Facile synthesis of nanostructured ZnO–rGO based graphene and its application in wastewater treatment

In the present work, graphene oxide GO is prepared by chemical exfoliation of graphite using Hummer’s method. A facile and green synthesis of ZnO–rGO nanocomposite is performed using aloe vera plant extract. The characterization tools (XRD, FTIR, FESEM, HrTEM, AFM) proved the formation of single phase of ZnO–rGO nanocomposite. Since the environmental contamination caused by Cd(II) ion is a world issue, it has a harmful effect, especially on the human health and environment. Subsequently, our goal in this work is to find an accurate method for detection and adsorption of toxic Cd(II). ZnO and ZnO–rGO nanocomposites are prepared for removing Cd(II), and on behalf of increasing its removal efficiency, GO is added. The results showed a great improvement in removal efficiency reached up to 90% at pH 6 after 90 min.


Introduction
Synthesis, properties and applications of graphenebased nanocomposites materials have attracted tremendous attention nowadays [1][2][3][4][5][6][7][8][9][10]. Thus, the insoluble graphene has limited its applications without functionalization by metal oxides, polymers and dopant. Among many methods of preparing the soluble graphene oxide and its reduced form, chemical reduction of graphite oxide is the most popular method. Chemical reduction methods generally use toxic and costly reducing agent, e.g., hydrazine hydrate [6][7][8]. In order to overcome this disadvantageous, some plant extracts are used for reduction the of graphene oxide [9,10]. These methods are low cost and eco-friendly. In this work, aloe vera plant extract is used to reduced graphene oxide and preparation of ZnO-rGO nanocomposite for wastewater treatment and removal of the heavy metal Cd.
Thus, the pollution of heavy metals in wastewater is one of the main challenges faced by the environment and affects human health [11]. Water polluted with heavy metals as Cd ions has the most harmful effect in environment owing to toxicity, long biological half-life, aggregation in many body parts and carcinogenic nature [12][13][14]. These organic pollutants mostly existing as trace quantities could accumulate through food chain and lead to serious health and environmental problems [15][16][17]. For example, the threshold limit value (TLVÒ) for cadmium dust and salts (as Cd) is 50 lg m -3 for an 8-h TWA, with a 15-min short-term exposure limit (STEL) of 200 lg m -3 . Hence, there are many efforts to invent a good approach for heavy metal removal from leftover water [18], for example membrane separation [19], chemical precipitation [20], coagulation and ion exchange [21] and adsorption [22]. Adsorption is considered as the most effective techniques for heavy metal removing from wastewater [23], because of its low price, simple operation, easy regeneration and wide application [24]. Now, many scientists give their attention on the adsorbents that have good ability for Cd(II) adsorption, such as metal oxides, activated carbon and chitosan. One of the most efficient materials for adsorption is considered to be metal oxides such as ZnFe2O4@TiO 2 [25] and CM-ZnO prepared by coffee ground [26], and Fe 2 O 3 has been paid great attention attributable to Ovacancy, good stability and low toxicity [27]. Among these metal oxides, ZnO nanoparticles have been used as adsorbent of heavy metals ions due to low cost, non-toxicity, chemical and thermal stability and easy preparation [28,29]. Removal efficiency of ZnO can be enhanced by adding reduced graphene oxide (rGO) [30]. Because of its large specific surface area, there are a large number of oxygenated function groups which are useful for adsorption process and are well dispersed in aqueous solutions [31][32][33]. Hence, the fabricated nanocomposites can be suggested for a deep investigation to be highlighted for disinfection and water treatment applications.
Herein, we prepared ZnO nanoparticles as well as ZnO-rGO nanocomposite by using aloe vera plant extract. Furthermore, the adsorption of Cd(II) has been tested on surface of ZnO and ZnO-rGO nanocomposite at different experimental parameters such as pH value and contact time, which enable us to determine the optimum condition for the purification of wastewater from Cd(II). Moreover, adsorption isotherm and kinetic models were studied.
2 Experimental techniques 2.1 Synthesis of the materials All chemicals are purchased from Sigma-Aldrich with purity more than 98% and used as it is without further purification.

