Adsorption Characteristics of Chlorophenols from Aqueous Solution

Article pubs.acs.org/jced

Adsorption Characteristics of Chlorophenols from Aqueous Solution onto Graphene Hong-Tao Fan,* Chong-Yang Zhao, Shuang Liu, and Hua Shen College of Applied Chemistry, Shenyang University of Chemical Technology, Shenyang, 100142, Liaoning China ABSTRACT: The adsorption of chlorophenols (CPs) on pristine graphene was investigated. The adsorption capacities of graphene were 88.1 mg g−1 for 2-chlorophenol (2-CP), 114.2 mg g−1 for 4-chlorophenol (4-CP), 155.3 mg g−1 for 2,4dichlorophenol (DCP), and 175.8 mg g−1 for 2,4,6trichlorophenol (TCP), respectively. The equilibrium was attained during 20 min. There was no significant effect on the adsorption amounts in the pH range of pH 1.01−8.09 for 2-CP, 1.08−8.01 for 4-CP, 1.04−7.03 for DCP, and 2.86−5.63 for TCP. The Langmuir and pseudo-second-order models supplied good fitting of the experimental data. Thermodynamic constants demonstrated that the process was endothermic and spontaneous.

1. INTRODUCTION Chlorophenols (CPs) have been widely used in industry, resulting in these chemicals having widespread distribution in the aquatic environment.1,2 CPs make up one of the watersoluble hazardous groups of toxic organic chemicals for living organisms and humans even at low concentrations, and are among the priority pollutants of major environmental concern.3,4 CPs contamination is a serious aqueous environmental problem. Therefore, the disposition of CPs from aqueous solution is a mission for the environmental researchers. To protect the aquatic ecosystems, various methods including electrochemical degradation,5 microbial degradation,6 oxidative degradation,7 ion exchange,8 reverse osmosis,9 and photocatalytic degradation,10 have been developed to treat the CPs-contaminated aqueous water in purification processes. As an alternative, adsorption is usually considered the promising choice due to its simplicity, efficiency, and versatility for the treatment of CPs at lower concentrations in polluted water.11 Several sorbents, including natural products,12,13 agricultural by-products,14,15 industrial by-products,16,17 polymers,18,19 fibers,20 fungus,21 mesoporous silica,22 carbon-based materials (such as granular activated carbon,23 carbon black,24 low-cost carbon,25 carbon nanotubes,26 and graphene27), have been utilized for removing the CPs from the aqueous solution. Graphene, a novel two-dimensional carbonaceous nanomaterial composed of sp2-hybridized graphitic carbon, attracts tremendous attention as a superior sorbents for the removal of pollutants.28,29 Recently, Yan et al.27 reported that the magnetic reduced graphene oxide composite with good properties had been used to remove CPs from aqueous solutions. Arriagada et al.30 applied the density functional theory to investigate the interaction between 4-CP and graphene, and test the possible application for the removal of 4-CP. Calculation results show that 4-CP binds with graphene through van der Waals interactions and π−π dispersion forces. Khashij et al.31 utilized graphene oxide to remove 4-CP in a contaminated water sample. However, it can be noticed that very few and isolated © 2017 American Chemical Society

fact are available on the adsorption characteristics, isotherm, kinetics, and thermodynamics of 2-CP, DCP, and TCP onto the pristine graphene. Therefore, the retention behavior of 2-CP, 4-CP, DCP, and TCP on the pristine graphene and the influences of exposure time, pH, initial concentrations, and temperature on the adsorption of CPs were tested. The parameters to the isothermal, kinetics, and thermodynamic properties of the adsorption process of CPs onto the surface of the pristine graphene were assessed.

2. EXPERIMENTAL SECTION 2.1. Reagents. Pristine graphene was purchased from Alfa Aesar. All the other reagents were analytical pure, obtained from Aladdin industrial corporation, and used as received. The desirable concentrations of CPs solutions were diluted from the individual solution obtained by the dissolution of CPs compounds (1 g) in 1000 mL of deionized water. 2.2. Batch Experiments. A 0.1 g sample of graphene was added into 25 mL of aqueous solution of CPs with the various concentrations (100−1000 mg L−1) at the various temperatures (298.15, 308.15, and 318.15 K), the various pH (2−10), and the various exposure time (5−60 min). Solution pH was monitored by adding 0.1 mol L−1 HCl and 0.1 mol L−1 NaOH solution as required. After equilibrium, the residual concentrations of CPs in solution were determined. The amount of uptake is calculated using qt =

