Recovery of hydroxytyrosol onto graphene oxide nanosheets: Equilibrium and kinetic models

Journal of Molecular Liquids 285 (2019) 213–222

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Recovery of hydroxytyrosol onto graphene oxide nanosheets: Equilibrium and kinetic models Selin Şahin a,⁎, Zeynep Ciğeroğlu b, Oğuz Kaan Özdemir c, Mehmet Bilgin a, Elaf Elhussein a, Özge Gülmez a a b c

Istanbul University-Cerrahpaşa, Engineering Faculty, Department of Chemical Engineering, Avcılar Istanbul, 34320, Turkey Uşak University, Engineering Faculty, Department of Chemical Engineering, Uşak, 64200, Turkey Yıldız Technical University, Department of Metallurgical and Material Engineering, Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 4 February 2019 Received in revised form 1 April 2019 Accepted 18 April 2019 Available online 19 April 2019 Keywords: Nanomaterials Graphene oxide Adsorption Biophenols Hydroxytyrosol Olive mill wastewater

a b s t r a c t In this study, a novel nanomaterial, graphene oxide (GO), which has never been used as an adsorbent in the separation of hydroxytyrosol (HT), has been utilized. GO nanosheets were synthesized by graphite oxidation naturally, which is known as modified Hummer's method. Then, the material obtained was characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) methods. Adsorption of HT from aqueous media onto synthesized GO was found to be N85% under optimum conditions (with 0.01 mg GO in 10 mL of solution at 150 rpm for 1440 min, where pH is 9). On the other hand, equilibrium (Langmuir, Freundlich, Temkin, Dubinin-Radushkevich and Redlich-Peterson) and kinetic (pseudo-first order, pseudo-second order, Elovich and intra-particle diffusion) models have been applied for analysis and representation of data. Thermodynamic findings point out that the related adsorption system is exothermic, applicable and spontaneous. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Polyphenols are the most extensive materials found in the plant world. Since these biologically active substances of plants are contended to be of great benefit to healthiness, the prospects of obtaining such compounds from agricultural waste by-products have been studied broadly. For this purpose, organic wastes from citrus [1,2], grape [3], apple [4], tea [5] and olives [6] were successfully used as raw materials. However, predominance in the relevant literature intended to separate mixtures of several ingredients rather than a target compound. This approach might be convenient for providing a mixture of compounds with similar characteristics in the nutritional and/or pharmaceutical industries. On the other hand, some of these individual components have a relatively high added value on its own [7]. For example, hydroxytyrosol (HT) (within the scope of this project, which is typically found in olive mill wastewater (OMW), is a very valuable antioxidant with its prospective activity in the prevention of certain cardiovascular and cancer ailments has been extensively investigated [8]. Many reports with this compound have indicated that the relevant substance has antiinflammatory and antiplatelet aggregation properties as well as its potential antioxidant capacity [9]. ⁎ Corresponding author. E-mail address: [email protected] (S. Şahin).

https://doi.org/10.1016/j.molliq.2019.04.097 0167-7322/© 2019 Elsevier B.V. All rights reserved.

Solvent extraction can be used to recover phenolic components from such media, but its selectivity is very low. Besides, an additional process such as purification might be necessary, resulting in extra cost and waste generation. Therefore, more selective and efficient, simpler and greener technologies have been developed. Adsorption is a method of separating target components from aqueous media, which is simple in design, operation, and scaling. Comparing to alternative methods, adsorption is insensitive to toxic substances, easy to regenerate and inexpensive with high capacity and speed [10]. Several adsorbent materials have been used in the recovery of biophenols from food and food by-products. Activated carbon [11–13], chitin and chitosan [14,15], molecularly imprinted polymers [7,16,17] and macroporous resins [18–21] used in the recovery of such natural products. Recent studies have shown that carbonaceous materials such as carbon nanotubes, porous carbon and graphene oxides with large specific surface area, abundant pore size distribution, and mass production feasibility have gained attention [22]. Eventually, developing advanced adsorbents with high adsorption yield, fast adsorption, and specific surface reactivity is of great significance. In recent years, graphene and/or chemically modified graphene oxide (GO) has been applied to remove heavy metal ions [23], dyes [24,25] and antibiotics [26]. However, GO has not yet been used in the adsorption of high-added value phenolic compounds except for the recovery of oleuropein from olive leaf extract [27].

