InstrumentsThe concentration of PZP in aqueous solutions was determined using an HPLC system (Agilent 1260, USA), an ultraviolet detector, and Chemstation software. pH measurements were performed using a pH meter (Mettler-Toledo, Switzerland) with a glass electrode. Ultrapure water used in the experimental studies was produced using a water purifier (Merck-Millipore Milli-Q, USA). Batch adsorption studies were carried out using a shaking water bath (WITEG WSB-30, Germany).ChemicalsPZP (≥ 98.0%) and tablets (Vopazzi, 200 mg) were obtained from Deva Holding Pharmaceutical Company (Istanbul, Turkey). Acetonitrile (≥ 99.9%), methanol (≥ 99.0%), ethanol (≥ 99%), NaOH solution (0.1 N), HCl solution (0.1 N), H3PO4 (85%), KH2PO4 (≥ 99.0%), and activated carbon were purchased from Sigma-Aldrich Chemie (Istanbul, Turkey).Determination of pHpzc value of activated carbonFirst, 500 mL of 0.1 M NaCl solution was prepared. Then, 50 mL of the salt solution was taken into 250 mL volume flasks and the pH of the solutions was adjusted with 0.1 M HCl or 0.1 M NaOH solutions to pH 2, 4, 6, 8, 10, 12. The initial pH of each solution was recorded. To each flask 0.050 g of activated carbon was added. The bottles were placed in a shaker and shaken for 48 h at a temperature of 25 °C and a shaking speed of 150 rpm. At the end of the shaking, each mixture was filtered through white tape filter paper and the pHf values of each sample were measured with a pH meter. The pHf values were plotted against pHi values35.Batch adsorption experimentsTo prepare the stock PZP solution (500 µg mL−1), 250 mg of PZP was precisely weighed and transferred to a 500 mL flask, and 100 mL of ultrapure water was added. The contents of the volumetric vial were sonicated for 30 min to dissolve the contents. The volume was then made up to the mark with ultrapure water. Batch adsorption experiments were carried out in 250 mL capped conical flasks using 50 mL of PZP solutions. In the batch adsorption study series, the effects of solution pH, initial concentration, adsorbent dose, and temperature on PZP removal efficiency were investigated. PZP solutions used in the experimental series were prepared at the desired concentrations by diluting from the stock solution. After adjusting the pH values of the prepared solutions, certain amounts of activated carbon were added to the solutions and placed in a thermostat water bath (WITEG, WSB-30). The solutions were shaken at 150 rpm in a water bath at the specified temperature. The samples were then filtered through a 0.22 µm syringe tip membrane filter, and PZP concentrations in the filtrates were determined by HPLC.The effect of pH on PCP removal efficiency was investigated for different pH values in the range 2–12, keeping all other parameters constant. The pH of the solutions was adjusted to the appropriate value using 0.1 N HCl or 0.1 N NaOH.In the second step, the effect of activated carbon dosage on the removal efficiency was investigated for different activated carbon dosages in the range of 0.2–0.6 g L−1, keeping all other parameters constant. The pH of the solutions was adjusted to the pH value determined in the previous step where the best removal efficiency was obtained before the addition of activated carbon.The effect of initial concentration on the removal efficiency was investigated for different concentrations of PZP in the range 25–100 mg L−1, keeping all other parameters constant.The effect of temperature on the removal efficiency was investigated for different temperature values in the range 20–50 °C, keeping all other parameters constant.In each experiment, only one parameter was varied and the other parameters were kept constant. After shaking for a predetermined period, 1 mL of sample was taken, filtered through an injector tip filter (0.22 μm membrane filter) and then the concentration of PZP in the filtrate was determined using an HPLC. All experiments were performed in triplicate and the results were averaged. The amount of PZP adsorbed per unit mass of activated carbon at time t (qt) and equilibrium (qe) was calculated from the following equations:$${q}_{t}=\left(\frac{{C}_{0}-{C}_{t}}{w}\right)\times V$$
(1)
$${q}_{e}=\left(\frac{{C}_{0}-{C}_{e}}{w}\right)\times V$$
(2)
