Facile Synthesis of Three-Dimensional MgâAl Layered Double

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Facile Synthesis of Three-Dimensional Mg−Al Layered Double Hydroxide/Partially Reduced Graphene Oxide Nanocomposites for the Effective Removal of Pb2+ from Aqueous Solution G. Bishwa Bidita Varadwaj,† Oluwaseun A. Oyetade,†,‡ Surjyakanta Rana,† Bice S. Martincigh,† Sreekantha B. Jonnalagadda,† and Vincent O. Nyamori*,† †

School of Chemistry and Physics, and ‡School of Health Sciences, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa ABSTRACT: A series of three-dimensional (3D) porous nanocomposites, comprised of partially reduced graphene oxide (pRGO) and CO32− containing Mg−Al layered double hydroxide, were synthesized in two steps. In the first step, graphene oxide (GO) was fabricated by a modified Hummers’ method, and, subsequently, in the second step layered double hydroxide (LDH) nanosheets were homogeneously grown on the surface of the GO sheets by an in situ crystallization approach, involving a facile coprecipitation technique. The alkaline medium used for the in situ growth of LDH on the GO surface resulted in the partial reduction of GO to pRGO, which was confirmed by XRD. XRD also revealed the successful formation of crystalline LDH nanosheets on the surface of pRGO, whereas FTIR spectroscopy confirmed the presence of different functional groups in the nanocomposites. Nitrogen adsorption−desorption studies of the prepared nanocomposites revealed them as high surface area porous materials. Electron microscopic techniques, like TEM and SEM, confirmed that the architectures of the prepared nanocomposites displayed an interconnected 3D network, where a number of LDH nanosheets were interwoven on the surface of pRGO. The elemental mapping and EDX analysis qualitatively confirmed the presence of all of the expected elements in the fabricated nanocomposites. Because of the unique 3D porous network and the presence of a large number of oxygen-containing functional groups, the prepared nanocomposites proved suitable for the adsorption of Pb2+ ions from aqueous solution with a maximum adsorption capacity of 116.2 mg g−1. Equilibrium was achieved after 180 min on conducting the adsorption experiments at pH 4.5. Desorption experiments established the possibility of recovering the metal ions as well as the regeneration of adsorbents for repeated use. KEYWORDS: layered double hydroxides, graphene oxide, nanocomposites, coprecipitation, adsorption

1. INTRODUCTION Over the last few decades, there has been increasing research interest in designing nanocomposites for various potential applications.1,2 The formulation of nanocomposites as effective adsorbents for water pollution remediation is among the various desired applications, because clean water is a basic need of life. A number of studies have reported carbon-based nanocomposites as effective adsorbents for water purification.3−5 Among the various carbon-based materials, graphene has emerged as a new-generation material, possessing a twodimensional (2D) sheet-like structure of sp2 hybridized carbon atoms.6 Graphene oxide (GO), a multiple oxygenated graphene layer with a one-atom thick honeycomb lattice structure, has received a great deal of attention for the effective removal of heavy metal ions from polluted water. Sitko et al.7 reported the high adsorption ability of GO toward a number of heavy metal ions, because of the presence of a number of oxygenic functional groups on its surface. The oxygen atoms present in the functional groups of GO contain lone pairs of electrons, © 2017 American Chemical Society

and therefore, by sharing those electron pairs, they can efficiently bind metal ions to form metal complexes. GO containing multiple oxygenic functional groups on its surface can easily form nanocomposites with other nanomaterials, which are good adsorbents. The synergistic activity of both components in the nanocomposites will boost their adsorption capacity. Layered double hydroxide (LDH) is a type of 2D inorganic layered material with brucite-like layers. LDH can be described by the generic formula [M2+1−xMx3+(OH)2]x+(An−)x/n·mH2O, where M2+ and M3+ represent divalent and trivalent metal cations, respectively, and An− are the charge compensating interlayer anions.8,9 Designing different types of LDH/LDH-based nanocomposites for use as novel adsorbents has also attracted much attention over the past few years.10,11 There are various reported Received: December 23, 2016 Accepted: May 5, 2017 Published: May 5, 2017 17290

DOI: 10.1021/acsami.6b16528 ACS Appl. Mater. Interfaces 2017, 9, 17290−17305

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ACS Applied Materials & Interfaces