Preparation of graphene oxide GO by Hummer's method
The steps of preparation of graphene oxide from graphite using Hummer's method are illustrated in detail in our pervious works [1][2][3][4][5]. Briefly, in the first step (low-temperature stage 0-5°C) graphite and concentrated H 2 SO 4 acid are used as intercalating agent, and in the second step (medium-temperature stage 35°C) KMnO4 is used as oxidizing agent.
Finally, H 2 O 2 and DI water are used as exfoliation assistant of the GO layers.

Preparation of aloe vera plant extract and reduced graphene oxide
The obtained aloe vera leaves are washed with water to eliminate dust and contaminated contents and dried at room temperature. 100 gm of aloe vera gel was taken and the extracted gel was collected and mixed well with water (150 mL) and continuously stirred at 60°C for 20 min. The solution is then cooled down and the resulting product is the leaf extract of aloe vera plant. It will be added to graphene oxide (GO) solution to reduce it to its reduced form. It is deserved to mention that the aloe vera contains mainly 98% water and polysaccharides which acts as active materials for reducing (GO).
To prepare reduced graphene oxide, 0.1 gm of graphene oxide is added to the 100 mL aloe vera solution at 60°C for 15 min and then sonication for 30 min.

Preparation of ZnO nanoparticles and ZnO-rGO nanocomposite
The preparation occurs by adding 0.3 M of zinc nitrate to 100 mL distilled water it will be equivalent to 8.9 gm, keep stirring for 15 min then increase the temperature to 60°C while adding drops of 0.1 M of NaOH. Let precipitate filter and dry. A white precipitate of ZnO nanoparticles is formed (see Fig. 1). ZnO-rGO nanocomposite is prepared by adding the reduced graphene oxide with aloe vera plant extract to ZnO nanoparticles dropwise at 60°C under constant stirring and then sonicate for 60 min. Let precipitate, wash and dry the resultant gray powder, and the formation of ZnO-rGO nanocomposite is confirmed by characterization tools XRD, FTIR, FESEM, TEM as will be discussed below.

Adsorption study of prepared nano-ZnO and ZnO-rGO nanocomposites
The adsorption performance of ZnO and ZnO-rGO nanocomposites was studied by batch sorption experiment. Initially, standard solution of heavy metal ion was prepared by dissolving 2000 mg L -1 of Cd (NO3)2 in distilled water under vigorous shaking by using electric shaker (ORBITAL SHAKER SO1). The adsorption of Cd(II) by using prepared nanoparticles is tuned by experimental parameters as solution pH and contact time. To study the effect of pH solution, 0.02 g of prepared sample is dissolved in 10 mL of standard solution. The pH of solution should adjust at different values (2-8) by using 0.1 M nitric acid and ammonium hydroxide solution. After an hour, the solution is collected and filtrated for measuring concentration of heavy metal after adsorption process by using ICP spectroscopy. After perceptive most favorable pH value for adsorption (i.e., 6), likewise, the optimum contact time can be fine-tuning, through dissolving 0.1 g of prepared samples in 100 mL of standard solution. The heavy metal [Cd(II)] concentration is measured after different time (10-120 min). The removal efficiency of ZnO and ZnO-rGO nanocomposite was considered by Eq. (1) where C 0 is the initial concentration of heavy metal solution (ppm) and C f is the final concentration of heavy metal solution (ppm).

Characterization
The X-ray powder diffraction (XRD) data are collected using a computer-controlled Bruker (D 8 discover) diffractometer with CuKa radiation k = 1.54056 Å . The measuring range was from 5°to 80°and the instrumental resolution was 0.004°-0.005°. Data were collected in step-scan mode with steps of 0.02°. Morphology of the produced materials was obtained by using a scanning electron microscope (SEM) model number JSM 6510 LV JEOL and transmission electron microscope (TEM) model number JEM 2100 JEOL. The atomic force microscopy (AFM) was obtained by using AFM-Agilent Technologies.
3 Results and discussion

