An investigation on kinetic and thermodynamic parameters of methylene blue adsorption onto graphene-based nanocomposite

Chemical Physics 535 (2020) 110793

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An investigation on kinetic and thermodynamic parameters of methylene blue adsorption onto graphene-based nanocomposite

T

Hoang V. Tran , Ly T. Hoang, Chinh D. Huynh ⁎

Department of Inorganic Chemistry, School of Chemical Engineering, Hanoi University of Science and Technology (HUST), 1st Dai Co Viet Road, Hanoi, Viet Nam

ARTICLE INFO

ABSTRACT

Keywords: Fe3O4/chitosan/graphene nanocomposite Adsorption Methylene blue Adsorption kinetic Thermodynamic parameters

In this work, adsorption of methylene blue (MB) onto graphene-based nanocomposite adsorbent has been evaluated on the adsorbent kinetic and thermal parameters sides. Specially, graphene (Gr) was prepared from waste dry battery electrode using a simple electrochemical method and prepared graphene has been used for fabrication of a Fe3O4/chitosan/graphene (FCG) nanocomposite by a simple co-precipitation method. Experimental results indicated the adsorption of MB onto FCG follows the pseudo-second order kinetic models as described: t/ qt = 0.02162 + 0.02076.t with R2 = 0.99852. Thermodynamic parameters of the adsorption of MB onto FCG were evaluated in temperature range from 283 K to 323 K and extracted data have provided values of ΔG0, ΔH° of −9.950 ÷ 6.434 kJ mol−1 and 27.348 kJ mol−1, respectively. The negative values of ΔG0 indicate the adsorption of MB onto FCG phenomenon occurred spontaneously. The positive value of ΔH° suggests the endothermic in nature and small value of ΔH° also implies a physical adsorption. These obtained results are useful to control the adsorption of MB onto graphene-based nanocomposite adsorbents in term organic dyes removal from solution.

1. Introduction Adsorption method is considered as a cheap, safety and efficiency technique for treatment of organic compounds and heavy metal ions in contamination water. Tradition adsorbents such as clay, zeolite, silica or bio-adsorbents such as plant wastes and crop wastes including rice husk, rice husk ash, bagasse fly ash or synthetic adsorbents such as active carbon, mesoporous carbon material, polymers, waste rubber tires or filter membranes were dispersed into polluted wastewater for adsorption of polluted organic compounds and heavy metal ions, but they were difficult recovered for regeneration and reuse. Furthermore, low adsorption capacity of tradition adsorbents are limited their applications [1–16]. Graphene is a two dimensional single atomic layer material of sp2 hybridized carbon atoms arranged in a honeycomb lattice structure with outstanding properties such as high surface area, high electrical conductivity, good chemical stability, and strong mechanical strength are unique features owned by graphene [17–21]. Graphene has excellent chemical and physical properties, including high specific surface area (~2630 m2 g) [5], chemical stability, excellent electrical conductivity and excellent thermal stability, therefore, graphene has received increasing attention in the area of adsorption [8,17,18,22–24]. In the literature, graphene and its derivatives including graphene (GR), graphene oxide (GO) and reduced graphene oxide (rGO) have been used to

⁎

improve the adsorption capacity because these graphene-based materials can create a three-dimensional (3D) structure of adsorbent materials therefore the porosity was improved leading to improve the adsorption efficiency and adsorption capacity (qmax, mg g−1) values of adsorbents for heavy metal ions or organic dyes removal via an adsorption processes. For example, chitosan/Fe3O4 nanocomosite has been utilized as adsorbent to adsorb of methylene blue (MB) with qmax was 149.2 mg g−1 [25] or adsorb Cr(VI) ions with qmax was 55.6 mg g−1 [11]. Meanwhile, with a small amount of GO (< 5 wt%) consisted in chitosan/Fe3O4/GO nanocomposite, this adsorbent has adsorbed MB with qmax of 180.83 mg g−1 [26] or Cr(VI) ion with qmax reached to 200 mg g−1[27], respectively. These data clearly demonstrate the role of graphene and its derivatives on enhancing adsorption capacity not only for organic compounds such as methyl violet, congo red, methylene blue removal [23,26,28,29] but also for adsorption of heavy metal ions such as Cr(VI), Cu(II), Hg(II) or Ni(II) [3,5–9,27,30–32]. However, there are very few works focused on adsorption kinetic and thermodynamic parameters, which play important role in controlling an adsorption process and improving the adsorption efficiency as well. Therefore, in this work, we describe the work on the kinetic and the thermodynamic parameters of adsorption process to well understand the mechanism adsorption process as well as towards controlling the adsorption process of organic dyes onto graphene-based adsorbent

