**8. Kinetics of biosorption**

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]:

$$q = \frac{V(Ci - Ce)}{M} \tag{1}$$

metal with active sites and is described by continuous interactions between adsorbed mol-

K = mg/g or l/mg; 1/n or n = Freundlich constant related to adsorption capacity; n = Freundlich

**Langmuir model**: Langmuir in 1918 published a model for describing gas or liquid adsorbed on solid material. It describes the monolayer sorption of metal with active sites and do not

\_\_\_\_\_\_\_\_

qe = Amount of metal ion removed (mg/g); Ce = Equilibrium concentration (mg/L); b = Langmuir constant related to affinity; qmax = maximum metal uptake (mg/g) under the

k, n = Freundlich and Langmuir constants (n value greater than 1.0 shows that sorption is

After biosorption of heavy metal from environment, its recovery is another crucial step which involves desorption of metal from biosorbent. According to previous literatures [105–107], various agents were used for this purpose which includes complexing agents

> , H2 SO4

acid). Before choosing the recovery agents, it should be kept in mind that chosen recovery agent should given least harm to physical properties of a biosorbent so that its efficiency of metal binding must remain in its original state to ensure its maximum efficiency for metal

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

involve interactions between adsorbed molecules [48, 49]. It is given by Eq. 3:

<sup>1</sup> (2)

http://dx.doi.org/10.5772/intechopen.72099

Biosorption of Heavy Metals

35

<sup>1</sup> <sup>+</sup> *bCe* (3)

, HCl), organic acids (acetic acid, citric

ecules [49, 103]. It is given by Eq. 2:

constant related to adsorption intensity.

favorable physical process) [49, 104].

**9. Desorption and recovery of metals**

(thiosulfate, EDTA), mineral acids (HNO<sup>3</sup>

given conditions.

binding [94, 106, 107].

**10. Conclusions**

form.

*qe* = *K Cen*

*qe* <sup>=</sup> *qmaxbCe*

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 of adsorbent.

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 stoichiometry are to be considered).

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

**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 metal with active sites and is described by continuous interactions between adsorbed molecules [49, 103]. It is given by Eq. 2:

$$
\eta e = K \mathcal{C} e\_n^1 \tag{2}
$$

K = mg/g or l/mg; 1/n or n = Freundlich constant related to adsorption capacity; n = Freundlich constant related to adsorption intensity.

**Langmuir model**: Langmuir in 1918 published a model for describing gas or liquid adsorbed on solid material. It describes the monolayer sorption of metal with active sites and do not involve interactions between adsorbed molecules [48, 49]. It is given by Eq. 3:

$$qe = \frac{q \text{maxbCe}}{1 + bCe} \tag{3}$$

qe = Amount of metal ion removed (mg/g); Ce = Equilibrium concentration (mg/L); b = Langmuir constant related to affinity; qmax = maximum metal uptake (mg/g) under the given conditions.

k, n = Freundlich and Langmuir constants (n value greater than 1.0 shows that sorption is favorable physical process) [49, 104].
