Effective Removal of Heavy Metals from Aqueous Solutions by

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Effective Removal of Heavy Metals from Aqueous Solutions by Graphene Oxide−Zirconium Phosphate (GO−Zr-P) Nanocomposite Sima Pourbeyram* Department of Chemistry, Payame Noor University, P.O. Box 19395-3697, Tehran, Iran ABSTRACT: In this research work, surface of graphene oxide was functionalized by zirconium and phosphate to form graphene oxide−zirconium phosphate (GO−Zr-P) nanocomposite, which is used for the removal of heavy metals from aqueous solutions. The GO−Zr-P nanocomposite was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), and zetapotential analysis. The sheets of GO−Zr-P was found to be strongly wrinkled, and the flat and layered structure of GO varied significantly after treatments with zirconium and phosphate. The effects of pH, contact time, and initial concentrations on the removal of heavy metals were studied. The results of batch experiments indicated that, at pH 6, maximum adsorption capacity can be achieved for Pb(II), Cd(II), Cu(II), and Zn(II), as 363.42, 232.36, 328.56, and 251.58 mg g−1, respectively. A removal efficiency of ∼99% was obtained after 20 min, via the dispersion of 150 mg of GO−Zr-P nanocomposite in 100 mL of 50 ppm heavy metals. The pseudo-second-order kinetic model provided excellent kinetic data fitting (R2 > 0.99) and data were fitted to both the Langmuir (R2 > 0.97) and Freundlich isotherm models (R2 > 0.99). The XPS results confirmed that the adsorption mechanism of zirconium and phosphate on the GO, as well as adsorption of metal ions onto the GO−Zr-P nanocomposite, was chemisorption, mainly through surface complexation. The results confirmed that GO−Zr-P nanocomposite could be a potential sorbent for effective and regenerable removal of heavy metals from aqueous solutions. urgency to find efficient and cost-effective adsorbents for the removal of heavy metals from wastewater. Graphene is a member of the carbonaceous nanomaterial family with a honeycomb structure that is sp2 − hybridized, with a thickness of one atom, and exhibits many outstanding physical and chemical properties.12 One significant branch of graphene-based materials is graphene oxide (GO), which is an oxidized form of graphene. Large surface area, large amounts of activated functional groups, good dispersibility in water, and a relatively easy preparation method and biocompatibility, demonstrate the possibility of applying GO as an excellent water treatment agent.13 Few-layered GO nanosheets,3 lowtemperature exfoliated graphene nanosheets,14 and GO aerogel15 are applied as sorbents for the removal of heavy metals from aqueous solutions. In recent years, for the effective removal of various environmental pollutants in wastewater, there have been efforts to develop graphene-based materials as novel adsorbents to abate pollutants such as heavy metals with superior adsorption capability. Sulfonated graphene nano-

1. INTRODUCTION Heavy metals are becoming increasingly prevalent, because of their release from various sources, including metal processing, batteries, plating, paints, fertilizers, waste disposal, and fuel burning.1 Heavy metals have a tendency to accumulate in living organisms, since they are not biodegradable, unlike organic contaminants. Toxic heavy metals of particular concern, in the treatment of industrial wastewaters, include lead, chromium, cadmium, mercury, arsenic, nickel, copper, and zinc.2 The harmful effects of heavy metals include several acute and chronic disorders, such as renal damage, emphysema, hypertension, testicular atrophy, and skeletal malformation in fetuses.3 There are many processes and common techniques, including precipitation,4 ion exchange,5 flotation,6 membrane,7 electrochemical treatment,8 and adsorption for removal of heavy metals from wastewater. Among them, adsorption, which is easy to perform, insensitive to toxic substances, flexible in design and operation, and reversible under certain conditions is considered as a fast alternative with great potential to treat water and wastewaters that contain heavy metals at low concentrations.9,10 The main disadvantage of the adsorption method is high price of the adsorbents, which increases the price of wastewater treatment.11 Therefore, it is there an © XXXX American Chemical Society

