Abstract
Functional magnetic Fe3O4@PPy microspheres were prepared and characterized by XRD, FTIR, SEM, TEM, and magnetometer, and the adsorption of Hg(II) onto Fe3O4@PPy was investigated. The results showed that the adsorption of Hg(II) onto Fe3O4@PPy dramatically increases within 5 min and reaches adsorption equilibrium at 200 min. The adsorption of Hg(II) increases with pH increased, and a removal efficiency (RE) of 90.5% was obtained at pH 7.2. The isotherm studies revealed that the adsorption of Hg(II) onto the Fe3O4@PPy fits well with the Langmuir isotherm model, and the calculated qm value of 232.56 mg/g. The adsorption process of Hg(II) onto the Fe3O4@PPy is well-fitted by the pseudo-second-order model with a high correlation coefficient (R2) of 0.999. The thermodynamic coefficients (ΔH°, ΔS°, and ΔG°) were calculated from the temperature-dependent adsorption isotherms and illustrated that the adsorption of Hg(II) on the Fe3O4@PPy was spontaneous and endothermic. Different desorption agents were used to recover Hg(II) adsorbed onto Fe3O4@PPy, and a satisfactory recovery percentage of 93.0% was obtained by using 0.1 M HCl and 0.05 M NaCl.
HIGHLIGHTS
Amino-functional magnetic Fe3O4@PPy microspheres were prepared and characterized.
The Hg(II) can be adsorbed by Fe3O4@PPy effectively.
The used Fe3O4@PPy could be regenerated by two-component desorbent of HCl and NaCl.
Graphical Abstract
INTRODUCTION
Mercury (Hg) is considered one of the most important environmental contaminants due to its bioaccumulation and strong biological toxicity. The presence of Hg can threaten human health even at a trace level (Zhang et al. 2017; Zhao et al. 2018). For all three species of mercury (elemental (Hg0), metallic (Hg(II)), and organic (MeHg)), Hg(II) and MeHg have higher toxicity to living organisms. Therefore, it is necessary to develop the efficient, fast, and economically feasible technology for Hg(II) removal (Yuan et al. 2014). Many physical, biological, and chemical techniques have been employed to remove the Hg(II) from wastewaters, such as membrane separation, ion exchange, chemical precipitation, coagulation, and adsorption (Zhang et al. 2012, 2020; Sharma et al. 2015; Wang et al. 2020). However, there are many problems with the current technology because of low efficiency, high cost, and complicated operation. Among these techniques, the adsorption method has been widely studied due to its low cost, simple operation, and high efficiency (Liu et al. 2018; Guo et al. 2019). Many adsorbents, such as activated carbon (Alomar et al. 2017; Huang et al. 2021), mesoporous silica (Antochshuk et al. 2003), chitosan (Sampaio et al. 2015), and magnetic nanocomposite (Kim & Park 2017; Xiao et al. 2019), have been used to remove the Hg(II) from the aqueous solution. Among these adsorbents, the amino-functionalized magnetic nanocomposite has gradually attracted the interest of researchers due to their superparamagnetic properties, biocompatibility, and easy to surface modification. The amino groups in nanocomposite can interact with metal ions through ion exchange or chelation and can also improve its adsorption features, such as selectivity and adsorption capacity (Zhou et al. 2019). The investigation of Wang et al. (2012) showed that the amino-functionalized magnetic composite microspheres have a good adsorption effect on the metal chromium ions in the solution. The core–shell Fe3O4@polypyrrole composite (Fe3O4@PPy) is a typical amino-functionalized magnetic nanocomposite which has been widely investigated in recent years (Peng et al. 2015; Tang et al. 2017). However, the applications of Fe3O4@PPy to remove Hg(II) from solution are rare.
In this work, Fe3O4@PPy composite microspheres are prepared and characterized by different techniques. After that, the Hg(II) adsorption behavior under different influence factors (pH, initial Hg(II) concentration, and contact time), the adsorption isotherms, kinetics, thermodynamics, and the reusability of Fe3O4@PPy were investigated. Finally, the optimal conditions and removal mechanism were determined.