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214

2. Materials and methods

Table 1 Gradient program and analysis conditions applied in HPLC. Conditions

2.1. Materials

Program

Agilent 1260 (Agilent, Waldbronn, ABD) (model) Agilent Eclipse Plus C18 RRHD 18 (3 mm × 5 mm; 1,8 μm) (column) Mobile phase A: Water+% 0,1 formic acid (v/v) Mobile phase B: Acetonitrile+% 0,1 formic acid (v/v) 276 nm (wavelength) 1 mL/dak (flow rate) 40 °C (column temperature) 20 μL (injection volume)

Time (min)

A (%)

B (%)

0.0

100

0

0.5

100

0

7.0 7.1 8.6 8.7

60 40 0 100 0 100 100 0

Ethanol, methanol, hexane, formic acid and acetonitrile were obtained from Merck, while Folin-Cioacalteu reagent, sodium carbonate, gallic acid, hydrochloric acid, sodium hydroxide, and hydroxytyrosol were from Sigma-Aldrich. Potassium persulfate, sodium nitrite and aluminum chloride were also supplied from Merck. 2.2. GO synthesis GO nanosheets were synthesized by graphite oxidation naturally, which is known as modified Hummer's method. 2.5 g graphite with a purity of 99% and a particle size of b200 μm were mixed in the sulfuric acid solution at certain times and temperature values. Then, 10 g

6 0

500

1000

1500

2000

2500

3000

3500

4000

4500

1 -4 -9 -14

a 1

0

0

500

1000

1500

2000

2500

3000

2500

3000

3500

4000

-1

-2

-3

b 1

0

0

500

1000

1500

2000

3500

4000

-1

-2

c Fig. 1. FTIR results of synthesized GOs by a) H2SO4 b) 125 mL H2SO4 + 14 mL HNO3 c) 125 mL H2SO4 + 14 mL H3PO4.

S. Şahin et al. / Journal of Molecular Liquids 285 (2019) 213–222 Table 2 Functional groups of graphites oxidized with several acids.

215

Table 3 Functional groups of GO synthesized with H2SO4 + NaNO3 mixture.

125 mL H2SO4

125 mL H2SO4 + 14 mL HNO3

125 mL H2SO4 + 14 mL H3PO4

125 mL H2SO4

125 mL H2SO4 + NaNO3

3381 cm−1 1624 cm−1 1409 cm−1 1317 cm−1 1096 cm−1 771 cm−1

3751 cm−1 3366 cm−1 2371 cm−1 1579 cm−1 1320 cm−1 1118 cm−1 996 cm−1 769 cm−1 707 cm−1

3745 cm−1 3572 cm−1 2762 cm−1 1617 cm−1 1371 cm−1 1042 cm−1 731 cm−1

3381 cm−1 1624 cm−1 1409 cm−1 1175 cm−1 1096 cm−1 771 cm−1 – – –

3411 cm−1 2794 cm−1 1719 cm−1 1577 cm−1 1409 cm−1 1223 cm−1 1036 cm−1 866 cm−1 678 cm−1

potassium permanganate was slowly added to the mixture, noting that the temperature does not exceed 5 °C. After the addition of potassium permanganate, the temperature was adjusted to 35 °C and stirred for 1 h. After 60 min, 170 mL of pure water was slowly added for dilutions reasons, taking care not to exceed 98 °C. Finally, the reaction was terminated by addition of 20 mL of hydrogen peroxide after the mixture was stirred at 98 °C for 30 min. The color of the solution turns to light brown with the addition of hydrogen peroxide. GO nanosheets precipitated by centrifugation were dried at 60 °C for 1 day under vacuum.

kinetics were determined, the effects of pH (3, 5, 6, 7, 9 and 11) and temperature (298, 308, 318, 328 and 338 K) on the adsorption process were investigated, respectively. Recovery (%) and adsorption capacity (qe) were calculated by the following equations:

2.3. GO characterizations

qe ¼

Since hydrogen bonding interactions are expected as the hydroxyl groups in the molecular structure of the biophenols promote the hydrogen bonding between the halide anion and phenol, it is important to determine the hydrogen bonds. Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer Inc., Wellesley, MA) was used to clarify this. X-ray diffraction (XRD) models were obtained with the Rigaku D/ max 2200 diffractometer (Japan) using Cu Ka irradiation (1.5406 A °) for structural characterization.