Received: February 23, 2016 Revised: April 22, 2016 Accepted: April 23, 2016

A

DOI: 10.1021/acs.iecr.6b00728 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research sheets,16 polypyrrole-reduced GO composite,17 functionalized graphene with potassium hexafluorophosphate,18 SiO2/graphene composite,19 EDTA-GO,20 and GO−hydrated zirconium oxide21 as novel nanoadsorbents have been used for the removal of heavy metals from aqueous solutions. However, most of these adsorbents are separated by using laborious processes such as high-speed centrifugation, membrane filtration, and an external magnetic field, which are not field-adaptable. Therefore, designing novel GO-based adsorbents for the removal of heavy metals in environmental application has paramount significance. Zirconium, as a metal cation that has thermal stability, lacks toxicity, and has a strong ionic and coordinative affinity for the groups containing oxygen,22 motivated us to synthesize the graphene oxide−zirconium phosphate (GO−Zr-P) nanocomposite as an adsorbent for the removal of heavy metals. To the best of our knowledge, the synthesis of the GO−Zr-P nanocomposite has not been reported yet. Based on the above considerations, we report a simple strategy for producing the GO−Zr-P nanocomposite as an adsorbent for the removal of Pb(II), Cd(II), Cu(II), and Zn(II) from aqueous solutions. The objectives of this study were to synthesize the GO−Zr-P nanocomposite and to characterize it via scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and zeta-potential analysis, as well as to describe the adsorption of heavy metals.

dropwise addition of 0.1 M zirconium chloride to the GO suspension under sonication. The yellow precipitate was centrifuged, washed with ethanol several times, and finally dried at 60 °C in a vacuum oven. GO−Zr suspension was prepared via the addition of 50 mg of GO-Zr powder to 50 mL of 0.1 M HCl, and sonication of the mixture for 20 min, to obtain a stable dispersion. The GO−Zr-P nanocomposite was synthesized via a dropwise addition of 0.1 M sodium dihydrogen phosphate to the GO−Zr suspension under sonication. The brown precipitate was centrifuged, washed with ethanol several times, and finally dried at 60 °C in a vacuum oven. 2.4. Removal of Heavy Metals. The adsorption of heavy metals was performed in a batch experiment. One hundred milligrams (100 mg) of GO−Zr-P nanocomposite was added to 100 mL of a solution containing Pb(II), Cd(II), Cu(II) and Zn(II), at desired initial concentrations, under stirring. The effect of pH on adsorption of heavy metals was studied in the pH range of 1−8 for a contact time of 20 min. The pH was adjusted via the addition of 0.1 M HCl or 0.1 M NaOH. The amount of heavy metals adsorbed per unit mass of the GO−ZrP nanocomposite was evaluated by using the mass balance equation:

q=

(C0 − Ce)V m

where q (mg g−1) is the amount adsorbed per gram of adsorbent, C0 and Ce are the initial and equilibrium concentrations of heavy metals in the solution (mg L−1), respectively, m is the mass of the adsorbent (g), and V (L) is the initial volume of the solution of heavy metals. 2.5. Desorption and Regeneration Experiments. For desorption, the metal ion-loaded adsorbent was collected, washed with water to remove the unadsorbed heavy metals, and then agitated with 10 mL of 3 M HCl for ∼10 min, and finally separated from the eluent. The final concentration of heavy metals in the eluent was determined by ICP-AES. To test the regeneration of the adsorbent, the adsorption−desorption cycle was repeated five times, using the same adsorbent.