MATERIALS AND METHODS
Chemicals and materials
FeCl3·6H2O (>99%), Hg(NO3)2 (>98.5%), and sodium acetate anhydrous (>99%) were purchased from Tianjin Chemical Reagent Third Factory, China. Pyrrole and glycol (>99.5%) were obtained from Shanghai Macklin Biochemical Co., Ltd, China. Absolute ethanol and polyethylene glycol (PEG)-6000 were from Tianjin Fuyu Fine Chemical Co., Ltd, China. HCl solution (36–38%), HNO3 (65%), and NaOH (>96%) were received from Tianjin Northern Tianyi Chemical Reagent Factory, China. All the chemicals were used without further purification. Distilled deionized water (18.4 MΩ cm) was used for material synthesis and aqueous experiments.
Methods of synthesis
Preparation of Fe3O4 microspheres
The Fe3O4 microspheres were prepared through a polyol reduction method (Peng et al. 2015; Figure 1). Firstly, 6 mM FeCl3·6H2O was dissolved in 40 mL glycol and stirred until evenly mixed. Subsequently, 42.9 mM sodium acetate anhydrous and 1.0 g of polyethylene glycol (PEG)-6000 were added and the mixed solution was stirred vigorously for 0.5 h. After that, the mixture was transferred to the 50 mL Teflon autoclave, and heated at 200 °C for 12 h. Then, the sample was collected with a permanent magnet, and washed with distilled water and ethanol. The above separation process is repeated three times. Finally, the resulting Fe3O4 microspheres were dried in a vacuum oven at 40 °C for 6 h.
Preparation of magnetic Fe3O4@PPy microspheres
Fe3O4 microspheres (0.10 g) were dispersed in HCl solution (40 mL, 0.1 M) under sonication, and then, the mixture was sealed in a three-necked flask and left to stand for 12 h. Subsequently, the sample was collected with the help of a magnet, washed with deionized water repeatedly to remove the residual HCl. After that, 10 mL absolute ethanol and 0.15 mL redistilled pyrrole were added into the flask under sonication and N2 atmosphere, and then, the mixture was sealed and left to stand at 5 °C for 12 h. Then, HCl (0.165 mL, 12 M) and FeCl3·6H2O solution (30 mL, 1.1 M) were slowly added into the above mixture in turn under 5 ± 1 °C and N2 atmosphere, stirring and sonication for 1 h. Finally, the precipitate was separated, washed with distilled water and ethanol, and then dried in a vacuum oven at 40 °C for 12 h (Bhaumik et al. 2011).
Instruments
The Fe3O4@PPy composite was characterized by X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.5406 Å) at 45 kV/40 mA, for 2θ values between 10° and 90° (D8 Advance, Bruker, Germany). The XRD patterns obtained were analyzed by Jade 9.1 software program. The infrared spectroscopic information was recorded by an IRPrestige-21 Fourier transform infrared spectroscopic spectrometer (FTIR, Shimadzu, Japan) for the functional group analysis. It was collected using pressed KBr discs with a resolution of 4 cm−1 over the range of 4,000–500 cm−1 on a Fourier transform infrared. The surface morphology of the composite was studied by scanning electron microscopy (SEM; Quanta 200E, FEI, USA) and transmission electron microscopy (TEM; JEM-2100F, JEOL, Japan). Magnetic measurements were performed with a superconducting quantum interference device (SQUID) magnetometer (MPMS XL-7, Quantum Design, USA). To determine Hg(II) ion removal by the adsorbent, the Hg(II) concentration in the remaining solution was measured with an inductively coupled plasma optical emission spectrometer (ICP-OES Agilent 5100, USA).
Adsorption investigation
Desorption investigation
1 mg Fe3O4@PPy with adsorbed Hg(II) ions was dispersed in 20 mL solution containing desorption agent and shaken in a conical flask for 3 h. After that, the adsorbent was separated, and the concentration of Hg(II) ions in solution was analyzed by ICP-OES for calculating Hg(II) the recovery rate.
