3.1. Characterization of bentonite clay@biochar@magnetic
Surface properties of biochar, bentonite, and bentonite@biochar@magnetic were determined by BET analysis. The SBET value of biochar, bentonite, and bentonite@biochar@magnetic were specified as 1164.3, 10.3, and 487.2 m2/g, respectively, which show that bentonite@biochar@magnetic has significant active area. Also, the pore volume of biochar, bentonite, and bentonite@biochar@magnetic was 1.28, 0.035, and 0.62 m3/g, respectively. Moreover, the pore size of biochar, bentonite, and bentonite@biochar@magnetic was 4.4, 12.5, and 5.1 oA. The results of pore size indicate that all materials have mesoporous structures.
SEM analysis also used to specify the morphology as well as bumps and holes on the adsorbent surfaces. As shown in Fig. 2, the number of pores and holes on the surface of bentonite@biochar@magnetic are more than bentonite. Also, there are many fine pores on the bentonite@biochar@magnetic surface, indicating that Bentonite@biochar@magnetic has more active sites than bentonite. Therefore, Bentonite@biochar@magnetic is more appropriate than bentonite for adsorption. After Hg2+ adsorption, the surfaces of bentonite and Bentonite@biochar@magnetic have clearly changed and most of the pores and bumps are covered, indicating the adsorption of Hg2+ on their surfaces.
Figure 2
Also, Fig. 2 indicates EDAX results for bentonite and Bentonite@biochar@magnetic sorbents before and after Hg2+ adsorption. As reported in Table 1, there are different elements in bentonite such as C (15.05%), O (40.95%), Mg (1.7%), Al (12.33%), and Si (29.98%). After adsorption of Hg2+ by bentonite, the percentage of C, O, Mg, Al, and Si was changed to 9.34%, 39.47%, 1.18%, 8.35%, and 19.4%, respectively. Also, 22.25% of Hg was observed in the bentonite structure after sorption, showing that Hg2+ is properly absorbed on the bentonite surface. Also, several elements are available in the bentonite@biochar@magnetic structure, which include C (49.83%), O (24.52%), Mg (0.58%), Al (2.35%), Si (5.7%), and Fe (17.02%). After Hg2+ adsorption, the percentage of these elements was changed to 14.92%, 19.86%, 0.33%, 1.61%, 5.41%, and 27.69%, respectively. Moreover, the percentage of Hg2+ was 30.19%, which is a significant percentage.
Table 1
The percentage of elements in the bentonite and bentonite@biochar@magnetic structures before and after Hg2+ elimination
Adsorbent | Elements | Weight (%) | Atomic (%) |
Bentonite | C | 15.05 | 33.17 |
O | 40.95 | 47.37 |
Mg | 1.7 | 1.29 |
Al | 12.33 | 8.45 |
Si | 29.98 | 19.74 |
Total | 100 | 100 |
Bentonite after Hg2+ adsorption | C | 9.34 | 17.65 |
O | 39.47 | 56.01 |
Mg | 1.18 | 1.1 |
Al | 8.35 | 7.03 |
Si | 19.4 | 15.68 |
Hg | 22.25 | 2.52 |
Total | 100 | 100 |
Bentonite@biochar@magnetic | C | 49.83 | 65.85 |
O | 24.52 | 24.33 |
Mg | 0.58 | 0.38 |
Al | 2.35 | 1.39 |
Si | 5.7 | 3.22 |
Fe | 17.02 | 4.82 |
Total | 100 | 100 |
Bentonite@biochar@magnetic after Hg2+ adsorption | C | 14.92 | 36.58 |
O | 19.86 | 36.56 |
Mg | 0.33 | 0.4 |
Al | 1.61 | 1.76 |
Si | 5.41 | 5.67 |
Fe | 27.69 | 14.6 |
Hg | 30.19 | 4.43 |
Total | 100 | 100 |
Table 1
Moreover, for determining the crystal phases in the biochar, bentonite, and Bentonite@biochar@magnetic structures, XRD analysis was employed (Fig. 3(a)). 2 peaks were seen at 21 and 43o in the AC structure, which can be attributed to the crystal phases of (002) and (110), respectively. The presence of these peaks can cause the layers to be in order (Priya & Sureshkumar, 2020; Rawal et al., 2018). Also, there are several peaks in the clay structure at 2Ɵ of 27.5, 35, 51, 56, 63.5, and 68.5o, which are attributed to muscovite, montmorillonite, quartz, and calcite (Sen & Gomez, 2011). Moreover, different peaks were seen in the bentonite@biochar@magnetic structure at 31.2, 35.5, 43, 57.5, and 63o, which are related to (220), (311), (400), (511), and (440), respectively.