2.2. Fabrication of GO. Graphene oxide (GO) was fabricated according to a modified Hummers’ method.21 Initially, a suspension containing graphite powder (0.5 g), NaNO3 (0.5 g), and concentrated H2SO4 (23 cm3) was stirred in an ice bath for 4 h. To the mixture was gently added KMnO4 (3 g) with constant stirring over 50 min. This suspension was then removed from the ice bath, and stirred for a further 2 h at 35 °C. To it was added double-distilled water (46 cm3), which was stirred for 20−30 min at 98 °C. H2O2 (20 cm3) was added slowly, and the mixture was stirred for one more hour. Another 50 cm3 of double distilled water was then added to the mixture, with constant stirring for a further hour at 25 °C. The precipitate was washed 3−4 times with 10% HCl and centrifuged many times with double-distilled water. The precipitate was then dried in a vacuum oven at 50 °C for 48 h to obtain GO. 2.3. Synthesis of Nanocomposites. A series of nanocomposites were synthesized via a simple low supersaturation coprecipitation method at a constant pH (10 ± 0.5). In a typical procedure, 0.1 g of GO powder in 100 cm3 water was ultrasonicated for 1 h and then stirred vigorously with a magnetic stirrer. A 100 cm3 mixed metal solution of Mg(NO3)2·6H2O and Al(NO3)3·9H2O, in which the Mg2+/Al3+ molar ratio was fixed at 2:1, was prepared in deionized water with a total metal ion concentration of 0.3 M. Another 100 cm3 alkaline solution of 0.2 M Na2CO3·10H2O and 0.6 M NaOH was also prepared. Both the mixed metal solution and the alkaline solution were added dropwise to the GO suspension, simultaneously. The pH throughout this process was monitored to be 10 ± 0.5. The gray colored suspension was then constantly stirred for 4 h and was subsequently aged in an oil bath at 80 °C for two more hours. The precipitate was filtered, washed with deionized water several times, and then dried at 100 °C for 24 h. The resulting gray colored powder was designated as 0.3 M Mg−Al LDH/pRGO. Following the same procedure, different composites x Mg−Al LDH/pRGO were synthesized, where x = [Mg2+] + [Al3+], and it varies as 0.1, 0.3, or 0.5 M. In addition, pristine Mg−Al LDH was also synthesized by the aforementioned procedure for comparisons without addition of GO. To see the effect of the alkaline medium on GO, one more sample was prepared. In brief, to the GO suspension was added an alkaline solution containing Na2CO3 and NaOH, followed by constant stirring for 4 h and then aging at 80 °C for 2 h. The resulting precipitate was filtered, washed, and dried at 100 °C for 24 h. The powder obtained was designated as pRGO. 2.4. Characterization Techniques. The Fourier-transform infrared (FTIR) spectra of the samples were recorded with a PerkinElmer Spectrum 100 spectrometer fitted with a universal ATR accessory, in the range 4000−500 cm−1. X-ray diffraction (XRD) patterns of the powdered samples were recorded in the 2θ range of 5−70° with a Rigaku MiniFlex 600 diffractometer, using Cu Kα radiation. The BET surface areas, pore volumes, and pore sizes of the prepared samples were determined by a multipoint N2 adsorption−desorption method at liquid N2 temperature (−196 °C) with a Micromeritics Tristar II 3020 instrument. Before analysis, samples were degassed at 110 °C and 10−6 Torr pressure for 5 h to clear the physically adsorbed moisture. Microstructural analyses of the synthesized samples were carried out by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The TEM micrographs of the fabricated samples were recorded by using a high-resolution transmission electron microscope (HR-TEM: JEOL JEM 2100). Images were taken with the help of a Megaview 3 camera. SEM, energy dispersive X-ray analysis (EDX) spectroscopy, and elemental mapping were performed with a JEOL, JSM 6100 microscope. The values of zeta potential were measured with a Malvern Zetasizer Nanoseries NanoZS instrument using a dip cell. The values were determined by adjusting the pH of the electrolyte (10 mM NaCl background solution) with appropriate amounts of 0.1 mol dm−3 HNO3 or NaOH. 2.5. Batch Adsorption Studies. Adsorption experiments were undertaken to investigate the effect of pH, contact time, and temperature on the sorption of Pb2+ onto GO, pRGO, and x Mg− Al LDH/pRGO composites. An approximate amount of 1 g of pure lead powder was weighed into 20 cm3 concentrated HCl acid, with the

methods for the synthesis of LDH, such as coprecipitation, homogeneous precipitation, hydrothermal synthesis, sol−gel technique, reverse microemulsion, and electrochemical synthesis.12,13 However, the low supersaturation coprecipitation route maintaining a constant pH is a facile method, where wellcrystallized LDHs are synthesized. Mg−Al LDH containing carbonate (CO32−) anions is a type of LDH, which showed its efficacy toward the adsorption of a variety of heavy metal ions, such as Pb2+, Cu2+, and Ni2+. For instance, Yu et al.14 reported the synthesis of a high surface area three-dimensional (3D) Mg−Al LDH by a solvothermal synthesis method, which was a very good adsorbent for removing As(V) and Cr(VI) metal ions from an aqueous medium. The high adsorption efficiency of the material was attributed to its 3D flower-like morphology, which furnished a high surface area. Our continued interest in synthesizing effective adsorbents for water pollution remediation15−17 motivated us to synthesize a series of nanocomposites comprised of CO32− containing Mg−Al LDH and GO, which are very good adsorbents on their own means. The synthesis of these composites involved two steps. In the first step, GO was fabricated, and, subsequently, in the second step, Mg−Al LDH nanosheets were homogeneously grown on the surface of GO sheets by an in situ crystallization approach, involving a facile low supersaturation coprecipitation technique. The alkaline medium used for the in situ growth of Mg−Al LDH on the GO surface resulted in the partial reduction of GO to partially reduced graphene oxide (pRGO). As is known, GO layers in aqueous medium behave as highly negatively charged sheets, because of the ionization of surface oxide groups. This surface negative charge of GO in aqueous medium is complementary to the inherent positive charge of the LDH sheets and therefore contributes to the growth of LDH on its surface. Lead is among various nonbiodegradable heavy metals that cause water pollution. It has long been considered as one of the most noxious heavy metals widely released from various industries. Even very low concentrations of Pb2+ ions in drinking water can cause damaging effects to human health like mental retardation, hypertension, kidney damage, and anemia.3 The synthesized Mg−Al LDH- and GO-based nanocomposites were found to be very good adsorbents for the effective removal of Pb2+ ions from aqueous solutions. Nanocomposites comprised of reduced graphene oxide and LDH for various energy-based applications are well documented.18−20 However, at present, the study of these nanocomposites for environmental pollution remediation is still in its infancy. The promising results obtained for the removal of Pb2+ ions by the synthesized nanocomposites suggest that these composites may offer great potential for the removal of other harmful pollutants from wastewater.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Graphite powder (10 nm) than those for GO and pRGO, and the pore sizes increased on increasing the amount of LDH in the composites. This may be because when the LDH platelets are grown in situ onto the GO matrix in the composite, they effectively separate the GO sheets, and this interweaving of LDH with GO leads to the large pore sizes. Therefore, the network organization of the layers results in nanocomposites with large surface areas that can offer more active sites for adsorption, leading to their enhanced pollutant adsorption capacity. The morphologies and microstructures of the prepared materials were thoroughly investigated by SEM (Figure 4).