Recoveryð%Þ ¼

ðC0 −Ce Þ x100 C0

ð1Þ

ðC0 −Ce ÞVi W

ð2Þ

qe: Adsorption capacity at equilibrium (mg-HT/g-GO) C0 and Ce: Initial and equilibrium concentrations of HT(mg/mL) Vi: Volume of HT solution (mL) W: Mass of GO (g) 3. Results and discussions 3.1. FTIR analysis results of GO

2.4. Determination of hydroxytyrosol Analyses of hydroxytyrosol were carried out through HPLC equipment. Analyzing conditions are given in Table 1. 2.5. Adsorption studies First, solutions of hydroxytyrosol in water at certain concentrations (5, 10, 25, 50, 75 and 100 mg/L) were prepared. In the batch adsorption process, GO (0.01 mg) and 10 mL hydroxytyrosol solution were mixed in 50 mL flasks until the adsorption equilibrium time in the shaker (150 rpm). Samples were taken from the solution at certain time intervals for determination of hydroxytyrosol level. After the adsorption

Particularly carboxyl groups (-COOH) should be formed in the oxidation to prevent the re-agglomeration of the graphene layers during the reduction. The method with the highest number of carboxyl groups was investigated by detection of functional groups formed on the surface of graphene layers as a result of the oxidation process via FT-IR analysis. Thus, optimum conditions were determined depending on the findings of different acid types and their amounts, different amounts of oxidizing agents, different waiting time and temperature values. FTIR tests of the GO products were performed in order to examine the change of functional groups on the surface depending on the acids (Fig. 1). Table 2 shows the peaks and the wavenumber values of the FTIR curves (Fig. 1) taken for different acids.

10 5

0

500

1000

1500

2000

2500

3000

3500

0 -5 -10 -15 -20 -25 -30 -35 Fig. 2. FTIR results of synthesized GOs by H2SO4 + NaNO3 mixture.

4000

4500

S. Şahin et al. / Journal of Molecular Liquids 285 (2019) 213–222

216

1 0

500

1000

1500

2000

2500

3000

3500

4000

0

-1

-2 Fig. 3. FTIR spectra of GO.

Table 2 shows the peaks of each of the common functional groups for each graphene oxide. Peaks over 3000–3700 cm−1 demonstrate the water molecules in GO structure. The peak seen at approximately 1340 cm−1 indicates the C\\O bond. The 1040 cm−1 peak is caused by C-OH vibration. Carbonyl (C = C stretching) on the edges of the graphene oxide can be attributable to the pean at around 1620 cm−1. The other peaks observed in the graphene oxide synthesized by the nitrite acid mixture are due to the nitrogen bond. The presence of carboxyl group was not observed for any of these three different mixtures. In addition, nitric acid and phosphoric acid additions lead to unwanted elements in the graphene oxide. Therefore, all other parameters were kept constant and graphene oxide was synthesized by using a sufficient amount of sodium nitrate and sulfuric acid mixture. FT-IR analysis of the synthesized graphene oxide is given in Fig. 2. As seen in Table 3, carboxyl groups at 1719 cm−1 were formed by the addition of sodium nitrate. Moreover, there are several functional groups due to acid and sodium interaction. Therefore, the washing procedure is hold more effectively in order to remove the concerned groups which are accepted as an excess. As given in Fig. 3, the strong peak seen at 3400 cm−1 is ascribed to O\\H vibration in GO structure. The peak seen at 1715 cm−1 originates from carboxylic acid absorption and/or from C = O groups at the edges of the graphene. The peaks at 1624 cm−1 and 1416 cm−1 might come from the asymmetric vibration of carbonyl and CH3, respectively. Finally, the peaks at 1175 cm−1 and 1032 cm−1 support the CH3 symmetric vibration and C\\O structure, respectively.

And at which temperatures functional groups formed on the surface of graphene layers during oxidation.

