3.1 Characteristic properties of graphene oxide
X-ray diffraction patterns of graphite, graphene oxide before and after adsorption was shown in Fig. 1. Graphite showed a strong peak at 2θ = 27.2o that is characteristic peak of graphite [23]. The diffraction peak at 2θ = 11.6o assigned the raw graphene oxide (before adsorption) peak and found appropriate as in the literature, which indicates forming of garphene oxide layers from graphite. d-spacing of graphite and graphene oxide was 0.328 nm and 0.759 nm, respectively. Increasing d-spacing between the layers can be attributed to the bounding of oxygen groups into the structure while synthesizing the graphene oxide from graphite. XRD pattern of graphene oxide after boron adsorption was similar to raw graphene oxide, which might be attributed to the low boron concentration used in the experiments.
Figure 1
FTIR spectra of graphite and graphene oxide are given in Fig. 2. FTIR spectra of graphite displayed a straight line due to polyaromatic layers in graphite structure. FTIR spectra of graphene oxide exhibited characteristic peaks. Graphene oxide spectra were showed a wide peak at 3340 cm− 1 indicated vibration band of OH group of water bounded on the structure. The absorption peaks at 1732 cm− 1 and 1620 cm− 1 could be attributed to vibration band of C = O group of carboxylic acid and C = C groups in graphene oxide structure. The peak between 1055 and 1230 cm− 1 assigned to vibration band of C-O belongs to epoxy and alkoxy groups. Stretching vibration of S-O group appeared at 831 cm− 1 belongs to sulphure in graphene oxide structure. FTIR spectra after boron adsorption did not show any significant change with the raw graphene oxide.
Figure 2
SEM images and EDX analysis of all samples were given in Fig. 3. As shown in graph, it was observed that the surface of graphite was rough and edges of the layers were sharp. After oxidation process, graphene oxide exhibited a porous structure and some folds, which indicated graphene oxide existance. Figure 3 shows the graphene oxide surface morphology after adsorption, which did not display any difference with raw graphene oxide due to low concentration of boron. Meanwhile, according to EDX results, C and O contents in graphite and graphite oxide structures in a good agreement.
Figure 3
3.2 Graphene oxide amount
The influence of adsorption dosage was investigated by changing graphene oxide amount between 0.05 and 0.4 g using 4 mg L− 1 initial boron concentration. Figure 4 shows change of boron removal with graphene oxide amount. Boron adsorption rapidly increased from 56–98% for 0.05 g and 0.2 g graphene oxide, respectively. A significant change was not observed using 0.4 g of graphene oxide (98.3%), which indicates that the active adsorption sites was fully occupied by boron molecules. Therefore, 0.2 g of graphene oxide was used for the further adsorption experiments. Meanwhile, a decline was achieved for the adsorption capacity, which attributed the gathering of the graphene oxide molecules by intermolecular interactions.
Figure 4
3.3 Solution pH
Solution pH is an important factor describes the surface properties of the adsorbent and also molecular structure of the adsorbate. Therefore, solution pH was adjusted from pH 2 to 12 and the adsorption capacities were given in Fig. 5.
Figure 5
As seen from the Fig. 5, maximum adsorption capacity was achieved at pH 6 (0.98 mg g− 1) and a slight decrease was observed until pH 10 (0.96 mg g− 1) and a sharp decrease was seen at pH 12 (0.37 mg g− 1). Boric acid forms borate anion in water with pKa value 9.2. According the solubility reaction below pH 8, boric acid (H3BO3) is the dominant species in water, while borate anion (B(OH)4−) is the dominant species above pH 10. Between pH 8 and 10, occurrence of H3BO3 species diminishes slowly and B(OH)4− species rises in water medium. Besides, hydroxyl and carboxylic acid groups on graphene oxide structure are protonated in acidic medium and the surface charge becomes positive. As the pH increases, the surface looses its protons and the surface becomes negatively charged. According to ionization of boric acid and protonation of graphene oxide surface, hydrogen bonding between oxygen and hydrogen molecules is the significant reason for the adsorption of boric acid in acidic medium. Boric acid slowly decreases; because of the borate anion begins to form in the solution as the pH increases, which causes a higher adsorption of boron. After pH 10, a significant decrease was observed due to electrostatic repulsive forces between the B(OH)4− and negatively charged surface of graphene oxide. Besides, competition between hydroxyl molecules (OH−) and B(OH)4− anions had a negative effect on boron adsorption; therefore, a sharp decrease was observed after pH 10.
3.4 Initial boron concentration and adsorption isotherms
One of the parameter effects adsorption capacities of graphene oxide is initial solution concentration. Boron solution concentrations were changed between 2 and 36 mg L− 1 and experiments were conducted using 0.2 g graphene oxide at room temperature during 24 h. As shown in Fig. 6, at low boron concentrations adsorption was achieved 98% (4 mg L− 1) and decreased to 40% for 36 mg L− 1 boron concentration. Certain amount of active adsorption sites occupied with boron molecules and after a point no further adsorption occurs on these sites. Therefore, it was easy to adsorb boron at low concentrations due to number of free adsorption sites. However, increasing boron concentration resulted with a decrease in adsorption, because of all adsorption sites covered by certain amount of boron molecules and rest of the boron molecules remained in the solution.
