Microalgal Bioremediation of Heavy Metals 213

transform infrared, spectroscopy (IR or FTIR), X-ray Absorption Spectroscopy (XAS) and Nuclear

Magnetic Resonance (NMR) to gain insight into the process and mechanisms of biosorption process

(Fomina and Gadd 2014, Michalak et al. 2013, Kiran et al. 2016).” It can be confirmed that surface

adsorption of the metals is found in the case of Cr (VI) bound at specific binding sites on the algal

biosorbent based on SEM and FTIR spectral analysis (Kiran et al. 2008).

To understand the procedure of the biosorption and optimize the processes, modeling and

simulation of the experimental data are generally done for which several models have been

developed (Volesky 2003a). The two regularly used models are Langmuir and Freundlich models, in

which the adsorption mechanism is demonstrated as a batch equilibrium isotherm curve to contrast

pollutant uptake proportion of different bio-sorbent and affinities for the metals (Mona et al. 2011a).

These equilibrium sorption models provide some normal information about a given process. As

the biosorbent adsorbs the metal to the equilibrium point, the value of metal uptake (qe) by the

bio-sorbent is plotted against the equilibrium (final) metal concentration (C). The Langmuir isotherm

assume a finite number of equal adsorption locations and the absence of lateral interrelation between

adsorbed species. The most regularly multilayer adsorption model is the Brunauer–Emmett–Teller

(BET) isotherm, which presume that the Langmuir equation is applied to every layer (Vijayaraghavan

and Yun 2008). Equilibrium data is right to Langmuir isotherm, and a linear plot is received from

this isotherm (Figure 12.2); and, the value of the Langmuir constant is calculated:

qe = QobCe + bCe

where;

qe = adsorption of metal (mg g^–1); Ce = residual metal (mg L^–1)

Qo (mg g^–1) and b (L mg^–1) are Langmuir constants exhibit the adsorption range and adsorption

energy (Mona et al. 2015a).

Freundlich isotherm considers a heterogeneous base of the adsorbent, and the equation is

Log qe = Log Kf + n Log Ce

“where;

qe = metal adsorbed (mg g^–1); Ce = residual metal ion concentration (mg L^–1); n = Freundlich

exponent; Kf = Freundlich constant indicating adsorbent capacity (mg g^–1 dry weight).”

And a linear plot of Log qe versus Log Ce explains the applicability of this isotherm (Figure

12.2) for the biosorbent (Gadd 2009).

There is another model, Brunauer Emmer and Teller (BET); in this model the 1st layer of

sorbent gets absorbed on the upper layer with the energy approximate to the heat of adsorption for

single layer sorption, and the next layer has same energy.

Ce

1

Ce

=

+

q

e

qmKL

qm

“where;

Ce = the equilibrium concentration of the adsorbate and qe is the adsorption capacity adsorbed at

equilibrium, qm is maximum adsorption capacity

KL = the Langmuir adsorption constant”

Response Surface Methodology (RSM) is one of the most widely used approaches adopted for

optimization of the parameters to get the maximum metal removal response from the biosorption

process (Mona et al. 2011a). Box-Behnken Design (BBD) and Central Composite Design (CCD) are

two models of Response Surface Methodology (RSM). Box-Behnken model (BBM) of RSM is used