Enhancing Sorption Capacities for Copper(II) and Lead(II) under

Article pubs.acs.org/jced

Enhancing Sorption Capacities for Copper(II) and Lead(II) under Weakly Acidic Conditions by L‑Tryptophan-Functionalized Graphene Oxide Mengting Tan,† Xiang Liu,*,† Wei Li,† and Hexing Li‡ †

The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China ‡ The Key Laboratory of the Chinese Ministry of Education in Resource Chemistry, Department of Chemistry, Shanghai Normal University, Shanghai 200234, China S Supporting Information *

ABSTRACT: L-Tryptophan functionalized graphene oxide (GO/LTrp) was successfully synthesized via the nucleophilic substitution reaction. Its structure was characterized and the performances of GO/L-Trp in the sorption of Cu(II) and Pb(II), including the influence of contact time, initial pH of solution, temperature of the system, and initial metal concentration, were explored. The studies indicated that GO/L-Trp exhibited enhanced sorption capacities for Cu(II) and Pb(II) at the initial pH of 5 and 4, respectively, due to the increased sorption sites. The sorption of Cu(II) and Pb(II) were nearly completed within 40 min and fitted better with Lagergren pseudo-second-order kinetic model, indicating that Cu(II) and Pb(II) sorption on GO/L-Trp were chemical interactions. Langmuir model was more suitable to describe the sorption processes and sorption capacities were 588 mg·g−1 and 222 mg·g−1 for Cu(II) and Pb(II), respectively. Thermodynamic studies revealed that sorption were exothermic and spontaneous. Moreover, it also showed that GO/L-Trp could be reused after desorption, suggesting potential application in wastewater treatment.

■. INTRODUCTION It is a worldwide environmental concern that wastewater pollution caused by the indiscriminate disposal contains toxic heavy metal ions. Wastewater from mining and the metallurgical and chemical manufacturing industries contains numerous metallic toxicants.1 Copper and lead, the most extensively used metal ions in industries, have posed risks to both ecological environment and human beings. It is necessary to remove these metal contaminants from wastewater before releasing due to their high toxicities. Traditional techniques for heavy metal removal include membrane filtration, precipitation, ionic exchange and sorption, and so forth.2,3 Among the above methods, the most promising process is sorption. Generally, the surface area of adsorbents have great impact on the sorption, although uptake of heavy metal ions is majorly attributed to chemical sorption or ionic exchange on specific sorption sites. Despite the planar and external surface sites of adsorbents can bind heavy metal ions and form outer-sphere complexes through ionic exchange and electrostatic interactions at earlier sorption stages,4,5 they are mainly surface coordination processes for heavy metal sorption on solids at low concentrations, which can be complexation reactions between heavy metal ions and surface binding sites after modeled thermodynamically. Consequently, the heavy metal sorption is greatly affected by the surface modification of adsorbents.6 Several adsorbents in the study of heavy metal removal include © XXXX American Chemical Society

activated carbon (AC) and carbon nanotube (CNT).7−9 However, these adsorbents have been suffering from either secondary pollution or low sorption capacities, especially under weakly acidic conditions. With a two-dimensional honeycomb carbon lattice structure, graphene has its potential as an advanced material owning to its outstanding thermal, mechanical, and structural performance and large surface area.10,11 As the exfoliation of graphite by oxidation, graphene oxide (GO) has a wide range of hydroxyl, epoxy and carboxyl groups attached to its surface.12 Besides the oxygenic functional groups which can bind metal ions, GO can also serve as a superb platform for loading other molecules.13−15 Its surface area in theory is 2630 m2/g, which is much larger than AC (898 m2/g) and CNT (213 m2/g). Because of its huge surface area, two-dimensional layered structure, and oxygenic functional groups, the low-cost production of functionalized GO on a large scale will lead to good adsorbents in purifying water.16 It is commonly recognized that the N atom of amine group can effectively bind heavy metal ions by providing lone pair of electrons.17 Therefore, the sorption capacities for Cu(II) and Pb(II) are expected to be enhanced by the amination of GO. Moreover, Received: January 5, 2015 Accepted: April 21, 2015