Also, VSM analysis was utilized to find the magnetic strength of bentonite@biochar@magnetic nanocomposite as shown in Fig. 3(b). The value of magnetic saturation for the nanocomposite was obtained as 80 emu/g, indicating that bentonite@biochar@magnetic nanocomposite has a paramagnetic property. This composite was able to remove from the suspension easily by a magnet.
Figure 3
Another important feature of a sorbent is its functional groups that play an essential role in uptake of pollutants. The results of FTIR analysis for determiming the functional groups on bentonite and bentonite@biochar@magnetic surfaces before and after Hg2+ sorption are displayed in Fig. 4. There are several functional groups on the bentonite surface such as hydroxyl, carboxyl and Si-O, which are important groups for adsorbing Hg2+. The peaks at 3411.9 and 3624.1 cm− 1 are attributed to OH and H2O tensile vibrations (Khaleghi et al., 2022). Also, a strong peak at 1022.23 cm− 1 shows the Si-OH tensile vibration (Sen & Gomez, 2011). After the Hg2+ adsorption by bentonite, the range and intensity of these peaks have not changed much and the range of peaks has shifted slightly, indicating the uptake of mercury by bentonite.
Also, Fig. 4(a) indicates the functional groups in the bentonite@biochar@magnetic surface. As shown, a strong peak at 3425.4 cm− 1 is related to -OH. Also, a peak was seen at 1655 cm− 1, which may be related to C = C. Moreover, a sharp peak at 1049cm− 1 is related to C-O, C = O or Si-O functional groups (Esmaeili and Tamjidi, 2020). Furthermore, several peaks were observed between 500–800 cm− 1, which are because of the functional groups such as Si-O-Al, Si-O-Si, Si-O-Mg, and Fe3O4 in the bentonite@biochar@magnetic surface. After adsorption of Hg2+ with bentonite@biochar@magnetic nanocomposite, some small peaks in the range of 1000–1600 cm− 1 were disappeared, which can be due to the uptake of Hg2+. Also, the intensity and range of the other peaks changed slightly, demonstrating that Hg2+ adsorption is physical and because of the weak vander waals forces.
Figure 4
Furthermore, the results of Raman analysis for activated carbon and bentonite@biochar@magnetic nano-composite are displayed in Fig. 4(b). For AC, 2 peaks at 1359 and 1589 cm− 1 are related to -sp3 carbon (D-band) and -sp2 carbon graphite (G-band) disturbances, respectively. These peaks depend on several parameters such as sp2 phase clustering, the rate of carbonization of materials, irregularity of bonds, ratio of sp2/sp3, porosity, size of crystals, and the presence of sp2 rings (Asimakopoulos et al., 2021; Yu et al., 2017). After modifying AC using bentonite and Fe3O4 nanoparticles, the intensity and position of peaks D and G changed, the peak D completely disappeared and the range of peak G shifted to 1579 cm− 1.
Besdies, Fig. 5 displays TEM images of bentonite and bentonite@biochar@magnetic nanocomposite. As is clear, bentonite and bentonite@biochar@magnetic have various morphologies with different particle sizes. Bentonite is on a micro scale, while bentonite@biochar@magnetic is on a nanoscale. The particle size of bentonite@biochar@magnetic composite is less than 25 nm, which can be due to the presence of magnetic nanoparticles.
Figure 5
3.2. Efficient parameters on uptake of Hg2+
The impact of pH on uptake of Hg2+ using two efficient adsorbents (bentonite and bentonite@biochar@magnetic) is illustrated in Fig. 6(a). pH is a very influential parameter on sorption because it causes ionization of the sorbent surface and affects the adsorption behavior. The experiments were done at ambient temperature, Hg2+ concentration of 10 ppm, sorbent concentration of 1 g/L, and time of one hour. As shown, the uptake efficiency increased quickly by increasing pH from 2 to 6 and after that, the uptake efficiency dwindled by enhancing pH from 7 to 9. At 2 < pH < 3 (strong acidic), there is intense competition between Hg2+ and H+ ions for bonding on the sorbent active sites. Also, at 3 < pH < 6 (weak acidic), Hg2+ ions can be easily deposited on the sorbent surface. Therefore, the uptake efficiency enhances sharply at pH 3 to 6. At pH > 6, intense competition occurs between OH− ions and Hg2+ for placement on the sorbent active sites, which prevents further sorption of Hg2+, resulting in a decrease in uptake efficiency (Wang et al., 2021).
