Many heavy metal-bearing wastewaters also contain their metal cyanide complex ions. Although the biosorption of single or multi-metal ions to various microorganisms has been extensively studied, very little attention has been given to the bioremoval and the expression of the adsorption equilibrium and kinetics of metal-metal cyanide complex ion systems. In this study, the simultaneous biosorption of iron(III) (ferric) cations and iron(Ill)-cyanide complex (ferricyanide) anions to Rhizopus arrhizus from binary mixtures was studied and compared with single metal and metal cyanide complex ion situation in a batch stirred system. The effects of initial pH and single and dual-component concentrations on the biosorption kinetics and equilibrium uptake of each component, both singly and in mixture were investigated. The working pH value for both species was determined as 2.0. Multicomponent biosorption studies were also performed at this pH value. The biosorption rates and equilibrium uptakes of iron(III) or iron(III)-cyanide complex ions increased by the presence of increasing concentrations of the other ion up to 200 mg l(-1) for iron(III) and up to 1000 mg l(-1) for iron(III)-cyanide complex ions. This situation showed a synergistic interaction between these ions. The Freundlich, Langmuir and Redlich-Peterson adsorption models were used to predict the mono-component equilibrium uptake and model parameters were estimated by the non-linear regression. It was seen that the mono-component adsorption equilibrium data fitted very well to the mono-component Langmuir and Redlich-Peterson models for both the components at moderate ranges of concentration. A modified synergistic Langmuir model was proposed for dual-component system and model parameters were also estimated by the non-linear regression. The pseudo second-order kinetic model was applied to single and multi-component experimental data assuming that the external mass transfer limitations in the system can be neglected and biosorption is sorption controlled. (C) 2002 Elsevier Science Ltd. All rights reserved.