3.2. XRD analysis results of GO X-ray diffraction curves (in the 0–90° range under 2° min−1 scanning speed) of the graphite, graphene oxide synthesized by the Hummer method, and the graphene particles reduced by thermal treatment are given in Fig. 4. The characteristic (002) diffraction peak for the graphite is seen in 2θ_26.5°. As a result of the calculations made on this (002) plane, the distance between the layers was found to be 3.38 Å for natural graphite. The peak of graphite (002) along with the oxidation shifted from 26.5° to approximately 12°. This change in angle values indicates that the distance between the layers increases, so the symmetry in the crystal structure is broken. The inter-layer distance from the plane (002) for GO was calculated similar to the graphite and found to be 8.27 Å. The reason for the relevant expansion of the distance between the layers is the hydroxyl, epoxy, carboxyl groups, and water molecules, which are formed during the oxidation process. Graphene is aggravated with the effect of weak Van der Waals forces after the reduction, leading to the formation of the hexagonal crystal structure. Therefore, a large peak formation in the range of 25 to 30° is observed for graphene. As a result of the recombination of graphene layers, the surface areas of the layers are reduced. So, the electrochemical properties of these graphene layers are negatively affected.

Fig. 4. X-ray diffraction curves of graphite, graphene oxide and graphene.

qe (mgg-1)

S. Şahin et al. / Journal of Molecular Liquids 285 (2019) 213–222

100 90 80 70 60 50 40 30 20 10 0

217

100 ppm 75 ppm 50 ppm 25 ppm 10 ppm 5 ppm

0

500

1000

1500

2000

Time (min)

Recovery (%)

a 100 90 80 70 60 50 40 30 20 10 0

100 ppm 75 ppm 50 ppm 25 ppm 10 ppm 5 ppm

0

500

1000

1500

2000

Time (min) b Fig. 5. Determination of equilibrium time in terms of a) adsorption capacity and b) adsorption yield.

3.3. Adsorption of hydroxytyrosol from aqueous solutions Process parameters such as the equilibrium time, initial concentration and pH were investigated for the recovery of HT from aqueous media. In order to determine the optimum values of the relevant parameters, solutions of different concentrations of HT in water were prepared. Therefore, data such as kinetics and isotherms of the relevant system were also evaluated.

3.3.1. Determination of equilibrium time and initial concentration 10 mL adsorption solution and 0.01 mg adsorbent (GO) were used in determining adsorption equilibration time. The amount of adsorbent per adsorbent (qe) and recovery (% yield) were analyzed in terms of HT for different time periods until the equilibrium was attained. Fig. 5a and b show the equilibrium time of the system in terms of adsorption capacity and recovery of HT. The equilibrium has been reached after 1260 min regarding both percentage of the adsorption and the loading value. Therefore, adsorption studies performed for 1440 min of adsorption time under 150 rpm shaking speed. The relevant values of the parameters were used for further kinetic and isotherm studies.

Fig. 5a and b also show the effect of different concentrations of HT on adsorption efficiency. Considering the adsorption capacity (Fig. 5a), the amount of HT adsorbed per gram GO is increased with the increase of the initial concentration. Hence, 100 ppm was selected as the initial HT concentration. 3.3.2. Determination of acidity of the media In order to determine the pH effect of the HT solution on the adsorption, solutions were prepared at different initial concentrations (Fig. 6). The pH of the solutions was adjusted to be between 3 and 11 via 0.1 N NaOH and 0.1 N H2SO4 solutions, respectively. Rise in the ambient pH value from 3 to 9 increased the adsorption loading value from 0.55 to 89.46 mg g−1. This finding shows how the acidity of the media is effective on the recovery of HT. The adsorption capacity of GO was 2.28 mg g−1 under the natural pH value of the olive mill wastewater (≈ pH:5). The increase in the adsorption efficiency by basic media might be explained by the fact that phenolic compounds are very difficult to dissolve in solutions with low pH values [28]. Therefore, the rest of the studies were carried out by adjusting the pH of the HT solution to 9. As known, the graphene oxide has a hexagonal structure. The structure also contains hydroxyl and carboxyl groups. The HT interact with

S. Şahin et al. / Journal of Molecular Liquids 285 (2019) 213–222

218

100 90 80

qe (mgg-1)

70

102.78 ppm 75.38 ppm

60

51.07 ppm

50

24.98 ppm 10.05 ppm

40

4.96 ppm

30 20 10 0 3

5

6

pH

7

9

11

Fig. 6. Effect of pH on the adsorption capacity.

