Facile Synthesis of Graphene Oxide for Multicycle Adsorption of

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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Facile Synthesis of Graphene Oxide for Multicycle Adsorption of Aqueous Pb2+ in the Presence of Divalent Cations and Polyatomic Anions Rishi Karan Singh Rathour,† Jayanta Bhattacharya,*,†,‡ and Abhijit Mukherjee†,§ †

School of Environmental Science and Engineering, ‡Department of Mining Engineering, §Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India

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S Supporting Information *

ABSTRACT: This study reports a low-cost, NaNO3-free synthesis of graphene oxide (GO) at room temperature, for the purpose of Pb2+ adsorption in single, binary, and ternary association. The GO displays single layered sheets of ∼1 nm thickness with specific surface area of 195 m2/g. The removal of Pb2+ shows an equilibrium time of 15 min with maximum adsorption of 178.5 mg/g, which is best fitted to pseudo-second-order kinetic and Langmuir adsorption isotherm model. A reduction in adsorption efficiency of Pb2+ by 9.52% in Pb2+−Ni2+ and 10.88% in Pb2+−Cd2+ was observed with an increase in concentration of Ni2+ and Cd2+ from 10 to 50 mg/L. Similarly, 19.31% drop in overall efficiency of Pb2+ was observed with increase in the initial concentration of ternary solution (Pb2+ ̵ Ni2+ ̵ Cd2+) from 10 to 50 mg/L. The presence of SO42− and NO3− up to 800 mg/L has little effect on Pb2+ adsorption, compared to Ni2+ and Cd2+. The GO shows only 11.8% decrease in Pb2+ adsorption after five consecutive adsorption−desorption cycles. This study signifies the economic potential of GO toward selective removal of Pb2+ from wastewater in the presence of divalent cations and polyatomic anions on the basis of its multicycle performance. yield and low cost of production.12−14,16 But, the chemical routes such as the Brodie−Staudenmaier5 uses KClO3 and nitric acid, which have intrinsic difficulties of a detonation risk, issue of harmful gases (e.g., NOX and ClO2), and production of carcinogenic substances.16,17 On the other hand, the KMnO4 based methods (Hummers, Modified Hummers, and Tour) suffer from the obligate metal pollution (Mn2+), generation of NOx gases (Hummer and its modified forms), and production of acidic wastewater.14,16 Therefore, there is a need for a viable route, which reduces the environmental risk and cost of GO production compared to previous methods without compromising the quality of GO.18 The treatment of industrial or natural wastewater containing numerous ionic species adds complexity into the adsorptive treatment because of the involvement of ionic competition.19 The selective removal of toxic metal is an important task toward the sustainable development of a process which helps ecological protection.18,20 The adsorption of Pb2+ ion on GO and its functionalized forms are commonly reported in the literature.8,11,21 But, the effect of the presence of coexisting metallic or other ionic species on Pb2+ adsorption by GO has been limitedly reported.22 Thus, in the perspective of wastewater treatment by GO, the knowledge of competition or inhibition posed by the ionic species is very important for

1. INTRODUCTION Lead (Pb2+) is a toxic heavy metal generated from the industrial effluents of metal fabrication, mining, batteries, paint-pigments, and landfill leachate.1,2 Pb2+ toxicity has been commonly associated with renal failure, malaise, dizziness, convulsions, cancer, and subtle effects on intelligence.3,4 Therefore, its maximum permissible limit in the industrial effluent and drinking water is fixed as 0.5, and 0.01 mg/L, respectively, by the government of India and the World Health Organization (WHO).5,6 At present, technologies such as ionexchange, coagulation, precipitation, adsorption, and membrane filtration have been used for remediation of Pb2+ and other heavy metals from wastewater.7,8 Amid these technologies, adsorption is the most promising in terms of economy and ease of operation.9 However, the search of suitable adsorbents, which exhibits the properties of a strong metal adsorption, nontoxic in nature, and economically viable has been ongoing since the past decade.10 Recently, graphene and its derivatives are seen as a favorable material for the adsorption of environmental pollutants.8,11 GO is a single atom thick sheet of carbon atoms with oxygenic functional groups on its basal plane and edge.12,13 Its special properties such as high specific surface area and abundant oxygenic groups on the surface make it suitable for the application in sensor, catalysis, biomedical, and wastewater treatment.11,14 Yet, the production of GO at large scale is still challenging.15,16 To date, the most promising route of GO synthesis is through chemical oxidation because of its high © XXXX American Chemical Society

Received: April 27, 2018 Accepted: July 20, 2018

A

DOI: 10.1021/acs.jced.8b00344 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. (a) XRD spectra of GO before and after Pb2+ adsorbed. (b) N2 adsorption−desorption isotherm of GO for BET specific surface area analysis.