(Co − Cu)V W

(1)

where qt is the uptake amount of CPs (mg g−1) at time t, C0 and Cu are the original and ultimate concentrations of CPs (mg Received: November 1, 2016 Accepted: January 26, 2017 Published: February 8, 2017 1099

DOI: 10.1021/acs.jced.6b00918 J. Chem. Eng. Data 2017, 62, 1099−1105

Journal of Chemical & Engineering Data

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L−1), respectively. V is solution volume (L). W is the mass of graphene (g). 2.3. Analytical Method of CPs. The concentrations of CPs were measured by the colorimetric method.32 The assay comprised 4-aminoantipyrine solution (2.0 mM, 1.0 mL), potassium ferricyanide solution (1.0 mL, 6 mM), and CPs solution (1.0 mL). Absorbance was obtained at 505 nm against a reagent blank (8 mL of water, 1 mL of 2 mmol L−1 ferricyanide solution, and 1 mL of 6 mmol L−1 4-aminoantipyrine solution) at room temperature. According to the Lambert−Beer law, the linear calibration curves with high correlation coefficient (r2 > 0.99) in the range of 0.50−5.00 mg L−1 for 2-CP and 4-CP, and 0.50−10.00 mg L−1 for DCP and TCP were obtained, respectively. The detection limits of 2-CP, 4-CP, DCP, and TCP were 0.0020, 0.0035, 0.0020, and 0.0030 mg L−1, respectively.

Figure 2. Effect of contact time on the adsorption equilibrium of CPs onto graphene: concentration of CPs = 800 mg L−1, pH = 5, temperature = 25 °C.

3. RESULTS AND DISCUSSION 3.1. Influence of pH. Solution pH which affects the solution chemistry of the phenolic compounds, the activity of the adsorbents, and the removal efficiency of the CPs, is a valuable constant. The pH dependence of CPs adsorption onto the graphene in the pH range of 2−10 is shown in Figure 1.

Figure 3. Adsorption capacities of graphene for CPs: pH = 5, time = 30 min, temperature = 25 °C. Figure 1. Effect of pH on the adsorption amounts of graphene: concentration of CPs = 800 mg L−1, time = 30 min, temperature = 25 °C.

of CPs from 100 to 700 mg L−1. This is due to an augment of the driving force of the concentration gradient with an increase of the concentrations of CPs, and showed little variation in the range of the original concentration of CPs from 800 to 1000 mg L−1 which is probably due to saturation of the sorbent surface with the enhancement of CPs concentration. The experimental values of saturated adsorption capacities of graphene for 2-CP, 4-CP, DCP, and TCP were 88.1, 114.2, 155.3, and 175.8 mg g−1, respectively. Comparison of the capacities for CPs onto the different sorbents from the literature with the values obtained of graphene for CPs is indicated in Table 1.12,13,15−17,20−25,27,34−37 As seen here, the capacities of pristine graphene for CPs were higher than most of the sorbents as indicated in Table 1, except for reduced graphene oxide and graphene oxide prepared from the ultrasonic method, pleurotus sajor caju and amino-modified ordered mesoporous silica. 3.4. Adsorption Kinetics. Since the adsorption amount of a sorbent depends upon the contact time, the study of the kinetics of adsorption is a vital factor. The adsorption is governed by the rate of arrival of the CPs at the surface and the proportion of the incident CPs which get the adsorption. Four of the most widely used kinetic models, that is, Lagergren’s pseudo-first-order, Ho and McKay’s pseudo-second-order,

The high stable adsorption amounts of graphene for 2-CP, 4CP, DCP, and TCP were found to occur in the pH range of 1.01−8.09, 1.08−8.01, 1.04−7.03, and 2.86−5.63, respectively, through π−π interactions and cation−π bonding.33 Whereas, the adsorption amounts decreased at higher pH because the π−π interactions and cation−π bondings between the CPs and graphene were suppressed with an increase of pH. Therefore, pH 5 was used in further trials. 3.2. Influence of Exposure Time. The dependence between exposure time and the adsorption amounts of CPs on graphene is shown in Figure 2. As seen here, the adsorption amounts of CPs increase during the first 20 min, which can be ascribed to the quantity of availabile binding sites on graphene, as well as the highest driving force for the mass transfer resulting in the rapid uptake of CPs at the beginning. Equilibrium seems to be achieved within 20−60 min because of the saturation of the available binding sites on graphene. 3.3. Influence of the Initial Concentrations. It can be seen from Figure 3 that the adsorption amounts of CPs increased with the enhancement of the original concentrations 1100