Figure 4. SEM micrographs of (a) GO, (b) 0.1 M Mg−Al LDH/ pRGO, (c) 0.3 M Mg−Al LDH/pRGO, and (d) 0.5 M Mg−Al LDH/ pRGO.

Curled and wrinkled nanosheets can be visualized from the SEM micrograph of GO (Figure 4a). The nanocomposites shown in Figure 4b−d are composed of LDH nanosheets uniformly grown vertically on the pRGO surface. The uniform vertical growth and cross-linking of the LDH nanosheets over the surface of pRGO resulted in a loose lamellar 3D porous architecture of the x Mg−Al LDH/pRGO nanocomposites. No morphological differences were observed among the nanocomposites with varying “x” values. Figure 5a shows a representative SEM image of 0.5 M Mg− Al LDH/pRGO, with the corresponding elemental mapping analysis (Figure 5b) and EDX profile (Figure 5c). Various elements present in 0.5 M Mg−Al LDH/pRGO are shown in different colors to categorize their positions within the nanocomposite. It can be seen that all of the expected elements, Mg, Al, C, and O, are uniformly distributed in the nanocomposite. EDX analysis also confirmed the presence of Mg, Al, C, and O as the major elements in the nanocomposite. Seconding the results obtained from SEM analysis, the HRTEM images of x M Mg−Al LDH/pRGO shown in Figure 6 also validate the vertical growth of LDH platelets on the pRGO surface. The layered structure of GO and pRGO can be seen from the micrographs (Figure 6a,b). It can be seen from the HR-TEM micrograph of 0.1 M Mg−Al LDH/pRGO that the 17295


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Figure 5. (a) SEM micrograph, (b) elemental mapping, and (c) EDX profile of 0.5 M Mg−Al LDH/pRGO.

adsorbents after 180 min, demonstrating a moderate rate of sorption of Pb2+ onto the studied sorbents. Of significant note is the increased metal sorption capacity demonstrated by the x Mg−Al LDH/pRGO nanocomposites as compared to the GO and pRGO nanomaterials at each period (Figure 9). The efficiency of the x Mg−Al LDH/pRGO nanocomposites toward Pb2+ removal noticeably increased on increasing the concentration of LDH in the composites (Figure 9). The highest efficiency exhibited by the 0.5 M Mg−Al LDH/ pRGO nanocomposite can be attributed to its high surface area and surface negative charge (Table 5; Figure 8). In this study, an agitation time of 24 h was employed for all other experiments to ensure the complete removal of Pb2+ from solution. 3.2.3. Kinetics Studies. Kinetics studies were performed to explain the transport of Pb2+ onto the various adsorbents and to understand the dynamics and rate of adsorption involved during adsorption. The pseudo-first-order, pseudo-secondorder, intraparticle diffusion, and Elovich models were tested with the experimental data obtained from the variation of contact time for each adsorbent. The nonlinear equations for these models are presented in Table 2. Table 6 illustrates the kinetics parameters for all of the models studied for the sorption of Pb2+ from aqueous solution onto GO, pRGO, and x Mg−Al LDH/pRGO nanocomposites. The adequacy of the model that describes the data is chosen on

suspensions are fairly stable within a pH range 3−9, and the surface charges become more negative on increasing the pH. Consequently, the enhanced metal cation adsorption observed can be explained by the strong electrostatic interactions between the negatively charged surfaces of the composites and positively charged Pb2+ ions. The sorption of Pb2+ onto GO and x Mg−Al LDH/pRGO composites can therefore be assumed to proceed via electrostatic interactions between Pb2+ and the negatively charged surfaces of the adsorbents. Hence, the adsorption of Pb2+ onto Mg−Al LDH, GO, and 0.5 M Mg− Al LDH/pRGO was significantly influenced by the surface areas of the sorbents and the surface charges present on the adsorbents at different pH values. 3.2.2. Effect of Contact Time. To examine the influence of contact time on the sorption of Pb2+ onto GO, pRGO, and x Mg−Al LDH/pRGO nanocomposites, experiments were conducted at pH 4.5 over a period of 5−1440 min. Figure 9 shows that at first there is a fast removal of Pb2+ as the contact time between the adsorbents and adsorbate increases, followed by a gentler increase until little or no further increase in removal was noticed. This trend was noticeable for all of the adsorbents. The availability of an increased number of active sites on the adsorbents results in rapid Pb2+ removal at the earliest stages (5−90 min) of the reaction. As the contact time increases, saturation of sites increases, resulting in a slow uptake of Pb2+ with time. A state of equilibrium was achieved for all 17296


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Figure 6. HR-TEM micrographs of (a) GO, (b) pRGO, (c) 0.1 M Mg−Al LDH/pRGO, (d) 0.3 M Mg−Al LDH/pRGO, and (e) 0.5 M Mg−Al LDH/pRGO.