RESULTS AND DISCUSSION
Characterization of Fe3O4@PPy microspheres
Figure 2 shows the XRD pattern of the Fe3O4@PPy microspheres. The diffraction peaks at 31.17°, 35.50°, 43.12°, 53.58°, 57.03°, and 62.63° correspond to (220), (311), (400), (511), and (440) crystal planes of the cubic anti-spinel structure, respectively. It is shown that magnetic Fe3O4 is not oxidized and Fe3O4@PPy microspheres are well crystallized in the preparation process of acidic proton and oxidative polymerization.
The FTIR spectrum of Fe3O4@PPy is presented in Figure 3. As shown in Figure 3, the strong absorption band at 579 cm–1 corresponds to the Fe–O stretching vibration of Fe3O4; the band at 1,556 cm–1 is attributed to the C = C vibrations of pyrrole rings; and the bands at 1,245, 1,033, and 773 cm–1 are attributed to C–N stretching vibration, C–H out-of-plane bending vibrations, and N–H in-plane bending vibrations, respectively. From these results, it can be inferred that the monomers have been polymerized successfully to be Fe3O4@PPy microspheres.
The SEM and TEM images of the Fe3O4@PPy microspheres are shown in Figure 4. From the SEM image in Figure 4, the thickness of the clear uniform shell is about 80 nm. It is clearly seen that the Fe3O4 core is encapsulated with polypyrrole coating, indicating the successful polymerization of pyrrole in the present reaction system and the formation of Fe3O4@PPy composite microspheres. It can be shown from TEM images that the Fe3O4@PPy microspheres with a uniform diameter size of about 290 nm have been prepared. The diameter is almost consistent with the Fe3O4@PPy prepared by previous study (Zhang et al. 2017). TEM images also show the magnification image of the Fe3O4@PPy microspheres retaining good dispersibility.
Figure 5 shows the saturation magnetization curves of Fe3O4@PPy microspheres. The saturation magnetization values of Fe3O4@PPy are 49.6 emu g–1, and the coercive force of Fe3O4@PPy microspheres is 28.85 Oe, indicating that the coated microspheres have good magnetic properties (Morel et al. 2008).
Effect of initial Hg(II) ions concentration on Hg(II) adsorption
The effect of initial ions concentration on the adsorption of Hg(II) by Fe3O4@PPy is shown in Figure 6. It can be seen that the RE and qe decreased and increased with initial Hg(II) concentration increasing from 5.0 to 60 mg/L, respectively. In particular, the qe increased sharply with the initial concentration increasing from 5 to 10 mg/L; however, the adsorption capacity increased slowly at initial concentration above 20 mg/L. This phenomenon may indicate that the adsorption sites of Fe3O4@PPy were not completely occupied at the low initial concentration of Hg(II); therefore, the adsorption capacity has not reached equilibrium. However, the number of free adsorption sites decreased with the Hg(II) ions concentration increases, resulting in part of Hg(II) ions cannot be combined with the adsorbent, and thus, the increase of qe becomes less obvious. Li et al. (2019) found that a novel hierarchical carbon/Fe–Mn composite readily fabricated from biomass was utilized as an adsorbent for Hg(II) removal. The composite exhibited high removal efficiency of 96.8%, and considerable adsorption capacity of 9.8 mg/g. Zabihi et al. (2010) fabricated porous carbons from walnut shells, which exhibited a high monolayer adsorption capacity of 151.5 mg/g for Hg(II) removal. In the results of this study, the maximum RE and qe were 93.86% and 232.55 mg/g, respectively, indicating that the Fe3O4@PPy can effectively adsorb the Hg(II).
Effect of the adsorbent dosage on Hg(II) adsorption
The effect of the Fe3O4@PPy dosage on Hg(II) removal is shown in Figure 7. As can be seen that the RE increases gradually with the increase of the adsorbent dosage. When the dosage is 0.25 g/L, the RE is 85.12%, and the RE reaches almost 90% at the dosage of 0.4 g/L. This phenomenon can be attributed to more available adsorption sites and the increase of adsorption contact area (Peng et al. 2015; Ma et al. 2020). On the other hand, the qe decreased with the increase of dosage. At low dosage, the number of Hg(II) ion in solution is much more than the binding sites of adsorbents. Therefore, most adsorption sites will be occupied by Hg(II). As the dosage increases, the number of adsorption sites increases rapidly. It indicates that some adsorption sites remained in an unsaturated state on Hg(II) adsorption, resulting in a decrease in the utilization rate of the Fe3O4@PPy (Mollahosseini et al. 2019).
