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The biosorption of cadmium metal ions by Saccharomyces cerevisiae and Kluyveromyces fragilis yeast cells was studied under various operating conditions in a batch system using shaker flasks and a 2-L double draft tube fluidized bioreactor system. The effects of key parameters such as solution pH, initial metal concentration, biosorbent concentration, and age of the biosorbent cells were investigated. The experiments were carried out at room temperature and pH values of 3.5, 4.5 and 5.0. Three biosorbent yeast cells concentrations (2.5, 5.0 and 7.5 g dry wt./L) were employed with aqueous solutions having initial cadmium concentrations of 50, 100 and 200 mg/L. The solution pH was found to play a critical role in the biosorption process and the metal uptake increased with an increase in pH value. The optimum pH was found to be 5.0. Also, increasing the metal concentration and decreasing the biosorbent concentration increased the final metal uptake per unit cell mass. At a sufficiently high cadmium concentration of 200 mg/L, the effect of initial metal concentration became the dominant factor, since the metal uptake became constant with respect to the initial metal concentration, regardless of pH value. The results showed that the effect of biosorbent concentration on metal uptake was less significant than those of the cadmium concentration and the pH. It was also found that the younger the yeast biosorbent, the higher the metal uptake capacity. Generally, the cadmium uptake was found to take place rapidly, whereby almost 90% uptake occurred within the first five minutes of exposure. The equilibrium of cadmium biosorption was investigated under the same operating conditions. The Langmuir and the Freundlich adsorption isotherms were used to fit the experimental data. However, both of these conventional adsorption models did not fit the experimental data accurately. The maximum uptake capacity was estimated to be close to 35 and 40 mg Cd/g DW cell for S. cerevisiae and K. fragilis, respectively at a pH value of 5.0.

Kinetics and Equilibrium of Cadmium Biosorption by Yeast Cells S. cerevisiae and K. fragilis