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Polylysine Functionalized Graphene Aerogel for the Enhanced Removal of Cr(VI) through Adsorption: Kinetic, Isotherm, and Thermodynamic Modeling of the Process Devendra Kumar Singh, Vijay Kumar, Sweta Mohan, and Syed Hadi Hasan* Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India S Supporting Information *

ABSTRACT: The amine functionalized graphene aerogel (GAFP) was prepared by using polylysine as an amine-rich cross-linker with the graphene oxide. The prepared GAFP aerogel was characterized by various analytical techniques including FTIR spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Raman spectroscopy, X-ray diffraction, and Brunauer−Emmett− Teller (BET). The as-prepared GAFP aerogel showed an excellent uptake capacity (170.64 ± 9.69 mg/g) for Cr(VI) which was much greater as compared to recently reported graphene-based adsorbents. The kinetics of Cr(VI) adsorption on the GAFP followed the pseudo-second-order model. The mechanism of adsorption was explored through various diffusion models such as Mckay et al., Waber-Morris, and Richenberg which revealed that the external diffusion and intraparticle diffusion governed the rate of Cr(VI) adsorption. The isotherm studies confirmed that the Cr(VI) chemically adsorbed on the GAFP in monolayer fashion which was consistent with the electrostatic interaction between oxyanions and protonated amine groups. The process of Cr(VI) adsorption was found to be thermodynamically spontaneous and endothermic.

1. INTRODUCTION Chromium is a metal of industrial importance which is known for its toxic nature. The wastewater generated from various industries such as textile, electroplating, and leather industries contains an immense quantity of Cr(VI) and Cr(III) which deteriorate the surface as well as groundwater.1 The Cr(VI) is more water-soluble and easily enters into living cells. Therefore, it is more hazardous than Cr(III).2 Cr(VI) is carcinogenic and mutagenic to human health and also affects other aquatic life forms of the ecosystem.3,4 The remediation of Cr(VI) from water is a big challenge because it is found as negative ions in water which poorly interact with the sediment and soil particles in the environment.5 Various methods such as chemical precipitation, ion exchange, membrane separation, and adsorption have been established for the removal of Cr(VI) from water.6,7 Among these, adsorption is an avenue to develop an economic and efficient method with no risk of secondary pollution.2,7 Various adsorbents including clays, activated carbon, waste biomass, inorganic silicates, and polymer nanofiber have been used as adsorbents for the remediation of Cr(VI).8−11 Nevertheless, most of the reported adsorbents are suffering for practical use due to either low adsorption capacity or no understanding of the kinetics, isotherms, and thermodynamics of the adsorption process. In recent years, various porous adsorbents including silica gel12−14 and metal−organic frameworks15,16 have been © XXXX American Chemical Society

extensively studied for the abatement of water contamination. Aerogel is another kind of porous material that possesses opened and interconnected pores. Therefore, it deserves special attention for being an adsorbent. The preparation of aerogel is mostly performed through freeze-drying the hydrogel obtained from different precursors.14 Graphene has been used as a nanoprecursor for the preparation of aerogels due to its remarkable chemical stability, high mechanical strength, and high theoretical surface area which integrates the properties of nanomaterials into the aerogel.17−19 The aqueous phase preparation of the graphene-based aerogel is a difficult task due to the low stability of the aqueous suspension of graphene. Therefore, researchers have preferred graphene oxide (GO) instead of graphene because it has oxygen-containing functional groups.20 Recently, a great effort has been devoted to enhance the physicochemical properties of the aerogel by intrinsic modifications and functionalization. For example, Wu et al. (2012) have doped the nitrogen and boron into the graphene aerogel which was demonstrated as a supercapacitor.21 In the present study, the aerogel was tailored according to the aqueous chemistry of the Cr(VI) to enhance the adsorption properties. In connection to this, polylysine has been selected as a cross-linking molecule for the preparation of graphene Received: February 18, 2017 Accepted: March 31, 2017