A

DOI: 10.1021/acs.jced.5b00015 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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taking the stability of adsorbent and secondary pollution into consideration, L-tryptophan (L-Trp) was chosen to modify GO. As an environmentally friendly material, L-Trp has the bulkiest side chain among 20 kinds of amino acid. The part of πconjugated hydrophobic indole ring, carboxyl and amine groups provide more binding sites after L-Trp interact with GO and made adsorbent hydrophobic favored to sorption. In this work, L-Trp functionalized graphene oxide was prepared by nucleophilic substitution reaction. As an efficient approach, this one-step strategy could be used to synthesize high quality of graphene oxide/L-tryptophan (GO/L-Trp) without any toxicity. The sorption performance and reuse of GO/L-Trp to remove Cu(II) and Pb(II) were investigated. GO/L-Trp was found to exhibit enhanced sorption capacity under low pH value and it could be used recycled.

A=

Co − Ce × 100% Co

qe =

Co − Ce V m

(1) (2)

−1

−1

where Co (in mg·L ) and Ce (in mg·L ) are the initial and final concentrations, respectively, of Cu(II) and Pb(II), m (in g) is the dry mass of GO/L-Trp, and V (in L) is the volume of Cu(II) and Pb(II) solution.

■. RESULTS AND DISCUSSION Characterization of GO/L-Trp. The interaction between GO and L-Trp has three possibilities: hydrogen bonding, electrostatic attraction between protonated amine and carboxylic groups, and nucleophilic substitution reaction between amine and epoxy groups. A previous study showed that this reaction was dominated by the nucleophilic substitution reaction.19 Amine groups acted as nucleophiles and attacked carbon atoms in epoxy groups, grafting the hydrocarbon chain of the L-Trp onto GO sheets. In order to identify the interaction between GO and L-Trp, the obtained GO/L-Trp was characterized by ATR-FTIR (Figure 1). CO, CO, and

■. EXPERIMENTAL SECTION Materials. Graphite was purchased from Qingdao Tianhe Graphite Co. Ltd., China. L-Tryptophan (L-Trp), potassium permanganate and other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and they were used without any further purification (see Supporting Information, Table S1). Synthesis and Characterization of Adsorbent. Graphene oxide (GO) was prepared by using an improved method oxidizing natural graphite with KMnO4.18 Subsequently, GO/LTrp was obtained as follows: 20 mL of ultrapure water was used to disperse 0.1 g of GO. Both 0.3 g of L-Trp and NaOH in equimolar amount were dissolved in 20 mL of ultrapure water and then added to the GO dispersion. After stirring for 24 h at rt, EtOH was used to treat the colloidal dispersion. Precipitate appeared was separated, washed, and ultimately dried at 50 °C. Wide angle patterns of GO and GO/L-Trp were recorded by the X-ray diffractometer (Bruker, Germany) with Cu Kα radiation. Raman spectra were collected by using an invia Raman spectrophotometer (Renishaw Ltd., U.K.). ATR-FTIR spectra were acquired by the Nicolet 6700 ATR-FTIR spectrophotometer (Thermo Fisher Scientific Inc., U.S.A.). Taking C 1 s = 284.6 eV as a reference, all surface electronic states and binding energy (BE) values were analyzed and calibrated by the Versa Probe PHI 5000 XPS. Residual metal ion concentrations were analyzed by TAS-990NFG flame atomic absorption spectrophotometer (Beijing general instrument Ltd., China) with a precision of 1 × 10−2 mg·L−1. Sorption Experiments. Aqueous solution of copper nitrate and lead nitrate were prepared, with initial concentrations at 10 mg·L−1 to 250 mg·L−1. Contact time surveyed was 5 min to 240 min and sorption temperatures were 293 K to 313 K. The sorption experiments performed were shown below. Equal mass (10 mg) of GO/L-Trp samples were added to 20 mL Cu(II) and Pb(II) solution with pH ranging from 2 to 8 in a set of sealed flasks. Then, 0.1 M HCl and NaOH were used to adjust the initial pH of solution. The flasks were stirred for a certain time in a constant bath temperature (20 °C). Subsequently, 0.22 μm microsyringe filters were used to filter all mixtures and flame atomic adsorption spectrophotometer was used to analyze the residual metal ion concentrations. In order to avoid discrepancies in the experimental results, all investigations were carried out in triplicate and experimental data reported were the average of at least three repetitive measurements with a 5% relatively standard deviation. The sorption percentage, A (in %), and sorption capacity, qe (in mg·g−1), were figured out according to eqs 1 and 2

Figure 1. ATR-FTIR patterns of (a) GO, (b) GO/L-Trp, and (c) a mixture of GO and L-Trp.

