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Bioremediation for Sustainable Environmental Cleanup

Figure 3.3. Kinetic curves obtained from the adsorption of nickel on peanut shells.

Once the ratio between the working concentration and the mass of the adsorbent to be used

was established, the kinetic tests begin with the objective of obtaining the kinetic constant and the

reaction order by applying different models.

As an example of the kinetic studies, the kinetic curves for 10, 20 and 30 mg Ni L^–1, using 0.3 g

of adsorbent, are presented in Figure 3.3. The equilibrium time for all systems was 30 min.

By processing these studies one can determine the order of the reaction and the kinetic constant.

For this, it is necessary to use pseudo-first order and pseudo-second order models. The mathematical

expression for the pseudo-first order kinetics model (Lagergren 1898) is widely used for adsorption

studies of liquids. The linear form is Eq. 3.3:

ln(qe – qt) = lnqe – k1 t

Eq. 3.3

where qt (mmol g^–1) is the amount of adsorbed solute at time t, and k (min^–1) is the constant velocity.

The pseudo-second order model was developed by Ho and McKay (1999). This model assumes

that adsorption follows a second-rate kinetic mechanism. It indicates that one sorbate molecule is

adsorbed on two active sites on the sorbent surface. The process can be expressed in a linear form

by the Eq. 3.4:

t

1

t

=

2 ⁺

t

Eq. 3.4

qt

k2 qe

qe

where, k2 (mmol g^–1 min^–1) is the velocity constant of pseudo-second order. The term (k2qe

2)

represents the initial adsorption rate.

In the case of peanut shells, the best fit was obtained for the pseudo-second order model

with regression coefficients higher than 0.998, while for the pseudo-first order fit the regression

coefficient in all cases was less than 0.92. In general, the reaction order for lignocellulosic materials

for metal removal responds to reaction of order 2. Several authors have demonstrated that the metal

ions adsorption process in lignocellulosic materials occurs by complexation, it occurs where the

carboxylic or carboxylate (R-COOH/R-COO-) and phenolic or phenolate (R’-OH / R’-O-) functional

groups simultaneously chelate the metal ions (Mn⁺), forming bidentate complexes R-COO-Me-

O-R’ (Neris et al. 2019, Guo et al. 2008, Piol et al. 2021, Boeykens et al. 2018).

Regarding the kinetic constant obtained, values between 600 and 15 g mmol^–1 were obtained.

The cyclic of these parameters (k and reaction order) is necessary to enter the Reactor App software.