The graphene oxide (GO) was obtained from the powder graphite by the route of Hummer’s method [21]. The sheets of GO are strongly hydrophilic and have some chemical and physical interactions between the layers and water molecules [21]. Graphite oxide can be completely exfoliated by simple sonication and by stirring the water/graphite oxide mixture for a long time in order to produce aqueous colloidal suspensions of graphene oxide sheets. After this processes, the surface of the obtained GO was chemically modified with 3-(trimethoxysilyl)propylamine (GO-APTMS) having a free amine group (-NH2) on the surface of GO. Finally, the amine-modified GO-APTMS was reacted with 3,5-di-tert-butylsalicylaldehyde to give the final hybrid material (HL).
1.1. Electronic properties of the graphene oxide (GO)
The UV-vis spectrum of GO in the aqueous dispersion is shown in Fig.1. Characteristic two absorption bands were observed in the spectrum of GO. A shoulder seen at ~310 nm corresponds to the n-π* transitions. The second characteristic band appeared at 237 nm can be attributed to the π-π* transitions of the aromatic C=C bonds which corresponds to the sp3 character in GO [20-23].
1.2. FT-IR
FT-IR spectra of the GO and hybrid material HL are given in Fig. 2. In the FTIR spectrum of GO, the broad peak at 3452 cm-1 comes from O-H vibrations of the -COOH and -OH groups located on the surface of the GO. The vibration peak at 2926 cm-1 is corresponded to aliphatic C-H stretching modes. The peak at the 1729 cm-1 can be attributed to the carbonyl (C=O) stretching band. In addition, the aromatic C=C vibrations were shown at the 1630 cm-1.
In FT-IR spectra of the obtained GO-APTMS, the vibration band at 3435 cm-1 can be attributed to O-H stretching mode. A weak peak at 1573 cm-1 was the stretch of NH2 groups which confirmed the successful grafting of APTMS to GO. The peak at 2930 cm-1 can be attributed to aliphatic C-H vibrations of the methylene in APTMS moiety and aliphatic groups on GO. The bending bands at 1118 and 1053 cm-1 may be assigned to the vibrations of Si–O–Si and Si–O–C, respectively. In the spectrum of the hybrid material (HL), there are two characteristic bands at 1115 and 1048 cm-1. The peak at 1632 cm-1 can be attributed to the azomethine group CH=N.
1.3. SEM and EDX
The EDX spectra of GO, GO-APTMS and HL materials are given in Fig. 3a-c. In EDX spectrum of GO (Fig. 3a), three elements C (70.35%), O (28.68%) and S (0.97%) were calculated in its ingredient. The very small amount of sulfur gradients in the syntheses may come from sulfuric acid used in the preparation reaction of the GO by Hummer’s method (Fig. 3b and 3c). The elemental compositions of GO-APTMS are 57.03% C, 29.12% O, 7.39% Si, 6.34% N and 0.12% S calculated in the EDX spectrum of GO-APTMS. According to the EDX spectra of GO-APTMS, it can be said that 3-(trimethoxysilyl) propylamine was attached on the surface of the GO. The compositions of the elements of HL are 61.83% C, 24.40% O, 6.94% Si, 6.38% N and 0.45% S calculated in the EDX spectrum of the material HL. While the percentages of C and N increasing, the percentages of the other elements decreased confirming the modification on the surface.