2. EXPERIMENTAL SECTION 2.1. Reagents. All reagents used had analytical purity without further purification. Graphite powder, zirconium chloride, hydrogen peroxide, lead nitrate, cadmium nitrate, copper nitrate, zinc chloride, potassium permanganate, sulfuric acid, and the other chemicals were obtained from Sigma. All water used was ultrapure water, which was purified by Millipore Milli-Q (18 MΩ cm). 2.2. Characterization. The morphology and structure of nanocomposite were explored by SEM (Model 1530VP, LEO, Germany) and TEM (JEOL, Tokyo, Japan). XRD analysis was performed on a Rigaku Model MiniFlex 600 X-ray diffraction analyzer (Rigaku, Japan). XPS was performed on an XPS system (Model Axis His-165 Ultra, Kratos, Shimadzu, Japan), with a monochromatized Al Kα X-ray source. The point of zero charge (PZC) of the nanocomposite was measured using a Zeta-Sizer (Malvern, Ltd., U.K.). The concentrations of heavy metals in solution were determined by inductively coupled plasma−atomic emission spectrometry (ICP-AES) (PerkinElmer, USA). 2.3. Synthesis of the GO−Zr-P Nanocomposite. GO was synthesized by the improved Hummer method.23 Briefly, a 9:1 mixture of concentrated H2SO4/H3PO4 was added to a mixture of graphite flakes (1 wt equiv) and KMnO4 (6 wt equiv). The reaction was then heated to 50 °C and stirred for 12 h. The reaction was cooled to room temperature and poured onto ice with 30% H2O2. For workup, the mixture was filtered through polyester fiber. The filtrate was centrifuged, and the supernatant was decanted away. The remaining solid material was then washed in succession with water, 30% HCl, and ethanol. The obtained GO was vacuum-dried overnight at room temperature. GO suspension was prepared via the addition of 50 mg of GO to 50 mL of 0.1 M HCl, and sonication of the mixture for 15 min, to obtain a stable dispersion. GO−Zr nanocomposite was synthesized via a

3. RESULTS AND DISCUSSION 3.1. Characterization of GO−Zr-P Nanocomposite. The XRD patterns of pristine GO, GO−Zr, and GO−Zr-P are presented in Figure 1. The GO pattern was dominated by a single broad peak at 2θ = 10.5°, which corresponded to an

Figure 1. XRD patterns of GO, GO−Zr, and GO−Zr-P nanocomposite. B


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Industrial & Engineering Chemistry Research

Figure 2. XPS survey scans of GO, GO−Zr, and GO−Zr-P. High-resolution scans of (a−c) O1s, (d, e) Zr3d, and (f) P2p peaks.

broad peak with a binding energy of 532.7 eV was associated with the C−O groups and the other peaks, with binding energies of 531.4 and 530.7 eV, can be assigned to the oxygen in the forms of Zr−O−C and Zr−O−H, respectively. After the adsorption of phosphate, the shape of the O1s peak was changed and deconvoluted into four component peaks, with binding energies of 532.7, 532.3, 531.4, and 531.1 eV, which were assigned to the oxygen in the forms of C−O, P−O−H, Zr−O−C, and Zr−O−P, respectively (Figure 2c). The noteworthy point was that the peak at 530.7 eV, corresponding to Zr−O−H, was shifted to 531.1 eV, corresponding to Zr− O−P, and the new high peak at 532.1 eV, corresponding to P− O−H, appeared, which confirmed completely covering of the Zr moiety by phosphate groups on the GO−Zr-P nanocomposite surface. The Zr3d peak of GO−Zr contained two separated peaks with binding energies of 183.3 and 181.3 eV (Figure 2d). It was found that, after the adsorption of phosphate, the chemical environment of Zr changed and the Zr3d binding energy shifted to 183.7 and 181.8 eV (Figure 2e). Before the adsorption of phosphate, hydroxyl groups existed in Zr moiety of GO−Zr. After adsorption, hydroxyl groups were replaced by phosphate anions and the Zr−O−P linkage was formed. Since the electronegativity value of P (2.19) was higher than that of H (2.1), the negative charge density of Zr in Zr−

interlayer distance of 0.84 nm. The expansion of the interlayer spacing, relative to the parent graphite (0.34 nm), was consistent with the oxidation of graphene sheets and intercalation of water molecules.24 In the XRD pattern of GO−Zr, the GO peak intensity decreased, the peak width increased, and the peak position shifted to 11.1°, because of distortion of the crystal structure of GO upon adsorption of Zr ions. The XRD pattern of GO−Zr-P nanocomposite indicated a new broad peak at 2θ ≈ 34°, and more distortion of the crystal structure of GO upon the formation of Zr−P moieties. XPS survey scans of GO, GO-Zr, and GO−Zr-P are presented in Figure 2. In the XPS spectrum of GO−Zr, in addition of GO characteristic peaks,25 three new peaks with binding energies of of 183.9, 331.9, and 344.5 eV appeared, corresponding to 3d, 3p3/2, and 3p1/2 peaks of Zr,26 which confirmed the adsorption of Zr on the GO surface. After the adsorption of phosphate, in addition to GO−Zr peaks, the appearance of one new peak at 133.7 eV corresponding to P2p,27 confirmed the adsorption of P on the GO−Zr surface. In order to investigate adsorption mechanism, high-resolution scans of O1s, Zr3d, and P2p peaks were also recorded and are presented in Figures 2a−f.The O1s broad peak of the GO with a binding energy of 532.5 eV was in agreement with the reported values25 (Figure 2a). It was found that, after the adsorption of Zr, the O1s peak was divided into three peaks (Figure 2b). The C