the hydroxyl group of the graphene oxide by hydrogen bonding. In addtion, HT was bonded to oxygen-containing groups of GO by hydrogen bonding. Besides, HT was bonded to aromatic moieties of GO through π-π interactions. HT has pKa (9.45). In Fig. 6, the adsorption capacity values of HT on GO showed sharply decrease with increasing pH lower than pKa of hydroxytrosol, may be associated with depronated HT. While pH increasing to 9, adsorption capacity increases. This event implied the electrostatic interactions exist thus most HT were deprotonated [29,30]. Furtheremore, schematic representation of interactions between HT and GO in Fig. 7. 3.3.3. Kinetic studies Pseudo-first order, pseudo-second order, Elovich, and intra-particle diffusion kinetic models were applied to the experimental data. Pseudo-first order kinetic model is as follows [31]: ln ðqe −qt Þ ¼ lnqe −k1 t

ð3Þ

qt: Adsorption capacity at any time (mg-HT/g-GO) k1: Rate constant of pseudo-first order model (min−1). Pseudo-second order kinetic model is as follows [32]: t 1 t ¼ þ ð4Þ qt k2 q2e qe

Rate constant k2 : (g mg−1 min−1)’dir.

of

pseudo-second

order

model

Elovich kinetic model is as follows [33]: qt ¼

ln ae be 1 þ lnt be be

ae: Initial sorption rate (mg g−1 min−1), be: Parameter for Elovich kinetic model (g mg−1),

ð5Þ

Intra-particle diffusion kinetic model is as follows [34]:

qt ¼ kp t 0:5 þ C

ð6Þ

C: boundary layer's thickness (mgg−1) kp: Rate constant of intra-particle diffusion kinetic model (mg g−1 min-0.5) Table 4 demonstrates the kinetic parameters obtained with various kinetic models at several initial HT concentrations. The fittings of the kinetic models were evaluated depending on the correlation coefficient (R2) values obtained from kinetic models and the constants defined above (k1, k2, kp, C, be and ae). Considering the correlation coefficient values, it can be concluded that the experimental data are consistent with the kinetic model of pseudo-first order. Furthermore, this compatibility was observed to be increased with the increase in the initial concentration of HT. Similarly, the adequacy of the experimental findings with the intra-particle diffusion kinetic model enhanced by the rise in the initial concentration of HT. R2 values N0.90 indicate that the Elovich model can be easily applied to any concentration. Among the applied kinetic models, the highest correlation coefficient was obtained with pseudo-second order kinetic model, indicating. That the step that determines the adsorption rate is chemical sorption [35]. 3.3.4. Adsorption isotherms Adsorption isotherms are one of the important factors for the design of the adsorption process. Actually, the adsorption isotherm explains how the adsorbent and adsorbed material interact with each other. In summary, it is one of the main factors for determination of adsorbent capacity. Thus, the isotherms provide information on the maximum capacity of GO for the adsorption of HT from olive mill wastewaters in the adsorption process. In this study, Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (D-R), and RedlichPeterson (R-P) models were used. Table 5 shows the equations of the isotherm models.

S. Şahin et al. / Journal of Molecular Liquids 285 (2019) 213–222

219

OH

HO

OH

OH

HO

OH

OH

HO

OH Fig. 7. A schematic representation of interactions between HT and GO.

Table 4 Kinetic parameters obtained with various kinetic models at several initial HT concentrations. Kinetic model

Pseudo-first oder Pseudo-second oder Intra-particle diffusion Elovich

Kinetic parameter k1x103 qe qt R2 k2x103 qe R2 kp C R2 be ae R2

qm_Adsorption capacity in monolayer (mg g−1) [35] Kl = Constant for Langmuir (L mg−1) Kf = Constant for Freundlich (mg1–1/nL1/ng−1) 1/n = Indication of adsorption intensity [36,37]

Initial concentration (ppm) 5

10

25

50

75

100

3.3 2.42 3.48 0.7987 2.2 3.77 0.9863 0.0948 0.5374 0.7393 1.4267 0.1043 0.9137

3.7 5.22 7.34 0.7958 1.1 7.94 0.9892 0.2022 1.0659 0.7575 0.6798 0.2158 0.9037

2.6 19.55 21.66 0.9759 0.18 24.94 0.9835 0.6178 0.4699 0.9544 0.2429 0.3747 0.9167