In a similar study to explore the effect of the adsorbent dosage on Hg(II) adsorption, Ghasemi et al. (2019) using polydopamine decorated SWCNTs found that the qe of 55 mg/g and the RE of 82% for Hg(II) removal can be achieved at the adsorbent dosage of 0.3 g/L. Consequently, it can be concluded that Fe3O4@PPy nanocomposite is a more effective adsorption media for the removal of Hg(II) from aqueous solution.
Effect of solution pH on Hg(II) adsorption
The RE at different pHs is presented in Figure 8. As can be seen that the RE of Hg(II) sharply increased from ∼8 to ∼86% as pH was raised from 2.1 to 6.0. Furthermore, when the pH was increased from 6.0 to 7.2, the RE increased gradually from ∼86 to ∼91%. It is attributed that the amino group of the pyrrole will protonate in strong acidic solution to form positive sites such as , resulting in electrostatic repulsion between the Hg(II) ions and the adsorbent. The dominant form of Hg(II) is in the form of stable compounds without electrostatic interaction (Zhang et al. 2012). Therefore, the adsorption capacity of Fe3O4@PPy is very low when the pH is less than 4.
At high pH, deprotonation occurred on the surface of Fe3O4@PPy, which reduced the electrostatic repulsion between the Hg(II) ions and the adsorbent. The amino groups of the pyrrole can combine with OH− in the solution to form negatively charged NH2OH, significantly enhancing the ability to adsorb Hg(II) (Deb et al. 2017). In the alkaline media, and complexes could be formed, which cannot be adsorbed by Fe3O4@PPy, because these complexes are water-insoluble (Mollahosseini et al. 2019).
Adsorption kinetics
Pseudo-first-order . | Pseudo-second-order . | Elovich . | ||||||
---|---|---|---|---|---|---|---|---|
k1 . | qe . | R2 . | k2 . | qe . | R2 . | a . | b . | R2 . |
0.015 | 36.97 | 0.811 | 2.24 × 10−3 | 174.2 | 0.999 | 4.64 × 107 | 0.160 | 0.980 |
Pseudo-first-order . | Pseudo-second-order . | Elovich . | ||||||
---|---|---|---|---|---|---|---|---|
k1 . | qe . | R2 . | k2 . | qe . | R2 . | a . | b . | R2 . |
0.015 | 36.97 | 0.811 | 2.24 × 10−3 | 174.2 | 0.999 | 4.64 × 107 | 0.160 | 0.980 |
As shown in Figure 9(a), the amount of Hg(II) adsorption on Fe3O4@PPy sharply increases within 5 min, and then reached equilibrium within approximately 200 min. The R2 (>0.999) of the pseudo-second-order model is higher than that of two other models, and the qe value (174.2 mg/g) is calculated from the pseudo-second-order model approximately equal to the experimental value (173.76 mg/g), indicating that the adsorption process may be chemisorption involving force through sharing or exchange of electrons between the Hg(II) and Fe3O4@PPy (Ghasemi et al. 2019). Elovich model can describe the chemisorption process on the highly heterogeneous adsorbent. The correlation coefficients of the Elovich model in this study is 0.980, indicating that there is a coordination bond between the Fe3O4@PPy and Hg(II) (Anbia et al. 2015).
Adsorption isotherms
The Langmuir, Freundlich, and D-R models were used for analyzing the adsorption data and to understand the adsorption mechanism.