Corresponding author. E-mail address: [email protected] (H.V. Tran).

https://doi.org/10.1016/j.chemphys.2020.110793 Received 17 February 2020; Received in revised form 24 March 2020; Accepted 5 April 2020 Available online 06 April 2020 0301-0104/ © 2020 Elsevier B.V. All rights reserved.

Chemical Physics 535 (2020) 110793

H.V. Tran, et al.

with the use of methylene blue (MB)- a cationic dye as a typical organic dye for examination. In addition, we also report here a new method for preparation of graphene from waste dry battery electrodes using a simple electrochemical method.

nanocomposite with mass ratio of Fe3O4:chitosan:graphene to be 43.48:2.17:54.35. These three volumes will be pipetted into a fresh flask. This mixture was homogenized using an ultrasound bath for 30 min, and NaOH solution (2 M) was dropped slowly into above solution under continuously stirring until pH9 and black precipitates will be occurred. The suspension was kept at room temperature without stirring for 2 h and the precipitate was collected by using an external magnet bar, then it was washed several times in distilled water until pH7. The black precipitate was dried at 80 °C for 18 h in a vacuum oven to obtain Fe3O4/chitosan/graphene (FCG) nanocomposite.

2. Experimental 2.1. Chemical and reagents Concentrated sulfuric acid (H2SO4 98 wt%), ammonium iron (II) sulfate hexahydrate ((NH4)2Fe(SO4)2·6H2O, 99 wt%), ethanol (C2H5OH, 96 v/v.%) were purchased from Duc Giang Chemical Co., Ltd. (Vietnam). Potassium sulphate (K2SO4, 99 wt%), iron(III) chloride hexahydrate (FeCl3·6H2O, 99 wt%), sodium hydroxide (NaOH, 99 wt %), acetic acid (CH3COOH, 99 wt%) and methylene blue (C16H18N3SCl; 99 wt%) were purchased from Xilong (China). All chemicals are A.R grade therefore they are used without any purification. Graphite rod electrodes were collected from waste dry zinc–carbon batteries (ZCB). To use as electrodes for graphene production, these graphite rod electrodes were polished and cleaned by acetone and ethanol. Chitosan (CS) was supported from a VTM co., Ltd, (Vietnam). A 2.5 %wt. CS solution was prepared by dissolving of 7.08 g CS powder into 250 mL of 1% v/v. acid acetic solution. This mixture was stirred at 500 rpm for 24 h at room temperature to obtain a homogeneous CS solution.

2.5. Adsorption of methylene blue onto FCG nanocomposite 0.02 g of FCG powder was added into 20 mL of methylene blue (MB) solution. Mixture was slowly shaked for 45 min. Then, FCG adsorbent was separated by using an external magnet or centrifugation. Concentration of MB solution will be analyzed on UV–Vis photometer using specific peak at 665 nm via using a calibration curve. To study adsorption kinetic, MB concentration in residue solutions after various adsorption times have been determined. For thermodynamic parameters study, the adsorption process was carried out in a water batch with controlling temperature. 2.6. Methods and data treatments

2.2. Electrochemical preparation of graphene

Surface morphologies of synthesized graphene and Fe3O4/chitosan/ graphene (FCG) nanocomposite were characterized using Field Emission Scanning Electron Microscope (FE-SEM) using a FE-SEM JEOL JSM-7600F. The magnetization measurement for FCG nanocomposite were carried out using a vibrating sample magnetometer (VSM, Lakeshore 7404) at room temperature with an applied magnetic field of 10 kOe. X-ray Diffraction (XRD) patterns graphene and FCG samples were obtained at room temperature by D8 Advance, Bruker ASX, using CuKα radiation (λ = 0.15406 nm) in the range of 2θ = 10° − 70°, and a scanning rate of 0.02 s−1. UV–Vis spectra were measured using Agilent 8453 UV–Vis spectrophotometer system with the wavelength in a range of 200–1200 nm. Amount of MB uptakes onto FCG adsorbent’s surface at equilibrium state (qe, mg g−1) is calculated as follows Eq. (1) [7,25,26]:

The whole electrochemical exfoliation experiments were performed at room temperature with a typical experiment as following described: Two waste graphite electrodes were employed as electrode materials and as a source for graphene production; they were immersed into a 200 mL of 0.1 M K2SO4 solution. For electrochemical expansion one electrode acts as cathode and other as the anode. Both electrodes were separated by a rubber sheet with a distance around 5 cm. The electrolysis was carried out with the help of alternating current (AC) source to with direct current (DC) output potential of E = 6 V and the overall electrochemical process were carried out for two hours. After completion of electrolysis process, the suspension was filtered by a vacuum filter. The remaining solid material was purified by successively washing with 50 mL of water, 50 mL of 30 v/v.% HCl, and 50 mL of ethanol. The solid obtained was vacuum-dried overnight at room temperature and weighed. Graphene was re-dispersed into distilled water by using an ultrasound bath for 1 h before further use.

qe =

V × (C0 ma

Ce )

(1)

The adsorption rate of MB on FCG adsorbent (v, mg L estimated as following Eq. (2) [25,26]:

2.3. Preparation of Fe3O4 nanoparticles

v=

Fe3O4 nanoparticles were prepared by co-precipitation method from a mixture salts containing of Fe2+ and Fe3+ ions with appropriate molar ratio of Fe2+: Fe3+ of 2: 1. For that, firstly, 0.866 g FeSO4·4H2O and 2.089 g FeCl3·6H2O salts were dissolved into 50 mL distilled water and obtained solution was heated to 80 °C. Secondly, a NaOH solution (2 M) was slowly dropped into above solution under continuously stirring until pH9 and Fe3O4 nanoparticles as black precipitates will be appeared. Reaction solution was kept at 80 °C for 2 h and then precipitate was separated by using an external magnet bar. The obtained solid was washed several times by distilled water until pH = 7. Finally, Fe3O4 was re-dispersed into water and kept at 4 0C to use.

dC = dt

Ct t

Co = 0

Ct

Co t

−1 −1

s

) can be

(2)

−1

where, C0, Ct, Ce (mg L ) are initial, time concentration and equilibrium concentration of MB in aqueous solution, respectively; ma is mass of used adsorbent dosage (g L−1); qe (mg g−1) is equilibrium adsorption amount. By determining the [MB] vs. adsorption time (t) then drawing the plot of adsorption rate (v) vs. adsorption time (t), the order of adsorption kinetic will be calculated. From Eq. (1), the adsorption equilibrium constant (Kp) has been identified as described Eq. (3) [4,7,25,26]:

Kp =

qe

(3)

Ce

The correlative of Kp with the standard free Gibbs energy change (ΔG0) is followed the Van’t Hoff equation, which is described by following Eq. (4) [4,7,25,26]:

2.4. Preparation of Fe3O4/chitosan/graphene (FCG) nanocomposite Fe3O4/chitosan/graphene (FCG) nanocomposite was prepared by co-precipitation method from three separated solutions including: (i) a suspension of Fe3O4 nanoparticles which was synthesized as describe above, (ii) a chitosan solution (2.5 %wt.) and (iii) graphene solution. Depending on concentration of above three solutions, the volumes of each solutions (i), (ii) and (iii) were calculated for preparation of FCG

dlnKp =

HT0 × dT RT 2

(4)

The second law of thermodynamics has been indicated by following Eq. (5) [4,7,25,26]: 2

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H.V. Tran, et al.

Fig. 1. (a) XRD of (i) graphene and (ii) Fe3O4/chitosan/graphene (FCG) nanocomposite; (b) VSM plot of FCG (inset: the magnetic separation of the FCG nanocomposite by using an external magnet); (c, d) FE-SEM of (c) graphene and (d) FCG, respectively.