Figure 6
According to initial solution parameters adsorption isotherm and isotherm models were analyzed. Figure 7 shows the isotherm of boron adsorption on graphene oxide surface. According to Giles et al. (1960), isotherm curves are classified into four main groups and each of them has subgroups [24]. Isotherm graph of boron adsorption defines L type isotherm model and belongs L2 type subgroup. L type isotherms define Langmuir isotherms, which implies monolayer adsorption on graphene oxide.
Figure 7
Langmuir, Freundlich, Temkin and Dubinin-Redushkevich isotherm models were used to analyze the isotherm data [16]. Langmuir isotherm model describes the co-energy adsorption sites on the adsorbent surface and as a result monolayer adsorption occurs on the same energy sites [25]. Freundlich isotherm explains heterogeneity of the surface that has different energy sites. Temkin isotherm model defines the reduction of adsorption heat, which is linear. Dubinin-Radushkevich isotherm model depends on the porosity of the adsorbent and takes into account the mean free energy of the adsorption.
To identify the conformity of isotherm models and experimental data, correlation coefficient and chi-square values were compared. To calculate the chi-square values, given equation below was used;
$${X}^{2}=\sum \left[\frac{{({q}_{cal}-{q}_{exp})}^{2}}{{q}_{cal}}\right]$$
8
The parameters calculated from the isotherm data were given in Table 1. Small values of X2 indicate that the isotherm model is in a good agreement with experimental data. According to correlation coefficients (R2) and chi-square (X2) values, Langmuir isotherm model (R2 = 0.99, X2 = 0.049) was best-fitted model of adsorption process, which indicates the monolayer adsorption of boron on graphene oxide. Langmuir adsorption capacity was found as 3.92 mg g-1. As shown in Fig. 7, calculated qe values were in a good correlation with calculated from Langmuir model among all isotherm models.
Table 1
Dimensionless constant separation factor (RL) could be calculated using Langmuir constant b (L mg-1). RL defines the adsorption process is favorable or unfavorable. RL is calculated as;
$${R}_{L}=\frac{1}{1+b{C}_{0}}$$
9
Calculated RL dimensionless separation factor is defined as; RL>1 unfavorable, RL =1 linear, 0 < RL <1 favorable and RL=0 irreversible. Correlation between initial solution concentration and dimensionless separation factor is depicted in Fig. 8. RL values of boron adsorption process changed between 0 and 1. Therefore, it is clear that adsorption of boron on graphene oxide was a favorable adsorption.
Figure 8
3.5 Contact time and adsorption kinetics
Contact time experiments were examined between 1 to 36 hours using 4 mg L− 1 initial boron concentration. Until 3h, no remarkable change was observed on boron adsorption (Fig. 9). However, after that point a significant increase was achieved. A plateau was reached between 24 h and 36 h, which attributed the adsorption equilibrium.
Figure 9
The kinetic modeling of boron adsorption was analyzed using various kinetic models [16].
Kinetic model parameters were calculated and given in Table 2. Pseudo-second-order kinetic model was the best fitting model among four models compared the correlation coefficients (R2 = 0.99). Pseudo-second-order kinetic model was derived based on the three assumptions; (i) the adsorption process is irreversible; therefore, desorption can be neglected, (ii) there is no concentration gradient of the adsorbate remaining in the solution, (iii) adsorption process is controlled by a chemical reaction [27, 28]. Meanwhile, calculated adsorption capacity from pseudo-second-order kinetic model (0.992 mg g− 1) was demonstrated a good correlation with the experimental value (0.986 mg g− 1).
Table 2
3.6 Adsorption thermodynamics
To understand the effect of temperature on boron adsorption, batch adsorption experiments were examined at 298, 308 and 318 K. It was observed that increasing temperature has a positive effect on boron adsorption on graphene oxide. Meanwhile, thermodynamic parameters were also calculated to better understanding of the adsorption behavior.
Change in standard enthalpy (ΔHo) was calculated according to Van’t Hoff equation [29];
$$ln\left(\frac{{C}_{e2}}{{C}_{e1}}\right)=\frac{{\varDelta H}^{o}}{R}\left(\frac{1}{{T}_{2}}-\frac{1}{{T}_{1}}\right)$$
14
Change in standard Gibbs free energy (ΔGo) was given as [29];
$${\varDelta G}^{o}=-RTlnK$$
15
$$K=\frac{{C}_{ads}}{{C}_{e}}$$
16
Change in standard entropy (ΔSo) was calculated [29];
$${\varDelta G}^{o}={\varDelta H}^{o}-T{\varDelta S}^{o}$$
17
Calculated thermodynamic parameters were given in Table 3. Positive value of ΔHo pointed out that the adsorption process has endothermic nature. Equilibrium constant values (K) showed a good correlation with ΔHo. Increasing temperature resulted with an increment in K values. ΔGo values were found negative suggesting that the boron adsorption process was spontaneously occurred at all temperatures. Positive values of ΔSo noticed that randomness at solid/liquid interface and degree of freedom of the adsorbed boron were increased.
Table 3