CC bonds appear at 1046 cm−1, 1731 cm−1, and 1619 cm−1, respectively, in Figure 1a and indicate that numerous oxygenic functional groups intercalated into GO layers. 1 Peaks correspond to the bending and stretching vibrations of NH bond are found at 1573 cm−1 and 3400 cm−1 after grafting LTrp to GO (Figure 1b),20,21 whereas they are not observed in the mixture of GO and L-Trp (Figure 1c). Information about the in-plane vibration and defects of sp2 carbon atoms, namely G and D bands, are provided by Raman spectroscopy, which is often used to characterize the structure of carbon materials. Figure 2 shows that the G and D bands of GO at 1316 cm−1 and 1589 cm−1 are correspond to the first order scattering of the E2g phonon from sp2 carbon atoms and the κ-point photons’ breathing mode of A1g symmetry related to the formation of sp3 disorder bands through oxidation process of the ones located at the plane of graphite sheets.22 Compared with GO, the G and D bands of GO/L-Trp at 1592 cm−1 and 1334 cm−1 are caused by the increase of defects and double bonds resonate.23 Moreover, defects’ quantities of carbon materials are often presented by the intensity ratio of D and G bands (ID/IG).19 Higher ID/IG ratio of GO/L-Trp (1.48) B



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As shown in Figure 4A, four peaks located at 532 eV, 496 eV, 400 eV, and 285 eV are attributed to O 1s, Na KLL, N 1s and C 1s signals, respectively. In Figure 4B, five fitting curves were obtained by the deconvolution of C 1s peak centered at 285 eV and their binding energies are 288.3 eV, 287.0 eV, 286.7 eV, 285.4 eV, and 284.6 eV, belonging to OCO, CO, C O, CN, and CC bonds, respectively.25,26 The new peak that appeared at 285.4 eV is attributed to the formation of C N bond by nucleophilic interaction between amine and epoxy groups. Influence of Initial pH of Solution. As one of the most important parameters dominating sorption, initial pH of solution has an impact on the speciation of ions in aqueous solution and electronegativity of binding sites on the surface of adsorbents. The influence of initial pH of solution on the sorption of Cu(II) and Pb(II) is illustrated in Figure 5. With

Figure 2. Raman spectra of (a) GO and (b) GO/L-Trp.

compared to GO (1.09) approves the introduction of sp3 defects after L-Trp functionalized with GO. Figure 3 shows the XRD patterns of GO and GO/L-Trp. A sharp and strong peak of GO appears at 10.7°, with the layer

Figure 5. Influence of initial pH of solution on the sorption of Cu(II) and Pb(II) on GO/L-Trp. Mass of GO/L-Trp = 10 mg, initial concentration of Cu(II) and Pb(II) = 10 mg·L−1, contact time = 4 h, temperature = 20 °C. Figure 3. XRD patterns of (a) GO and (b) GO/L-Trp.

pH increasing, the sorption percentages increase, and they are 98% and 95% when pH are 5 and 4 for Cu(II) and Pb(II) (see Supporting Information, Table S2). Figure 6 shows that at pH ≤ 8, the predominant M(II) (M= Cu/Pb) species are M2+, M(OH)+ and M(OH)2. However, according to the solubilityproduct constant KSP of Cu(OH)2 and Pb(OH)2 at room temperature, Cu(OH)2 begins to appear at pH = 6.2 and Pb(OH)2 does not exist at pH ≤ 8 at 10 mg·L−1 of initial metal ion concentration. Although the positively charged COOH, OH, and NH2 groups caused by protonation reaction and

spacing of 8.2 Å. This shift is caused by the bonding effect and intercalation of oxygenic functional groups. On the other hand, after being functionalized with L-Trp, an increase in layer spacing of 9.8 Å is monitored in 9.0°, indicating L-Trp has intercalated into the layers of GO. Moreover, the peak of GO/ L-Trp in 23.6° indicates that GO is slightly reduced when reacted with L-Trp, which makes adsorbent hydrophobic and favored to sorption.24