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Elovich, and the intraparticle diffusion (ID) were considered to explicate the data. The linear expression of Lagergren’s equation is38

Table 1. Comparison of the Adsorption Capacities of Various Sorbents for Chlorophenols adsorption capacities (mg g−1) sorbents bituminous shale chitosan rice-straw derived ash industrial wastes fly ash hollow fibers Pleurotus sajor caju amino-modified ordered mesoporous silica granular activated carbon carbon black coir pith carbon magnetic reduced graphene oxide carbon-coated monoliths activated carbon from milk vetch activated carbon from coconut coir pith graphene oxide from ultrasonic reduced graphene oxide from ultrasonic graphene oxide from Hummers reduced graphene oxide from Hummers pristine graphene

2-CP

4-CP

3.1

DCP

TCP

4.2 0.14

52.0 50.3 1.1 26.1 159.5 275.4

57.4

132.5 1.7

25.0 189.0

35.4 373.2 338.8

109.9 343.6 17.7 63.8

273.7

75.0

102.3

93.7 87.0

117.5

169.6

208.7

37

32.1

37

49.9

37 155.3

t /qt = 1/k 2qe 2 + t /qe −1

175.8

(3)

−1

where k2 (g mg min ) is the velocity constant of Ho and McKay’s equation. The linear plots of t/qt versus t were plotted (Figure 4b).The linear expression of the Elovich model is described as40 qt = (1/β) ln(αβ ) + (1/β ) ln t −1

−1

(4)

−1

where α (mg g min ) and β (g mg) are the constants of the Elovich equation. The linear plots of qt versus ln t were plotted (Figure 4c). The linear expression of the ID model is described as41

36 37

114.2

where k1 (min ) is the velocity constant of Lagergren’s equation, qe is the amount of adsorbate adsorbed at saturation, and qt (mg g−1) is the amount of adsorbate adsorbed at time t (min). The linear plots of log(qe − qt) versus t were plotted (Figure 4a). The linear expression of Ho and McKay’s equation is indicated as39

23 24 25 27

134.5

(2)

−1

12 13 15 16 17 20 21 22

34 35

19.0

88.1

log(qe − qt ) = log qe − k1t /2.303

ref

qt = k pit 0.5 + Cpi

(5)

where kpi is the velocity constant of ID in stage i (mg g−1 min−0.5), Cpi gives the thickness of the boundary layer.42 The linear plots of qt versus t0.5 were plotted (Figure 4d). The r2 values of Lagergren’s equation (r2 = 0.9021 for 4-CP, 2 r = 0.8881 for 2-CP, r2 = 0.8231 for DCP, and r2 = 0.8309 for TCP) and Elovich equation (r2 = 0.8149 for 4-CP, r2 = 0.8847

this study

Figure 4. Plots of pseudo-first-order (a), pseudo-second-order (b), Elovich (c), and intraparticle diffusion (d) models for the adsorption of CPs onto graphene. 1101



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Table 2. Calculated Kinetic Parameters for the Adsorption of CPs onto Graphene adsorbates

pseudo-first-order model

2-CP

k1 = 0.057 min−1 qeq = 18.7 mg g−1 r2 = 0.8881 k1 = 0.056 min−1 qeq = 25.3 mg g−1 r2 = 0.9021 k1 = 0.047 min−1 qeq = 34.4 mg g−1 r2 = 0.8231 k1 = 0.037 min−1 qeq = 36.3 mg g−1 r2 = 0.8309

4-CP

DCP

TCP

pseudo-second-order model k2 = 6.48 × 10−3 g mg−1 qeq = 90.1 mg g−1 r2 = 0.9998 k2 = 4.71 × 10−3 g mg−1 qeq = 116.3 mg g−1 r2 = 0.9997 k2 = 3.15 × 10−3 g mg−1 qeq = 158.7 mg g−1 r2 = 0.9996 k2 = 3.35 × 10−3 g mg−1 qeq = 175.4 mg g−1 r2 = 0.9999

min−1

min−1

min−1

min−1

Elovich model α = 3596.5 mg g−1 β = 0.113 g−1 mg r2 = 0.8847 α = 2195.4 mg g−1 β = 0.0789 g−1 mg r2 = 0.8149 α = 1652.5 mg g−1 β = 0.0542 g−1 mg r2 = 0.8460 α = 6694.6 mg g−1 β = 0.0571 g−1 mg r2 = 0.8562

min−1

min−1

min−1

min−1

intraparticle diffusion model kp1 = 9.23 mg g−1 min−1 r2 kp2 = 0.652 mg g−1 min−1 r2 = 0.9895 kp1 = 13.6 mg g−1 min−1 r2 kp2 = 1.19 mg g−1 min−1 r2 = 0.9885 kp1 = 19.7 mg g−1 min−1 r2 kp2 = 1.25 mg g−1 min−1 r2 = 0.9779 kp1 = 17.8 mg g−1 min−1 r2 kp2 = 1.88 mg g−1 min−1 r2 = 0.9726

