**10.3. Effect of initial metal concentration**

The mass transfer resistance between the liquid and solid phases can be overcome by the initial concentration of metal ion [175]. The biosorption capacity (quantity of biosorbed metal ions per unit weight of the biosorbent) of the biosorbent increases initially with the increase in metal ion concentration and then reaches a saturation value. However, the biosorption efficiency of the biosorbent decreases with increase in metal ion concentration. The higher biosorption efficiency at low metal concentration is due to the complete interaction of ions with the available binding which sites results in higher rates of efficiency. At higher concentrations, the number of metal ions remaining unbound in the solution is high due to the saturation of available binding sites [176]. The effect of different initial concentration (25–500 mg/L) of Cd ions on the biosorption of *Hypnea valentiae* was studied. It was found that highest biosorption efficiency (86.8%) was observed with a Cd concentration of 25 mg/L from simulated wastewaters [177]. The biosorption efficiency of the cashew nut shell decreased from 86.03 to 76.17% with the increase in copper ion concentration from 10 to 50 mg/L [178].
