and 8.0 respectively.In a binary system, the optimum pH for sorption of Cr (VI) remained the same (2.0) while for Ni (II) it changedto 5.0. Although both the biomasses had a higher adsorption capacity for Cr (VI) and Ni (II) at single-component situation due to the initial pH of the solution, the equilibrium uptakes of Cr (VI) and Ni (II) in thebinary mixture were found to be decreasing because of the antagonistic interaction between the components.It was observed that the co-ion effect on the equilibrium uptake became more pronounced as pH levelincreased for Cr (VI) and decreased for Ni (II). Adsorption isotherms were developed for the dual-metal ionsystem at these two pH values and expressed by the mono-component Langmuir and Freundlich adsorptionmodels. The model parameters were estimated by the non-linear regression.
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