JEOL (JEM-2100) with an operating voltage of 200 kV. Raman spectroscopy by HORIBA JOBIN YVON (model T64000). Thermogravimetric analysis (TGA) in Q50 (TA Instruments) between 30 and 600 °C with the heating rate of 10 °C/min, under oxygen flow. The Fourier transform infrared (FTIR) instrument of Thermofisher (Nicolet 6700) was used to collect the spectra of GO between 400 and 4000 cm−1. The UV−vis spectrophotometer of Shimadzu (model UV1800) was used. The Brunauer−Emmett−Teller (BET) surface area and pore size of GO was determined from AutosorbiQ (Quantachrome instrument, USA). 2.3. Synthesis of GO. The oxidization of graphite was performed in 98% H2SO4 with KMnO4 at room temperature without using NaNO3 and H3PO4 as previously reported.24 In general, graphite powder (1.00 g; 1 wq.) was mixed in 25 mL of H2SO4 (98%) for 15 min and then the KMnO4 (4.00 g; 4 wq.) was added slowly (0.25 wq. per increment). After complete addition of KMnO4 the mixture was left for 10 h, before addition of 200 mL of ultrapure water for prolonged period of around 2.5 h, followed by 15 min stirring, and adding of 5 mL of H2O2. At the end GO was purified by continuous washing with 1 N HCl and ultrapure water until neutral pH was achieved. The final dry weight of graphite oxide was achieved to be 1.72 g. The single layered GO was obtained by ultrasonication of the powder graphite oxide into ultrapure water. 2.4. Batch Adsorption Experiments. All the batch experiments of Pb2+ adsorption on GO were studied in the sterile sealed polypropylene bottles. The kinetics of Pb2+ adsorption was observed between 0 and 120 min, on three different initial Pb2+ concentrations (50, 100, 200 mg/L) at a fixed dose of GO (1.00 g/L), with a temperature of 303 K, stirring speed of 170 rpm, and without pH control. The effect of initial pH (from 2 to 6), on the adsorption of Pb2+ was monitored by keeping initial Pb2+ concentration at 100 mg/L, dose of GO to 1.00 g/L, temperature at 303 K, and stirring at 170 rpm. The dose optimization was done by adding different weights of GO from 0.1 to 2.00 g/L and keeping the other parameters such as pH 5.0, initial Pb2+ concentration = 80 mg/

the improvement or development of GO-based adsorbent for application.23 This study reports a NaNO3 free synthesis of GO at room temperature. Excluding the use of NaNO3 helps in halting NOX gas generation. The utilization of only four weight equivalent (wq.) of KMnO4 for one wq. of graphite has the benefit of cost reduction as well as avoiding over oxidation. More strikingly, this new synthesis route paves a way for GO synthesis in a more ecofriendly manner, which can be used in other fields. Particularly, the use of GO for adsorption of Pb2+ from the synthetic wastewater and to study the effects of the presence of coexisting cations (Cd2+ and Ni2+), and anions (SO42− and NO3−) in binary and ternary solutions makes the work unique. This aspect is necessary for the field applications of GO in wastewater treatment and has been experimented for the first time in our knowledge.

2. MATERIALS AND METHODS 2.1. Materials. The extra pure fine graphite powder (≤50 μm) was obtained from Loba chemise, India. Potassium permanganate (KMnO4), H2SO4 (98%), H2O2 (30%), HCl (36%), C2H5OH (100%), (NaOH), Pb(NO3)2, Ni(NO3)2· 6H2O, and 3CdSO4·8H2O were procured from Merck India, and used devoid of additional refinement. The stock solution (1.00 mg/mL) of Pb2+, Ni2+, and Cd2+ were prepared in 1% HNO3 by adding suitable quantity of Pb(NO3)2, Ni(NO3)2· 6H2O, and 3CdSO4·8H2O, respectively. The source of NO3− and SO42− were from NaNO3 and Na2SO4, respectively. The 0.1 N NaOH and HCl were used for pH maintenance. The SMART2PURE (Thermo Fisher Scientific) ultrapure water (conductivity < 18 MΩ/cm) was used during experiments. 2.2. Characterization. The X-ray diffraction (XRD) characterization of GO was performed in the range 2θ = 5− 40° with the help of Cu Kα radiation (λ= 1.5405 Å), by PAN analytical (X̀ pert Powder). The microscopic characterization of GO was done by atomic force microscope (AFM) from Agilent Technologies (model 5500), field-emission scanning electron microscope (FESEM) from Carl-Zeiss MERLIN (GEMINI-2), with energy-dispersive X-ray spectroscopy (EDXS) facility and high-resolution transmission electron microscope (HRTEM) B