The scanning electron microscope (SEM) images of GO, GO-APTMS and HL materials were given in Figs. 4a and 4b. According to the SEM images, the individual GO sheets were found to have a thickness of 100 mm that much larger than the thickness of single layer graphene. This increase in the thickness is due to the introduction of the oxygen-containing functional groups. It can also be noted that the GO sheets were thicker at the edges. This is because the oxygen-containing functional groups were mainly combined at the edges and the surface of GO. GO sheets were firmly suspended and did not bend according to the SEM images. However, SEM image of HL (Fig. 4b) reveals the extended sheets of lateral dimensions in the length of 1 mm. On the other hand, SEM image displays two-dimensional crumpled nano-sheets with a few stacked layers
1.4. XRD and TEM
The structural changing of GO to HL were observed from X-ray diffraction patterns as in (Fig. 5a,b). The sharp diffraction peak at 2θ = 9.45° (d=0.935 nm) and narrow small peak at 2θ= 44.5524° correspond to the spacing of the GO in the plane (Fig. 5a). After the synthesis of APTMS-GO, while disappearing the sharp peak of GO, a new sharp peak revealed at 2θ= 6.2° (d=1.42 nm). The other new broader and weaker diffraction peaks appeared at 2θ=10.95° (d=0.19 nm) and 22.13° (d= 0.4 nm) indicated that the attachment of the functional silane groups occurred successfully on the surface of GO nano-sheets. In XRD pattern (Fig. 5b) of HL, the sharp diffraction peak at 2θ=10.5274° belongs to the graphene oxide. The weak peaks at 45.0124 and 51.4285° can be attributed to the bonded compounds in the hybrid material.
The morphological structures of the materials GO, APTMS-GO and HL were also characterized by transmission electron microscope (TEM) (Figs. 6a-c). The elastic corrugations and the scrolled or folded edges often result in different brightness on the surface of the GO (Fig. 6a). The results indicate that GO has a high surface area and is a mesoporous material. The mesoporous structure and low surface area may be due to the agglomerations of GO sheets during the drying treatment because of the van der Waals forces between each single sheet of GO. In TEM image (Fig. 6b) of APTMS-GO, the black dots on the surface of the GO can be attributed to the amino silane groups. The TEM image of the hybrid material was taken at 0.2 mm. In the hybrid material (Fig. 6c), several black aggregations on the surface of the GO-APTMS may be assigned to the imine group.
1.5. Thermal Properties
Thermal properties of all the materials were analyzed by thermogravimetric (TGA) and differential thermal analyses techniques (DTA) under N2 atmosphere in the range of 20-900 °C. Thermal curves of the materials have given in Figs. 7k-n. In TGA curve of the powder graphite, the adsorbed water molecules desorbed between 50-100 °C. The absorbed water molecules and the degradation of some substituents were lost in the 100-200 °C temperature range. After this temperature range, the mass loss of the graphite continues up to 850 °C (Fig. 7a) with the degradation of organic backbone. In TGA curve of the graphene oxide (Fig. 7b), the adsorbed water molecules are move away in 40-50 °C range.
Organo-silanized substituents and absorbed water were decomposed up to 200 °C in the thermal curve of the GO-APTMS (Fig. 7c). Decomposition of the organic backbone continued up to 900°C. The TG/DTA/DTG curves of the hybrid material (Fig. 7d) showed that the decomposition process occurs at four steps. At first and second steps, the adsorbed and decomposed water molecules from the substituents such as silanol and oxygen containing structures. At the other steps, the organic parts of the material HL decomposed continued up to 800 °C. The thermal stability of the hybrid material is lower than the other graphene derivatives.
1.6. Adsorption-Desorption and Isotherms
The hybrid material (HL) was used as adsorbents for the removal of Zn(II) and Co(II) cations from aqueous solutions. For the adsorption measurements, the aqueous solutions of Zn(II) or Co(II) (25 ppm, 25 mL) were mixed with the hybrid material (HL) at room temperature as a batch process. At the end of each experiment, HL was filtered, and the metal content was analyzed with ICP-OES. The results were given as the average of the replicates. The effect of pH, concentration, contact time and temperature on the adsorption capacity of the hybrid material (HL) were explored. The amount of metal ions adsorbed per unit mass of the hybrid material (HL), maximum absorption capacity Q (mg/g) was determined by the following equation (Eq. 1)
Q = [(C0-C). V] / m (1)
where C0 and C (mg/L) are the liquid-phase concentrations of metals before and after adsorption respectively. V is the volume of the solution (L) and m is the mass of hybrid material (HL).