Figure 7. Adsorption of Pb2+ onto Mg−Al LDH, GO, pRGO, and x Mg−Al LDH/pRGO. (a) Effect of pH [conditions: 25 cm3 of 100 mg dm−3 Pb2+ solution, 24 h contact time, 20 mg dose of adsorbent, agitation speed 150 rpm, temperature 20 °C] and (b) Pb2+ speciation as a function of pH in aqueous solution. Numerical values of log β for the metal hydroxide complexes used in the calculation of the speciation curves were obtained from Critical Stability Constants compiled by Smith and Martell,51 and plots were obtained with the aid of HySS software.52

present on the surface of the adsorbents.7,16 As the amount of LDH increases, the active sites on the adsorbents will increase, resulting in an increased uptake (qeq) of Pb2+ from aqueous solution (Table 6). Similarly, the boundary thickness (l) of LDH-containing nanocomposites was noticed to increase considerably as the amount of LDH increased (Table 6). This trend denotes that adsorption onto all adsorbents was influenced by the boundary layer of the adsorbents.15 Hence, a multistep process could be said to be involved in the removal of Pb2+ onto these sorbents. Adsorption usually occurs through four processes: (i) diffusion of the adsorbate molecule from the bulk solution onto the surface of the adsorbent, (ii) passage of the adsorbate through the liquid attached to the surface of the

the basis of the lowest values of the sum of squared residuals (SSR) and residual squared errors (RSE).16 Hence, the experimental data obtained for GO fit the pseudo-secondorder model, while the Elovich model provided best fit for data obtained for pRGO and all x Mg−Al LDH/pRGO nanocomposites (Table 6). Similarly, the qeq values (a representation of the uptake of Pb2+ per gram of sorbent) for the pseudosecond-order model produced closer values to the experimental values (Table 6). This model assumes a bimolecular interaction, involving the sharing/exchange of electrons between the adsorbate and adsorbent, is responsible for adsorption.16 Hence, adsorption of Pb2+ onto these adsorbents is assumed to proceed via ionic interactions with the hydroxyl groups 17297


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adsorbent, (iii) diffusion of adsorbate through the pores of the adsorbent, and (iv) interaction of the adsorbate with the active sites on the adsorbent.15 Hence, the sorption of Pb2+ onto GO, pRGO, and x Mg−Al LDH/pRGO nanocomposites could be said to be a multistep process. 3.2.4. Effect of Temperature. Changing the adsorbate temperature has been reported to affect the ability of an adsorbent to remove a particular adsorbate.54 This is due to the variation in the solution viscosity, adsorptive forces responsible for adsorption, rate of diffusion of the adsorbate molecules, and the textural properties of the adsorbents.16,54,55 Hence, the influence of temperature was investigated by varying the adsorbate temperature within the range of 293−318 K for all adsorbents. An increase in the adsorption capacity (qe) of GO and pRGO is observed as the temperature of the adsorbate is increased (Figure 10a,b). An increase in the adsorbate temperature results in a decrease of the solution viscosity, an increase in the rate of diffusion across the external boundary, an increase in the porosity and pore volume of the adsorbents, and activation of active sites on the sorbents.16,54,55 These factors were responsible for the increased Pb2+ uptake onto the aforementioned sorbents, due to the increased mobility of Pb2+ onto the active sites of these sorbents. This trend therefore suggests an endothermic nature of adsorption,16,54,55 and implies the effectiveness of these sorbents for the removal of metal ions at point source, because effluents are normally discharged at above ambient temperatures.15,50 In the case of 0.5 M Mg−Al LDH/pRGO, there is virtually no change in Pb2+ sorption with change in temperature, which demonstrates the applicability of the sorbent at both low and high temperatures (Figure 10e). An increase in the adsorbate temperature could also lead to the decrease in the adsorptive forces responsible for interaction between the adsorbent and adsorbate.16 This was observed for the 0.1 and 0.3 M Mg−Al LDH/pRGO composites (Figure 10c,d). This effect was much more noticeable for the 0.1 M Mg−Al LDH/pRGO nanocomposite, hence resulting in a decrease of qe as the adsorbate temperature was increased.

Figure 8. Variation of the zeta potential with pH for Mg−Al LDH, GO, and 0.5 M Mg−Al LDH/pRGO.

Figure 9. Effect of contact time on the adsorption of Pb2+ onto GO, pRGO, and x Mg−Al LDH/pRGO [conditions: 25 cm3 of 100 mg dm−3 Pb2+ solution, pH = 4.5, 20 mg dose of adsorbent, temperature 20 °C, agitation speed 150 rpm].

