Fully competitive biosorption of chromium(VI) and iron(III) ions from binary metal mixtures by R. arrhizus: Use of the competitive langmuir model

Abstract

The uptake of heavy metal ions by microbial biomass has been extensively studied but very little attention has been given to the biosorption of multi-component systems. In this study, a process of fully competitive biosorption of chromium(VI) and iron(III) ions to Rhizopus arrhizus from binary metal mixtures is described and compared to single metal-ion situations in solution. Effects of the presence of chromium(VI) and iron(III) ions together on the biosorption of chromium(VI) and iron(III) ions were investigated in terms of initial rates of biosorption, maximum uptake capacity and equilibrium isotherms. Both chromium(VI) and iron(III) were more effectively adsorbed to the biomass at very low values of pH. The optimum initial pH for the biosorption of chromium(VI) and iron(III) ions by R. arrhizus was determined as 2·0. The initial biosorption rates and the adsorptive capacity of the biomass for chromium(VI) and iron(III) ions increased with increasing temperatures in the range 25–45°C and 25–35°C, respectively. These properties were used in the bioremoval of chromium(VI) and iron(III) ions simultaneously from binary mixtures. The instantaneous, equilibrium and maximum uptake of chromium(VI) and iron(III) was reduced by the presence of increasing concentrations of the other metal. In particular, the long-term uptake of chromium from the solution in the presence of iron was greater than the uptake of iron under the same conditions. In the single-ion situation, the adsorption isotherms were developed at different pH and temperature values and it was seen that the adsorption equilibrium data fit the non-competitive Langmuir model. The ‘best fit’ parameters for the non-competitive chromium(VI) and iron(III) biosorption at pH 2·0 and at 25°C were determined to be q s = 58·12 mg g −1 and b = 0·047 litre mg −1; q s = 34·73 mg g −1 and b = 0·066 litre mg −1, respectively. For the multi-component adsorption equilibrium, the competitive adsorption isotherms were also developed and the competitive Langmuir model was shown to be consistent with the observed uptake of multi-metal ions.