A

DOI: 10.1021/acs.jced.7b00188 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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where Ci is the initial concentration (mg/L) of Cr(VI) and Ct is the concentration (mg/L) of Cr(VI) remaining in the solution after adsorption. W is the weight (g) of GAFP added in the solution, and V is the volume (L) of Cr(VI) solution taken.

aerogel because it is a choice of a biocompatible molecule for the environmental application. The polylysine not only acts as a cross-linking molecule but also functionalizes the aerogel matrix by introducing the amine group. Therefore, we have prepared the amine-functionalized graphene aerogel (GAFP) using GO and polylysine. The prepared GAFP aerogel showed an excellent enhancement in the uptake capacity for Cr(VI) as compared to recently reported graphene-based adsorbents (Table S1).22−33 The process parameters of the adsorption were optimized. The detailed kinetics, isotherm, and thermodynamic studies for the adsorption process were also conducted.

3. RESULTS AND DISCUSSION 3.1. Characterization of GAFP Aerogel. The chemical interactions of GAFP constituents, viz., GO and polylysine, were investigated by the FT-IR spectroscopy. The FT-IR spectra of the GO, polylysine, and GAFP are presented in Figure 1 in which the spectrum of GO showed the peaks at

2. MATERIALS AND METHODS 2.1. Materials. Graphite powder (mesh size 150 μm), potassium permanganate (KMnO4, >99.00%), phosphoric acid (H3PO4, 99.00%), and sulfuric acid (H2SO4, 99.99%) were purchased from Sigma-Aldrich. Potassium dichromate (K2Cr2O7, 99.0%) was purchased from Alfa Aesar. Hydrochloric acid (HCl, 35.4%), nitric acid (HNO3, 72%), hydrogen peroxide (H2O2, 30%), and sodium hydroxide (NaOH, 97%) were procured from SD Fine Chemicals Pvt. Ltd. Polylysine (95%) was purchased from Bimal Pharma Pvt. Ltd. All of the chemicals were used without further purification. Throughout the experiments, deionized double-distilled water (DW) was used for the preparation of standards and solutions. The separate stock solutions of Cr(VI) of concentration 1000 mg/L were prepared by dissolving the 2.85 g of K2Cr2O7 into the 1000 mL of water. Further, the solutions of required concentration were prepared by using stock solution. 2.2. Preparation of GAFP Aerogel. The GO was prepared by the modified Hummer’s method.34 0.01 g of asprepared GO was suspended into 50 mL of DW using probe ultrasonicator (Sonics, Vibra Cell). 0.2 g of polylysine was dissolved into the 10 mL of DW. Then, the polylysine solution was added slowly into the GO suspension at 60 °C without stirring that immediately formed the hydrogel, and the experiment was left undisturbed for next 20 min at the same temperature. To prepare aerogel, the obtained graphene hydrogel was frozen at −50 °C and placed into the lyophilizer for 6 h which removed the trapped water leaving the porous matrix of GAFP. 2.3. Batch Adsorption Studies. The experiments of Cr(VI) adsorption were performed in the 100 mL Erlenmeyer flasks. For this purpose, 10 mL of Cr(VI) solution of known concentration was transferred into the flask. Then, a calculated amount of GAFP was also transferred into the flasks. Thereafter, the flasks were placed on the orbital shaker at a speed of 100 rpm. Then, the adsorption mixture was centrifuged to remove the GAFP, and supernatant was collected for the measurement of unadsorbed Cr(VI). The concentration of the Cr(VI) in the samples was measured by an atomic absorption spectrophotometer (AAS, Shimadzu AA6300) having a cathode lamp as a light source of wavelength 357.78 nm. The cathode lamp was operated at 10 mA lamp current with the slit width of 0.7 nm. The background correction was performed by using deuterium lamp. The uptake capacity (q) of GAFP for Cr(VI) was calculated from the equation given below:35 q=

(C i − Ct )V W

Figure 1. FT-IR spectra of GO, polylysine, and GAFP.