3.3 33.23 45.24 0.8989 0.23 47.85 0.9981 1.1902 7.4212 0.8352 0.1199 1.5720 0.9543

1.8 50.02 66.14 0.8356 0.012 68.49 0.9887 1.6585 8.0251 0.9204 0.0899 1.904 0.9573

2.3 72.99 89.46 0.9339 0.0074 95.24 0.9906 2.3582 7.983 0.9503 0.0640 2.203 0.9406

BT ¼ RT

 b

ð7Þ

b = Constant conserning the sorption heat for Temkin (J mol−1) AT = Constant for Temkin (L g−1) R = Universal gas constant (8.314 J mol−1 K−1) T = Absolute temperature (K)

Ea ¼ 1

 ð−2βÞ1=2

ð8Þ

S. Şahin et al. / Journal of Molecular Liquids 285 (2019) 213–222

220 Table 5 Isotherm models and equations. Isotherm model

Equation

Langmuir

Ce Ce 1 ¼ þ qe qm Kl qm 1 logqe ¼ logK f þ logCe n qe = BT ln AT + BT ln Ce 2 qe = qme−βε AC e qe ¼ 1 þ BC ge

Freundlich Temkin Dubinin-Radushkevich (D-R) Redlich-Peterson (R-P)

Ea = Mean adsorption energy (kJ mol−1)

When the results are evaluated in general, we can say that all isotherms are sufficient for the applied system except for the Langmuir.

2 −2

β = Activation coefficient (mmol j ) ε = Constant for D-R (mol2Kj−2) [38] A = Constant for R-P (L g−1) B = Constant for R-P (L mg−1)g g = Constant for R-P [39] Table 6 summarizes the parameters of various isotherm equations for HT adsorption on GO. Isotherms give information about the interaction between adsorbent and adsorbate. Langmuir and Freundlich isotherms are the two most popular models in the adsorption systems. In the Langmuir isotherm, it is stated that adsorbed molecules are adsorbed on the monolayer surface. According to the Freundlich equation, the adsorption areas on the surface of an adsorbent are heterogeneous [40]. The adequacy of HT on GO into Langmuir and Freundlich isotherms was evaluated according to the correlation coefficient findings. As it is well known, the correlation coefficient to 1 is an indicator that the satisfaction increases. Freundlich model was observed to be more suitable for adsorption of HT from aqueous media (Table 6). As seen in Table 6, the relevant adsorption system conforms to the Temkin isotherm. This is an indication of the reduction of adsorption heat for the target component on the GO surface [41]. As the surface of the nanomaterial is covered with hydroxytyrosol, the adsorption temperature decreases. The Dubinin-Radushkevich isotherm informs whether the adsorption process is physical or chemical. If the Ea value calculated from the related equation is N8 kJ mol−1, the adsorption occurs chemically [38]. The energy released in our system was found to be 500 kJ mol−1. The experimental results were also found to be consistent with the Redlich-Peterson equation (Table 6). R-P isotherm combined the Freundlich and Langmuir isotherms in one equation. This isotherm results were similar to that of Freundlich isotherm in high concentrations. Fig. 8 also demonstrates the compatibility of isotherm models with experimental data, but for Langmuir isothem model.

3.3.5. Thermodynamic studies Thermodynamic parameters must be obtained to determine whether an adsorption process is endothermic, exothermic, or spontaneous. In the thermodynamics of adsorption, the enthalpy change (ΔHo) refers to the heat absorbed by a reaction under constant pressure. As is known, if the enthalpy change is positive, the adsorption process is endothermic. Otherwise, it indicates that the process is exothermic. Depending on the Langmuir constant (Kl), standard molar Gibbs free energy (ΔGo) is calculated according to the given equation below: ΔG0 ¼ −RT InK

ð9Þ

Under constant temperature conditions, the standard entropy change (ΔSo) is calculated as follows: ΔG0 ¼ ΔH0 −TΔS0

ð10Þ

The thermodynamic parameters of the concerned system are given in Table 7. If the entropy change is found to be less than zero, the adsorption process is thought to occur spontaneously. In this system, the Gibbs free energy change in the adsorption of HT on GO was found negative for all concentration and temperature values. Therefore, we can say that the adsorption is applicable and spontaneous [38]. A negative quantity in ΔH0 shows that the adsorption process is exothermic, while the negative value in ΔS0 is an indication that the process is an enthalpy- driven process as well as the interaction between HT and GO [42]. 140 120

Table 6 Comparative results of various isotherm parameters.