Table 6. Kinetics Parameters for the Adsorption of Pb2+ onto GO, pRGO, and x Mg−Al LDH/pRGO Nanocomposites model experimental pseudo-first-order

pseudo-second-order

intraparticle diffusion

Elovich

a

parametera

GO

pRGO

0.1 M Mg−Al LDH/ pRGO



qexp/mg g−1 k1/10−2 min−1 qeq/mg g−1 RSE SSR k2/10−3 g mg−1 min−1 qeq/mg g−1 RSE SSR kid/mg g−1 min−0.5 l/mg g−1 RSE SSR α/mg g−1 min−1 β/g mg−1 RSE SSR

70.38 0.067 65.79 4.475 641.0 1.520 69.82 1.962 123.2 3.078 26.04 30.11 2992 235.2 0.138 4.910 771.5

65.05 0.050 59.41 4.723 713.8 1.185 63.65 1.964 123.5 2.756 27.34 24.86 2039 58.08 0.129 4.131 15.55

77.84 0.100 71.40 6.091 1187 2.146 75.30 3.045 296.7 3.420 47.99 34.69 3972 146.4 0.151 3.449 35.71

89.64 0.137 84.21 4.291 589.1 2.764 87.82 1.569 78.73 4.024 58.40 43.98 6382 357.4 0.167 4.637 52.15

121.6 0.229 115.67 6.894 1521 3.749 119.6 3.168 321.2 5.556 96.64 64.79 2385 382.6 0.182 4.022 517.7

RSE, residual standard error; SSR, sum of squared residuals. 17298


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Figure 10. Effect of change in temperature on the adsorption of Pb2+ onto (a) GO, (b) pRGO, (c) 0.1 M Mg−Al LDH/pRGO, (d) 0.3 M Mg−Al LDH/pRGO, and (e) 0.5 M Mg−Al LDH/pRGO nanocomposites [conditions: 25 cm3 of 10−100 mg dm−3 Pb2+ solution, pH = 4.5, 24 h contact time, 20 mg adsorbent dose, agitation speed 150 rpm].

3.2.5. Adsorption Isotherms. Two-parameter (Langmuir,28 Freundlich,29 Temkin,30 and Dubinin−Radushkevich31) and three-parameter (Sips,32 Toth,33 Redlich−Peterson,34 and Khan35) isotherms were applied to understand the mechanism of adsorption and the mode of interaction between Pb2+ and the surfaces of the adsorbents. The nonlinear equations of the models are presented in Table 3. Adsorption equilibrium data

were obtained by varying the initial Pb2+ concentration over a temperature range of 293−318 K. The adequacy of the models that best describe the equilibrium data was assessed from the lowest SSR and RSE values.15,50 Table 7 gives the isotherm parameters for the models that fit the equilibrium data for GO and pRGO. The isotherm parameters obtained for the x Mg−Al LDH/pRGO composites 17299


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ACS Applied Materials & Interfaces Table 7. Isotherm Parameters for the Adsorption of Pb2+ onto GO, pRGO temp/K adsorbent GO

isotherm

parametersa

293

303

313

318

Langmuir

qm B RSE SSR KF N RSE SSR qm B N RSE SSR qm B RSE SSR qm B N RSE SSR

65.10 0.337 1.756 24.67 24.84 3.977 6.283 315.8 65.62 0.343 1.029 1.864 24.33 62.94 0.733 2.717 59.06 60.19 0.732 0.723 1.767 21.86

66.84 0.389 2.844 64.69 27.22 4.201 6.895 380.3 68.28 0.407 1.087 2.973 61.87 64.76 0.979 1.883 28.38 63.41 1.027 0.873 1.720 20.71

69.01 0.543 2.742 60.16 30.91 4.536 7.964 507.4 69.9 0.554 1.059 2.890 58.45 66.25 1.256 3.192 81.49 63.51 1.551 0.657 0.575 2.314

70.51 0.741 3.037 73.80 33.84 4.76 7.345 461.6 74.11 0.731 1.241 2.662 49.60 69.30 1.519 2.33 43.31 67.12 1.883 0.764 1.107 8.585

Freundlich

Sips

pRGO

Langmuir

Sips

a

RSE, residual standard error; SSR, sum of squared residuals.

Table 8. Isotherm Parameters for the Adsorption of Pb2+ onto x Mg−Al LDH/pRGO Nanocompositesa temp/K

a

adsorbent

isotherm

parameters

293

303

313

318

0.1 M Mg−Al LDH/pRGO

Langmuir


Langmuir


Langmuir

qm b RSE SSR qm b RSE SSR qm b RSE SSR

74.95 1.789 4.829 186.5 84.82 20.02 24.78 138.6 116.2 9.098 23.79 137.3

64.82 0.522 1.855 27.54 83.89 9.215 16.96 85.82 116.5 13.21 32.64 196.4

54.17 0.197 0.570 2.599 79.35 2.575 5.411 31.44 117.2 20.43 43.44 272.4

45.58 0.156 1.292 13.36 78.96 1.554 4.604 20.63 121.6 50.66 52.16 349.0

RSE, residual standard error; SSR, sum of squared residuals.

to fit their experimental data for the adsorption of Pb2+ onto GO- and LDH-containing adsorbents.7,56−58 An increase in Pb2+ uptake (qm) was observed as the adsorbate temperature was increased from 293 to 318 K for GO, pRGO, and the 0.5 M Mg−Al LDH/pRGO nanocomposite (Tables 7 and 8). The adsorptive strength (b) of these sorbents noticeably increased as the adsorbate temperature was increased. This trend demonstrates a strong interaction between Pb2+ and the active sites on these sorbents as the adsorbate temperature is increased. 16 However, increasing adsorbate temperature resulted in a decreased uptake of Pb2+ when 0.1 and 0.3 M Mg−Al LDH/pRGO nanocomposites were used. The Langmuir maximum adsorption capacities (qm) obtained in this study demonstrate favorable comparison with others reported for similar adsorbents (Table 9).7,56,58−60 Hence, the activities of both

are given in Table 8. Of the two-parameter isotherms, the equilibrium data obtained for GO and pRGO were best described by the Langmuir model, while for the threeparameter isotherms, the Sips model gave a good representation of the data. The equilibrium data obtained by using the x Mg−Al LDH/pRGO composites could only be described by the Langmuir model (Table 8). The Langmuir isotherm assumes that adsorption occurs on a homogeneous surface by monolayer adsorption without interaction between adsorbed entities, while the Sips isotherm is a combination of the Langmuir and Freundlich isotherms.32 Hence, the sorption of Pb2+ onto the surface of GO, pRGO, and x Mg−Al LDH/pRGO nanocomposites could be assumed to be homogeneous, occurring on uniform and equivalent sites, without interaction between adjacent adsorbate ions.28,54 The Langmuir isotherm has also been predicted by various authors 17300