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Figure 3. TEM images of (A) GO, (B) GO−Zr, and (C) GO−Zr-P nanocomposite.

O−P was lower than that in Zr−O−H, resulting in an increase in the binding energy of Zr3d.24 According to EDX results, the Zr:P atomic ratio was ∼1. Therefore, according to the above-mentioned results, the adsorption of phosphate on the surface of GO−Zr could be represented schematically as follows:

Figure 3 shows the TEM images taken from the (A) GO, (B) GO-Zr, and (C) GO−Zr-P nanocomposite. In Figure 3A, GO sheets exhibited a layered structure with a smooth surface without wrinkles. TEM demonstrated that the GO sheets had one to several layers. In the TEM image of the GO−Zr nanocomposite, the structure of the GO sheets remained almost the same, but the nanocomposite had a rougher surface, compared to the pure GO, and its surface was wrinkled (Figure 3B). The TEM image of the GO−Zr-P nanocomposite revealed a significantly different morphology. In this image, Zr−P nanoparticles appeared as dark dots ∼1−2 nm in diameter (see Figure 3C). The nanoparticles were well distributed, with a high density on the GO surface, with interparticle distances in the range of ∼3−5 nm. Figure 4 displays the SEM images of GO (Figure 4A), GO− Zr (Figure 4B), and the GO−Zr-P nanocomposite (Figures 4C and 4D). It can be seen that GO had a layered structure with a relatively smooth surface. After formation of the GO−Zr nanocomposite, the coverage of GO surface by Zr moiety was not observed; however, the sheets of GO were strongly wrinkled, so that the flat and layered structure of GO was destroyed significantly. The SEM image of the GO−Zr-P nanocomposite revealed more roughness and wrinkles than the GO−Zr sheets. By increasing the image magnification well distributed nanoparticles were visualized (Figure 4D). The diameter and dispersion of nanoparticles were in good agreement with those obtained from TEM. The dependence of charge of the GO sheets to pH was investigated by zeta potential measurements (Figure 5). As expected, GO sheets were negatively charged over the entire studied pH range (pH 1−8). The hydroxyl and carboxylic functional groups at the edges of the GO sheets developed negative charges due to deprotonation. In the case of the GO− Zr nanocomposite at the pH range of pH 1−4, the surface

Figure 4. SEM images of (A) GO, (B) GO−Zr, and (C, D) GO−Zr-P nanocomposite.

Figure 5. Zeta potential of GO, GO−Zr, and GO−Zr-P nanocomposite, as a function of pH.

charge was positive, because of the presence of Zr ions on the edges of the GO sheets. At pH >4, there was a rapid decline of surface positive charge and the pHzpc of nanocomposite was ∼5. At high pH (pH >8), the zeta potential diagrams of GO−Zr D


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Industrial & Engineering Chemistry Research and GO became similar, because by increasing pH, the bounded Zr ions were hydrolyzed and released from the surface. Surface of GO−Zr-P nanocomposite was slightly positive at pH 1. In the range of pH 2−6, the slight negative charge of the surface was slowly increased. At pH >6, a rapid rise of surface negative charge was observed due to deprotonation of phosphate groups, as well as a gradual detachment of Zr−P nanoparticles from the surface at pH >8. 3.2. Removal of Heavy Metals by the GO−Zr-P Nanocomposite. The removal of heavy metals was carried out by batch-type experimentation from aqueous solution at room temperature. The amount of Pb(II) and Cd(II) (100 ppm) adsorbed by 100 mg of GO−Zr-P nanocomposite, as a function of time, is shown in Figure 6. The results revealed that

Figure 7. Variation of qe by initial concentration of heavy metals obtained from 100 mL of 5−400 ppm Pb(II), Cd(II), Cu(II), or Zn(II), by 1 mg mL−1 of the GO−Zr-P nanocomposite after 15 min.