1.7. Effect of pH
The maximum adsorptions were provided at pH = 9 for Zn(II) (24.624 mg/g) and Co(II) (23.766 mg/g). During the adsorption process, the oxygen-containing functional groups on the surface of the hybrid material are formed anionic layer with H+ ions releasing in the solution. The adsorption of Zn(II) and Co(II) is gradually subjected to ion exchange with the hydrogen cation on the surface of hybrid material (HL) when pH are increased [22, 23]. The effect of pH on the adsorption is shown in Fig. 8.
1.8. Effect of contact time
The effect of the reaction time on the adsorption capacity of the hybrid material (HL) was also investigated at pH = 9 in the 0-120 min. range. The dependence on time is shown in Fig. 9. For both Co(II) and Zn(II), the adsorption rate is extremely high for the first 10 min with the adsorption of 21.26 and 23.12 mg/g, respectively. The adsorption capacity of HL for Co(II) reaches a maximum at 50 min with 24.951 mg/g and remains constant. The maximum absorption occurs at 70 min with 24.762 mg/g for Zn(II). The adsorption equilibrium is mainly due to high geometrical affinity between heavy metal ions and functional hybrid material. The slightly faster adsorption of Co(II) may be better coordination interactions between the HL and metal ion.
1.9. Effect of the initial metal concentration on the adsorption capacity
Effect of the initial metal concentration on the adsorption capacity was investigated at pH = 9 for a period of 50 min for Co(II) and 70 min for Zn(II) (Fig. 10). The adsorption capacity of HL presents an increasing trend with concentration increasing. The maximum adsorption occurs at 691.004 ppm for Zn(II) and 623.786 ppm for Co(II) and remains almost constant above these concentrations. The concentration increase results in a slightly better adsorption for Zn(II).
1.10. Effect of Temperature
The effect of temperature on the adsorption capacity was investigated in the 0-60 °C range (Fig. 11). Until 25 °C, the sharp adsorption increase was observed for Zn(II) and Co(II) ions. The maximum adsorptions were obtained at 24.92 and 24.37 °C for Zn(II) and Co(II), respectively. In the 20-60 °C range, there is no considerable change in the adsorption capacity of the HL.
1.11. Reusability of the adsorbent
Regeneration process is very important in the industry. The reusability of the hybrid material was investigated 10 times. The obtained results are given in Fig. 12. After each experiment, the adsorbed metal ions were removed from the HL by using HNO3 (4M) as the desorption agent. According to the adsorption capacity of the hybrid material for each cycle, the organo-silanized hybrid material showed effective adsorption properites up to seven times (Fig. 12). After seven times using, the adsorption capacity of the hybrid material decreased suggesting the deformation of the HL.
1.12. Analysis of real samples and analytical performance of the method
The standard addition methods were used for the removal of Co(II) and Zn(II) ions from drinking water samples by the hybrid material. The internal standard addition method by spiking the samples with Co(II) and Zn(II) ions in the fixed concentrations were used. The obtained results were given in Table 1. The recovery performance of was obtained as 98.65-101.75% for Zn(II) adsorption while 98.55-104.0% for Co(II). The results showed that there is a good correlation between the values obtained and approved. The analytical performance data for the pre-concentration studies for Co(II) and Zn(II) ions are listed in Table 2. The obtained data indicate that the Co(II) and Zn(II) ions can be detected as low concentration as 2.13 and 3.43 ng/mL, respectively. The precision of the method was found as 1.9% for Zn(II) and 3.8% for Co(II). In the preconcentration studies, a linear curve was observed in the range of 0.7-26.0 ng/mL for Zn(II) and 0.5-22 ng/mL for Co(II). The enrichment factor was found as 441 for Zn(II) and 675 for Co(II).
Table 1. Determination of Zn(II) and Co(II) ions in real water samples by standard addition methods.