The fitted curves and parameters of the three models are shown in Figure 10 and Table 2. The calculated R2 values of three models indicated that the adsorption followed the Langmuir model very well. The calculated qm value of 239.76 mg/g is very close to the experimental value of 232.55 mg/g. The results suggested that the binding energy of the Fe3O4@PPy surface was uniform, namely the Fe3O4@PPy surface exhibited the identical adsorption activity. Generally, the E values range from 8 to 16 kJ mol−1 and less than 8 kJ mol−1 correspond to the chemical adsorption and physical adsorption mechanism, respectively (Ghasemi et al. 2019). The E value calculated in this study is 9.13 kJ mol−1, indicating that the chemical adsorption is the dominating mechanism in the Hg(II) adsorption process.
Langmuir model . | Freundlich model . | D-R model . | ||||||
---|---|---|---|---|---|---|---|---|
qm . | KL . | R2 . | 1/n . | KF . | R2 . | β . | E . | R2 . |
239.76 | 0.614 | 0.997 | 0.258 | 3.31 × 105 | 0.755 | 6.0 × 10−3 | 9.13 | 0.973 |
Langmuir model . | Freundlich model . | D-R model . | ||||||
---|---|---|---|---|---|---|---|---|
qm . | KL . | R2 . | 1/n . | KF . | R2 . | β . | E . | R2 . |
239.76 | 0.614 | 0.997 | 0.258 | 3.31 × 105 | 0.755 | 6.0 × 10−3 | 9.13 | 0.973 |
Adsorption thermodynamics
T (K) . | ΔG° (kJ mol−1) . | ΔH° (kJ mol−1) . | ΔS° (J mol−1K−1) . |
---|---|---|---|
298 | −2.17 | 40.05 | 141.84 |
303 | −2.99 | ||
308 | −3.67 | ||
313 | −4.37 | ||
318 | −5.02 |
T (K) . | ΔG° (kJ mol−1) . | ΔH° (kJ mol−1) . | ΔS° (J mol−1K−1) . |
---|---|---|---|
298 | −2.17 | 40.05 | 141.84 |
303 | −2.99 | ||
308 | −3.67 | ||
313 | −4.37 | ||
318 | −5.02 |
From Table 3, it can be seen that the values of ΔG° are more negative with temperature increasing from 298 to 318 K, indicating that the Hg(II) adsorption on Fe3O4@PPy is spontaneous. The positive value of ΔH° indicated that the adsorption process is endothermic. The positive value of ΔS° suggested that the adsorbed Hg(II) on the Fe3O4@PPy surface are organized in a more random fashion compared with those in the aqueous phase. Generally, the value of ΔH° for the physical and chemisorption is in the range of 2.1–20.9 kJ mol−1 and 80–200 kJ mol−1, respectively (Li et al. 2019). Therefore, the adsorption of Hg(II) by Fe3O4@PPy has both physical adsorption and chemisorption.
Mechanism speculation
For the Hg(II) adsorption onto the Fe3O4@PPy, the pH is an important factor affecting adsorption by changing the species of mercury ions in the aqueous solution and Zeta potentials of Fe3O4@PPy. Figure 11 shows Zeta potential values and isoelectric point (IP) of Fe3O4@PPy. As can be seen from Figure 11, the IP value of Fe3O4@PPy is about 3.75. Fe3O4@PPy owned more negative charges and negative charges would be carried on their surface in our experimental pH 4–12, showing more potential to adsorb Hg(II) by electrostatic attraction.
Desorption experiment
In order to recover mercury, avoid secondary pollution and study the reusability of Fe3O4@PPy, the desorption experiment was studied. The previous studies have shown that the addition of HCl will protonate Fe3O4@PPy (Kim & Park 2017) and makes the adsorption slower (Ghasemi et al. 2019; Mohammadnia et al. 2019). In addition, NaCl in solution will also inhibits Hg(II) adsorption because the adsorbed Hg(II) can react with Cl− to form complexes soluble in water and the NaCl molecule can occupy the part of adsorption sites of the adsorbent (Wang et al. 2010). Therefore, the desorption investigation was carried out by using two-component desorbent composed of different concentrations of HCl and NaCl (0.05 M). These experimental results are shown in Figure 12.