Figure 6
To investigate the impact of temperature, several tests were done in differet temperatures (25-50oC), while other parameters like time of 60 min, mixing rate of 700 rpm, Hg2+ concentration of 10 ppm, and sorbent concentration of 1 g/L were considered constant. According to Fig. 6(b), with enhancing temperature from 25 to 50 oC, the Hg2+ uptake efficiency dwindled from 98.55 to 92.59% for bentonite@biochar@magnetic nanocomposite and 94.72 to 86.56% for bentonite, respectively, indicating that 25oC is the best temperature. The bond between the Hg2+ ions and the sorption sites weakens with enhancing temperature and thus the uptake efficiency decreases (Fardmousavi & Faghihian, 2014; Li et al., 2020).
The pollutant concentration is another important factor on the uptake process. To this end, bentonite and Bentonite@biochar@magnetic adsorbents were utilized to eliminate Hg2+ ions in different concentrations of Hg2+, while other parameters such as temperature of 25oC, sorbent concentration of 1 g/L, time of 60 min, and stirring speed of 700 rpm were kept constant. As shown in Fig. 6(c), the uptake efficiency of Hg2+ ions dwindled from 98.55 to 47.72% by enhancing the Hg2+ concentration from 10 to 100 ppm, respectively, which can be due to intense interaction between Hg2+ ions and active sites of the adsorbent. Therefore, for bentonite and Bentonite@biochar@magnetic nanocomposite, 10 mg/L Hg2+ concentration was considered the optimal value.
Moreover, the impact of sorbent dosage (0.25-5g/L) on the elimination of Hg2+ using bentonite and bentonite@biochar@magnetic adsorbents was studied at Hg2+ concentration of 10 ppm, laboratory temperature, pH 6, and mixing rate of 700 rpm. As exhibited in Fig. 6(d), the uptake efficiency of Hg2+ enhances with raising sorbent dose from 0.25 to 1.5 g/L, because more active sites and more functional groups will be available, leading to enhanced uptake efficiency. At sorbent dosage of 1.5 g/L, the uptake efficiency of Hg2+ using bentonite and Bentonite@biochar@magnetic adsorbents were obtained 98.78% and 97.44%, respectively. At sorbent dose > 1.5 g/L, the uptake efficiency of Hg2+ remained constant because of the saturation of active sites (Liu et al., 2020).
Furthermore, impact of sorption time (5-160 min) on elimination of Hg2+ using bentonite and bentonite@biochar@magnetic sorbents was shown in Fig. 6(e). According to the outcomes, in the early times, the Hg2+ sorption rate was high and the uptake efficiency by both sorbents enhanced by enhancing time, which can be due to the rapid binding of Hg2+ ions to active sites of adsorbents. At times greater than 80 min, the sorption efficiency remained constant. After saturation of active sites, the uptake efficiency reached equilibrium (Lilhare et al., 2021). Therefore, the utmost adsorption efficiency of Hg2+ ion by bentonite and bentonite@biochar@magnetic nanocomposite in 80 min were obtained as 98.78% and 97.67%, respectively.
3.4. Adsorption kinetics, isotherms and thermodynamic
Sorption kinetics of Hg2+ using bentonite and bentonite@biochar@magnetic adsorbents in different contact times (5-160 min) are displayed in Fig. 8. Also, kinetic variables are given in Table 2. The experimental sorption capacity for bentonite and bentonite@biochar@magnetic adsorbents were determined as 6.638 and 9.867 mg/g, respectively. The sorption capacities using the QFO model for bentonite and bentonite@biochar@magnetic adsorbents were determined as 9.3 and 10.73 mg/g, while using the QSO model were 4.85 and 7.12 mg/g, respectively. These amounts show that the value of Wexp for both adsorbents are closer to Wcal in the QSO model. Also, the higher amount of R2 for the QSO model compared to the QFO model demonstrates that the QSO model can better describe the sorption kinetic and sorption data fit better with the QSO model.