G0 = H 0

T × S0 =

3.4 Å of graphite as thin layer (JCPDS 89-8487) [34,35]. Meanwhile, XRD spectra of FCG (Fig. 1a, curve ii) shows six characteristic peaks of Fe3O4 at Bragg angles of 30.37°, 35.66°, 43.10°, 53.55°, 57.16° and 62.84° corresponding to the reflections of (2 0 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) facets of Fe3O4 crystal particles with inverse spinel cubic structure sample (JCPDS file, PDF No. 65-3107) [8,18,27]. Magnetization hysteresis loop of FCG nanocomposite (Fig. 1b) shows no remanence and coercivity, suggesting the FCG nanocomposite is a superparamagnetic material. Fig. 1b also indicates the saturation magnetization values (Ms) of FCG was 41.01 emu g−1. Noticeably, the FGC sample exhibited typical superparamagnetic behavior and high saturation magnetization value (Ms), therefore this FGC adsorbent can be easily removed from solutions and recycled by applying an external magnetic field (inset in Fig. 1b). FESEM of graphene (Fig. 1c) shows exfoliated layered graphene sheets, which exhibited the large, flexible, freely oriented and loosely stacked graphite sheets disassembled from graphite. Whereas, FESEM of FGC nanocomposite (Fig. 1d) clearly shows abundant of Fe3O4 nanoparticles have been immobilized on graphene sheets. Fig. 1d also shows porous structure of FGC nanocomposite.

(5)

R × T × lnKp

Using Eq. (3) to Eq. (5), the adsorption equilibrium constant (Kp) is depended to adsorption temperature (T) and values of thermodynamic parameters (ΔH0, ΔS0) as follows [33]:

dlnKp =

HT0

RT 2

lnKp = lnKp =

× dT

H0 1 ×T R q S0 ln Ce = R e 0

−1

H0 R×T 0

(6) −1

0

−1

−1

where ΔG (J mol ), ΔH (J mol ) and ΔS (J mol K ) are the standard free Gibbs energy change, the standard enthalpy change and the standard entropy change of the adsorption process, respectively. R is gas constant (R = 8.314 J mol−1 K−1), Ce is the equilibrium concentration of MB (mg L−1), qe is the equilibrium MB concentration on FCG adsorbent (mg g−1) and T is working temperature (K). q From a linear fitting of ln Ce vs. 1 (Eq. (6)), the ΔH0 and ΔS0 values T e will be calculated, respectively. The positive value of ΔH0 implies an endothermic nature of adsorption while the negative indicates an exothermic adsorption process. Endothermic adsorption processes are required high temperature to obtain higher adsorption capacity (qmax) than that low temperature, meanwhile exothermic adsorption processes are required a low temperature for operating the adsorption [7,15,25–31].

3.2. Equilibrium and kinetic of MB adsorption onto FCG To demonstrate the adsorption capacity of each component in FCG nanocomposite for MB removal, four adsorption experiments were conducted by MB adsorption onto graphene, Fe3O4 nanoparticles, chitosan, and FCG adsorbents. Fig. 2a shows low MB removal efficiency when Fe3O4 nanoparticles and chitosan were used as adsorbents. In contrast, very high removal efficiencies (~100%) were found when using of graphene or FCG as adsorbents. Inset in Fig. 2a shows directly

3. Results and discussions 3.1. Materials characterizations Fig. 1a (curve i) shows XRD pattern of synthesized graphene with a reflection (0 0 2) peak at 2θ = 26.5° corresponding to d spacing of 3

Chemical Physics 535 (2020) 110793

H.V. Tran, et al.

Fig. 2. (A) Amount of MB up-taken onto FCG adsorbent at equilibrium state (qe, mg.g-1) during adsorption process (inset: residue equilibrium MB concentration of after different contacting time); (b) Methylene blue (MB) removal efficiency by various adsorbents: (i) graphene; (ii) Fe3O4 nanoparticles; (iii) chitosan and (iv) FCG nanocomposite (inset: color of MB solution as (C0) original and after adsorbed by corresponding adsorbents); (c, d) Adsorption kinetic of MB onto FCG is described following: (c) pseudo-first order or (d) pseudo-second order kinetic models, respectively. Table 1 Adsorption kinetic parameters of MB onto FCG nanocomposite adsorbent. Experimental equilibrium adsorption capacity, qe(Exp) (mg g−1)

47.35

Pseudo-first-order

Pseudo-second-order

k1 (min−1)

qe(Cal) (mg g−1)

