3.1 Characterization
The physic-chemical property of GO and TAIGO was confirmed by SEM, AFM and FTIR. The morphology of the TA and TAIGO was shown in Fig. 2. It could be found that the GO and TAIGO exhibited layered structures, which could be accounted for the polar groups on them (Mi et al., 2019). The formation of GO and TAIGO was also determined by AFM in the tapping mode (Fig. 3). It was found that GO showed a height profile of thickness 1 nm, while the TAIGO was 2 nm. The results of AFM indicated that the GO was prepared, and the TA was immobilized on the GO successfully. The immobilization TA could be account for the SN2 nucleophilic reactions, in which the epoxy ring on GO was destroyed by the phenolic hydroxyl group on TA (Lei et al., 2011). Besides, the π-π interaction and van der Waals force on sites of the aromatic rings were the other possible formation mechanisms (Patil et al., 2009; Li et al., 2010).
To further study the changes in the groups, the FTIR tests were conducted. As shown in Fig. 4, there were many functional groups in GO, such as the hydroxyl groups at 3429 cm− 1, carbonyl groups at 1717 cm− 1 and epoxy groups at 1118 cm− 1. With the immobilization of TA, the ring-opening reactions of epoxy groups were getting intense, and the epoxy groups on the TAIGO were significantly weakened. Besides, it was noticed that the introduction of TA could also be confirmed in the FTIR of TAIGO, in which some characteristic peaks of TA could be observed.
3.2 Adsorption performance
For comparing the difference in removal efficiency between GO and TAIGO, the adsorption experiments in the actual desulfurization wastewater were conducted. In contrast, exhibited in Fig. 5, the removal efficiency of GO and TAIGO in the desulfurization wastewater A was 80.23%, 90.39%, and 65.44%, 78.06% in the desulfurization wastewater B. The TAIGO showed a better adsorption performance than GO, which could be accounted for the inducted abundant of adjacent phenolic hydroxyls on TA. In addition, the results of specific surface area and zeta potential were presented in Table 2, which indicated that the TAIGO exhibited a better pore structure and higher charged properties, which may further enhance the removal performance of Hg2+ in wastewater.
Table 2. The specific surface area and zeta potential of GO and TAIGO.
Material
|
Specific surface area
(m2·g-1)
|
Zeta potential
(mV)
|
GO
|
430
|
-42.8
|
TAIGO
|
562
|
-48.5
|
Table 3. Kinetic parameters for the adsorption on TAIGO adsorbent.
Pseudo-first-order
|
Pseudo-second-order
|
K1
|
qe (mg/g)
|
R2
|
K2
|
qe (mg/g)
|
R2
|
0.111
|
10.08
|
0.691
|
0.047
|
23.81
|
0.9995
|
3.3 Effect of operation parameters
To better and deeply understand the factors affecting adsorption performance, the simulated desulfurization wastewater prepared by Hg2+ solution was used instead of the actual wastewater in the following experiment.
3.3.1 Effect of contact time
Figure 6 showed the effect of reaction time on Hg2+ removal efficiency. It could be found that the removal efficiency increased quickly in the initial 10 min and the adsorption process was completed within 15 min. It can be inferred that the adsorption process was rapid, and it took a short time for adsorption completely. The Hg2+ concentration in the desulfurization wastewater was relatively high at the initial stage of adsorption, which increased the adsorption driving force. Besides, large amounts of adsorption sites of TAIGO were available within 15 min. After a period of time, the adsorption sites were occupied, leading to the slow growth in removal efficiency.
3.3.2 Effect of TAIGO dosage
The amount of adsorbent was a vital index for industrial application. The effect of adsorbent dosage was studied by adding different amounts of TAIGO into 250 mL Hg2+ containing solution. The reaction time was 120 minutes to ensure the adsorption process proceeded entirely. As shown in Fig. 7, as the dosage of TAIGO increased from 5 mg to 10 mg, the Hg2+ removal efficiency increased from 56.28–92.1% and gradually remained stable with the further addition of TAIGO to 20 mg. As the adsorbent addition increased, the adsorption sites of Hg2+ gradually increased, which enhanced the binding probability between the TAIGO and Hg2+. However, with the continued increase of the adsorbent dosage, no remarkable variations in removal efficiency were observed. It can be inferred that the adsorption process had reached the saturated state, and the surface of TAIGO adsorption sites was overlapped, which restrict the further improvement in removal efficiency.