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ACS Applied Materials & Interfaces LDH and GO toward metal ion removal can be enhanced by forming composites.

Table 10. Thermodynamic Parameters for the Adsorption of Pb2+ onto GO, pRGO, and x Mg−Al LDH/pRGO Nanocomposites

Table 9. Comparison of the Langmuir Maximum Capacity (qm) for the Adsorption of Pb2+ onto Various Adsorbents adsorbents graphene oxide Mg/Al LDH magnetic chitosan/ graphene oxide graphene nanosheets Mg/Fe LDH GO pRGO 0.1 M Mg−Al LDH/pRGO 0.3 M Mg−Al LDH/pRGO 0.5 M Mg−Al LDH/pRGO

conditions −3

pH 5.0, Ci 1 mg dm , 2 mg dose, 120 min, 25 °C pH 5.7, Ci 15 mg dm−3, 24 h, 30 °C pH 5.0, 20 mg dose, 60 min, 30 °C pH 4.0, Ci 5−80 mg dm−3, 12.5 mg, 15 h, 30 °C pH 6.0, Ci 159.57 mg dm−3, 24 h, 25 °C pH 4.5, Ci 10−100 mg dm−3, 20 mg, 120 min, 20 °C pH 4.5, Ci 10−100 mg dm−3, 20 mg, 120 min, 20 °C pH 4.5, Ci 10−100 mg dm−3, 20 mg, 120 min, 20 °C pH 4.5, Ci 10−100 mg dm−3, 20 mg, 120 min, 20 °C pH 4.5, Ci 10−100 mg dm−3, 20 mg, 120 min, 20 °C

qm/mg g−1 1119

7 57

76.94

58

35.70

59

78.73

60

65.1

this study this study this study this study this study

74.95 84.82 116.2

GO

refs

66.16

62.94

adsorbents

pRGO




3.2.6. Thermodynamic Parameters of Adsorption. The process of adsorption is temperature-dependent and is associated with various thermodynamic parameters, such as the changes in Gibbs energy (ΔG°), entropy (ΔS°), and enthalpy (ΔH°). These parameters were estimated from eqs 3 and 461 for the sorption of Pb2+ onto GO, pRGO, and x Mg−Al LDH/pRGO nanocomposites. ΔG° = − RT ln K

T/K

ΔG°/kJ mol−1

ΔH°/kJ mol−1

ΔS°/J K−1 mol−1

293 303 313 318 293 303 313 318 293

−24.77 −25.61 −27.41 −28.72 −26.61 −27.85 −29.48 −30.57 −29.25

303 313 318 293

−26.27 −24.10 −23.45 −35.54

−111.4

−277.9

303 313 318 293

−34.15 −31.82 −30.98 −34.37

−103.9

−229.7

303 313 318

−35.89 −38.22 −41.33

33.04

193.7

30.31

191.4

62.29

323.8

solution interface. Hence, adsorption onto GO, pRGO, and 0.5 M Mg−Al LDH/pRGO nanocomposite is entropy-driven, but enthalpy-driven for the 0.1 and 0.3 M Mg−Al LDH/pRGO nanocomposites. Similarly, the interaction between the adsorbate cation and the adsorbents could be inferred from the heats of adsorption (ΔH°) obtained for each adsorption process. Interaction between the adsorbate and adsorbent is termed physical (physisorption) when the ΔH° value is between 2.1 and 20.9 kJ mol−1.15,64 However, the process is chemical (chemisorption) when the heat of adsorption is between 80 and 200 kJ mol−1.15,64 Hence, a chemical interaction could be assumed to be responsible for adsorption between Pb2+ and 0.1 and 0.3 M Mg−Al LDH/pRGO nanocomposites. On the other hand, a physio-chemical process, indicating that adsorption was facilitated by both processes, was assumed for GO, pRGO, and 0.5 M Mg−Al LDH/pRGO composites, because the ΔH° values were higher than for a physisorption process, but lower than for chemisorption.15,16 In this study, adsorption was therefore assumed to occur via the formation of strong ionic bond (chemisorption) or van der Waals forces (physisorption) between the adsorbate cation and the adsorbents. The thermodynamic parameters obtained indicate the spontaneity of the adsorption process, hence implying the effectiveness of all sorbents for the remediation of wastewater containing heavy metal ions as pollutants. 3.2.7. Desorption Studies. Desorption experiments give insights on the recovery of metal ions on the spent adsorbents and the possible regeneration of the adsorbent for reuse. This process helps to minimize the disposal of spent adsorbents, hence reducing the production of secondary pollutants. In this study, two solutions, 0.1 mol dm−3 HCl and 0.01 mol dm−3 EDTA, were applied as eluents, to ascertain the best eluent for regeneration of the spent adsorbents. Desorption experiments were performed by agitating an approximate amount of 100 mg