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The surface functional groups introduced during the oxidation of graphite was investigated by FTIR analysis and presented in Figure 2a. The FTIR spectra of graphite gives a

L, temperature of 303 K, and stirring speed (170 rpm) constant. The influence of stirring speed was also investigated at pH 5.0, temperature of 303 K, dose of GO of 1.00 g/L, and initial Pb2+ concentration of 100 mg/L. The adsorption isotherm was performed between 293 and 313 K at different initial concentrations of Pb2+ (5−200 mg/L), and keeping the parameters pH (5.0), dose of GO (1.00 g/L), stirring (170 rpm) constant. The effect of the presence of cations (Cd2+ and Ni2+) and anions (SO42− and NO3−) on Pb2+ adsorption was analyzed by varying the concentrations of Pb2+, Cd2+, and Ni2+ from 10 to 50 mg/L and SO42− and NO3−, from 50 to 800 mg/ L. The binary and ternary studies were performed at pH 5.0, stirring at 170 rpm, dose of GO at 1.00 g/L, and temperature at 303 K. The residual concentration of Pb2+ in the solution throughout the experiment was analyzed by filtering the post adsorption solution with 0.22 μm Teflon filter, and the filtrate was analyzed by AAS (model no. ice3300, Thermofisher). The accuracy limit of Pb2+ detection and quantification by AAS was found to be 0.43 and 1.3 mg/L, respectively. The adsorption capacity (q) of GO toward Pb2+ and percentage removal (%R) of Pb2+ by GO was calculated from the following, eqs 1 and 2, respectively: qe =

(Co − Ce) × V W

ij C − Ce yz zz100 %R = jjj o j Co zz k {

Figure 2. (a) FTIR spectra of graphite, GO before and after Pb2+ adsorption. (b) Raman spectra of graphite and GO.

straight line indicating the absence of functional groups on its surface. After oxidation of graphite the following functional groups, namely, O−H stretching vibrations (3414 cm−1),19 CO stretching vibrations (1727 cm−1),26 CC from unoxidized sp2 region (1625 cm−1),17 C−O−C asymmetric vibration (1401 cm−1),19,27 O−C−O vibration (1221 cm−1),17 and C−O (1053 cm−1)24,28 were found on the GO surface, which also indicates the oxidation of graphite in the proposed condition. The spectra of GO after Pb2+ adsorption, displays an obvious enhancement of the peak at 1812.65 cm−1 and may be due to the protonation of the carboxyl group,19 whereas, the CC peak shift from 1625 to 1619 cm−1 could be induced by the binding of Pb2+ to the aromatic network of GO through the cation−π interaction.19 Hence, on the basis of FTIR peak shifts and appearance of new peaks, it can be said that the adsorption of Pb2+ on GO was through the electrostatic and cation−π interaction. Raman spectra of graphite (Figure 2b) exhibits two major peaks at Raman shift of 1582 (G band) and 2712 cm−1 (2D band),29 whereas, the peak at 1348 cm−1 (D band) is due to the presence of natural defects/disorders in the graphite.29,30 After the oxidation of graphite, significant broadening of both the D and G bands indicate the introduction of defects on the graphite plane.27 The peak shift of G band (from 1582 to 1592 cm−1) in GO is ascribed to the occurrence of an isolated double bond, which resonates at the upper frequency.17,24,29,31 The proportion of intensity of D to G band (ID/IG) is commonly applied for determining the lattice disorder introduced during the oxidation of graphite, which could directly be linked to the structural crystalline size (La: the average size of sp2 carbon atoms). The La calculated from the equation: La = 44 × (ID/IG)−1, gives ID/IG of 0.72 and 0.92 for graphite and GO, respectively, and the corresponding La values were 61.11 and 47.82. This result indicates that during the oxidation of graphite the defects were introduced; therefore, the size of sp2 domains was reduced in GO.26,27 Figure 3 demonstrates, the microscopic images of the GO from FESEM, HRTEM, and AFM analysis. The FESEM image (Figure 3a) of the GO reveals some crumples and wrinkles, implying the presence of extremely thin (single layer) sheets. Similarly, the HRTEM image also shows folds in the sheet

(1)

(2)

where qe (mg/g) is the equilibrium adsorption capacity of GO toward Pb2+; C0 and Ce (mg/L) are the initial and final concentration of Pb2+ in the solution, respectively. W (g) is the weight of GO, and V (L) is volume of Pb2+ solution. 2.5. Desorption Experiments. The desorption of Pb2+ from the GO surface was achieved by washing the spent adsorbent with 0.5 N HNO3, and then the GO (collected at the 0.22 μm Teflon filter) was repeatedly washed with ultrapure water until the filtrate reached neutral pH. Finally, the GO was collected from the filter and dried at 65 °C in hot air oven for use in the next adsorption cycle. Five adsorption− desorption cycles were performed for testing.

