2.5 Investigation and study of the curcumin absorption process
To investigate the process of absorption of curcumin solution on the synthesized adsorbents, 100 ml of the curcumin solution was taken as a sample and the adsorbent was added to it and placed on the shaker at room temperature for 2 hours. After a while, the sample was taken and centrifuged for better separation. The concentration of curcumin solution in the samples taken at different times and were measured using a UV light spectrometer.
Finally, using equation (1), the absorption percentage of curcumin solution and using equation (2), the adsorption capacity of the synthesized magnetic metal-organic framework nanocomposite was calculated and determined.
%Adsorption= (C0 - Ct / C0)*100 (1)
qt = (C0-Ct) V/m (2)
Where C0 and Ct are the initial concentration of curcumin solution and the concentration of curcumin solution at time t respectively, V is the volumes of curcumin solution per liter, m the amount of adsorbent (gr) and qt are the adsorption capacity of the magnetic metal-organic framework nanocomposite in terms of (mg.g-1) [26-29]. Finally, the effect of parameters such as adsorbent dose, adsorbent type and concentration of curcumin solution on the adsorption of curcumin solution by synthesized magnetic metal-organic framework nanocomposites was studied.
To investigate the behavior of the synthesized adsorbent, the relationship between the absorbing sample and the adsorbent surface and sample interaction with the adsorbent requires the equations that are used to investigate this adsorption behavior. There are several ways to look at absorption isotherms. These isotherms include the Langmuir, Freundlich and the Temkin. In this study, two isotherms of Langmuir and Freundlich with the following equations have been used to investigate the ruling isotherm to the absorption process [29-31].
The Langmuir adsorption isotherm model is examined and calculated with Equation 3 and its variables are presented in Table 6. The Langmuir adsorption isotherm is based on the single-layer adsorption of the adsorbent, which here is the curcumin solution, on the adsorbent surface, indicating that no reaction occurs between the absorbed sample and adsorbent molecules.
Ce/qe = 1/KL. qL + Ce/qL (3)
In this equation, Ce is the concentration of curcumin solution at equilibrium (mg.l-1), qeis the amount of material absorbed per unit mass of the adsorbent, qL the maximum absorption capacity is synthesized on the nanocomposites of the magnetic metal-organic framework, and KL is the Langmuir constant which refers to the energy of absorption.
The Freundlich isotherm model is examined and calculated with Equation 4 and its variables are presented in Table 6. Frondelich adsorption isotherm is synthesized on the adsorbent based on multi-layered and heterogeneous adsorption of the adsorbent. The closer value of 1/n to zero showing more heterogeneous absorption and if the value is 0.1>1/ n> 1, the absorption process is optimal [32].
Ln qe = Ln KF + (1/nF) Ln Ce (4)
In the above relation Ce the concentration of curcumin solution at equilibrium (mg.l-1), qe the amount of material absorbed per unit mass of the adsorbent, 1/n the heterogeneous factor that indicates the type and severity of absorption and KF is the constant isotherm of the Frondelich.
The third isothermal model studied is Temkin isotherm with linear equation 5, this calculation model and its variables are presented in Table 6.
qe = B1 Ln KT+B1 Ln Ce (5)
In the above relation Ce the concentration of curcumin solution at equilibrium (mg.L-1), qethe amount of substance absorbed per unit mass of the adsorbent, KT isotonic absorption coefficient of Temkin and B1 is the slope of the diagram of this isotherm [32-36].
Absorption kinetics is used to determine the mechanism and rate of the kinetic of the synthesized magnetic metal-organic framework nanoparticle and to control the absorption processes [37]. There are several different kinetic models used in this study to better understand the experimental data of the process of absorption of curcumin solution by magnetic metal-organic framework nanocomposite synthesized from quasi-first and second-order kinetics and intramolecular penetration to quantify the quantitative absorption kinetics. The quasi-first-order model occurs with an equation of 6 expressing penetration into the single layer, and the changes in adsorption value over time are proportional to the number of unoccupied adsorbent sites [38-40].
Ln (qe – qt) = Ln qe – k1t (6)
In equation 6, K1 equilibrium constant quasi-first order velocity (min-1), qe the amount of curcumin absorbed in equilibrium (mg/g) and qt is the amount of curcumin absorbed at time t (mg/g). The quasi-second order model with Equation 7 represents the chemical adsorption rate with a slowing speed which controls surface absorption where the square of the number of unoccupied sites in the adsorbent is proportional to the occupancy rate of the absorbed sites [41].
t / qt = 1 / K2qe2 + t / qe (7)
In this regard, K2 is the equilibrium constant of the second-order kinetic and in terms of (g/mg .min). The parameters qe and qt, indicate the amount of curcumin absorbed in equilibrium and the amount of curcumin absorbed at time t and in terms of (mg/g) respectively. Finally, by comparing these three kinetic models, it can be estimated which synthesized metal-organic framework nanocomposite follows which kinetics model and its adsorption rate are calculated [42-45].