3.3.3 Effect of pH
The pH value of the wastewater not only affects the existence of functional groups of the adsorbent but also changes the existence form of Hg2+ in the desulfurization wastewater (Wang et al., 2015). Thus, it is essential to investigate the effect of pH on the adsorption performance for Hg2+. In Fig. 8, as the pH value increased from 3 to 12.3, the removal efficiency increased and then decreased. The optimum removal efficiency was obtained at pH of 9. This phenomenon could be ascribed to the following reasons. When the pH value of wastewater was lower than 7, most of the mercury ions in the desulfurization wastewater existed in the valence state of Hg2+, and the excess H+ ions would bind with the adsorption sites of TAIGO, resulting in a decline in adsorption capacity towards Hg2+. It was noted that the adjacent phenolic hydroxyls on TA were sensitive to the existence of H+, and the lower pH value was conducive for chelating reaction with Hg2+ in desulfurization wastewater (Huang et al., 2009). As the pH value increased to 9, the ionization degree of phenolic hydroxyls located in TA was promoted, leading to the enhancement of the chelation and electrostatic effect between TAIGO and Hg2+ (Sun et al., 2020; Ma et al., 2005). However, when the pH value further increased above 9, the Hg2+ in the desulfurization wastewater was easily converted into insoluble Hg(OH)2, which was directly related to the decrease of adsorption capacity.
3.3.4 Effect of ion components
To evaluate the adsorption performance of TAIGO in the complex wastewater component, the adsorption experiment in different ion components was conducted. In the actual desulfurization wastewater, the concentration of the Cl− exhibited a wide range from 4000 to 20000 mg/L, which may affect the adsorptive capacity for Hg2+. The effect of chloride ion (Cl−) on the adsorption efficiency was shown in Fig. 9 (a). It can be observed that the TAIGO showed an excellent absorption performance (88.2%) at a low concentration of the Cl− (12000 mg/L). However, as the chloride ions concentration increased, the adsorption efficiency of TAIGO was gradually decreased. It could be ascribed that the excessive chloride ions in the slurry were likely to accumulate on the surface of TAIGO, leading to a decline in adsorbent surface charge. Besides, it was reported that the Hg2+ tended to from Hg(OH)2 to HgCl(OH) and HgCl42− in a solution containing chloride ions, and may further hinder the absorption performance (Castro et al., 2011).
Besides, the effect of SO32− was also investigated in this work. Although the content of SO32− was relatively low in desulfurization wastewater, it may cause the migration and conversion of Hg2+ (Wu et al., 2019). Figure 9 (b) gave the effect of concentration of the SO32−on removal efficiency. It can be observed that as the concentration of SO32− increased to 700 mg/L, the removal efficiency remained at 88.2%. However, when the concentration is more significant than 700 mg/L, the inhibitory effect of SO32− on the adsorption efficiency was gradually strengthened, and the adsorption efficiency could be reduced to 75.6%. It was noted that the SO32− was an essential factor that caused the Hg2+ in the desulfurization wastewater to be reduced to Hg0 and released into the atmosphere (Ma et al., 2018). Interestingly, with the increase of the SO32− concentration, more Hg2+ would residue in wastewater. The fact was that the Hg2+ would accelerate the free radical reactions, causing the SO32− oxidized to SO42−. That is, less Hg2+ could be involved in the chemical adsorption reaction, which may cause a low adsorption efficiency. Similar results had also been found in other transition metals (Wu et al., 2004). For SO42−, as the increase of concentration increased, the removal efficiency showed no remarkable change. It suggested that the existence of SO42 has little impact on removal performance in the desulfurization wastewater system.
In addition, with the development of the desulfurization wastewater concentration technology, the concentrations of Ca2+, Mg2+ and Na+ were in an extensive variation range. Therefore, the effect of Ca2+, Mg2+ and Na+ concentrations was considered, and the results were displayed in Fig. 10. With the concentrations of Ca2+, Mg2+, and Na+ raised from 2000 to 6000 mg/L, there was no significant decline in the adsorption efficiency. The main reason is that the lack of d or f electron orbit may cause the weakening of the interaction force between Hg2+ and ions. That is, the interaction between Hg2+ and TAIGO was stronger than Ca2+ and Mg2+ ions, and the presence of the Ca2+, Mg2+ and Na+ would not affect the adsorption efficiency of TAIGO (Sun et al., 2014).