= 0.9849

= 0.8542

= 0.9308

= 0.8915

Figure 5. Plots of Langmuir (a), Frendlich (b), and Dubinin−Radushkevich (c) isotherms for the adsorption of CPs onto graphene.

for 2-CP, r2 = 0.8460 for DCP, and r2 = 0.8562 for TCP) were less than that of Ho and McKay’s equation (r2 = 0.9997 for 4CP, r2 = 0.9998 for 2-CP, r2 = 0.9996 for DCP and r2 = 0.9999 for TCP) (Table 2). This implied that the Lagergren’s and Elovich kinetics equations did not fit to the data. It was clear from that the amounts of CPs on graphene (90.1 mg g−1 for 2CP, 116.3 mg g−1 for 4-CP, 158.7 mg g−1 for DCP, and 175.4 mg g−1 for TCP) from the Ho and McKay’s equation agreed well with the experimental values. The results demonstrated that the Ho and McKay’s equation was able to satisfactorily fit the adsorption data in the whole data range, indicating that the adsorption of CPs onto graphene was determined by chemical adsorption.43,44 The above-mentioned models could not distinguish the diffusion mechanism, and the ID model was applied to fit the data. There are two straight lines which do not go through the origin (Figure 4d). These also imply that not only does the ID

result in the rate-determining step but also other processes may govern the velocity synchronously.45,46 3.5. Adsorption Isotherms. The data were fitted by Langmuir, Freundlich, and Dubinin−Radushkevich (D−R) isotherms. The Langmuir isothermal linear equation is represented as47 Ce/qe = 1/(qmax b) + Ce/qmax

(6)

The Freundlich isothermal linear expression is expressed by48 log qe = log kF + (1/n)log Ce

(7)

The D−R isothermal linear expression is descried as ln qe = ln qs − kadε 2

49

(8) −1

where qe is the adsorption amount (mg g ), Ce is the equilibrium concentration of adsorbates (mg L−1), b (mL mg−1) is the constant, and qmax is the saturation capacity (mg 1102



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g−1), KF and n are the Freundlich constants, kad is the D−R isotherm constant (mol2 kJ−2); qs is the maximum capacity (mg g−1); ε is obtained as follows: ε = RT ln(1 + 1/Ceq)

indicating that chemical adsorption acts as a significant part during the adsorption. 3.6. Thermodynamic Study. Values of standards Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) are the actual indicators for practical application of a process. The adsorption amounts of CPs at 298.15, 308.15, and 318.15 K, have been used to obtain thermodynamic parameters for the system. The thermodynamic parameters can be obtained as the following equations:

(9) −1

−1

where R is gas constant (8.314 J mol K ); T is the absolute temperature (K). Ceq is the equilibrium concentration of adsorbates (mol L−1). E (kJ/mol) is the free energy change and the value of E is favorable for estimating the type of sorption reaction,50 and obtained from the D−R parameter kad as follows: E = −(2kad)−1/2

ΔGo = −RT ln K 0

ln K 0 =

(10)

The straight lines are given by plotting Ce/qe versus Ce (Figure 5a); by plotting ln Ce versus ln qe (Figure 5b); and by plotting ln qe versus ε2 (Figure 5c). The Langmuir isotherm model was the best-fitting isotherm compared to the Freundlich and D−R isotherms for the adsorption of CPs onto graphene (Table 3). The high

ΔS o ΔH o − R RT

(11)

(12)

where T is the temperature (K) and R is the gas constant. ΔH° and ΔS° were obtained from eq 12 (Figure 6).