1726.52 and 1619.72 cm−1, which corresponded to the υs(CO) of the carboxylic group and υs(CC) of the graphene sheet, respectively. The peak of GO observed at 1726.52 cm−1 vanished in the spectrum of GAFP due to the expected amide bond formation with the amine group of the polylysine. The two peaks at 1646.58 and 1561.36 cm−1 corresponded to the υs(CO) and υb(N−H) of amide bond respectively present in the GAFP due to the amide bond formation as well as the amide bond present in the polylysine chain. The peaks at 1646.58 and 1561.36 cm−1 of GAFP spectrum were not welldistinguishable due to the overlapping of a peak at 1619.72 cm−1 (υs(CC)) of the GO. Moreover, the comparison of all these three FT-IR spectra suggested that the GAFP exhibited the peaks of GO spectrum at 1225.78, 2852.89, and 2923.94 cm−1 with the peaks of polylysine at 1357.78, 1453.94, 2852.89, and 2923.94 cm−1. To further illustrate the bonding and functionalization, XPS analysis was conducted on the GO and GAFP. The wide survey spectra of GO and GAFP are compared in Figure 2a. Unlike GO, one additional peak appeared on the spectrum of GAFP corresponding to the bonding energy of N 1s electron which explicitly advocated the presence of polylysine in the GAFP matrix. The core level C 1s spectrum of GO (Figure 2b) represented the Gaussian peaks corresponding to the CC or C−C (285.01 eV), C−O (287.10 eV), and −COOH (288.79 eV). However, the peak at 288.79 eV did not appear in the core level C 1s spectrum of GAFP (Figure 2c) because the −COOH has formed the expected amide bond with the polylysine. Consequently, a Gaussian peak at 287.52 eV (OC−N)

(1) B



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Figure 2. XPS spectra (a) wide scan spectra of GO and GAFP. (b) Core level C 1s spectrum of GO. (c) Core level C 1s spectrum of GAFP. (d) Core level N 1s spectrum of GAFP.

3.2. Effect of pH on the Uptake Capacity of GAFP. The pH of the solution plays a vital role in the adsorption phenomena of the heavy metals. Therefore, the adsorption experiments were carried out by varying the pH from 2 to 10 while keeping the initial Cr(VI) concentration 90 mg/L, GAFP dose 0.5 g/L, and temperature 30 °C. The result is shown in Figure 4, which clearly showed that the uptake capacity decreased slowly on increasing the pH from 2 to 6, but it decreased sharply on increasing the solution pH above 6. These effects of pH on the uptake capacity for Cr(VI) can be clarified by considering the existence of various oxyanions of the Cr(VI), viz., Cr2O7−, CrO42−, and HCrO4− and pHzpc of the GAFP.37 The pHzpc of the GAFP was found to be 6.2. Therefore, the surface of GAFP acquired positive charge at the pH below 6.2 where the amount of positive charge increased on decreasing the solution pH. That is why the positively charged GAFP surface was favorable for the adsorption of oxyanions at lower pH. Therefore, it is quite clear to visualize that the amine groups of polylysine formed a quaternary ammonium cation which facilitated the electrostatic attraction toward the Cr2O7−, CrO42−, and HCrO4− species of Cr(VI) (Figure 5). The surface of the GAFP became negatively charged on increasing the pH above 6 which possessed a repulsion force for the oxyanions of Cr(VI). 3.3. Effect of Contact Time on the Uptake Capacity of GAFP. The equilibrium time for the adsorption of Cr(VI) was investigated by obtaining the effect of contact time on the uptake capacity at three different doses of the GAFP such as 0.3, 0.5, and 0.7 g/L. The experiments were performed initial Cr(VI) concentration 90 mg/L, pH 2, and temperature 35 °C. The results are represented in Figure 6 which showed initially rapid adsorption at all tested GAFP doses due to the easy