Structural characterization
The XRD of GO and ZnO-rGO is shown in Fig. 2a, 1 Steps for the preparation of ZnO-rGO nanocomposite surface of GO, which facilitate the hydration and exfoliation of GO sheets in aqueous media as reported before [1,2]. In XRD patterns of ZnO-rGO nanocomposite, the beaks of ZnO are clearly seen sharp and broad with hkl identified as shown in Fig. 2b. The main diffraction peaks can be assigned to the hexagonal structure, space group P 63 m c (186) (COD 2300113). Unlike the X-ray pattern of graphene oxide, the nanocomposite of ZnO-rGO does not show any peak at 2h = 11°in the XRD pattern. This may be attributed to the oxygen-containing groups of GO that are removed and GO is reduced to graphene oxide rGO by the aloe vera plant extract. The diffraction peak of rGO is usually observed at 2h = 24°(002) usually weak and very broad due to lack of crystallinity of graphene. The XRD patterns of the ZnO-rGO (002) usually do not appear because the diffraction peaks of nanocrystals ZnO are much stronger than (002) of rGO and it is marked by arrow in Fig. 2b. The average crystallite size can be determined from the Debye-Scherrer's equation [2] (b) (a) Fig. 2 a XRD pattern of graphene oxide GO. b XRD pattern of ZnO nanoparticles and ZnO-rGO nanocomposite where W f is the Bragg reflection peak width at half maximum intensity and excluding instrumental broadening, D is the average crystallize size and hD is the Bragg angle and k is the wavelength of the incident radiation (k Cu = 1.54056 Å ). The average crystallize size of ZnO nanoparticles is 64 nm and 72 nm for ZnO-rGO nanocomposite. The FTIR spectra measured from 4000 to 400 cm -1 using compressed ballet of KBr. The peaks around 3400-3500 cm -1 are vibration of OH due moisture. FTIR spectra of GO, ZnO-rGO nanocomposites are shown in Fig. 3. In the transmission curve of GO, the characteristic peaks at 3430 and 1710 cm -1 correspond to the stretching vibrations of O-H and C=O bands. The stretching peak of C-C was at 1052 cm -1 and the vibration peak of C=C was at 1618 cm -1 . For the ZnO-rGO, the characteristic vibration peaks around 575 cm -1 which is assigned to the stretching mode of the Zn-O. The peak at 1600 cm -1 corresponds to C=C is exists and indicating the existence of graphene nanosheet as in the previous work [2,5]. The results of FTIR spectra show that the ZnO-rGO nanocomposite is formed in the synthesis.

Morphological characterization
The SEM images are shown in Fig. 4a, b which confirms the formation of the material in the nanoscale; the average crystallite size is about 70 nm. Figure 4c shows the AFM of ZnO-rGO nanocomposite. The analysis of these protrusion results showed a distribution of Z max and Z min to be 109 nm and 40 nm, respectively, and the mean is around 70 nm. The TEM images are shown in Fig. 4d-f; the graphene sheets appear as wrinkled paper while the nanorod crystal of ZnO is attached and distributed on the surface.