(3)

ΔH ° ΔS° + (4) RT R −1 −1 where R is the universal gas constant (8.314 J K mol ) and T is the absolute temperature in Kelvin. K is the distribution adsorption coefficient, calculated from the product of qm and b, obtained from Langmuir plots (Tables 7 and 8). The calculated K value was made dimensionless by multiplying by 1000.61−63 A plot of ln K against 1/T was obtained, and the thermodynamic parameters, ΔH° and ΔS°, were calculated from the slope and intercept of the line, respectively.61−63 Table 10 presents the calculated thermodynamic parameters obtained for the adsorption of Pb2+ onto GO, pRGO, and x Mg−Al LDH/pRGO nanocomposites. The negative ΔG° values demonstrate the spontaneity of the adsorption process for all adsorbents investigated (Table 10). These values were observed to increase as the adsorbate temperature is increased for GO, pRGO, and the 0.5 M Mg−Al LDH/pRGO nanocomposite and to decrease with temperature for the 0.1 and 0.3 M Mg−Al LDH/pRGO nanocomposites (Table 10). Hence, the removal of Pb2+ onto GO, pRGO, and 0.5 Mg−Al LDH/pRGO nanocomposite was favored at high temperatures. Positive ΔH° and ΔS° values obtained for these sorbents denote an endothermic process of adsorption and an increase in the disorderliness at the solid/solution interface, resulting in a favorable adsorption process (Table 10). In contrast, negative ΔH° and ΔS° values for the 0.1 and 0.3 M Mg−Al LDH/ pRGO nanocomposites indicate an exothermic process of adsorption and a decrease in the disorderliness at the solid/ ln K = −

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LDH/pRGO nanocomposites. The figure shows considerable removal of Pb2+ onto the recovered adsorbents. This trend is consistent with the percentage adsorbed onto pristine adsorbents (Figure 9). To re-examine the efficacy of the adsorbents, the reused composites were taken through a process of regeneration with HCl to examine the stability of the materials after reuse. Figure 12 shows the FTIR spectra of regenerated (a) 0.3 M Mg−Al

of the Pb-loaded adsorbent in a 10 cm3 aliquot of each eluent for 30 min in a thermostated water bath preset at 20 °C. After agitation, the suspensions were filtered, and the concentrations of the desorbed metal ions were determined by ICP-OES. Table 11 presents the percentage desorbed from a loaded adsorbent by using either HCl or EDTA as the eluent. Table 11. Percentage Desorption of Pb2+ by Using HCl or EDTAa desorption/% adsorbent

EDTA

HCl

GO pRGO 0.1 M Mg−Al LDH/pRGO 0.3 M Mg−Al LDH/pRGO 0.5 M Mg−Al LDH/pRGO

64.88 54.86 47.09 63.61 45.35

83.92 87.30 71.20 82.67 89.09

a

Conditions: 10 cm3 of either solution, 100 mg of Pb-loaded adsorbent, agitation speed 150 rpm, equilibration time 30 min, and temperature 20 °C.

Although a fairly good desorption efficiency was obtained when 0.01 mol dm−3 EDTA solution was applied as the eluent, better adsorbent regeneration was achieved with 0.1 mol dm−3 HCl. These results demonstrate the ability of HCl to effectively regenerate spent adsorbents and recover the adsorbate for reuse in many other industrial applications, such as electroplating. Hence, HCl-regenerated adsorbents were washed with deionized water to remove any adsorbed Pb2+ on their surfaces, dried in the oven at 80 °C, and reapplied for Pb2+ adsorption, to investigate the extent of reusability without mechanical failure. In this experiment, 20 mg of regenerated adsorbents was weighed into 25 cm3 of a solution containing an initial Pb2+ concentration of 100 mg dm−3. The solutions were placed in a thermostated water bath and agitated at 20 °C for 24 h. After agitation, the final concentrations of Pb2+ were determined from the filtrates, and the extent of reusability was estimated by using eq 2. Figure 11 shows the percentage of Pb2+ adsorbed onto the regenerated adsorbents of GO, pRGO, and x Mg−Al

Figure 12. FTIR spectra of regenerated (a) 0.3 M Mg−Al LDH/ pRGO and (b) 0.5 M Mg−Al LDH/pRGO nanocomposites.

LDH/pRGO and (b) 0.5 M Mg−Al LDH/pRGO nanocomposites. The spectra indicate that the characteristic functional groups of these composites are still present, demonstrating the stability of these materials after regeneration. These results therefore illustrate that the x Mg−Al LDH/ pRGO nanocomposites were effectively regenerated by using HCl as the eluting agent and that the materials could be reused after regeneration without loss in efficacy. Additionally, possible leaching of the LDH constituents into the adsorbate solution was tested. For this, the concentrations of Mg2+ and Al3+ were determined in the filtrates obtained after desorption. The experiments showed no traces of Mg2+ and Al3+ in the filtrates, demonstrating that both metal cations remained intact in the brucite layers of the LDH composites. 3.3. Analysis of Real Samples. Wastewater and industrial effluents contain a variety of substances, including several cations and anions. The removal of metal ions from wastewater onto adsorbents could be altered due to the presence of interfering substances, such as salts, in solution. It is therefore imperative to investigate the efficacy of adsorbents in real-life scenarios, to understand the effect of competing substances on metal ion sorption. Four water samples were therefore collected from various locations across Durban, South Africa. First, experiments were performed to determine the efficiency of x Mg−Al LDH/ pRGO nanocomposites in dilute solutions, that is, where initial Pb2+ concentrations are in low amounts (≤1 mg dm−3). The initial Pb2+ concentration in all samples was within 0.05−1 mg dm−3. In these samples, our results demonstrate 100% removal of Pb2+ onto each composite, demonstrating the ability of the adsorbents to remove metal ions in low concentrations. These water samples were then spiked to contain initial Pb2+ concentrations in the ≤5 mg dm−3 range, because the