appeared in Figure 2c due to the amide bond formed between GO and polylysine and present in the polymer chain of polylysine.36 Figure 2c also showed the Gaussian peaks at the 285.01 and 285.98 eV corresponding the GO carbon skeleton and C−NH2 moiety of polylysine, respectively. The core level N 1s spectrum of GAFP (Figure 2d) demonstrated Gaussian peaks at 400.71 and 401.66 eV which corresponded to the N− CO and C−NH2, respectively. Figure 2d showed that the intensity of N−CO peak was significantly higher than the C−NH2. It was due to the fact that some of the C-NH2 moiety of polylysine formed amide bond (N−CO) with the carboxylic group of the GO. The morphology and porous attributes of the GO and GAFP were analyzed using SEM micrographs. The SEM micrograph of GO (Figure 3a) showed the flaky wrinkled sheets of the GO. The SEM micrograph of GAFP is presented in Figure 3b which showed a quite uniform morphology of the aerogel in the large scale. The aerogel possessed the arbitrarily interconnected GO sheets which formed a three-dimensional network. The close observation revealed that the hierarchical pores of various size (from 20 μm to several nm) were distributed throughout the GAFP aerogel. The pore walls were not continuous. Therefore, the pores were well-interconnected which was a significant characteristic of the aerogel for being an excellent adsorbent. The energy-dispersive X-ray spectroscopy (EDS) spectra were collected on the surface of GO and GAFP to measure the elemental composition. The EDS analysis of GO is represented in Figure 3c which showed that the carbon and oxygen atoms were present in the GO. The EDS spectrum of the GAFP (Figure 3d) advocated the presence of high nitrogen content as a signature of polylysine with the carbon and oxygen atoms of GO. C



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Figure 3. SEM images of (a) GO and (b) GAFP. EDS spectra of (c) GO and (d) GAFP.

Figure 4. Effect of pH on the uptake capacity.

availability of binding sites and the high concentration gradient of Cr(VI). The rate of adsorption got slower with the lapse of time because of an extent of decreased availability of free binding sites as well as the concentration gradient of Cr(VI) with the proceedings of adsorption.38 Finally, the equilibrium was established at 55 min for all tested GAFP doses. Therefore, the equilibrium time did not depend on the ratio of GAFP doses to Cr(VI) concentration. Furthermore, the uptake capacity of GAFP decreased from 189.90 ± 12.15 to 170.64

Figure 5. Schematic representation of the electrostatic interaction of oxyanions of Cr(VI) with the protonated amine groups.

± 11.09 mg/g on increasing the dose from 0.3 to 0.5 g/L. But, on further increasing the dose from 0.5 to 0.7 g/L, the uptake capacity decreased remarkably from 170.64 ± 11.09 to 122.91 D



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Figure 6. Effect of contact time on the uptake capacity at different GAFP doses.

± 6.85 mg/g. It was because the metal-to-binding site ratio decreased on increasing the GAFP dose where Cr(VI) became insufficient above the dose 0.5 g/L.39 3.4. Kinetic Studies. The kinetic analysis would give the insight into the adsorption process which is primarily used for designing the effective adsorption model. The kinetics of the Cr(VI) adsorption was explored through well-established kinetic models such as pseudo-first-order and pseud-secondorder. The mathematical formula of pseudo-first-order kinetic model can be represented as follows:40 log(qe − qt ) = log(qe) −

ks t 2.303

(2) Figure 7. Kinetic models plot at different GAFP doses: (a) pseudofirst-order and (b) pseudo-second-order.

where qt and qe represent the uptake capacity (mg/g) at time t and at equilibrium, respectively. ks is the equilibrium rate constant. The pseudo-first-order linear plot log(qe − qt) vs t (min) is presented in Figure 7a which shows a remarkable deviation of the kinetic data from the linearity for all tested GAFP doses. The model parameters related to the pseudo-first-order are listed in Table 1. The poor correlation coefficient (R2) of the data and great deviation of the calculated uptake capacity from the experimental uptake capacity advocated that the pseudofirst-order kinetic model cannot give an appropriate kinetic approximation for the adsorption of Cr(VI) on the GAFP. Therefore, the experimental data were further explored by the pseudo-second-order kinetic model. The linear form of the pseudo-second-order model can be represented as follows:41 t 1 1 = + t qt k′2 qe qe (3)