As shown in Figure 12, the recovery rate of Hg(II) increased with the increasing concentration of HCl. The recovery rate of Hg(II) was only 31.89% with 0.01 M HCl and 0.05 M NaCl. Furthermore, the recovery rate of 93.03% can be achieved with the HCl and NaCl concentration of 0.1 and 0.05 M, respectively. The experimental results showed that the regeneration by HCl and NaCl is available, and the Fe3O4@PPy appeared the excellent stability and outstanding recycle possibility. The mercury desorbed in this study can be treated centrally to avoid secondary pollution of the water environment, such as making dry batteries, etc.
Comparison with other adsorbents for Hg(II) adsorption
The comparison of Hg(II) adsorption performance between Fe3O4@PPy and other adsorbents are listed in Table 4. It can be seen that Fe3O4@PPy has an excellent adsorption ability for Hg(II), indicating that Fe3O4@PPy has great potential in wastewater treatment applications, especially for Hg(II) removal.
Adsorbent . | Experimental parameters . | qmax (mg/g) . | References . | |||
---|---|---|---|---|---|---|
Adsorbent dosage (mg/L) . | Initial concentration (mg/L) . | pH . | Equilibrium time (min) . | |||
GO/Fe3O4–Si–Pr–SH | 200 | 20 | 7 | 60 | 129.7 | Mohammadnia et al. (2019) |
SWCNTs/Fe3O4@PDA | 200 | 20 | 7 | 60 | 249.07 | Ghasemi et al. (2019) |
SWCNT-SH | 250 | 30 | 5 | 60 | 131 | Bandaru et al. (2013) |
Carboxylate functionalized bentonite | 200 | 25 | 5.5 | 300 | 113 | Anirudhan et al. (2012) |
Porous carbon | 80 | 1 | 6 | 180 | 9.8 | Li et al. (2019) |
Fe3O4@PPy | 250 | 10 | 6 | 200 | 232.55 | This work |
Adsorbent . | Experimental parameters . | qmax (mg/g) . | References . | |||
---|---|---|---|---|---|---|
Adsorbent dosage (mg/L) . | Initial concentration (mg/L) . | pH . | Equilibrium time (min) . | |||
GO/Fe3O4–Si–Pr–SH | 200 | 20 | 7 | 60 | 129.7 | Mohammadnia et al. (2019) |
SWCNTs/Fe3O4@PDA | 200 | 20 | 7 | 60 | 249.07 | Ghasemi et al. (2019) |
SWCNT-SH | 250 | 30 | 5 | 60 | 131 | Bandaru et al. (2013) |
Carboxylate functionalized bentonite | 200 | 25 | 5.5 | 300 | 113 | Anirudhan et al. (2012) |
Porous carbon | 80 | 1 | 6 | 180 | 9.8 | Li et al. (2019) |
Fe3O4@PPy | 250 | 10 | 6 | 200 | 232.55 | This work |
CONCLUSIONS
The magnetic Fe3O4@PPy composite microsphere exhibited great potential on Hg(II) ion removal from solution. The adsorption data was well fitted in the Langmuir and pseudo-second-order kinetic models, and the calculated maximum adsorption capacity was 232.56 mg/g. Thermodynamics studies show that the adsorption of Hg(II) on Fe3O4@PPy is an endothermic and spontaneous process, and the adsorption of Hg(II) by Fe3O4@PPy has both physical adsorption and chemisorption. The Hg(II) ion loaded Fe3O4@PPy could be regenerated by two-component desorbent composed of 0.1 M HCl and 0.05 M NaCl and the maximum recovery of 93.0% was obtained which suggested that the Fe3O4@PPy had good potential to capture and recover Hg(II) ion from aqueous solution.
ACKNOWLEDGEMENTS
The study was supported by the National Natural Science Foundation of China (52074176, 51774200, and 51904174); the Natural Science Foundation of Shandong Province (ZR2020ME106); 2019 Science and Technology Plan of Qingdao West Coast New District (2019-48); ‘Qun xing’ programs of SDUST (QX2018M43); Graduate Tutor Guidance Ability Improvement Program of Shandong Province (SDYY18080)/SDUST; Shandong Province Key Research and Development Project (2019GGX103035); and Young Science and Technology Innovation Program of Shandong Province (2020KJD001).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.