3. RESULTS AND DISCUSSION 3.1. Characterization of GO. The XRD analysis of GO in Figure 1a, reveals the 002 reflection at 2θ = 10.04°, corresponds to c-axis expansion of 0.87 nm, which was previously calculated as 0.33 nm at 2θ = 26.49° in graphite as available in Supporting Information (Figure S1).24,25 Thus, it can be surmised that only the 4.00 g of KMnO4 was sufficient to oxidize 1.00 g of graphite into GO at room temperature conditions in the absence of NaNO3 and H3PO4. Furthermore, in Figure 1a, the XRD spectra of GO after the adsorption of Pb2+ gave rise to the new peaks at 2θ = 20.87°, 26.77°, and 33.30° attributed to the Pb3O4, and 23.35° and 29.77° for Pb0.8O2 (JCPDS ref no. 98-007-6341 and 98-001-7587), indicating the adsorption of Pb2+ on the GO surface. The UV−vis spectra of GO (Figure S2) display peaks at 231.5 and ∼300 nm corresponding to ππ* and nπ* transition, as previously reported.24 The BET specific surface area of GO was found to be 195 m2/g, by N2 adsorption−desorption isotherm (Figure 1b). Whereas, the pore volume and pore radius, estimated from Barret−Joyner−Halenda (BJH) analysis was 0.4432 cm3/g and 23.19 Å, respectively. C



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adsorbed GO (respective image is given in Figure S3) displays the elemental presence of Pb with C and O, justifying the adsorption of Pb2+ on GO,16 as previously discussed in XRD and FTIR analysis. Thermogravimetric analysis of GO (Figure S4) shows the combustion of oxygenic functional groups from its surface at elevated temperature; therefore, continuous weight loss was observed throughout 30550 °C. The initial weight loss from 30 to 180 °C can be attributed to loss of water molecules,33 while the weight loss between 180 and 220 °C corresponds to the release of CO, CO2, and trapped moisture.24 The final weight loss after 220 °C until the complete decomposition of GO is attributed to the loss of stable oxygen and carbon from the basal plane.17,24 3.2. Effect of Contact Time. The change in adsorption of Pb2+ with contact time is presented in Figure 4a. The percentage removal of Pb2+ increases with contact time, and after 15 min no further change in Pb2+ adsorption was observed, which indicates the establishment of adsorption equilibrium. Therefore, an extended time of 120 min was proposed for subsequent experiments of Pb2+ adsorption on GO. The fast adsorption kinetics of Pb2+ at the initial stage is due to obvious presence of a large number of vacant adsorption sites on the GO which later are occupied by Pb2+ as a result of saturation from the adsorption process. The percentage removal of Pb2+ on GO was 83.15, 41.93, and

Figure 3. (a) FESEM, (b) HRTEM, and (c) AFM analysis of GO; (d) EDX spectra of GO after Pb2+ adsorption.

(Figure 3b), with a 6-fold symmetric SAED pattern (inset Figure 3b), indicating good crystallinity and single layer characteristic of GO.17,32 The AFM image (Figure 3c) shows the thickness of GO ∼ 1 nm, reiterating the presence of single layer GO.17,27,32 In Figure 3d, the EDX spectra of Pb2+

Figure 4. Time dependent kinetics (a), pseudo-first-order kinetic model (b), pseudo-second-order kinetics model (c), and intraparticle diffusion model (d), of Pb2+ adsorption on GO. D



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25.5%, at the initial Pb2+ concentration of 50, 100, and 200 mg/L, respectively. This fast adsorption of Pb2+ on GO signifies its potential application toward efficient treatment of Pb2+ laden water. Table 1, shows a comparison of the GO with