Langmuir

Freundlich

Temkin

D-R

R-P

Parameter

Value

qm Kl R2 Kf n R2 AT BT R2 ε qm E R2 A B g R2

−44.444 −0.064 0.4582 2.096 0.626 0.9365 0.553 42.844 0.9506 2 × 10−6 61.94 500 0.8562 7.39 202.99 0.54 0.9896

qe (mg/g)

Isotherm model

100 80 60 40 20 0 0

2

4

-20

6

8

10

12

14

Ce (ppm) Langmuir

Freundlich

Temkin

Dubinin-Radushkevich (D-R)

Redlich-Peterson (R-P)

Experimental Data

Fig. 8. Adsorption isotherms of hydroxytrozol on GO. Adsorption conditions: initial pH value: 9, adsorption time: 1260 min, and adsorption temperature: 25 °C.

S. Şahin et al. / Journal of Molecular Liquids 285 (2019) 213–222 Table 7 Thermodynamic parameters at different temperature values and initial concentrations. Initial concentrations (ppm)

5

10

25

50

75

100

Temperature (K)

ΔG0 (kJ mol−1)

298 308 318 328 338 298 308 318 328 338 298 308 318 328 338 298 308 318 328 338 298 308 318 328 338 298 308 318 328 338

−2384.3 −1474.9 −565.7 343.6 1252.9 −2657.3 −1794.1 −930.9 −67.7 795.5 −4617.6 −4139.6 −3661.6 −3183.6 −2705.6 −5396.2 −4745.9 −4095.5 −3445.1 −2794.7 −5022.4 −4483.8 −3945.2 −3406.6 −2867.9 −4991.3 −4218 −3444.7 −2671.4 −1898.1

ΔH0 (kJ mol−1)

ΔS0 (J kmol−1)

−29,481.40

−90.93

−28,380.67

−86.32

−18,861.97

−47.80

−24,778.21

−65.04

−21,072.66

−53.86

−28,035.64

−77.33

4. Conclusions When the results of characterization analyzes such as FTIR and XRD were evaluated, it was proved that the graphite powder successfully synthesized to graphene oxide by the Modified Hummer's method. Our synthesized nanomaterial has proven its effectiveness as an advanced adsorbent with 85% yield. The pH of the adsorption medium has been observed as a very important parameter affecting the recovery of HT amount. Increasing the pH from 3 to 9 increased the amount of adsorbed substance per GO from 0.55 to 89.46 mg. In the aqueous HT system, experimental data fit all the applied kinetic models. The highest correlation coefficient has been achieved with pseudo-second order kinetic model. All the isotherms except Langmuir have been found to be sufficient for the concerned adsorption system, remarking that the adsorption process expresses the heterogeneous surface. Acknowledgement The authors thank The Scientific & Technological Research Council of Turkey (TÜBİTAK) for financial support for this research project (Grant number: 117M848). References [1] K. Grohmann, J.A. Manthey, R.G. Cameron, B.S. Buslig, Purification of Citrus Peel Juice and Molasses, (n.d.). doi:https://doi.org/10.1021/jf9903049. [2] M. Scordino, A. Di Mauro, A. Passerini, E. Maccarone, Highly purified sugar concentrate from a residue of citrus pigments recovery process, LWT- Food Sci. Technol. 40 (2007) 713–721, https://doi.org/10.1016/j.lwt.2006.03.007. [3] D. Kammerer, J. Gajdos Kljusuric, R. Carle, A. Schieber, Recovery of anthocyanins from grape pomace extracts (Vitis vinifera L. cv. Cabernet Mitos) using a polymeric adsorber resin, Eur. Food Res. Technol. 220 (2005) 431–437, https://doi.org/10. 1007/s00217-004-1078-z. [4] D.R. Kammerer, R. Carle, R.A. Stanley, Z.S. Saleh, Pilot-scale resin adsorption as a means to recover and fractionate apple polyphenols, J. Agric. Food Chem. 58 (2010) 6787–6796, https://doi.org/10.1021/jf1000869.

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