Samples
|
Zn(II)
(mg.L-1)
|
Recovery (%)
|
Co(II)
(mg.L-1)
|
Recovery (%)
|
|
Added
|
Found
|
|
Added
|
Found
|
|
|
0.00
|
nd
|
-
|
0.00
|
nd
|
-
|
|
20.00
|
20.28 ± 0.9a
|
101.40
|
20.00
|
20.80 ± 0.36
|
104.00
|
Certified Water
|
40.00
|
39.46 ± 0.7
|
98.65
|
40.00
|
39.42 ± 0.49
|
98.55
|
|
60.00
|
61.05 ± 0.3
|
101.75
|
60.00
|
61.73 ± 0.86
|
102.88
|
|
80.00
|
80.73 ± 0.5
|
100.91
|
80.00
|
80.09 ± 0.05
|
100.11
|
|
0.00
|
nd
|
-
|
0.00
|
nd
|
-
|
Tap Water
|
20.00
|
20.02 ± 0.70
|
100.10
|
20.00
|
20.37 ± 0.95
|
101.85
|
(Drinking Water)
|
40.00
|
40.67 ± 1.20
|
101.67
|
30.00
|
30.72 ± 1.02
|
100.54
|
|
60.00
|
60.98 ± 1.23
|
101.63
|
60.00
|
61.07 ± 0.65
|
101.78
|
|
80.00
|
81.01 ± 0.98
|
101.26
|
80.00
|
80.70 ± 0.76
|
100.87
|
nd: not detected; a: meanvalue± standart deviation based on three replicate measurements.
Table 2. Performance characteristic of preconcentration procedure.
Parameters
|
Zn(II)
|
Co(II)
|
Precision (R.S.D.)
|
1.9 %
|
3.8 %
|
Detection limit (3s)
|
2.13 ng/mL
|
3.43 ng/mL
|
Linear calibration range
|
0.7-26.0 ng/mL
|
0.5-22.0 ng/mL
|
Reg. equation (after preconc.)
|
AA = 0.0019 Zn(II) + 0.0008 ng/mL
|
AA = 0.0025 Co(II) - 0.0003 ng/mL
|
Con. regression equation
|
AA = 0.0034 Zn(II) + 0.0043 µg/mL
|
AA = 0.0049 Co(II) + 0.0037 µg/mL
|
Enrichment factor
|
0.0019 ng/mLx1000µg/mL/0.0043=441
|
0.0025 ng/mL x 1000µg/mL/0.0037= 675
|
1.13. Langmuir adsorption isotherms
The adsorption isotherms were used to investigate the interaction types between the adsorbents and metal ions. Possible interaction between the hybrid material (HL) and the metal ions [Zn (II) or Co (II)] is in the form of coordinated-covalent bonding. Therefore, it fits with Langmuir adsorption model. The model suggests that the molecules or ions are adsorbed at a fixed number of well-defined sites. During the batch experiments, the Langmuir adsorption isotherm was used to evaluate adsorption properties. Eq. (2) expresses the Langmuir adsorption isotherm:
Ceq/Q=1/(Qmax x b)+Ceq/Qmax (2)
where q is the adsorbed amount of Zn(II)or Co(II) (mg/g), Ceq the equilibrium Zn(II)/Co(II) concentration (mg/mL), b the Langmuir constant (mL/mg), and Qmax is the maximum desorption capacity (mg/g). Fig. 13 and Table 3 illustrate the equilibrium constants of Langmuir isotherm. The correlation coefficients of Langmuir isotherm (R2) were determined as 0.9841 for Zn(II) and 0.998 for Co(II). The experimental and theoretical maximum adsorption values show a good correlation between each other (Table 3).
Table 3. Langmuir adsorption constants for the hybrid material.
|
Experimental
|
Langmuir constans
|
HL
|
Qmax (mg/g)
|
Qmax(mg/g)
|
b
|
R2
|
Zn(II)
|
627.963
|
633.347
|
3.2
|
0.9841
|
Co(II)
|
691.004
|
587.6785
|
2.4
|
0.998
|