Figure 4. XPS spectra of (A) GO/L-Trp and (B) C 1s of GO/L-Trp. C



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Figure 6. Influence of pH on distribution of Cu(II) and Pb(II) species in aqueous solution.

since there are no deep pores to traverse.29 Because of the increase of competition between M(II) and H+ released from the surface of GO/L-Trp and the decrease of vacant binding sites over time, the sorption rate gradually decreased. In the sorption kinetic studies, Lagergren pseudo-first-order and pseudo-second-order model are often used to reveal the mechanism of sorption process, which can be described as eqs 3 and 4

the competing of H+ and M(II) in acidic condition lead to low sorption percentages,27 the number of COONa increases after GO reacted with L-Trp and COONa on GO/L-Trp plays a big role in enhancing sorption capacities. The OH,  NH2, and COOH groups on GO/L-Trp are negatively charged with the pH increase, and its sorption capacity toward Cu(II) and Pb(II) is strengthened.28 Sorption Kinetic Studies. Figure 7 shows that Cu(II) and Pb(II) sorption proceeded very fast originally, with 92% and

ln(qe − qt) = −k1t + ln qe

(3)

t t 1 = + 2 qt qe k 2q e

(4)

(

)

where qt (in mg·g−1) and qe (in mg·g−1) are the equilibrated sorption capacities, k1 (in min−1) and k2 (in g·mg−1·min−1) are rate constants of the pseudo-first-order and pseudo-secondorder kinetic models, and t (min) is contact time. As shown in Figure 8 and Table 1, we found that the coefficient of determination (R2) of pseudo-second-order model for Cu(II) and Pb(II) are 0.998 and 0.999, both of which are higher than the values obtained by the pseudo-firstorder model, indicating that the sorption of Cu(II) and Pb(II) on GO/L-Trp can be described by the Lagergren pseudosecond-order kinetic model and their sorption rates are determined by the chemical sorption.30 Sorption Isotherm Studies. It is a vital step to find out the most suitable isotherm model to describe the sorption process by fitting experimental data to different models. As the most frequently used isotherm models, the Langmuir and Freundlich models are used to analyze the experimental data of Cu(II) and Pb(II) sorption in the present study, and they can be described as eqs 531 and 632

Figure 7. Effect of contact time on the sorption of Cu(II) and Pb(II) on GO/L-Trp. Mass of GO/L-Trp = 10 mg, initial concentration of Cu(II) and Pb(II) = 10 mg·L−1, temperature = 20 °C, pH = 5 and 4 for Cu(II) and Pb(II), respectively.

97% being removed within 40 min (see Supporting Information, Table S3). However, the rate of sorption slowed down with time increasing. Large surface area and nonporous layered structure of GO/L-Trp contribute to the high sorption rate in the initial 40 min and strong driving force is not needed

Figure 8. Largergren (A) pseudo-first-order and (B) pseudo-second-order kinetic plots for the sorption of Cu(II) and Pb(II) on GO/L-Trp. D



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Table 1. Lagergren Pseudo-First-Order and Pseudo-Second-Order Kinetic Models for Cu(II) and Pb(II) Sorption on GO/L-Trp pseudo-first-order metal Cu(II) Pb(II) a

k1

qe,cal

min−1

mg·g−1

0.0619 0.0184

11.447 2.92

pseudo-second-order k2

qe,cal

qe,expa

g·mg−1·min−1

mg·g−1

mg·g−1

0.0087 0.0771

20.921 18.1

19.310 19.3

R2 0.963 0.951

R2 0.998 0.999

Standard uncertainty u is u (qe,exp) = 0.1 mg·g−1.

qe =

qmKLCe 1 + KLCe

ln qe = ln KF +

contributes to the increase of sorption capacities for Cu(II) and Pb(II), which suggests that GO/L-Trp has great potentiality in the treatment of high concentration metallic wastewater. Comparison of qe,exp, qL, and qF for Cu(II) and Pb(II) are listed in Table 2 and it is clear that qL is closer to qe,exp than qF.