Activity in water treatment
The uptaking behavior of Cd(II) ions by ZnO and ZnO-rGO was investigated at different experimental parameters as pH as well as contact time. Figure 5 shows the effect of pH solution on adsorption behavior. It is clearly that ZnO and ZnO-rGO follow same trend which is that as pH increased, the physical adsorption of Cd(II) ion increased. The lower adsorption of heavy metal at lower pH value is related to competition between H ? and Cd 2? over the available active sited of adsorbent [34]. As pH value increased up to 6 H ? decreased and more active sites become available for Cd(II) adsorption [35]. Otherwise, at basic pH (i.e., 8), OHions exist in solution. Consequently, Cd (OH) 2 is formed [36]. So, Cd(II) ions were removed by adsorption in addition to precipitation. For that reason, the optimal pH was preferred to be 6.
Another parameter that has a durable effect on adsorption of heavy metal ions is contact time. Figure 6 displays the relation between removal efficiency of Cd(II) ions by using ZnO and ZnO-rGO nanocomposite at different contact time over ranges (10-120 min).
It is obvious that there is a continuous rise in adsorption with increasing contact time till it reaches equilibrium after 80 and 90 min for ZnO and ZnO-rGO nanocomposite, respectively. Rapid adsorption at the beginning is due to the availability of a large number of active sites [37]. Removal of Cd(II) by using ZnO was reached equilibrium before ZnO-rGO nanocomposite can be related to the number of available active sites in ZnO surface which is less than that in ZnO-rGO nanocomposite. Removing efficiency performance of ZnO-rGO is preferred than ZnO. As it reached 90% after 90 min, this can be explained by unique structure and great oxygen number-covering functional groups on GO surface makes it fine dispersed in water [38]. Finally, the optimum conditions for adsorption of Cd(II) are pH 6 for 90 min by using ZnO-rGO nanocomposite. Studying adsorption isotherm as well as kinetics is the only way for discovering the mechanisms followed in adsorption process [39]. There are two isotherms used to explain the adsorption which are Langmuir and Freundlich isotherms. Langmuir and Freundlich isotherms are expressed through Eq. (3) and (4), respectively.
where q e and q m (mg g -1 ) are the adsorption capacity at equilibrium and maximum adsorption, respectively, and K l (L mg -1 ) is the affinity binding constant, while K f and n are physical constants signifying   The Langmuir isotherm is related to monolayer adsorption at homogeneous sites and equivalent adsorption energies [40]. Freundlich isotherm is described heterogeneous surfaces [41]. By fitting Fig. 7a, b, it is found that Langmuir model is more fit, and on the other hand, the correlation coefficient (R 2 ) for Freundlich (0.83667) is relatively smaller than that for Langmuir (0.94009). Therefore, adsorption of Cd(II) on surface of prepared sample occurred through monolayer adsorption and uniformly.
Heavy metal adsorption kinetics are often studied by using pseudo-first-order and pseudo-secondorder kinetics. Pseudo-first-order kinetics describes the physisorption. Physisorption is weak occurs without chemical bonding, and only van der Waals forces exist. Consequently, this adsorption is considered to be reversible. The pseudo-second-order kinetic deals with chemisorption adsorption which is occurred through two reactions. The first reaction extents equilibrium quickly. The second reaction leaks slowly and reaches equilibrium after long time [41]. In chemisorption, there is bond formation between adsorbates and adsorbents through electron sharing. So, it is stronger than physisorption. Another type of kinetic models is the intraparticle diffusion kinetic model. Weber and Morris model is studying this type of kinetic, which accepts that intraparticle diffusion model is the single rate-determining stage; meanwhile, the mass removal of adsorbate is well thought out a rapid process [42,43].
To explain the adsorption kinetic mechanism, there are three models of kinetics were studied: Pseudo-first-order model: ln q e À q t ð Þ¼ln q e À k 1 2:303 t ð5Þ Pseudo-second-order model: Interparticle diffusion model: where k 1 is the pseudo-first-order, k 2 is the pseudosecond-order and k 3 is the interparticle diffusion rate constants. It can be decided that further most of the heavy metal ion adsorption on surface of ZnO-rGO nanocomposites had close fitting with the pseudosecond-order kinetic model.
Refer to Figs. 8, 9 and the correlation coefficients that have values (0.826, 0.9820 and 0.9630) for pseudo-first-order, pseudo-second-order and intraparticle kinetic models, respectively. Table 1 shows comparison between our results and the reported literature. As shown, there were a remarkable change and improvement of Cd(II) adsorption by using the suggested nanocomposite (ZnO-rGO) (see Fig. 10).

Conclusion
ZnO and ZnO-rGO nanocomposites were prepared successfully by simple green synthesis method using aloe vera plant extract. The characterization tools (XRD, FTIR, SEM, TEM, AFM) prove the correct synthesis. The prepared samples are used as adsorbent for heavy metal ions Cd(II). The removal efficiency was increased from 80 to 90% by adding rGO to ZnO. The optimum conditions for adsorption of Cd(II) was achieved at pH6 after 90 min and by using ZnO-rGO nanocomposite. For isotherm and kinetics studied, we conclude that the adsorption of Cd(II) on the surface of prepared nanosamples occurred through monolayer adsorption described by Langmuir model by chemisorption reaction as it is, following the pseudo-second-order isotherm.