would be a better approximation of the kinetics followed by the Cr(VI) adsorption on the GAFP. Therefore, the rate of adsorption at any time t would be proportional to the (qe − qt)2 where qe−qt term represents the free binding sites at time t. 3.5. Adsorption Mechanism Based on Diffusion Models. 3.5.1. Mass Transfer Studies. In the adsorption process, adsorbates move from the liquid phase to the adsorbent solid surface by four consecutive steps which are as follows:42 1. The adsorbate transportation from the bulk solution toward the boundary film. 2. Adsorbate diffusion from boundary film to adsorbent surface, i.e., external diffusion. 3. Adsorbate shift from adsorbent surface toward the interparticle space and pores, i.e., intraparticle diffusion. 4. Adsorption−desorption on the active sites of the adsorbent. Among these steps, external diffusion and intraparticle diffusion are slow; thus, they would command the rate of adsorption. The influence of the external diffusion on the rate of the adsorption can be calculated by the Mckay et al. model of external diffusion which is represented as follows:43

where k′2 is an equilibrium rate constant for the pseudosecond-order kinetic model. The pseudo-second-order linear plots of the experimental kinetic data are represented in Figure 7b which showed the significant linearity with all GAFP doses. The intercept and slope obtained from Figure 7b were used for the calculation of model parameters related to the pseudo-second-order model which are listed in Table 1. The high values of R2 and concurrence of calculated uptake capacity and experimental uptake capacity suggested that the pseudo-second-order model E



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Table 1. Kinetic Parameters for the Adsorption of Cr(VI) at Different GAFP Doses model parameters

0.3 g/L

0.5 g/L

0.7 g/L

0.13 170.64 ± 11.09 391.74 0.837

0.10 122.91 ± 6.85 157.40 0.911

0.094 196.07 0.985

0.131 136.98 0.993

3.83 0.987

2.93 0.986

41.96 −52.83 0.988 9.35 105.37 0.944

30.92 −31.06 0.986 5.75 82.40 0.968

Pseudo-first-order K1 (min−1) qe(exp.) (mg/g) qe(cal.) (mg/g) R2 K2′ qe (mg/g) R2 βt × 10−7 (cm2/s) R2 Kid.1 (mg/g·min0.5) C1 R2 Kid.2 (mg/g·min0.5) C2 R2

0.11 189.87 ± 12.15 363.08 0.909 Pseudo-second-order 0.086 222.22 0.988 Mass Transfer 2.46 0.983 Interparticle Diffusion 40.34 −39.74 0.996 14.39 90.06 0.964

⎛C ⎛ mk ⎞ ⎛ 1 + mk ⎞ 1 ⎞ ⎟ − ⎜ ⎟β S t ln⎜ t − ⎟ = ln⎜ ⎝ 1 + mk ⎠ ⎝ mk ⎠ t s 1 + mk ⎠ ⎝ Ci

to the intraparticle diffusion step and showed a comparatively slow diffusion of Cr(VI) toward the binding sites present in the pores and intraparticle spaces. Moreover, the third straight lines represented the equilibrium state. Thus, it is quite clear that the adsorption of Cr(VI) has more than one step which would influence the overall rate of adsorption. 3.5.3. Richenberg Model. As clear from the intraparticle diffusion study, the Cr(VI) adsorption is a multistep process which is controlled by more than one mechanism. The contribution of different steps on the rate determination was investigated by using the Richenberg model which can be written as follows:45