Additionally, the following formula was used to determine the proportion of PZP adsorbed:$$Adsorption (\%)=\left(\frac{{C}_{0}-{C}_{e}}{{C}_{0}}\right)\times 100$$
(3)
The initial concentration (C0, mg L−1), the equilibrium concentration (Ce, mg L−1), the concentration at the time of t (Ct, mg L−1), the volume of solution (V, L), and the mass of activated carbon (w, g)36,37,38,39,40.Equilibrium modeling of the adsorption processAdsorption isotherms provide qualitative information about the nature of the interaction between the solute and the surface, in addition to the exact correlation between the adsorbate concentration and the amount of surface deposition at a given temperature. When an ideal correlation is obtained for equilibrium curves that can describe the relationships between the PZP adsorbed at a given temperature and the PZP concentrations remaining in solution, a suitable sorption system design is obtained. In this study, Langmuir, Freundlich, and Temkin isotherm models, which are the three most commonly used models for equilibrium modeling of PZP adsorption on activated carbon, were used. The Langmuir isotherm is based on the physical and/or physical interactions between the solute and vacant sites on the adsorbent surface. The linear equation of the Langmuir isotherm model is expressed as follows36,37,38,39,40.$$\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{q}_{m}{K}_{L}}+\frac{{C}_{e}}{{q}_{m}}$$
(4)
In this equation, KL is the Langmuir adsorption constant (L mg−1) and qm is the monolayer adsorption capacity of activated carbon (mg g−1). The affinity between adsorbate and adsorbent can be determined by the dimensionless separation factor (RL). RL is calculated by the following formula:$${R}_{L}=\frac{1}{1+{K}_{L}{C}_{0}}$$
(5)
where C0 is the initial adsorbate concentration of the solution (mg L−1). If the RL value is greater than 1, the isotherm shape is unfavorable. If the RL value is equal to 1, the isotherm shape is linear. If the RL value is between 0 and 1, the isotherm shape is favorable. If the RL value is equal to 0, it is irreversible.In contrast to the Langmuir model, The Freundlich model implies multilayer adsorption on the adsorbent surface. The linearized equation of the Freundlich isotherm model is expressed as follows:$$\text{ln}{q}_{e}=\text{ln}{K}_{F}+\frac{1}{n} \text{ln}{C}_{e}$$
(6)
In this equation, KF (L mg−1) is the Freundlich constant and n is a parameter describing the favorability of the adsorption process; if n > 1, PZP adsorption on the adsorbent is expected to occur at high concentrations.Temkin isotherm model describes adsorption processes occurring on heterogeneous surfaces, taking into account the assumption that the heat of adsorption decreases linearly with coverage due to adsorbate-adsorbent interactions. In addition, Temkin isotherm assumes chemical adsorption through strong electrostatic interactions between the adsorbent surface and adsorbate molecules. Temkin isotherm model is given in Eq. (7) and its linear form in Eq. (8):$${q}_{e}=\frac{R T}{{b}_{T}} \text{ln}{{(K}_{t}C}_{e})$$
(7)
$${q}_{e}={B}_{t} \text{ln}{K}_{t}+{B}_{t} \text{ln}{C}_{e}$$
(8)
$${B}_{t}=\frac{RT}{b}$$
(9)
where b is Temkin isotherm constant (J mol−1), Kt is Temkin isotherm equilibrium binding constant (L g−1) and R is universal gas constant, respectively. A positive value of Bt indicates that adsorption is endothermic41.The Dubinin-Radushkevich isotherm model is another empirical model formulated for the adsorption process that initially follows the pore-filling mechanism. It is usually applied to express the adsorption process occurring on both homogeneous and heterogeneous surfaces. The nonlinear expression of the Dubinin-Radushkevich isotherm model can be represented as Eqs. (10) and (11)42:$${q}_{e}= {q}_{s }\text{exp}(-{K}_{DR}{\varepsilon }^{2 })$$
(10)
$$\varepsilon =RT\text{ln}\left(1+ \frac{1}{{c}_{e}}\right)$$
(11)
where qs (mg g−1) is a constant related to the adsorption capacity; KDR (mol2 kJ−2) is a constant related to the average free energy of adsorption; R (J mol−1 K−1) is the gas constant; and T (K) is the absolute temperature. The linear form of the Dubinin-Radushkevich isotherm model is given in Eq. (12) below.$$\text{ln}{q}_{e} =\text{ln}{q}_{s} -{K}_{D} {\varepsilon }^{2}$$