Figure 8A, removal efficiency of the heavy metals (50 ppm) increased as the amount of the GO−Zr-P nanocomposite increased. When the amount of the GO−Zr-P nanocomposite was increased to 150 mg, ∼99% removal efficiency was achieved after 20 min at room temperature. The maximum contaminant limit for most of the heavy metals in drinking water is in the ppb range, as reported by World Health Organisation.28 Therefore, the relationship between amount of the adsorbent dosage and the removal efficiency in the presence of 50 ppb of heavy metals was also studied. As shown in Figure 8B, as above, the removal efficiency of the heavy metals increased by increasing the amount of the GO−Zr-P nanocomposite, but adsorbent saturation was not observed, even after 60 min. In the presence of 50 mg of the GO−Zr-P nanocomposite, ∼98% removal was achieved after 30 min at room temperature. Therefore, the removal of heavy metals at lower concentrations took more time than that at higher concentrations. The as-synthesized GO−Zr-P nanocomposite exhibited excellent adsorption, compared with the most previously reported graphene-based adsorbents (Table 1). The excellent adsorption was attributed to chemical adsorption through strong surface complexation, as well as the large specific surface area of the GO−Zr-P nanocomposite.29−34 3.2.1. pH. One of the most important factors in a liquid− solid adsorption procedure is the pH of the aqueous phase. In order to prevent the formation of metal hydroxide precipitates, the pH ranges in this study were chosen as pH 1−8; the obtained results are shown in Figure 9. It was found that the adsorption of all heavy metals by the nanocomposite was significantly affected by the solution pH. According to Figure 9, it was noticed that the plots of all heavy metals have approximately similar variation trends, and removal efficiency increased obviously with increasing pH. Smaller adsorption at low pH can be due to the protonation of the phosphate groups, which prevent the approaching of heavy metal cations to the adsorption sites. At the range of pH 3−6, high and relatively constant adsorption capacity was observed. At this range of pH, with a rather low proton concentration, active functional groups of the adsorbent can form complex with heavy metals. However, at pH >6, the removal efficiency of all heavy metals

Figure 6. Variation of qe with time, obtained from 100 mL of 100 ppm Pb(II), Cd(II), Cu(II), or Zn(II) by 1 mg mL−1 of GO, GO−Zr, and GO−Zr-P nanocomposite.

the adsorption happened in two different steps. During the first 10 min, the adsorption increased rapidly. After that, adsorption increased gradually and finally reached equilibrium after 20 min. After adsorption, a tendency of the nanocomposite to agglomerate and precipitate was observed. The excellent dispersibility of the GO−Zr-P nanocomposite and the strong tendency of metal-adsorbed nanocomposites to precipitate opened the pathway for the removal of heavy metals from water solution. For comparison, adsorption of Pb(II) and Cd(II) on GO−Zr and GO was also studied. The results showed that, although Pb(II) and Cd(II) were adsorbed on GO, the maximum adsorption capacity was much lower than that for the GO−Zr-P nanocomposite. On the other hand, on the GO−Zr nanocomposite, no adsorption of heavy metals was observed under the same conditions. The effect of initial concentration of the heavy metals was studied by using 100 mg of the GO−Zr-P nanocomposite for solutions with different concentrations of heavy metals. Figure 7 shows that the amount of heavy metals adsorbed on the nanocomposite increased by increasing initial concentrations of the heavy metals in the range of 10−200 ppm, and then, the sorbent was finally saturated by a relatively constant amount of the heavy metals (∼200 ppm). The results of batch experiments indicated that maximum adsorption can be achieved for Pb(II), Cd(II), Cu(II), and Zn(II) at pH 6 as 363.42, 232.36, 328.56, and 251.58 mg g−1, respectively. Moreover, the relationship between the adsorbent dosage and the removal efficiency was also investigated. As shown in E


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Figure 8. Effect of GO−Zr-P nanocomposite dosage on removal efficiency, with respect to time, from 100 mL of (A) 50 ppm and (B) 50 ppb Pb(II), Cd(II), Cu(II), or Zn(II) after 15 min.