kinetics rate constant, respectively. ki (mg/g*min0.5) is the intraparticle rate constant and Ci is the intercept of stage i, representing boundary layer thickness. The calculated constants from the linear plot of pseudo-firstorder (Figure 4b) and pseudo-second-order (Figure 4c) kinetic models are reported in Table 2. The correlation coefficient (R2) of pseudo-first-order is quite low (between 0.79 and 0.95), and the large deviation in experimental and calculated qe suggest that Pb2+ adsorption on GO does not fit to pseudofirst-order kinetic model, whereas, the high value of R2 (>0.99) and low discrepancy between experimental and calculated qe directs that Pb2+ adsorption on GO follows pseudo-secondorder kinetics, which implies that the adsorption is controlled by a chemisorption mechanism.26,44,45 Furthermore, the intraparticle diffusion model was also tested to judge the mass transfer mechanism of Pb2+ on GO. The plots of qt vs t0.5 in Figure 4d, are neither a straight line nor they pass through the origin, signifying that intraparticle diffusion is involved in the adsorption but not the solitary rate controlling step.45 The increase of Ci from 35.97 to 42.69 mg/g (Table S1), with increase in the initial Pb2+ concentration from 50 to 200 mg/L, advocates that the boundary layer diffusion effect would be encouraged with an increase in initial concentration of Pb2+ solution. 3.4. Effect of Adsorbent Dose. The adsorption of Pb2+ on GO was monitored at different doses between 0.1 and ̵ 2.00 g/L, and the results are presented in Figure 5a. The %R of Pb2+ was increased from 27.08 to 97.26%, with an increase in the dose of GO from 0.1 to 1.00 g/L, but a further increase in GO dose (from 1.00 to 2.00 g/L) shows only ∼2.4% increase in the removal efficiency from the 1.00 g/L level. Hence, considering an economic standpoint, an optimum dose of 1.00 g/L was selected for the removal of Pb2+ in subsequent experiments. The increase in adsorption efficiency from 27.08 to 97.26% with a dose of GO can be ascribed to the increase of total available adsorption sites. 3.5. Effect of pH. Solution pH has vital effect on the adsorption of heavy metals because it affects the solubility of metals (Me2+ ↔ Me(OH)+↔ Me(OH)2 ↔ Me(OH)3+), and association−disassociation of functional groups from the surface of the adsorbent.25,44 In Figure 5b, the plot of change in %R of Pb2+ vs pH, indicates that maximum removal is at pH 6. But, near pH 6.0, the formation of lead hydroxides (as white color precipitate) started;46 hence, aiming for Pb2+ adsorption, all the further experiments were performed at pH 5. The removal of Pb2+ is enchanced with solution pH, because as a rule, at lower pH, protonation of the surface functional group easily takes place, which restricts the Pb2+ adsorption. As a consequence, there is a low adsorption of Pb2+, whereas, with increase in solution pH, the surface of GO becomes more deprotonated leading to additional negative charge on the surface, thus facilitating more adsorption of Pb2+ on the GO.24,25 The proliferation in Pb2+ removal at higher pH may

Table 1. Comparison of the Adsorption Equilibrium Time, Maximum Adsorption Capacity (qm) and pH of Present Study with Others adsorbent

equilibrium time (min)

qm (mg/g)

pH

20 240 30 100

132.01 73.52 86.40 3.37

5.3 6 6 7

34 35 36 37

80 300

33.10 173.00 293.00 36.00 1409.00 125.00 746.20 138.9 819.7 363.42 387.6

5

38

4.5 5.5 5 5 6.1 6 3

39 40 26 41 33 42 43

5

present study

DTC-GO PS@Fe3O4@GO KC clay montmorillonite clay (Mt) Mt-TOA Na-birnessite TMA-birnessite GAC1240 β-SrHPO4 graphene oxide GOs GO5 beads HPA-GO GO−Zr−P DTPA/MGO composites GO

15 240 20 720 60 20 1440 15

178.5

ref

other experimented adsorbents, particularly noticeable being the fast kinetics and adsorption capacity. The effect of stirring speed (figure not given) was also monitored and it was deduced that there is no noticeable change on the adsorption of Pb2+ by GO. 3.3. Adsorption Kinetics. The understanding of adsorption kinetics is very important in designing the adsorption treatment system. Therefore, the kinetics of Pb2+ removal was evaluated at three initial Pb2+ concentrations (50, 100, and 200 mg/L), between 0 ̵ 120 min. The experimental data of Pb2+ adsorption were fitted against pseudo-first-order, pseudosecond-order, and the intraparticle diffusion model, the linear forms of which are expressed in eqs 3, 4, and 5, respectively. log(qe − qt ) = log qe −

k1 t 2.303

(3)

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

(4)

qt = k it 1/2 + Ci

(5)

where qe (mg/g) is adsorption capacity at equilibrium time, qt (mg/g) is the adsorption capacity at time t. k1 (1/min) and k2 (g/mg*min) are pseudo-first-order and pseudo-second-order

Table 2. Parameter Acquired from the Pseudo-First-Order and Pseudo-Second-Order Kinetic Model Fitting pseudo-first-order kinetic model

pseudo-second-order kinetic model

concn (mg/L)

qe, expt (mg/g)

K1 (min−1)

qe, calc (mg/g)

R2

K2 (g/mg·min)

qe, calc (mg/g)

R2

50 100 200

41.574 41.928 51.635

0.0128 0.0079 0.0146

5.2408 2.8862 8.8344

0.8216 0.9506 0.7924

0.0184 0.0351 0.0096

41.563 41.964 51.335

0.999 0.999 0.998

E



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Figure 5. Effect of adsorbent dose (a) and pH (b) on the adsorption of Pb2+ on GO.