Table 2
Kinetc parameters of Hg2+ ion sorption for bentonite and bentonite@biochar@magnetic adsorbents
Kinetic model | Variable (unit) | Bentonite | Bentonite@biochar@magnetic |
QFO | We,cal (mg/g) | 9.3 | 4.85 |
K1 (min− 1) | 0.049 | 0.053 |
R2 | 0.964 | 0.958 |
We,exp (mg/g) | 6.64 | 9.87 |
QSO | We,calc (mg/g) | 10.73 | 7.13 |
K2 (g/mg min) | 0.086 | 0.016 |
R2 | 0.996 | 0.998 |
We,exp (mg/g) | 6.64 | 9.87 |
Figure 8
Table 2
Also, the isothermal behavior of the uptake of Hg2+ ions from polluted water using bentonite and bentonite@biochar@magnetic sorbents was studied by the Langmuir and Freundlich isotherms. To this end, several tests were done in various Hg2+ concentration (10–100 ppm) and the outcomes are plotted in Fig. 9 and presented in Table 3. Accordingly, the Wmax value for Hg2+ ions elimination using bentonite and bentonite@biochar@magnetic sorbents were 60.98 and 66.66 mg/g, respectively, indicating that the uptake capacity of bentonite has been increased by improving its surface with AC and Fe3O4 nanoparticles. The values of R for bentonite (0.034–0.259) and bentonite@biochar@magnetic nanocomposite (0.015–0.129) demonstrates that the sorption process of Hg2+ is favorable. Also, the value of n for bentonite and bentonite@biochar@magnetic sorbents were 2.58 and 3.07, indicating the physical nature of the uptake process. Due to the higher amounts of R2 for bentonite and bentonite@biochar@magnetic, Hg2+ uptake followed the Langmuir isotherm. The R2 values for two models were greater than 0.9, indicating the high reliability of the models with laboratory data (Al-Ghouti et al., 2022).
Table 3
Equilibrium variables of Hg2+ sorption using bentonite and bentonite@biochar@magnetic
Equilibrium models | Variable | Bentonite | Bentonite@biochar@magnetic |
Langmuir | Wmax (mg/g) | 60.98 | 66.66 |
K (L/mg) | 0.286 | 0.675 |
R2 | 0.973 | 0.998 |
R | 0.034–0.259 | 0.015–0.129 |
Freundlich | n | 2.58 | 3.07 |
Kf (L/g) | 15.28 | 24.72 |
R2 | 0.944 | 0.927 |
Figure 9
Table 3
The utmost adsorption efficiency and adsorption capacity of Hg2+ ions using bentonite and bentonite@biochar@magnetic nanocomposite were compared with other researches as reported in Table 4.
Table 4
Comparison of uptake efficiency and uptake capacity of different adsorbents in Hg2+ elimination
Adsorbent | Uptake efficiency (%) | Uptake capacity (mg/g) | Conditions | Reference |
polymerised saw dust | 94 | 20.4 | 4 h | (Raji and Anirudhan 1996) |
Bentonite-Based Monolith. | 63.9 | 0.187 | 200 min | (Azzahra and Masrura, 2021) |
Starch/polyethyleneimine magnetic mesoporous silica | 97.87 | 244.87 | 150 min | (Yang et al., 2021) |
Zeolite | 91.84 | 0.7 | 12 h | (Prasetya et al., 2020) |
Thiophenol-thiophene polymer | 99 | 62.5 | 90 min | (Albakri et al., 2021) |
Glycine/magnetic/alginate beads | - | 3.59 | - | (Lilhare et al., 2021) |
Polypyrrole-chitosan nanocomposite | - | 40 | - | (Duan et al., 2018) |
Scrophularia striata stems | 94 | 35.14 | 180 min | (Dehghani et al., 2020) |
Bentonite | 97.67 | 60.98 | 80 min | This work |
Bentonite@biochar@magnetic | 98.78 | 66.66 | 80 min | This work |
Table 4
Moreover, the thermodynamic variables for Hg2+ removal by bentonite and bentonite@biochar@magnetic sorbents in various temperatures are displayed in Fig. 10 and its outcomes are given in Table 5. Accordingly, ΔG° negative values were obtained for both sorbents, illustrating that the uptake of Hg2+ using bentonite and bentonite@biochar@magnetic sorbents is spontaneous. Also, the amounts of ΔH° for Hg2+ adsorption using bentonite and bentonite@biochar@magnetic sorbents were − 30.83 and − 59.37 KJ/mol, respectively, demonstrating that the adsorption of Hg2+ is exothermic. Moreover, the amounts of ΔH° for Hg2+ adsorption using bentonite and bentonite@biochar@magnetic sorbents were − 79.29 and − 163.38 J/mol.K, indicating that random uptake of Hg2+ ions by bentonite and bentonite@biochar@magnetic sorbents is reducing.
Table 5
Thermodynamic constants for removal of Hg2+ using bentonite and bentonite@biochar@magnetic nanocomposite
Adsorbent | Gº (KJ/mol)∆ | ∆Hº (KJ/mol) | ∆Sº (J/mol.K) |
298 | 303 | 308 | 313 | 318 | 323 |
Bentonite | -7.15 | -6.74 | -6.4 | -6.13 | -5.77 | -5 | -30.83 | -79.29 |
Bentonite@AC@ magnetic | -10.45 | -9.95 | -9.54 | -7.92 | -7.06 | -6.78 | -59.37 | -163.4 |
Figure 10
Table 5