Table 1. Comparison of the Parameters of Some Graphene-Based Adsorbents for Some Metal Ions sorbent

metal ion

pH

graphene oxide, GO GO nanosheets EDTA-GO GO sulfonated magnetic GO GO aerogel GO

Cd(II) Pb(II) Pb(II) Zn(II) Cu(II) Cu(II) Cu(II) Zn(II) Cd(II) Pb(II) Pb(II) Pb(II) Pb(II) Cd(II) Cu(II) Zn(II)

6 3−5 6.8 7 4.68 4−6 3−7 5−8 4−8 3−7 5 7 6 6 6 6

magnetic chitosan/GO GO/chitosan GO−Zr-P

initial concentration 20 ppm 40 ppm 5−300 ppm 40 ppm 73.71 ppm 40−80 ppm 1 ppm 1 ppm 1 ppm 1 ppm 50 ppm 0.05−200 0.05−200 0.05−200 0.05−200

ppm ppm ppm ppm

time 15 h 20 min 20 min 6h 60 min 120 min 120 min 120 min 120 min 60 min 20 20 20 20

min min min min

qmax (mg g−1)

ref

106.3 35.46 479 246 62.73 29.59 294 345 530 1119 76.94 99 363.42 232.36 328.56 251.58

3 14 20 29 30 31 32 32 32 32 33 34 this study this study this study this study

decreased significantly. At a higher pH range, the surface-bound Zr ions gradually released from the nanocomposite surface and the heavy metals were probably adsorbed only on the unmodified GO surface. 3.2.2. Ionic Strength. Generally, inner-sphere surface complexation is affected by pH values, whereas outer-sphere surface complexation or ion exchange is influenced by ionic strength. Electrolytes often form outer-sphere complexes through electrostatic forces. Therefore, if the adsorption of metal ions occurs through the formation of inner-sphere surface complexes or chemisorption, the adsorption will not be dependent on the ionic strength.35 In order to study the effect of ionic strength, the adsorption of heavy metals from solutions containing 0, 0.01, 0.1, and 1 M of KCl, CH3COONa, and KNO3 was investigated. The results showed, in the studied range, by increasing ionic strength the removal efficiency of heavy metals did not decrease, indicating the formation of inner-sphere surface complexes of metal ions on the GO−Zr-P nanocomposite. 3.3. Adsorption Isotherms. The analysis of the isotherm data is important to describe how the adsorbates interact with adsorbents, affording the most important parameter for

Figure 9. Effect of pH on the removal efficiency of 1 mg mL−1 of the GO−Zr-P nanocomposite for 100 mL of 100 ppm Pb(II), Cd(II), Cu(II), or Zn(II) after 20 min.

F


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Industrial & Engineering Chemistry Research designing a desired adsorption system. Two equilibrium models, including Langmuir isotherms (eq 1) and Freundlich isotherms (eq 2), were chosen to describe the adsorption process:36,37 Ce C 1 = e + qe qm klqm

log(qe) =

(1)

1 log(Ce) + log(k f ) n

(2)