Table 3. Adsorption Isotherm Constants of Pb2+ Adsorption on GO temperature (K) isotherm model

parameter

293

303

313

Langmuir

qm (expt) (mg/g) qm (calc) (mg/g) KL (L/mg) RL R2 qm (calc) (mg/g) KF (mg/g) (L/mg)1/n 1/n R2 qm (calc) (mg/g) BT AT (L/mg) R2

150.07 151.44 0.783 0.006−0.203 0.998 244.39 50.12 0.406 0.742 180.78 21.19 34.77 0.977

160.01 164.67 0.484 0.005−0.168 0.993 232.81 45.08 0.445 0.737 160.02 24.29 18.16 0.986

165.20 176.81 0.357 0.005−0.171 0.971 260.91 41.52 0.518 0.695 150.42 29.35 9.46 0.978

Freundlich

Temkin

also be ascribed to low competition between Pb2+ and H+ ions for the same adsorption site. 3.6. Adsorption Isotherm. The adsorption isotherm experiments were performed to scrutinize the behavior of Pb2+ adsorption on GO at equilibrium time on the temperatures 293, 303, and 313 K. The isotherm result was fitted on three models, that is, Langmuir, Freundlich, and Temkin adsorption isotherms. Langmuir adsorption isotherm assumes that the adsorption sites on the adsorbents are identical and limited, the adsorption of adsorbate molecules on the adsorbent takes place in monolayer form without lateral contact or hindrance between the adsorbed molecules.47,48 The nonlinear and linear equation of Langmuir isotherm model can be written as in eqs 6 and 7, respectively. qe =

where C0 (mg/L) is the initial concentration of Pb2+. The RL value tells about the type of process, that is, when RL > 1.00, unfavorable; RL = 1.00, linear; 0 < RL < 1.00, favorable; and RL = 0, irreversible. Freundlich adsorption isotherm model is based on the assumption that the adsorbent surface is heterogeneous and the distribution of heat of sorption on the surface is not uniform; that is, some specific sites will be occupied first.25,43,47,48 The nonlinear and linear forms are expressed in eqs 9 and 10, respectively. qe = KFCe1/ n log qe = log KF +

qmKLCe 1 + KLCe

(6)

Ce 1 1 = + Ce qe KLqm qm

(7)

1 log Ce n

(10)

where qe (mg/g) is the Pb2+ concentration adsorbed on the surface of GO at equilibrium. The constant of adsorption capacity (KF (mg/g)/(L/g)1/n) and heterogeneity factor (n) were calculated from the intercept and slope of log qe vs log Ce plot. Temkin adsorption isotherm was established on the assumption that the heat of sorption of all the molecule decreases linearly with surface coverage of the adsorbent.47,48 The nonlinear and linear form is expressed in eqs 11, and 12, respectively.

where qe (mg/g) and qm (mg/g) are, respectively, the amount of Pb2+ adsorbed on the GO at equilibrium and its maximum saturated monolayer capacity. KL (L/mg) is Langmuir constant related to the adsorption energy. Ce (mg/L) is the Pb2+ concentration in solution at equilibrium. Furthermore, another crucial parameter known as the separation factor (RL) was calculated with the help of Langmuir constants by using eq 8. 1 RL = 1 + KLCo

(9)

qe =

RT ln A TCe bT

qe = BT ln A T + BT ln Ce

(8) F

(11) (12) DOI: 10.1021/acs.jced.8b00344 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Figure 6. Linear fit (a) Langmuir, (b) Freundlich, (c) Temkin; (d) model fitting of adsorption isotherm of Pb2+ on GO.

where ΔG (kJ/mol) is standard Gibbs free energy. T (K) and R = 8.314 J/mol·K, are the absolute temperature and the universal gas constant, respectively. Kc = qe/Ce is the equilibrium partition coefficient, qe (mg/g) and Ce (mg/L) are the equilibrium concentration of Pb2+ on solid and liquid phase, respectively. ΔS (J/mol·K) and ΔH (kJ/mol) are the entropy and enthalpy change, respectively. The negative values of ΔG for the temperatures 293, 303, and 313 K (Table 4), indicate that the adsorption of Pb2+ on

