**10. Conclusions**

**Initial metal ion concentration**: The initial concentration provides an important driving force to overcome all mass transfer resistance of metal between the aqueous and solid phases [102]. Increasing amount of metal adsorbed by the biomass will be increased with initial concentration of metals. Optimum percentage of metal removal can be taken at low initial metal concentration. Thus, at a given concentration of biomass, the metal uptake increases with increase

**Metal affinity to biosorbent**: Physical/chemical pretreatment affects permeability and surface charges of the biomass and makes metal binding groups accessible for binding. It can be manipulated by pretreating the biomass with alkalis, acids detergents and heat, which may

Before going in the details of studying kinetics of biosorption, one should understand the quality of a biosorbent. For observing the quality of a biosorbent, two factors should be considered (i) how much metal ion is attracted by the biosorbent, (ii) to which extent metal ions are retained on biosorbent in an immobilized form. The metal uptake by the biosorbent can be calculated by checking the difference in initial quantities of metal ions in medium to that remained in the medium after biosorption takes place. This is studied by the following Eq. 1 [48, 49, 94]:

q = amount of metal biosorbed by biomass (mg/g); V = Volume of metal solution (L); Ci = Initial concentration of metal (mg/L); Ce = Concentration of metal (mg/L) at equilibrium; M = Mass

Units = milligrams of solute sorbed per gram of dry biosorbent material (when engineering process – mass balance calculations are to be considered) or mmol/g (when the mechanism or

According to Abdi and Kazemi [49], in order to observe biosorption kinetics of any heavy metal, sorption performance of a biosorbent must be taken into consideration. For it, a biosorption isotherm should be studied. A biosorption isotherm is the plot of uptake of metal (q) versus equilibrium solute concentration in the solution (Cf). For studying the isotherm plots, parameters including temperature, pH and ionic strength are kept constant whereas metal concentration is varied. Literature showed that confusion prevails regarding pH because it is common believe that pH of a medium changes during whole process of biosorption. Biosorption isotherms are typically described by two models (i) Freundlich and (ii) Langmuir. These models are two - parameters models which are vastly used to describe the equilibrium

**Freundlich model**: Freundlich and Kuster in (1907) published first mathematical equation to describe the isotherm. It is a non-liner sorption model. It involves monolayer sorption of

*<sup>M</sup>* (1)

in initial concentration [48].

34 Biosorption

**8. Kinetics of biosorption**

stoichiometry are to be considered).

of adsorbent.

increase the amount of metal uptake [48, 94].

*<sup>q</sup>* <sup>=</sup> *<sup>V</sup>*(*Ci* <sup>−</sup> *Ce*) \_\_\_\_\_\_\_\_

state for adsorption of metal ions experimental work [48].

Biosorption is eco-friendly and cheap method of removing metals from the environment. Previous researches conducted during last five decades provided vast amount of information about different types of biosorbents and their mechanism of metal uptake. More research is needed to explore new biosorbents from environment. A deep insight is required not only on method of metal removal, but also its efficient recovery so that it can be obtained in usable form.