(5)

1 ln Ce n

(6)

−1

where Ce (in mg·L ) is the concentration of equilibrium, qe (in mg·g−1) is the equilibrated sorption capacity, KL (in L·mg−1) and qm (in mg·g−1) are Langmuir parameters relevant to the energy and capacity of sorption, and n and KF (in mg1−n·Ln·g−1) are Freundlich parameters relevant to the intensity and capacity of sorption. Isotherms of Cu(II) and Pb(II) sorption on GO/L-Trp are illustrated in Figure 9. Fitting sorption equilibrium data to these

Table 2. Comparison of qe,exp, qL, and qFa for Cu(II) and Pb(II) initial concentration of Cu(II)/ Pb(II) −1

Cu(II)

Pb(II)

mg·L

qe,exp

qL

qF

qe,exp

qL

qF

50 100 150 200 250

66.8 128.6 184.4 229.2 276.1

67 127.7 183.9 232.7 274.5

68.9 122.1 176.3 230.9 286.3

27.8 52.6 70.4 87.4 98.8

28.5 51.3 70.7 86.4 99.9

28.8 49.2 68.5 86.4 104.2

a

qe,exp, sorption capacity of the experiment; qL, sorption capacity of Langmuir model; qF, sorption capacity of Freundlich model. Standard uncertainty u is u(qe,exp) = 0.1 mg·g−1.

Table 3. Comparison of Fitting Results by Langmuir and Freundlich Models for Cu(II) and Pb(II) Sorption on GO/LTrp Langmuir model metal

qm mg·g

Figure 9. Sorption isotherms of Cu(II) and Pb(II) on GO/L-Trp. Mass of GO/L-Trp = 10 mg, contact time = 40 min, temperature = 20 °C, pH = 5 and 4 for Cu(II) and Pb(II), respectively.

Cu(II) Pb(II)

−1

588 222

Freundlich model R2

KL −1

L·mg

0.008 0.004

KF ·L ·g

1−n

mg 0.997 0.991

n

8.48 1.95

n

R2

1.29 1.33

0.995 0.989

−1

Moreover, R2 of Langmuir model shown in Table 3 are higher than that of Freundlich model, indicating that Langmuir model is more precise in evaluating sorption equilibrium, which

two models, isotherm parameters were obtained and shown in Figure 10. It is obvious that the increase of initial metal ion concentration leads to the decrease of sorption percentage but

Figure 10. (A) Langmuir and (B) Freundlich isotherms for the sorption of Cu(II) and Pb(II) on GO/L-Trp. E



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suggests that sorption on GO/L-Trp is monolayer coverage. The comparison of sorption capacities between AC, CNT, GO, and functionalized GO for Cu(II) and Pb(II), gathered from the references, are shown in Table 4.33,34

Table 5. Thermodynamic Parameters for Cu(II) and Pb(II) Sorption on GO/L-Trp at Different Temperaturesa metal Cu(II)

Table 4. Comparison of Cu(II) and Pb(II) Sorption Capacities of AC, CNT, GO and Functionalized GO qmax adsorbent SWCNTs Acidified MWCNTs AC AC cloths GO/PAMAMsa EDTA-GO CNTs immobilized by calcium alginate GO GO/Fe3O4 GO/L-Trp a

Cu(II)

Pb(II)

mg·g−1

mg·g−1

28.8 36 68.68 67.9 117.5 21.25 588

30−80 85 109.7 210 568.18 479 ± 46

222

Pb(II) reference a

7 8 9 5 33 27 34 16 15 this work

Thermodynamic Studies. Information on the inherent energy changes in the sorption process are provided by thermodynamic studies. In this work, the impact of temperature on Cu(II) and Pb(II) sorption were explored at 293 K, 303 K, and 313 K. Equations 7 and 8 shown below were used to analyze the entropy (ΔS), Gibbs free energy (ΔG), and standard change in enthalpy (ΔH).