3.4 Adsorption kinetics
Adsorption kinetics provided a vital information index about the mechanism of Hg2+ adsorption onto TAIGO, which was necessary to depict the adsorption behaviour of TAIGO. In this study, the pseudo-first-order and pseudo-second-order kinetic model was applied to fit the adsorption dates of Hg2+ adsorption on the TAIGO.
The pseudo-first-order and pseudo-second-order kinetics model were represented by the following equation:
(3)
(4)
Where qe and qt (mg/g) represent the amounts of ions adsorbed at equilibrium state and time t (min). The k1 and k2 g/(mg·min) are the rate constants of two kinetic models.
Figure 11 shows the fitting curves for the adsorption of Hg2+ with TAIGO under natural pH. The correlation coefficient (R2 = 0.9995) of the pseudo-second-order kinetics model was larger than pseudo-first-order, which indicated that the chemical adsorption played a significant role in the adsorption process of Hg2+ and the physical adsorption promoted the adsorption performance (Anbia et al., 2016).
Besides, the parameters k1 and k2 were an essential index for evaluating absorbent performance. Table.3 listed the adsorption rate constants based on the pseudo-first-order and pseudo-second-order for Hg2+ adsorption. Furthermore, the comparative analysis between TAIGO and other reported GO-based materials was studied. It could be seen from Table 4, and the TAIGO exhibited an excellent absorption performance than most materials and a shorter equilibrium adsorption time needed.
Table 4
Comparison of adsorption capacities with various adsorbents for heavy metals.
Adsorbents
|
Target pollutant
|
Kinetic
|
Adsorption capacity(mg/g)
|
Equilibrium time (min)
|
Ref.
|
Thiol-functionalised silica-coated magnetite
|
Hg2+
|
PSO
|
9.5
|
15
|
(Hakami et al., 2012)
|
EDTA functionalized graphene oxide nanoparticles
|
Hg2+
|
PSO
|
18.6392
|
160
|
(Sun et al., 2020)
|
Magnetic nanoparticle–graphene oxide
|
Se4+
|
-
|
4.99
|
0.2
|
(Fu et al., 2014)
|
Persimmon tannin/graphene oxide composites
|
Ge4+
|
-
|
117.38
|
-
|
(Zhang et al., 2019)
|
Magnetite/reduced graphene oxide nanocomposites
|
Pb2+
|
PSO
|
13.87
|
180
|
(Qi et al., 2015)
|
Tannin-immobilized graphene oxide
|
Hg2+
|
PSO
|
23.81
|
15
|
This paper
|
The probable Hg2+ removal mechanism was shown in Fig. 12. For the TAIGO, in desulfurization wastewater, the considerable surface area of GO was acting as a large net to capture the Hg2+ and served as the carriers for TA. Also, the functional groups in GO could improve the removal performance for Hg2+. Besides, the multiple ortho-position phenolic hydroxyl structure of TA functioned as a multi-base ligand to complex reaction with Hg2+. A stable five-membered ring chelate with Hg2+ was formed, which was the form of oxygen anions. The third phenolic hydroxyl group in the pyrogallol structure could further promoted the dissociation effect of the other two phenolic hydroxyl groups. Thus, the stability of Hg2+ based complexes could be formed, and the Hg2+ was effectively removed from desulfurization wastewater.
3.5 Regeneration performance
The regeneration performance was a crucial factor in evaluating the absorbent's economics, so it is necessary to conduct the regeneration experiment. In this experiment, a strongly acidic environment was created to complete the regeneration process, the 0.1 mol/L HCl was applied to elute the absorbent. In Fig. 13, the removal efficiency has remained at 88% after three recycles. As discussed in the effect of pH, the excess H+ would lead to the protonated effect on TAIGO, the Hg2+ would be replaced, and excellent desorption efficiency could be achieved. However, the decomposition of adsorbents would occur in the process elute, leading to a slight efficiency decline in the third absorption that the pristine one. The good regeneration performance of TAIGO indicated that it was an economical and efficient absorbent, which could be further used in the practical desulfurization wastewater.