R2

k2 (min−1)

qe(Cal) (mg g−1)

R2

0.13319

36.36

0.9798

0.0199

48.17

0.9985

−1

qe(Exp) = experimental equilibrium adsorption capacity (mg g ). qe(Cal) = calculated equilibrium adsorption capacity (mg g−1).

color of MB solution before (bottle was labeled C0) and after adsorption by graphene, Fe3O4 nanoparticles, chitosan and FCG (with the bottles were labeled (i), (ii), (iii) and (iv), respectively) in high agreement with calculated data. These obtained results can be attributed to highly attractive to MB of graphene[23,26,28,29]. Equilibrium concentrations of MB (Ce, mg L−1) after different adsorption time (min) (Fig. 2b, inset) shows that adsorption speed increased rapidly in the several first minutes, which can be attributed to highly free surface area of FCG adsorbent in beginning time. After a period of time, this free area was reduced because of adsorbed MB molecules was filled onto FCG surface. Using Eq. (1), experimental equilibrium adsorption capacity (qe(Exp), mg g−1) were calculated and results are shown in Fig. 2b. It is clearly indicated that the adsorption process is reached equilibrium in 40 min and the experimental adsorption capacity, qe(Exp) = 47.35 mg g−1 can be found in Fig. 2b. The best approximation between qe(Exp) (mg g−1) and the calculation adsorption capacity (qe(Cal)), mg g−1) and high the correlation factor R2

calculated from each kinetic model are important factors to find out a suitable kinetic model to describe the adsorption of MB onto FCG nanocomposite. In order to evaluate the adsorption mechanism and the kinetic adsorption of MB adsorption onto the FCG nanocomposite, the pseudo-first-order and pseudo-second-order kinetic models were used.

Pseudo

first

Pseudo

second

ordermodel: ln(q e ordermodel:

q t ) = lnq e

(7)

k1t

t 1 t = + qt qe k2 qe2

(8) −1

where qe and qt are the amounts of MB adsorbed (mg g ) at equilibrium and at any time t (min), t is the adsorption time (min), k1(min−1) and k2 (g mg−1 min−1) are the pseudo-first and pseudo-second orders rate constant, respectively. From the experimental results (Fig. 2a), a curve describes relative between ln(q e q t ) vs. adsorption time t (min) and t vs. adsorption time t (min) are plotted as shown in Fig. 2c and qt

Fig. 2d, respectively. By calculating, the results are as follows for the 4

Chemical Physics 535 (2020) 110793

H.V. Tran, et al.

chitosan composite[25], 2D nanolamellar Fe3O4[36], activated carbon [37], modified pumice stone[38] or bone charcoal[39] (Table 2).

Table 2 Adsorption kinetics of methylene blue onto Fe3O4/chitosan/graphene nanocomposite (FCG) from aqueous solution. Adsorbents

[MB], (mg L−1)

k2 (g mg−1 min−1)

qe(Exp)(*), (mg g−1)

Refs.

Magnetic chitosan composite Magnetic chitosan and graphene oxide (MCGO) Magnetic graphene oxide Fe3O4/chitosan/graphene nanocomposite (FCG) Vegetal fiber activated carbon (AC1: ZnCl2 + H3PO4/24 h) Vegetal fiber activated carbon (AC2: HNO3/24 h) 2D nanolamellar Fe3O4

32 50 100 50 50

0.0016 0.0309 0.0124 0.0058 0.0199

33.2 132.6 179.3 588.2 47.35

[25] [26]