Figure 11. Percentage Pb2+ adsorbed on HCl-regenerated adsorbents: (a) GO, (b) pRGO, (c) 0.1 M Mg−Al LDH/pRGO, (d) 0.3 M Mg− Al LDH/pRGO, and (e) 0.5 M Mg−Al LDH/pRGO nanocomposites [conditions: 25 cm3 of 100 mg dm−3 Pb2+ solution, pH = 4.5, 24 h contact time, 20 mg adsorbent dose, agitation speed 150 rpm]. 17302


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ACS Applied Materials & Interfaces concentrations of Pb2+ were low in the real water samples. All nanocomposite materials demonstrated excellent efficiency for Pb2+ removal from both spiked and unspiked water samples. These results therefore demonstrate the efficacy of the composites for Pb2+ removal even at low concentrations. Second, the influence of competing substances (cations/ anions) in water samples was investigated to determine the effectiveness of Mg−Al LDH/pRGO nanocomposites toward Pb2+ removal in real-life scenarios. In this experiment, the concentrations of some divalent metal ions, such as Cd2+, Zn2+, and Cu2+, was increased in the real water samples so as to contain an initial concentration of 5 mg dm−3. The samples were also spiked to contain an initial Pb2+ concentration of 2 mg dm−3. Similarly, the concentration of NaCl and MgSO4 was increased to 0.1 mol dm−3 in all water samples. Real water samples usually contain a variety of substances; hence, they serve as excellent indicators to determine the effectiveness of Mg−Al LDH/pRGO nanocomposites in the presence of several competing substances. For this, about 50 mg of each composite was weighed into a 25 cm3 aliquot of the water sample and agitated in a thermostated water bath preset at 20 °C for 24 h. After agitation, the suspensions were filtered, and the final concentration of Pb2+ was determined in the filtrates. The percentage efficiency of the adsorbents was assessed to investigate their efficacy for Pb2+ removal in the presence of competing substances. Our results demonstrate a moderate (60−72%) percentage removal for Pb2+ in the presence of these competing cations and anions. The moderate removal yields of Pb2+ onto the nanocomposites could be attributed to competition for active sites by other cations in solution. The decreased Pb2+ sorption could also be attributed to salt interference, due to the formation of complexes as a result of chlorides or sulfates in solution.65 Although the effectiveness of the Mg−Al LDH/ pRGO nanocomposites for Pb2+ removal decreased in the presence of several competing substances, they still functioned within reasonable limits, thereby demonstrating their suitability for metal ion removal in wastewater and industrial effluents.

nanocomposites containing 0.1 and 0.3 M Mg−Al LDH. Thus, among all of the adsorbents tested in this work, 0.5 M Mg−Al LDH/pRGO was found to be the most effective for the removal of Pb2+ ions from aqueous solutions. The negative surface charge at the desired pH along with the high surface area of the nanocomposite are the factors responsible for its high sorption efficiency. Desorption experiments demonstrated that regeneration of the adsorbents for reuse and the recovery of the metal ion for other possible applications was feasible. In addition, the nanocomposites performed well on real samples in the presence of competing ions. Therefore, these nanocomposites appear to possess all of the necessary characteristics for efficacious heavy metal ion adsorbents.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +27 31 260 8256. Fax: +27 31 2603091. E-mail: [email protected]. ORCID

G. Bishwa Bidita Varadwaj: 0000-0002-6170-2554 Sreekantha B. Jonnalagadda: 0000-0001-6501-8875 Vincent O. Nyamori: 0000-0002-8995-4593 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the financial support of the University of KwaZulu-Natal (UKZN) and the National Research Foundation, South Africa. We are also thankful to the UKZN Nanotechnology Platform for its support.

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REFERENCES

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4. CONCLUSIONS In this work, we have fabricated a series of 3D Mg−Al LDH/ pRGO nanocomposites, where Mg−Al LDH nanosheets were assembled via a low supersaturation coprecipitation method over the surface of GO. The alkaline medium used during the synthesis procedure of the nanocomposites partially reduced GO to pRGO. A systematic investigation of the prepared materials revealed that LDH nanoplates were interwoven vertically onto the pRGO surface forming 3D open porous architectures with high surface areas. The suitability of these nanomaterials to act as heavy metal ion adsorbents was tested for the removal of Pb2+ from aqueous solutions through batch adsorption experiments. Adsorption studies were conducted at pH 4.5, and equilibrium was achieved after 180 min. The kinetics data were described by the pseudo-second-order and the Elovich models. An increase in Pb2+ uptake onto the LDH-containing nanomaterials markedly improved as the amount of LDH increased. The Langmuir isotherm best described the equilibrium data, thereby demonstrating monolayer coverage of Pb2+ onto the adsorbent active sites. The adsorption onto all adsorbents was spontaneous and feasible, demonstrating an endothermic adsorption for GO, pRGO, and 0.5 M Mg−Al LDH/pRGO. However, an exothermic adsorption process was noticed for the 17303


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