(4)

where Ct (mg/L) represents the Cr(VI) concentration after time t (min) and Ci (mg/L) denoted the initial Cr(VI) concentration. m (g/L) is the mass of GAFP used per unit volume of the solution, and k is the Langmuir constant calculated by Langmuir isotherm. Ss (cm−1) is the specific surface area of GAFP present in per unit volume, and βt (cm2/ s) is the external diffusion coefficient. The linear plots for the external diffusion of Cr(VI) are represented in Figure 8a which showed the suitability of the Mckay et al. model with the kinetic data of current sorption system. The significant linearity of the experimental data at all GAFP doses undoubtedly recommended that the external diffusion has influenced the rate of adsorption. The βt for the external diffusion of the Cr(VI) from boundary film to GAFP surface were calculated which were found to be 2.46 × 10−7, 3.83 × 10−7, and 2.93 × 10−7 for adsorbent doses of 0.3, 0.5, and 0.7 g/L, respectively (Table 1). 3.5.2. Intraparticle Diffusion Model. The possibility of intraparticle diffusion of Cr(VI) and its contribution to the overall rate of adsorption was investigated by the widely accepted Weber-Morris model. The mathematical equation of the Weber-Morris model in its linear form can be represented as follows:44 qt = k idt 0.5 + C

G=1−

6 exp( −Bt ) π2

(6)

where G = qt/qe. Bt is the mathematical function of G which can be calculated for each value of G by the following equation: Bt = −0.4977 ln(1 − G)

(7)

Thus, obtained values of Bt were plotted against time which are presented in Figure 8c. The plots showed significant linearity with R2 values 0.980, 0.987, and 0.993 at the GAFP doses 0.3, 0.5, and 0.7 g/L, respectively. The Richenberg model believes that the linear fit of the plot passes from the origin when the rate of adsorption only depends on the intraparticle diffusion, but if the straight line fails to hold the origin, then the external diffusion is influencing the overall rate of adsorption.46 The straight line plot for the current adsorption system did not cross from the origin. Thus, intraparticle diffusion as well as external diffusion influenced the rate of adsorption of Cr(VI). 3.6. Effect of the Initial Concentration and Temperature on the Uptake Capacity. To investigate the effect of temperature and Cr(VI) concentration on the uptake capacity of GAFP, the experiments were carried out at temperatures of 25, 35, and 45 °C by varying the concentration from 60 to 140 mg/L. The results are represented in Figure 9 which clearly showed remarkable enhancement of uptake capacity on raising the Cr(VI) concentration from 60 to 90 mg/L beyond which the enhancement in uptake capacity was rather marginal. Therefore, the 90 mg/L Cr(VI) concentration was adequate to

(5) 0.5

where kid (mg/g·min ) represents the intraparticle diffusion rate constant. Figure 8b represented the intraparticle diffusion profile of Cr(VI) adsorption. The intraparticle diffusion plot showed three distinct linear parts which clearly demonstrated the involvement of more than one step in the adsorption process. The parameters for different straight lines plot are listed in Table 1. The straight lines appearing in between 5 and 25 min represented the external diffusion step which involved the transfer of mass from the boundary film to the solid surface. The second straight line between 25 and 50 min was attributed F



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Figure 9. Effect of the initial Cr(VI) concentration on the uptake capacity at different temperatures.

Langmuir, and Dubinin−Radushkvich (D-R) model were applied to the equilibrium data. 3.7.1. Freundlich Isotherm. Its mathematical equation can be presented as follows:11 1 log qe = log k f + log Ce (8) n where n is the Freundlich parameter of adsorption intensity and kf represents the Freundlich isotherm constant. The plot of log qe vs log Ce (Figure 10a) showed very poor linearity of the experimentally obtained data at all selected temperatures. The values of kf and n were evaluated from the intercept and slope obtained from Figure 10a which are summarized in Table 2. Therefore, the Freundlich isotherm did not fairly describe the isotherm of the Cr(VI) adsorption on the GAFP. The value of n from 1 and 10 is an indicative of the favorability adsorption process.48 Therefore, the values of n recommended that the current adsorption system was favorable for the removal of Cr(VI). 3.7.2. Langmuir Isotherm. The linear equation of the Langmuir model can be represented as follows.49

Figure 8. (a) Mass transfer plot at different GAFP doses. (b) Intraparticle diffusion plot at different GAFP doses. (c) Richenberg model plot at different GAFP doses.

