MATERIALS AND METHODS
2.1.7. Adsorption and desorption 1. Pretreatment of adsorbent
The amberlite polymeric adsorbent was purchased as a wet product embedded with sodium chloride and sodium carbonate salt to retard the bacterial growth. This salt must be washed out from the resin prior to use for adsorption. Moreover, for the concentration of hydrophobic compounds, the amberlite resin must be pretreated with a water-miscible organic solvent. Therefore, the amberlite resins were pretreated prior to use to ensure efficient adsorption of anthocyanin from the crude extract. The amberlite resins (XAD2, XAD4, and XAD7) were pretreated by soaking in ethanol for 24 h. After removal from ethanol, the resins were washed with distilled water and soaked in 1M NaOH for 5 h to remove monomers trapped inside the resin pores. The resins were washed with distilled water and kept in an oven at 60
°C for 24 h before use for adsorption (Yang et al., 2016). Activated charcoal was washed with hot water to remove air bubbles from pores to ensure anoxic conditions. However, the presence of oxygen may improve adsorption efficiency, but it also promotes an undesired polymerization reaction (Soto et al., 2008). Bentonite was washed with water repeatedly before use for adsorption.
2.1.7.2. Adsorption and desorption tests
Pretreated adsorbents (1 g) and 25 mL of purple rice bran extract were mixed in 250 mL flasks. Flasks were kept in a shaker at a rate of 45 rpm (4.71 rad/s) for 24 h at room temperature (28 ± 2 °C) (Buran et al., 2014). After adsorption, the adsorbents were separated from the crude extract solution through filtration. For desorption testing, resins were washed with 25 mL distilled water. After washing, the resin was mixed with 50 mL of 95 % ethanol in flasks. The flasks were kept in a shaker at 45 rpm (4.71 rad/s) for 24 h at room temperature (28
± 2 °C) for desorption. Adsorption/desorption ratio and capacities were calculated using the following equations:
Adsorption ratio, (%)= o e 100
o
C C
E C
(2.12)
Adsorption capacity, (%)= ( )
(1 )
i
e o e
q C C V
M W
(2.13)
Where, E is the adsorption ratio (%); qe is the adsorption capacity (mg/g dry resin) at equilibrium, Co (mg/L) is the initial concentration of anthocyanin in the extract; Ce (mg/L) is the equilibrium concentration of anthocyanin in the extract. After the completion of the adsorption process, the resin was separated from the extract and the anthocyanin content of the extract was measured as equilibrium concentration anthocyanin. Vi (mL) is the volume of the crude extract; W (g) is the mass of the resin and M is the moisture content of the resin (% w/w)
Desorption ratio, % 100
( )
d d
o e i
D C V
C C V
(2.14)
Desorption capacity, mg/g)(
(1 )
d d
C Vd
q M W
(2.15)
Recovery, ad (%) d d 100
o i
R C V
C V (2.16)
Where, D (%) is the desorption ratio; Cd (mg/L) is the concentration of anthocyanin in the desorption solutions; Vd is the volume of the desorption solution (mL); Vi is the volume of the initial sample solution (mL); Co and Ce are the same as those defined above, and qe (mg/g) is the adsorption capacity at adsorption equilibrium. Rad is recovery percentage (%).
2.1.7.3. Adsorption kinetics
Pretreated adsorbent (1 g) was mixed with 25 mL of purple rice bran extract in a flask and placed in a shaker at 45 rpm (4.71 rad/s) at ambient temperature (28 ± 2 °C). Aliquots of 1 mL were drawn from the solution at regular intervals of 30 min, up to 210 min, in order to identify the time at which the adsorption process attains equilibrium. Adsorption kinetics was assessed using the pseudo-first and pseudo-second-order kinetic models, which used to predict the mechanism of adsorption process (Chen et al., 2016)
Pseudo-first order model
q
t q
eq e
e k t1 (2.17)log( ) log 1
2.303
e t e
q q q k t (2.18)
Pseudo-second order model
2 2
1 2 e t
e
k q t q k q t
(2.19)
2 2
1
t e e
t t
q q k q (2.20)
Where, k1 and k2 are rate constant of the pseudo-first and pseudo-second order models, respectively. qe (mg/g) is the adsorption capacity of resins at equilibrium; qt (mg/g) is the concentration of anthocyanin absorbed at time t.
2.1.7.4. Adsorption isotherm
Pretreated adsorbent (1 g) was added to 25 mL purple rice bran extract of different concentrations of anthocyanin (15-40 mg/L). Adsorption was conducted at room temperature (28 ± 2 °C) in a shaker at 45 rpm (4.71 rad/s). The equilibrium adsorption isotherms for anthocyanin were determined using various adsorption isotherms models as shown in Table 2.2. The adsorption capacity of adsorbent at various equilibrium conditions (qe,mg/g) and the equilibrium concentration of anthocyanin (Ce, mg/L) was the input variable.
Table 2.2: Adsorption isotherms models
SL. No. Model Nonlinear form Plot Two parameter model
1 Langmuir m e
e
L e
q q C K C
e Vs e
e
C C
q
2 Freundlich 1/n
e f e
q K C
log Vs log q
eC
e3 Hill m en
e n
d e
q q C
K C
log Vs log
( )
e
e
m e
q C
q q
Three parameter model 4 Redlich–Peterson
1
r e n r e
q K C e A C
ln r ee 1 Vs ln e
K C C
q
5 Toth
( )1/
T e e
t e
q K C a C n
ln e Vs ln e
T
q C
K
6 Koble–Corrigan
1
n t e
e n
e
q A C
BC
--- 2.1.7.5. Diffusion model
In the present work, the adsorption mechanisms of anthocyanin were investigated using a model that takes into account the influence of adsorption on solute transport in the adsorbent.
In order to investigate this mechanism of adsorption for anthocyanin on various adsorbents, an intra-particle diffusion model-based mechanism was used. According to a functional relationship of the intra-particle diffusion model, the uptake varies almost proportionately with the half-power of time, t0.5, rather than t. The most-widely applied intra-particle diffusion equation for the bio-sorption system is given by Weber. and Morris (1963):
0.5
t t
q k t
(2.21)Where, qt is the adsorption capacity of adsorbent (mg/g), kt is the intra-particle diffusion rate constant (g/mg min0.5) and ψ is intercept.
2.1.7.6. Thermodynamic parameters of adsorption
The only XAD7 resin was considered for the investigation of adsorption thermodynamics for anthocyanin. Adsorption thermodynamics of resin was found to reflect the in-depth information regarding structural and inherent energy change of adsorbent (Gao et el., 2013). It also provided the mechanisms involved in the adsorption process. During adsorption, the enthalpy change was calculated using the Clausius–Clapeyron equation, which was linearized as:
2
(ln L)
d K H
dT RT
(2.22)
ln L H ln o
K K
RT
(2.23)
The Gibbs free energy change can be calculated as follows:
ln
LG RT K
(2.24)
The adsorptive entropy was calculated with the following equation S ( H G)
T
(2.25)
Where, ΔH (kJ/mol) is enthalpy change; ΔS (J/mol K) is entropy change; ΔG (kJ/mol) is Gibbs free energy change; R (8.314 J/mol K) is the ideal gas constant; T (K) is the temperature; KL is the equilibrium distribution coefficient, which is the ratio of the amount adsorbed on the solid to the equilibrium concentration in solution; Ko is constant.