(12)
Kinetic modeling of the adsorption processBatch adsorption data were analyzed by pseudo-first-order, pseudo-second-order, and intra-particle diffusion models and kinetic parameters were calculated. The pseudo-first-order kinetic model describes adsorption based on solids’ sorption capacity, which essentially assumes that a PZP molecule sorbs to a sorption site. The pseudo-first-order rate equation that is linearized can be expressed by the formula given below.$$\text{ln}\left({q}_{e}-{q}_{t}\right)=\text{ln}{q}_{e}-{k}_{1} t$$
(13)
In this equation, k1 is the pseudo-first-order adsorption rate constant (1 min−1), qe and qt are the adsorption capacity (mg g−1) at equilibrium and time t, respectively30,31,32,33.A pseudo-second-order equation to define adsorption kinetics in terms of adsorption capacity can be expressed as follows:$$\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}{q}_{e}^{2}}+\frac{1}{{q}_{e}} t$$
(14)
In this equation, k2 is the so-called pseudo-second-order adsorption rate constant (g mg−1 min−1), qe and qt are the adsorption capacity (mg g−1) at equilibrium and t is time, respectively30,31,32,33.The intraparticle diffusion model equation can be expressed by the formula given below.$${q}_{t} = {K}_{B} {t}^{1/2}+C$$
(15)
where Kp is the IPD rate constant (mg g min−0.5) and C is a constant describing the initial adsorption capacity30,31,32,33.Thermodynamic modeling of the adsorption processThermodynamic parameters such as enthalpy change (ΔH°), Gibbs free energy change (ΔG°), and entropy change (ΔS°) were determined to decide whether the process was spontaneous or not and to investigate the effect of temperature on the adsorption performance. The Gibbs free energy change can be determined from the equation given below:$${\Delta G}^{0}=-RT\text{ln}{K}_{e}$$
(16)
In this equation, Ke is the equilibrium constant, R is the gas constant (8.314 J mol−1 K−1), and T is the absolute temperature (K).$${K}_{e}=\frac{{q}_{e}}{{C}_{e}}$$
(17)
In this equation, Ce and qe represent the equilibrium concentration of PZP in solution (mg L−1) and, on the adsorbent (mg g−1), respectively30,31,32,33.The equation given below expresses the relationship between ΔG°, ΔH°, and ΔS°:$${\Delta G}^{0}= {\Delta H}^{0}-T {\Delta S}^{0}$$
(18)
After manipulating Eq. (13), a linear equation for the calculation of thermodynamic parameters is obtained.$$\text{ln}{K}_{e}=-\frac{{\Delta G}^{0}}{RT} = \frac{{\Delta S}^{0}}{R}-\frac{{\Delta H}^{0}}{RT}$$
(19)
Analytical method and validationAn HPLC method was developed for the quantification of PZP in aqueous solutions. The conditions of the developed chromatographic method are presented below: In the chromatographic method, an Extend C18 column (250 mm × 4.6 mm i.d. 5 µm) was used for separation at a constant temperature of 25 °C. The mobile phase was 10 mM KH2PO4 solution (pH: 2.0 with orthophosphoric acid) and acetonitrile (20:80, V/V). Isocratic elution with a flow rate of 1.2 mL min−1 and eluent detection was performed at a wavelength of 253 nm using an ultraviolet detector. The retention time of PZP was determined as 10.65 min. The developed analytical method was validated for the parameters “specificity, system suitability, linearity, precision, accuracy, sensitivity, and robustness” according to ICH Q2 (R1) guidelines43,44.Standard solutions (the range of 5–30 μg mL−1) were injected into the HPLC system to test its linearity into the HPLC system on three consecutive days. The chromatographic responses (peak areas) from the instrument were recorded for each concentration of the standard solutions. A calibration curve was drawn with concentrations on the x-axis and chromatographic responses (peak areas) on the y-axis. Data from the analytical method were used for regression analysis using the least squares method. The linearity of the method was determined by the absolute mean recovery, relative standard deviation, and R2 of the calibration curve. The sensitivity of the HPLC technique was assessed by determining the limits of detection (LOD) and limits of quantification (LOQ).
Adsorptive removal of anticarcinogen pazopanib from aqueous solutions using activated carbon: isotherm, kinetic and thermodynamic studies