Ce C 1 = 0 + 0e qe qb q

(9)

0

where q is the isotherm predicted uptake capacity and b is the Langmuir parameter related to the binding energy. The Langmuir isotherm profiles for the Cr(VI) adsorption at three temperatures are shown in Figure 10b which have good linearity with the equilibrium data at all temperatures. The model parameters were estimated from the intercepts and slopes obtained from the plots Ce/qe vs Ce (Figure 10b) which are listed in Table 2. The considerably high values of R2 and fine concurrence between q0 and experimental uptake capacity undoubtedly recommended the appropriateness of the Langmuir isotherm for the current adsorption system. Thus, binding sites were energetically equivalent where Cr(VI) adsorbed in the monolayer. Moreover, it is observed from Table 2 that the value of q0 increased by raising the temperature from 25 to 35 °C which means that the adsorption of Cr(VI) was favored by the increase in temperature. Therefore, q0 reduced on further raising the temperature from 35 to 45 °C

saturate the adsorption sites present in the GAFP dose of 0.5 g/ L. The effect of temperature is visualized from Figure 9 that the uptake capacity corresponding the initial Cr(VI) concentration 90 mg/L increased from 134.73 ± 7.81 to 170.64 ± 9.69 mg/g on raising the temperature from 25 to 35 °C. The uptake capacity of GAFP decreased from 170.64 ± 9.69 to 142.31 ± 8.11 mg/g on further raising the temperature from 35 to 45 °C. This is may be due to the increased randomness in the solution phase that becomes a limiting factor and neutralizes the positive effect of temperature. 3.7. Isotherm Studies. The understanding of equilibrium relationship between extent of adsorbate present on the solid surface and remaining adsorbate in the liquid phase is very crucial to design the adsorption system.47 The values of certain isotherm parameters are able to describe the nature of the adsorption and surface characteristics of the adsorbent. Hence, three commonly used isotherm models such as Freundlich, G



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adsorption, whether physical or chemi-adsorption. Hence, the D-R isotherm was employed which can be presented in the form of a mathematical equation as follows:50 ln qm = ln X m − βF 2

(10)

where qm is the uptake capacity in m·mol/g. Xm is the maximum uptake capacity predicted by the isotherm in m·mol/ g. F represents the Polayni potential of adsorption which can be calculated from the following equation: ⎛ 1⎞ F = RT ln⎜1 + ⎟ Ce ⎠ ⎝

(11)

where T is the temperature of adsorption equilibrium and R is the gas constant. β is a representative of the adsorption free energy (E) per mole of adsorbate which can be correlated by the following equation.

1 −2β

E=

(12)

Figure 10c showed the linear plots of D-R isotherm at the temperatures 25, 35, and 45 °C for the Cr(VI) adsorption. The plots showed the significant degrees of linearity with the experimental isotherm data as the values of R2 are considerably enough (Table 2) for all tested temperatures. Therefore, the DR isotherm is applicable for the elucidation of the obtained experimental data. The adsorption energy E was measured by eq 13 using values of β obtained from the slopes ofFigure 10c which are presented in Table 2. The values of E were found between 8 to 16 kJ/mol which clearly pointed toward the chemical adsorption of Cr(VI) on the GAFP.51 3.8. Thermodynamics Studies. The thermodynamic viability of the Cr(VI) adsorption was investigated by measuring the Gibbs free energy change (ΔG0), entropy change (ΔS0), and enthalpy change (ΔH0). The following thermodynamic equations were taken into account for the calculation of these thermodynamic parameters for the Cr(VI) adsorption.52 kc = Figure 10. Equilibrium studies at different temperatures. (a) Freundlich isotherm plot, (b) Langmuir isotherm plot, and (c) D-R isotherm plot.