(7)

ΔG = − T ΔS + ΔH

(8)

ΔH

ΔS

ΔG

kJ·mol−1

J·mol−1·K−1

kJ·mol−1

293 303 313 293 303 313

−13.3

−10.9

−22.9

−47.2

−10.1 −9.99 −9.89 −9.07 −8.60 −8.13

Standard uncertainty u is u(T) = 0.1 K.

The decrease of randomness at the liquid/solid interface, which was caused by the interaction between different charged metal ions and active sites, can be inferred by the negative values of ΔS of Cu(II) and Pb(II) sorption. Reusability of GO/L-Trp. The stability and potential regeneration of GO/L-Trp were studied. Desorption was conducted by washing GO/L-Trp bound Cu(II) and Pb(II) out with 20 mL HCl (pH ∼ 2) and rinsing GO/L-Trp with ultrapure water after sorption. Then, GO/L-Trp was dried at 50 °C and reused. Several sorption−elution cycles were made and the recovery value of GO/L-Trp was examined. The sorption capacities of GO/L-Trp after recycling for Cu(II) and Pb(II) are illustrated in Figure 12, which shows that the sorption

PAMAMs: polyamidoamine dendrimers.

⎛q ⎞ ΔH ΔS ln⎜ e ⎟ = − + c RT R ⎝ e⎠

T K

where T (in K) is the absolute temperature of the system and R (8.3145 J·mol−1·K−1) is the universal gas constant. The values for −ΔH/R and ΔS/R, which are equal to the slope and intercept of the straight line in Figure 11, were obtained by plotting ln(qe/ce) against 1/T. The related thermodynamic parameters were also calculated and listed in Table 5. Sorption processes are exothermic and spontaneous, which can be indicated by the negative values of ΔH and ΔG.

Figure 12. Recycling of GO/L-Trp for the sorption of Cu(II) and Pb(II). Mass of GO/L-Trp = 10 mg, initial concentration of Cu(II) and Pb(II) = 10 mg·L−1, temperature = 20 °C, contact time = 40 min, pH = 5 and 4 for Cu(II) and Pb(II), respectively.

capacities decrease slightly from 19.0 mg·g−1 to 18.7 mg·g−1 and 19.6 mg·g−1 to 18.6 mg·g−1 with the utilization frequency increase (see Supporting Information, Table S4). The decline of sorption capacities are less than 5% after reused for three times. The results indicate that GO/L-Trp, as an adsorbent, can be recycled in the removal of Cu(II) and Pb(II).

■. CONCLUSIONS GO/L-Trp was successfully synthesized by nucleophilic substitution reaction and characterized. It showed high removal efficiencies that were over 95% at pH 5 and 4 for Cu(II) and Pb(II), respectively. Moreover, Cu(II) and Pb(II) sorption on GO/L-Trp were almost completed within 40 min and followed Lagergren pseudo-second-order kinetic model. The sorption for Cu(II) and Pb(II) were typical monomolecular layered and sorption capacities were 588 mg·g−1 and 222 mg·g−1 at 293 K, respectively. Thermodynamic studies revealed that sorption

Figure 11. Plot of ln(qe/ce) versus 1/T for the sorption of Cu(II) and Pb(II) on GO/ L-Trp. Mass of GO/ L-Trp = 10 mg, initial concentration of Cu(II) and Pb(II) = 10 mg·L−1, contact time = 40 min, pH = 5 and 4 for Cu(II) and Pb(II), respectively. F



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were exothermic and spontaneous. The reusability of GO/LTrp indicated that it could be an effective adsorbent for toxic heavy metal removal.

■

ASSOCIATED CONTENT

S Supporting Information *

Tables of basic information on reagents, influence of initial pH of solution, effect of contact time on sorption, and sorption capacities of GO/L-Trp after recycling. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00015.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This research was financially supported by the Open Project of the Key Laboratory of the Chinese Ministry of Education in Resource Chemistry and the National Natural Science Foundation of China (21206057). Notes

The authors declare no competing financial interest.

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REFERENCES

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