Chapter 2 LITERATURE REVIEW
2.10 Adsorption Technology in Wastewater Treatment
2.10.5 Adsorption Isotherm
Adsorption isotherms, or adsorption properties and equilibrium data, describe how contaminants interact with adsorbent materials and are therefore crucial in optimizing the usage of adsorbents.
The most acceptable correlation for the equilibrium curve must be established in order to optimize the design of an adsorption system to remove pollutant from solutions. For reliable prediction of adsorption parameters and quantitative comparison behavior for different materials within any given system, an exact mathematical description of equilibrium adsorption capacity is required (Crini et al., 2018).
Adsorption isotherm indicates the relationship between the adsorbate in the liquid phase and the adsorbate adsorbed on the surface of the adsorbent under equilibrium at constant temperature.
These are important to describe the interaction of adsorbate molecules with adsorbent surface.
Isotherms are empirical relations which are used to predict how much solute can be adsorbed by adsorbent. The study of the isotherm is essential in assessing the adsorption efficiency of the adsorbent. This study is also useful in optimizing the operating conditions for effective adsorption (Rahman, 2014). Generally, the amount of material adsorbed is determined as a function of the concentration at a constant temperature, and the resulting function is called an adsorption isotherm.
The applicability of the isotherm equation is compared by judging the correlation coefficient, R2. The shape of adsorption isotherms gives qualitative insight about the sorption process and the degree of surface coverage by the adsorbate which is useful for the assessment of the viability of the process for a particular application. It helps to choose the most suitable adsorbent and for the determination of sorbent dosage required for the process. Brunauer classified the shape of isotherm into five basic types for gaseous and liquid phase application and are given in Figure 2.6 (Brunauer et al., 1938).
Figure 2.6: Brunauer et al. 1938)
28 Type I isotherm is for very small pores or microporous adsorbents. Adsorption occurs by filling of the micropores. The adsorbate uptake rate depends on the accessible micropore volume rather than total internal surface area.
Type II and Type IV isotherms are observed for non-porous or microporous adsorbents with unrestricted monolayer-multilayer adsorption. At first the adsorption volume rapidly increases at low relative pressures of less than 0.01 due to interaction of the adsorbate molecules with the higher energetic region followed by the interaction with less energetic region. When the monolayer formation of the adsorbed molecules is complete, multilayer formation starts to take place
corresponding to the y an
abrupt rise indicates the bulk condensation of adsorbate gas to liquid.
Type III and Type V
adsorbate interactions than adsorbate-adsorbent interaction.
The equilibrium of the biosorption process is usually described by fitting the experimental data with isotherm models (Rahimizadeh and Liaghat, 2015). The two well-known equilibrium adsorption isotherm models are Langmuir and Freundlich isotherm models. Commonly used single-component adsorption isotherm models are summarized in Table 2.4.
Table 2.4: Commonly used single-component adsorption models (Rahimzadeh and Liaghat, 2015)
Adsorption isotherms are developed by exposing a given amount of adsorbate in a fixed volume of liquid to varying amounts of adsorbent. Typically, in a container, a known amount of adsorbent is added to each container and agitated intermittently for the desired time periods. At the end of the test period, the amount of adsorbate remaining in solution is measured. The adsorbent phase concentration after equilibrium is computed using the equation:
Isotherm Equation Advantages Disadvantages
Langmuir Interpretable
Parameters Not structured;
monolayer sorption
Freundlich Simple Expression Not structured; no
leveling off
Langmuir-Freundlich Combination of the
above equations Unnecessarily complicated
Redke and Prausnitz Simple Expression Empirical; uses three parameters
Reddlich-Petterson Approaches Freundlich
at high concentrations
No special advantages Dubinnin-
Radushkevich (volume adsorbed)
Independent to
temperature No limited behavior in
29
e = ( o e) / (1)
Where, qe = equilibrium adsorption, mg/g; Co = initial concentration of adsorbate, mg/L; Ce = final concentration of adsorbate after adsorption, mg/L; m= mass of adsorbent, g; and V = volume of wastewater, L.