−1

where qe (mg g ) is the amount adsorbed at equilibrium, Ce (mg L−1) the equilibrium concentration of the heavy metals, qm (mg g−1) the Langmuir monolayer adsorption capacity, kl (L mg−1) the Langmuir constant, kf ((g kg−1 (g m−3))n) the Freundlich parameter, and 1/n the adsorption intensity. The adsorption isotherms of Pb(II) and Cd(II) on the GO−Zr-P nanocomposite at different initial concentrations were given in Figure 10. The Langmuir and Freundlich isotherm parameters for all heavy metals, calculated from the linear plots, are given in Table 2. The high R2 value of the Freundlich isotherms (>99%) suggested that the adsorption performance of heavy metals onto the GO−Zr-P nanocomposite could be explained reasonably by the Freundlich model. The n value of heavy metals adsorption onto the GO−Zr-P nanocomposite indicated that the adsorption was favorable under the studied conditions. The high correlation coefficient values (R2 ≈ 97%) revealed that the experimental data of adsorption of all heavy metals were also fitted well by the Langmuir model. In other words, the adsorption process occurred at the functional groups/ binding sites on the surface of the GO−Zr-P nanocomposite, which was regarded as monolayer adsorption. It was found that the adsorption capacity of the GO−Zr-P nanocomposite was significantly higher than the capacity of the most other adsorbents. The remarkably high adsorption capacity of the GO−Zr-P nanocomposite was mainly attributed to the large amount of phosphate groups immobilized on the high-surfacearea GO-Zr matrix. These results confirmed that the GO−Zr-P nanocomposite can be considered as a novel adsorbent for the removal of heavy metals from aqueous solutions. 3.4. Adsorption Kinetics. The adsorption kinetic was evaluated using pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-order kinetic model is given as the following equation: log(qe − qt ) = log(qe) −

k1t 2.303

Figure 10. (A) Adsorption isotherms of Pb(II) and Cd(II) on the GO−Zr-P nanocomposite (1 mg mL −1 ) at different initial concentrations (5−200 ppm) after 15 min. Fitting of isotherms data with (B) linear Langmuir models and (C) linear Freundlich models for the removal of Pb(II) and Cd(II).

(3)

The pseudo-second-order kinetic model can be expressed as follows: t 1 t = + 2 qt qe k 2qe

Table 2. Langmuir and Freundlich Adsorption Isotherm Parameters for Pb(II), Cd(II), Cu(II), or Zn(II) Adsorption on GO−Zr-P Nanocomposite at pH 6 and Room Temperature

(4) −1

Pb(II)

where qe is the equilibrium adsorption amount (mg g ), qt is the adsorption amount (mg g−1) at time t (min), k1 and k2 are pseudo-first-order and pseudo-second-order rate constants (g mg−1 min−1), respectively.24 The experimental (exp) and calculated (cal) parameters of the above kinetic model parameters are summarized in Table 3. The pseudo-secondorder kinetic model (R2 > 0.99) fitted well with the adsorption data of heavy metals adsorption on the GO−Zr-P nanocomposite rather than pseudo-first-order kinetic model. Furthermore, the calculated qe value of the pseudo-secondorder kinetic model was close to experimental data rather than G

qm (mg g−1) KL (L mg−1) R2

384.61 0.12 0.98

n KF R2

4.76 139.86 0.99

Cd(II) Langmuir 263.15 0.07 0.97 Freundlich 2.81 49.09 0.99

Cu(II)

Zn(II)

340.24 0.11 0.98

262.02 0.09 0.96

4.21 130.32 0.99

3.75 74.12 0.99


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Table 3. Adsorption Kinetic Parameters for Adsorption of Metal Ions on GO−Zr-P Nanocomposite, at pH 6 and Room Temperature Pseudo-First-Order Pb(II) Cd(II) Cu(II) Zn(II)

Pseudo-Second-Order 2

qe,exp

K1

qe,call

R

363.42 232.36 328.56 251.58

0.13 0.14 0.11 0.14

202.07 141.45 172.22 120.13

0.94 0.92 0.89 0.96

K2

qe,call

R2

0.008 0.004 0.007 0.004

386.43 229.27 333.32 214.47

0.99 0.99 0.99 0.98

potential applicability of the sorbents. It was observed that desorption of all metal ions from the adsorbent increased by decreasing the pH of the solution. The desorption of Pb(II) went from 0% to 90% quickly, when the pH was reduced from pH 6 to 0. The GO−Zr-P nanocomposite was effectively regenerated (∼100%) by desorption of adsorbed metal ions by treatment of metal-ions-loaded nanocomposite with 3 M HCl for ∼10 min. The GO−Zr-P nanocomposite was easily separated from the system via centrifugation. The high adsorption efficiency performance was maintained after being used for at least five cycles. The results indicated that the GO− Zr-P nanocomposite had the potential to be reused in several treatment cycles and can make its place as a cost-effective adsorbent in the future. 3.7. Real Sample Analysis. The proposed method was applied to samples of industrial wastewater from a chemical industry and two rivers of Miandoab, Zarrineh-Rud, and Simineh-Rud. After the addition of 50 ppm of Pb(II), Cd(II), Cu(II), and Zn(II), the removal of metal ions by the GO−Zr-P nanocomposite was studied. Removal efficiencies (%) obtained from the average of three independent adsorption tests at pH 6 and room temperature are presented in Table 4. The results showed the presence of some amount of metal ions in the Zarrineh-Rud that were in agreement with the permissible values of Iranian Legislation.

