**10.6. Effect of agitation speed**

The increase in agitation speed increases the biosorption capacity of the biosorbent by minimizing its mass transfer resistance. While the added turbulence enhances the sorption of the metal ions [184], it may also lead to the destruction of the physical nature of the biosorbent. A moderate speed ensures the best homogeneity for the suspension with a high capacity of biosorption. High agitation speeds result in the occurrence of vortex phenomenon which results in the loss of the homogenous nature of the suspension. Excessive turbulence may also reduce the time of interaction between the biosorbate and biosorbent, thus decreasing the extent of biosorption [183]. The optimum speed of agitation for the biosorption of Cd and Zn by *Aspergillus niger* was found to be 120 rpm [183]. With an increase in agitation speed from 0 to 80 rpm, the biosorption efficiency also increased from 32.4 to 65% [62].