2.10.5.3 Langmuir Isotherm
Langmuir isotherm describes quantitatively the formation of a monolayer of adsorbate on the outer surface of the adsorbent, and after that no other adsorption takes place. It is based on the view that every adsorption site is identical and energetically equivalent. It also assumes that the ability of molecule to bind and adsorbed is independent of whether or not the neighboring sites are occupied.
That is there will be no interactions between adjacent molecules on the surface and immobile adsorption. Irving Langmuir in 1916 derived a simple adsorption isotherm, on theoretical considerations based on kinetic theory of gases. This is named as Langmuir adsorption isotherm.
The Langmuir model was developed based on the following assumptions (Alam, 2018): (a) the surface containing the adsorbing sites is homogeneous; (b) the adsorbing molecule adsorbs into an immobile state with all adsorption sites considered to be equivalent; (c) molecules form a mono- layer coverage only; and (d) there is no interaction between adsorbate molecules on adjacent sites.
The Langmuir adsorption isotherm is defined as:
(2)
Where, qe is the amount adsorbed per amount of adsorbent at equilibrium (mg/g); Ce is the equilibrium concentration of adsorbate (mg/L); qm is monolayer (maximum) adsorption capacity (mg/g) and KL is Langmuir constant related to energy of adsorption (L/mg)
In linear form,
(3)
A linear plot of 1/qe against 1/Ce suggests the applicability of the Langmuir isotherms. The values of qm and KL were determined from the slope and intercepts of the plot.
30 Figure 2.7: Typical Langmuir Isotherm Curve (Naat et al., 2018)
Hall et al. (1966) proposed a dimensionless separation factor or equilibrium parameter, RL, as an essential
which is defined as:
(4)
Where, Co= reference fluid-phase concentration of adsorbate (mg/l) (initial concentration), KL = Langmuir constant (L/ mg)
Value of RL indicates the shape of the isotherm accordingly as shown in Table 2.5 below. For a single adsorption system, Co is usually the highest fluid-phase concentration encountered.
Table 2.5: Characteristics of adsorption Langmuir isotherm (Gopalakrishnan et al., 2013) Separation factor, RL Characteristics of adsorption Langmuir
isotherm
RL> 1 Unfavorable
RL = 1 Linear
0 <RL< 1 Favorable
RL = 0 Irreversible
Limitation of Langmuir theory temperatures.
(b) When the pressure is increased or temperature is lowered, additional layers are formed. This has led to the modern concept of multilayer adsorption.
31 2.10.5.4 Freundlich Isotherm
The Freundlich equation is an empirical relationship describing the sorption of solutes from a liquid to solid interface. Freundlich model is expressed as,
qe= KF Ce1/n (5)
Where, qe is the amount adsorbed at equilibrium (mg/g); Ce is the equilibrium concentration of adsorbate (mg/L); KF is the measure of adsorption capacity; and n is the adsorption intensity.
Linear form of Freundlich equation is,
log(qe) = log(KF)+1/n log(Ce (6)
A plot of log qe against log Ce gives a linear line with a slope of 1/n and intercept of log . From the experimental data, KF values increase with increasing temperature.
Value of n represents favorability of adsorption condition (Baseri et al., 2012) and ranges between 0 and 1. This indicates the degree of non-linearity between solution concentration and adsorption as follows:
Table 2.6: Characteristics of adsorption Freundlich isotherm (Muriuki, 2015)
n Favorability
n = 1 Moderate adsorption
n > 1 Good adsorption
n < 1 Poor adsorption
The value n also indicates the relative distribution of the energy and the heterogeneity of the adsorbate sites. The more heterogenous the surface, the closer the value of n is to 0, and the more the adsorption is unfavorable (Nimibofa et al., 2017).
32 Figure 2.8: Typical Freundlich Isotherm Curve (Yong et al., 2016)
Limitation of Freundlich theory
(1) The theory holds good only at low pressure
(2) Freu is very high.