Table 3. Isothermal Parameters for the Adsorption of CPs onto Graphene at 298.15 K adsorbates

Langmuir isotherm

Freundlich isotherm

2-CP

qmax = 94.3 mg g−1

KF = 17.7

b = 0.031 L g−1

n = 3.67

R2 = 0.9931

R2 =0.9592

qmax = 121.9 mg g−1

KF = 17.8

b = 0.028 mL g−1

n = 3.14

R2 = 0.9948

R2 = 0.9330

qmax = 169.5 mg g−1

KF = 25.1

b = 0.054 mL g−1

n = 2.81

R2 = 0.9941

R2 = 0.9761

qmax = 212.8 mg g−1

KF = 12.5

This "italic" element has trailing space which should be removed b = 0.020 mL g−1 R2 = 0.9909

n = 1.98

4-CP

DCP

TCP

R2 = 0.9726

Dubinin− Radushkevich isotherm kad = 0.0028 mol2 kJ−2 qs = 155.2 mg g−1 E = 13.36 kJ mol−1 R2 = 0.9821 kad = 0.0034 mol2 kJ−2 qs = 232.7 mg g−1 E = 12.13 kJ mol −1 R2 = 0.9600 kad = 0.0033 mol2 kJ−2 qs = 404.4 mg g−1 E = 12.31 kJ mol −1 R2 = 0.9761 kad = 0.0048 mol2 kJ−2 qs = 720.4 mg g−1

Figure 6. Variation of equilibrium constant as a function of temperature (1/T).

The overall ΔG° values were all negative, corresponding to the feasibility and spontaneous nature during the process (Table 4). The positive values of the enthalpy change (11.34 kJ mol−1 for 2-CP, 11.98 kJ mol−1 for 4-CP, 15.95 kJ mol−1 for DCP, and 16.34 kJ mol−1 for TCP) indicated that the adsorption was exothermic. The positive values of entropy change (ΔS°) were 58.01 J mol−1 K−1 for 2-CP, 57.48 J mol−1 K−1 for 4-CP, 68.44 J mol−1 K−1 for DCP, and 73.77 J mol−1 K−1 for TCP, respectively, which corresponded to an increase in the degree of randomness at the absorbents and adsorbates interface during the adsorption of CPs due to the delivery of hydration molecules.52 On the basis of the thermodynamics constants, the process was spontaneous.

E = 10.21 kJ mol−1 R2 = 0.9888

4. CONCLUSIONS The kinetic studies indicated that equilibrium between graphene and the aqueous solution in the adsorption of CPs on graphene was achieved during 20 min following the Lagergren’s equation. The adsorption data could be well followed by Langmuir isotherm with the saturation capacities of 88.1 mg g−1 for 2-CP, 114.2 mg g−1 for 4-CP, 155.3 mg g−1 for DCP, and 175.8 mg g−1 for TCP. The optimal pH was in the range of 1.01−8.09 for 2-CP, 1.08−8.01 for 4-CP, 1.04−7.03 for DCP, and 2.86−5.63 for TCP. The calculation of thermodynamic parameters demonstrated that the adsorption was spontaneous and exothermic. Further, the positive values of ΔS° reflected the random nature of the process at the solid−

determination coefficient values (R2 > 0.99) were obtained for the Langumuir model indicating a consistency with the experimental parameters and confirming the monolayer adsorption of CPs onto the surface of graphene. The values of qmax were obtained as 94.3 mg g−1 for 2-CP, 121.9 mg g−1 for 4-CP, 169.5 mg g−1 for DCP, and 212.8 mg g−1 for TCP at 298.15 K, respectively. E values of 2-CP, 4-CP, DCP, and TCP at 298.15 K obtained from D−R isotherms were found to be 13.36, 12.13, 12.31, and 10.21 kJ mol−1, respectively. The bonding energy of chemical mechanisms is > 8 kJ mol−1,51 1103



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Table 4. Thermodynamic Constants for Adsorption of CPs onto Graphene thermodynamic constants adsorbates

temperature (K)

K0

ΔGo (kJ mol−1)

ΔHo (kJ mol−1)

ΔSo (J mol−1 K−1)

2-CP

288.15 298.15 308.15 288.15 298.15 308.15 288.15 298.15 308.15 288.15 298.15 308.15

9.362 11.14 12.73 6.756 8.010 9.346 4.916 5.839 7.581 7.863 9.635 12.25

−5.36 −5.98 −6.52 −4.58 −5.16 −5.73 −3.82 −4.37 −5.19 −4.94 −5.62 −6.42

11.34

58.01

11.98

57.48

15.95

68.44

16.34

73.77

4-CP

DCP

TCP

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solution interface and the affinity of graphene for the adsorption of CPs.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hong-Tao Fan: 0000-0003-4128-9982 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was financially supported by NSFC (21477082). REFERENCES

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Adsorption Characteristics of Chlorophenols from Aqueous Solution

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