40 60 40 60 20 30 40 50 60 300 50

0.059 0.099 0.069 0.091 0.028 0.012 0.017 0.014 0.012 0.063 0.000287

11.159 18.182 9.398 17.123 19.330 28.710 6.756 8.474 11.494 5.000 41.221

Modified Pumice Stone Bone charcoal Biomass Obtained fromthe Algae D. Antarctica

3.3. Thermodynamic parameters of the MB adsorption onto FCG nanocomposite It is well known the adsorption processes are strongly depended onto working temperature and these processes are controlled by thermodynamic parameters including the standard enthalpy change (ΔH0, J mol−1), the standard entropy change (ΔS0, J mol−1 K−1) and the standard free Gibbs energy change (ΔG0, J mol−1) of the adsorption processes. The depending of the qe (mg g−1) onto working temperature is described in Fig. 3a. It is found that qe increases when increasing of the adsorption processes temperature. From values of qe, Ce we have calculated Kp (Eq.3) values and a relative curve between equilibrium constants (Kp) vs. working temperature according to Van't Hoff law was plotted in Fig. 3b and the values of the ΔG0, ΔH0 and ΔS0 are given in in Table 3. As shows in Table 3, positive values of ΔH° suggested that the adsorption of MB onto FCG is endothermic in nature. This result implies values of qe, Kp will be high when the adsorption process be operated at high temperature. In addition, the calculated value of ΔH0 was 27.348 kJ mol−1, which implies a physical adsorption because of in a range of 80–200 kJ mol−1 implies a chemisorption [41]. Meanwhile, the negative values of ΔG0 at all temperatures suggested that the adsorption phenomenon occurred spontaneously [25–28]. The increased negative values of ΔG0 along with the increased temperatures (table 3) confirmed that the accumulation phenomenon of MB onto FCG was more favorable at a higher temperature with the results were in accordance with above experimental data in Fig. 3a. Table 3 also shown the positive values of ΔS0, which suggested that the randomness increased at the solid–liquid interface during the adsorption process.

[29] This work [37]

[36] [38] [39] [40]

(*) qe(Exp) = experimental equilibrium adsorption capacity (mg g−1).

pseudo-first-order model:

ln(q e

q t ) = 3.59347

(9)

0.13319t; (R2 = 0.9798)

The experimental value of equilibrium adsorption capacity qe = 47.35 mg g−1 does not agree with the calculated ones (qe −1 ), in addition, the correlation coefficient value (Cal) = 36.36 mg g (R2 = 0.9798) is low for the adsorption data (Table 1). These results imply that the first-order rate equation is not suitable to describe this adsorption process. In contrast, the results are as following for the pseudo-second-order model: (Exp)

t = 0.02162 + 0.02076t; (R2 = 0.99852) qt

4. Conclusions Through the experimental process for preparing of graphene–based adsorbent then researching adsorption kinetic and thermodynamic parameters in adsorbing of methylene blue (MB) onto Fe3O4/chitosan/ graphene (FCG) nanocomposite adsorbent, the work has obtained the following results:

(10) −1

This calculated value of qe (qe(Cal) = 48.17 mg g ) is high approximation with the experimental ones (qe(Exp) = 47.35 mg g−1) and a good linearity (R2 = 0.9985), imply the adsorption kinetic follows the pseudo-second-order model. Presence results are highly agreement with previous reported in term adsorption kinetic order, however, higher obtained rate coefficient (k2, g mg−1 min−1) and equilibrium adsorption capacity qe(Cal) (mg g−1) in our work indicated high adsorption affinity of FCG to MB molecules than that other adsorbents such as magnetic

1) Graphene can be simply produced from waste dry battery electrodes using an electrochemical method; 2) Produced graphene has been used for preparation of Fe3O4/chitosan/graphene (FCG) nanocomposite and it has been used as adsorbent for efficiency removal of MB from solution;

Fig. 3. (a) Effect of temperature on the equilibrium adsorption capacity (qe, mg g-1) of FCG. Working conditions: V = 50 mL; [MB] = 50 mg.L−1, adsorbent dosage (ma) = 0.02 g and contact time: 40 min; (b) Plot of the adsorption equilibrium constant (Kp) vs. T-1. 5

Chemical Physics 535 (2020) 110793

H.V. Tran, et al.

Table 3 Thermodynamic parameters of MB adsorption onto FCG nanocomposite. Working conditions: V = 50 mL; [MB] = 50 mg L−1, adsorbent dosage (ma) = 0.02 g, contact time: 40 min and working temperatures from 10 to 50 °C. T (K)

Ce (mg/l)

qe (mg/g)

KD (l/g)

ΔGo (kJ mol−1)

ΔHo (kJ mol−1)

ΔSo (J mol−1 K−1)

283 303 313 323

3.048 1.282 1.069 0.863

46.95 48.72 48.93 49.14

15.404 38.003 45.772 56.935

−6.434 −9.164 −9.950 −10.854

27.348

119.170

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