that of pseudo-first-order kinetic model. Thus, the pseudosecond-order kinetic model was appropriate for the entire adsorption process. 3.5. Adsorption Mechanism. To confirm the mechanism of adsorption of metal ions on the GO−Zr-P nanocomposite, the high-resolution O1s XPS spectrum of GO−Zr−P-Cd was recorded and is presented in Figure 11. Compared with the

Table 4. Removal Efficiency (%) Obtained from Average of Three Independent Adsorption Tests at pH 6 and Room Temperature after Addition of 50 ppm of Metal Ions to Real Samples

Figure 11. High-resolution O1s XPS spectrum of GO−Zr-P-Cd.

spectrum of adsorbent, the spectrum of Cd-loaded adsorbent can be deconvoluted into five peaks, with binding energies of 532.8, 532.3, 531.7, 531.4, and 531.2 eV, which can be assigned to the oxygen in the forms of C−O, P−O−H, P−O−Cd, Zr− O−C, and Zr−O−P, respectively. The XPS results provided clear evidence that, after the adsorption of Cd(II), the environment of oxygen of the phosphate groups were changed significantly. The noteworthy point was that the intensity of the P−O−H peak was reduced significantly and a new high peak, corresponding to P−O−Cd, appeared. Therefore, it may be speculated that the adsorption of Cd(II) was mainly through chemical interaction, such as complexation with phosphate groups on the GO−Zr-P nanocomposite surface. The same results were obtained in the case of the other metal ions. As a consequence, possible mechanism for the adsorption of metal ions (M) can be represented schematically as follows:

Pb(II) Cd(II) Cu(II) Zn(II)

industrial waste (%)

Zarrineh-Rud (%)

Simineh-Rud (%)

111.2 106.7 125.0 110.9

110.4 99.5 117.1 105.4

99.2 99.6 101.5 98.8

4. CONCLUSION The adsorptive property of the GO−Zr-P nanocomposite toward divalent heavy metals, Pb(II), Cd(II), Cu(II), and Zn(II) were investigated. The GO−Zr-P nanocomposite was prepared using phosphate as an adsorbent, Zr ion as a binder, and graphene oxide (GO) as a template by a facile method. The surface morphology and structure of the GO−Zr-P nanocomposite were investigated by XRD, XPS, TEM, SEM, and zeta-potential analysis. The results of batch experiments indicated that maximum adsorption can be achieved in a broad range of pH (∼3−6) for all heavy metals. The maximum adsorption capacity of Pb(II), Cd(II), Cu(II), and Zn(II) on the GO−Zr-P nanocomposite at pH 6 were 363.42, 232.36, 328.56, and 251.58 mg g−1, respectively. Adsorption isotherms and kinetic studies suggested that the adsorption of heavy

3.6. Desorption Study. Regeneration is one of the most important aspects that should be considered in the fabrication of the sorbents. Simple regeneration procedure enhances the H


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metals on the GO−Zr-P nanocomposite was monolayer coverage and adsorption was controlled by chemical adsorption involving the strong surface complexation of heavy metals with the phosphate groups on the surface of GO−Zr-P. The adsorption experiments showed that the dispersibility of GO− Zr-P in water changed remarkably after complexation of heavy metals. After adsorption, the tendency to agglomerate and precipitate was observed. Potential application of the GO−Zr-P nanocomposite in analytical chemistry as a solid sorbent for a preconcentration of trace elements and in heavy-metal ion pollution cleanup resulting from its maximum adsorption capacity, which was much higher than those of many of the currently reported sorbents.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +98-481-2227171. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Payame Noor University of Miandoab providing facilities for this work. REFERENCES

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Effective Removal of Heavy Metals from Aqueous Solutions by

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