where BT = RT/bT is related to heat of adsorption. T (K) and R = 8.314 J/K/mol are the absolute temperature and the universal gas constant, respectively. BT and AT are Temkin isotherm and equilibrium binding constants, respectively. The plot of qe vs ln Ce provides the value of BT and AT. Table 3 summarizes the adsorption isotherm constants of Langmuir, Freundlich, and Tempkin isotherm. The adsorption of Pb2+ on GO fitted better to the Langmuir (Figure 6a) than the Freundlich (Figure 6b) and Temkin (Figure 6c) isotherm model. The Langmuir model displays a higher R2 (>0.99) value, better curve fitting to experimental data, and good agreement between the calculated and experimental qm values. The RL values of Pb2+ adsorption on GO throughout the study varied between 0 and 1, suggesting the favorability of adsorption.49 The value of Freundlich constant 1/n was below 1, showing the adsorption was favored.44 It is noteworthy that the adsorption of Pb2+ was greater than 94% for the initial Pb2+ concentration of 125 mg/L, whereas, for the concentration up to 50 mg/L, the percentage removal was ∼99%, indicating the excellent adsorption potential of GO toward removal of a wide concentration range of Pb2+ from an aqueous solution. 3.7. Thermodynamics. The effect of temperature on Pb2+ adsorption by GO was evaluated to understand the nature of the adsorption process. The influence of temperature on the adsorption can be expressed by thermal characteristic parameters47 as given in eqs 13 and 14. ΔG = − RT ln Kc ln Kc =

ΔS ΔH − R RT

Table 4. Thermodynamic Parameters of Pb2+ Adsorption on GO temperature (K)

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (J/mol·K)

293 303 313

−7.39 −9.92 −12.44

66.48

252.125

GO was spontaneous.47,49 The ΔG < 40 kJ·mol−1, suggests that the adsorption of Pb2+ on GO was through varieties of physical processes, that is, hydrogen bonding, hydrophobic interaction, cation−π interaction, and coordination interaction, etc.49 The value ΔH > 0 indicates the adsorption process was endothermic.39 Hence, the adsorption of Pb2+ on GO would be more favorable at higher temperature (Figure 7). Furthermore, the positive values of ΔS specify an increase in randomness, which leads to good affinity of Pb2+ on GO.43

(13)

4. INTERFERENCE STUDY The adsorption of Pb2+ on GO was examined in the presence of divalent cations (Ni2+ and Cd2+) and polyatomic anions (SO42− and NO3−) in the binary and ternary mixture solutions.

(14) G



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known that the affinity of metal ions toward an adsorbent increases with an increase in electronegativity and decreases with hydrated radius.50−52 Here, the higher affinity of Pb2+ compared to the counterpart metal ions could be attributed to the higher electronegativity, which follows the order of Pb2+ (2.33) > Ni2+ (1.91) > Cd2+ (1.69) and the smaller hydrated radius (Å), that is, Pb2+ (4.01) > Ni2+ (4.04) and Cd 2+ (4.26). Therefore, the order of heavy metal ion uptake by GO in the binary and ternary solution follows the order of Pb2+ > Ni2+ > Cd2+. It is also concluded that the removal efficiency of Pb2+ by GO decreases with increase in both the initial concentration of Pb2+ and interfering metal ions, that is, Cd2+ and Ni2+. The effects of polyatomic anions on Pb2+ removal in the binary and ternary solution system are shown in Figures S5 and S6, respectively. The %R of Pb2+ in the binary solution of Pb2+− SO42− and Pb2+−NO3− drops from 99.71 to 94.21% and 99.75 to 94.78%, respectively, with the increase in the concentration of both the anions from 50 to 800 mg/L as shown in Table S4. However, in the case of ternary solution of Pb2+−SO42−− NO3−, a reduction in %R of Pb2+ varied from 99.81 to 96.92, (Table S5), with an increase in concentration of both the anions from 50 to 400 mg/L; therefore, an overview of the adsorption of Pb2+ in binary and ternary anionic solution suggests that the SO42− and NO3− have a lesser inhibitory effect on Pb2+ adsorption by GO than that of Ni2+ and Cd2+.

Figure 7. Relationship between ln K and 1/T. Conditions: GO = 1.00 g/L, Pb2+ = 200 mg/L, pH = 5, stirring speed = 160 rpm.