CAe Ce

(13)

ΔG = −RT ln kc ln kc =

(14)

ΔS ΔH − R RT

(15)

where CAe and Ce denote the amount (mg/L) of Cr(VI) present in the solid phase and liquid phase, respectively. R represents the gas constant (8.314 J/mol), and T represents the temperature in Kelvin. The ΔG0 for the adsorption of Cr(VI) on the GAFP was calculated at temperatures of 20, 25, 30, and 35 °C which are listed in Table 3. The negative values of ΔG0 greatly supported

which might be because of augmented molecular vibrations at the comparatively higher temperature. The value of b also followed the same pattern as q0 for the temperature change which again suggested that 35 °C could be favorable temperature for the Cr(VI) adsorption. 3.7.3. D-R Isotherm. Since the Freundlich and Langmuir isotherm parameters do not explain the nature of the

Table 2. Parameters of Langmuir, Freundlich, and D-R Isotherms for the Adsorption of Cr(VI) at Different Temperatures Freundlich

Langmuir

D-R

temperature

kf(mg/g)

n

R2

Q0 (mg/g)

b (L/mg)

R2

Xm (mmol/g)

E (kJ/mol)

R2

25 °C 35 °C 45 °C

92.46 127.05 102.80

8.60 7.55 9.25

0.92 0.78 0.81

136.61 175.74 166.66

0.96 2.44 2.06

0.99 0.99 0.99

2.71 3.50 2.83

8.34 10.93 10.52

0.983 0.980 0.986

H



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Table 3. Thermodynamic Parameters for the Adsorption of Cr(VI) on the GAFP temperature (° C) 20 25 30 35

ΔG (kJ/mol) −1.14 −2.70 −4.93 −7.26

± ± ± ±

0.06 0.14 0.26 0.41

ΔH (kJ/mol)

ΔS (kJ/mol/K)

118.77

0.41

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00188. Description of used characterization techniques, BET analysis, Raman spectra, XRD, pHzpc measurement, and comparison of uptake capacity with the previously reported graphene based adsorbent (PDF)

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the adsorption of Cr(VI) to be spontaneous. Moreover, the negativity of ΔG0 values augmented with the rising temperature which fairly recommended that the higher temperature would support the adsorption process. The ΔH0 and ΔS0 for the Cr(VI) adsorption were calculated from the slope and intercept of the van’t Hoff plot (Figure 11) and listed in Table 3. The

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Phone: +915426702861. Mob.: +91-9839089919. Fax: +91-5422368428. ORCID

Syed Hadi Hasan: 0000-0001-6705-7907 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors D.K.S., V.K., and S.M. acknowledged the MHRD, New Delhi, India for providing the financial assistance. The authors also acknowledge the Central Instrumentation Facility (CIF), Indian Institute of Technology (BHU), Varanasi, India.

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REFERENCES

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Figure 11. van’t Hoff plot for the adsorption of Cr(VI) on the GAFP.

positive values of ΔH0 recommended the endothermic adsorption of Cr(VI) on the GAFP which showed agreement with the result obtained from isotherm studies. ΔS0 for the adsorption of Cr(VI) was also positive due to the amplified randomness in the solid−liquid interface as a consequence of adsorption and subsequent desorption.53 3.9. Conclusions. In summary, an amine-functionalized GAFP aerogel was prepared for the enhanced removal of Cr(VI). GAFP showed an adsorption capacity of 170.4 ± 9.69 mg/g for Cr(VI) which is much greater than recently reported graphene-based adsorbents. The electrostatic attraction between protonated amine groups and oxyanions of the Cr(VI) was served as a driving force for the adsorption of Cr(VI). Thus, GAFP would be an excellent alternative of the conventional adsorbents for the abetment of Cr(VI) from drinking water as well as from industrial effluents. The adsorption of Cr(VI) followed the pseudo-second-order kinetics where the adsorption occurred through two rate determining mechanisms, i.e., external diffusion and intraparticle diffusion. The isotherm studies revealed that Cr(VI) adsorbed in monolayer fashion on the GAFP. The energy of adsorption calculated by the D-R isotherm suggested that the Cr(VI) adsorption on the GAFP was chemisorption. The thermodynamic parameters ΔG0, ΔH0, and ΔS0 indicated that the adsorption of Cr(VI) on the GAFP is a spontaneous and endothermic process. Therefore, the removal of Cr(VI) from water by GAFP would be beneficial because it is a kinetically and thermodynamically feasible process. I



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