The binary (Pb2+−Cd2+, Pb2+−Ni2+, Pb2+−SO42− and Pb2+− NO3−) and ternary (Pb2+−Ni2+−Cd2+ and Pb2+−SO42−− NO3−) mixture solutions were prepared by mixing together individual ion solutions of known concentrations. The adsorption study of Pb2+ in the binary and ternary mixture solution system was conducted at pH 5; temperature, 303 K; stirring, 170 rpm; dose of GO, 1.00 g/L. The initial concentration of the Pb2+ in the binary system of the cationic study was fixed to 50 mg/L, while the concentration of Ni2+ and Cd2+ varies between 10 and 50 mg/L. However, in the case of the ternary system of cations the initial concentration of all the metal ions varies between 10 and 50 mg/L. The value of %R of Pb2+ in the presence of Ni2+ and Cd2+ in the binary and ternary system is shown in Tables S2 and S3, respectively. In Figure 8a the %R of Pb2+ by GO in the binary metal solutions is displayed. The %R of Pb2+ in Pb2+−Ni2+ solution changes from 99.3 to 89.78% and for Pb2+−Cd2+ solution from 99.3 to 88.42%, with the change in the initial concentration of Ni2+or Cd2+ from 10 to 30 mg/L, whereas, in the ternary solution (Figure 8b), the %R of Pb2+ varies between 99.43 and 91.66, 96.99 and 87.72 and 92.42 and 80.12%, when the initial concentration of Pb2+ was 10, 30, and 50 mg/L, and Ni2+ and Cd2+ was shuffled between 10 and 50 mg/L, respectively. The significant decrease in the removal efficiency suggests an inhibition effect posed by the coexisting divalent cations. This antagonistic behavior of Ni2+ and Cd2+ toward adsorption of Pb2+ on GO in the binary or ternary solution system can be explained on the basis of the fundamental properties such as electronegativity and hydrated radius of metal cations. It is

5. DESORPTION STUDY The reusability of any adsorbent is very important for the economic treatment of wastewater. Therefore, desorption of Pb2+ from the surface of GO was performed by washing the GO with 0.5 M HNO3. The result shows that HNO3 weakens the interaction between GO and Pb2+, which might result in release of Pb2+ from the surface of GO. After desorption, the regenerated GO was dried and reused for subsequent Pb2+ adsorption. The adsorption efficiency of regenerated GO toward Pb2+ shows a decrease of only 11.8% (from 99.3 to 87.5%), even after five consecutive adsorption−desorption cycles (Figure 9). This result suggests that GO can be used for the fast and repeated adsorption of Pb2+ from an aqueous solutionyet another reason for its potential for similar applications. 6. CONCLUSION A low-cost, NaNO3-free synthesis of graphene oxide (GO) at room temperature, for the purpose of Pb2+ adsorption in single, binary, and ternary associations is reported. The study led to the following conclusions:

Figure 8. Effect of Ni2+ and Cd2+ presence on the adsorption of Pb2+ by GO in (a) binary and (b) ternary mixture system. H



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ORCID

Jayanta Bhattacharya: 0000-0001-8644-2915 Funding

All the authors are thankful to central research facility, Indian Institute of Technology, Kharagpur, India, for providing the material characterization facilities. The scholarship of RKS Rathour is supported by the Ministry of Human Resource and Development India, a Government of India wing of research and development. Notes

The authors declare no competing financial interest.

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Figure 9. Recycling of GO for Pb2+ adsorption. Parameter of the adsorption is GO dose = 1.00 g/L, pH = 5, initial Pb2+ concentration = 100 mg/L, stirring = 170 rpm, and temperature = 30 °C.

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• The synthesis route shows a viable, low cost, NaNO3free method of GO preparation at room temperature. • The exclusion of NaNO3 from the reaction mixture eliminates the generation of NOx gas in the workplace as well as into the environment. • The adsorption of Pb2+ on GO is quite high and fast, which opens up a potential area of GO application for rapid wastewater treatment. • The kinetics of Pb2+ adsorption on GO follow the pseudo-second-order kinetics, implying that the adsorption takes place through chemisorption. • The isotherm and thermodynamic analysis shows that the Pb2+ is homogeneously adsorbed on the surface of GO, and the whole process is endothermic and spontaneous. • Finally, the as synthesized GO can be used for the multiple cycle adsorption of Pb2+ and also in the presence of Ni2+, Cd2+, SO42− and NO3−.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00344. The XRD spectra of graphite (Figure S1), UV−vis spectra of graphene oxide (Figure S2), TEM image of Pb2+ adsorbed GO (Figure S3), thermogravimetric analysis of GO (Figure S4), effect of SO42− and NO3− presence in binary system on the adsorption of Pb2+ by GO (Figure S5), effect of SO42− and NO3− presence in ternary system on the adsorption of Pb2+ by GO (Figure S6), intraparticle diffusion model constants for Pb2+ adsorption on GO (Table S1), adsorption of Pb2+ in binary solution Pb2+−Cd2+ or Pb2+−Ni2+ (Table S2) and the adsorption of Pb2+ in ternary solution of Pb2+− Cd2+−Ni2+ (Table S3) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91−3222−283702. I



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