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Comparison of the adsorption preference using superparamagnetic Fe3O4-SH nanoparticles to remove aqueous heavy metal contaminants
Accepted Manuscript Title: Comparison of the adsorption preference using superparamagnetic Fe3 O4 -SH nanoparticles to remove aqueous heavy metal contaminants Authors: Sen Lin, Lili Liu, Yong Yang, Wei Zhang, Cheng Lian, Kuangfei Lin PII: DOI: Reference:
S0263-8762(17)30405-7 http://dx.doi.org/doi:10.1016/j.cherd.2017.07.027 CHERD 2768
To appear in: Received date: Revised date: Accepted date:
18-5-2017 14-7-2017 17-7-2017
Please cite this article as: Lin, Sen, Liu, Lili, Yang, Yong, Zhang, Wei, Lian, Cheng, Lin, Kuangfei, Comparison of the adsorption preference using superparamagnetic Fe3O4SH nanoparticles to remove aqueous heavy metal contaminants.Chemical Engineering Research and Design http://dx.doi.org/10.1016/j.cherd.2017.07.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
TITLE Comparison of the adsorption preference using superparamagnetic Fe3O4-SH nanoparticles to remove aqueous heavy metal contaminants AUTHORS AND ADDRESSES Sen Lin1, Lili Liu1, Yong Yang2, Wei Zhang1, Cheng Lian3*, Kuangfei Lin1* 1 State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, East China University of Science and Technology, Shanghai, 200237, China 2 College of Engineering, Peking University, Beijing, 100084, China 3 School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China Email: [email protected]; [email protected] *Corresponding authors
Graphical abstract
Highlights: 1. Fe3O4-SH has excellent adsorption performances for heavy metals. 2. The effect of thiolation on magnetic recoverability has been confirmed. 3. The adsorption selectivity and interference among heavy metals were confirmed.
ABSTRACT
In this study, thiolated magnetic iron oxide nanoparticles (Fe3O4-SH) were prepared and characterized in detail, which exhibited benign superparamagnetism and magnetic recoverability. And the adsorption and desorption processes for various heavy metals, including Pb2+, Ni2+, Zn2+ and Cu2+, had been investigated comprehensively both in single and mixed systems using the Fe3O4-SH nanoparticles as adsorbents. The results showed that heavy metal ions could be removed rapidly depending on the complexation with the SH-groups on the Fe3O4-SH surface. Besides, the adsorption was all sensitive to pH and ionic strength while the nano-adsorbents had the highest adsorption capacity and rate for Ni2+ according to the adsorption kinetics and thermodynamics. In addition, the corresponding competitive adsorption processes and desorption experiments reflected that Fe3O4-SH had the strongest affinity and adsorption selectivity for Pb2+, followed by Ni2+ and finally by Cu2+ and Zn2+. Generally, the present study might provide a serviceable direction for the future efforts towards the application of Fe3O4-SH nano-adsorbents in industrial wastewater treatments involving multiple heavy metal contaminants.
Key words: Fe3O4-SH; ionic strength; competitive adsorption; desorption; adsorption selectivity; magnetic recovery
1. INTRODUCTION
With the growth spurts of industries worldwide, including metal plating facilities, mining operations, tanneries, batteries, pesticides, etc., a mass of wastewater containing heavy metal contaminants is directly or indirectly discharged into the environment increasingly every year, which has severely threatened human health and ecological security (Hasselriis and Licata, 1996; Rani et al., 2014; Yadav, 2010). Unlike organic contaminants, heavy metals are not biodegradable and tend to accumulate in living organisms. So far, a variety of heavy metals have been confirmed to be teratogenetic, mutagenic and carcinogenic, whose severe harm on human health has regularly reviewed by international organizations such as the WHO, etc. (Babich and Stotzky, 1985; Jarup, 2003; Sharara et al., 1998; Tubafard et al., 2008). Although these adverse health effects have been known for a long time, the exposure to heavy metal pollution continues and even keeps increasing in some areas nowadays (Li et al., 2014; Sures, 2004; Tchounwou et al., 2012; Wang et al., 2013). Hence, it is still a great challenge to remove heavy metal contaminants from these industrial wastewater with safe and efficient technologies. Hitherto, various technologies have been developed to treat aqueous heavy metal contaminants, such as chemical precipitation, electrolysis, ion exchange, adsorption, membrane separation, bio-concentration, etc. (Fu and Wang, 2011; Fu et al., 2012; Fu et al., 2014; Hashim et al., 2011; Wan Ngah
and Hanafiah, 2008). In these technologies, adsorption has been widely used due to its high efficiency and economy as well as the convenience of subsequent processing (Areco et al., 2012; Bailey et al., 1999; Ge et al., 2012; Ihsanullah et al., 2016; Lin et al., 2012b; Qu, 2008; Uddin, 2017). Up to now, the superparamagnetic Fe3O4 nanoparticles have attract great attention due to the high adsorption capacities and the magnetic recoverability, which overcomes the instinctive hardness of nano-adsorbents to separate and regenerate from wastewater after use (Lin et al., 2017b; Lin et al., 2012a). However, the bare Fe3O4 is easily oxidized and prone to agglomeration in wastewater, so the effective surface modifications are essential to protect the magnetic nano-adsorbents (Gomez-Pastora et al., 2014; Lin et al., 2017c; Singh et al., 2011). Therein, thiolation has been a fairly common functionalization to enhance the stability and efficacy of the bare Fe3O4 nanoparticles as heavy metal adsorbents in wastewater. Nowadays, vast studies have focused on the adsorption performances of the Fe3O4-SH nanoparticles for different heavy contaminants (Bach et al., 2012; Jung et al., 2011; Odio et al., 2016; Villa et al., 2016; Yantasee et al., 2007). While the researches on concrete adsorption processes using the Fe3O4-SH nanoparticles as adsorbents, referring to the properties of competition and interference among different heavy metal ions in mixed system, are still far from sufficient. However, the exploration is of great significance for the practical application of the Fe3O4-SH nanoparticles in industrial wastewater treatments (Gomez-Pastora et al., 2014). Therefore, in our study the adsorption competition and interference among different heavy metal ions were investigated extensively as well as the adsorption performances. The work started
with the preparation and characterization of the Fe3O4-SH nanoparticles by grafting ammonium hydroxide (3-mercaptopropyl)-trimethoxysilane (MPTMS) onto the bare Fe3O4 surface. Following, the adsorption performances of the prepared Fe3O4-SH for various typical heavy metals ions were examined comprehensively, including Pb2+, Ni2+, Zn2+ and Cu2+, representing the typical heavy metal contaminants. Meanwhile, the adsorption selectivity and competition among these heavy metals were also surveyed in detail dependent on the adsorption and desorption experiments in single and coexisting systems, which is essential for offering valid guidance for the application of the Fe3O4-SH nanoparticles in wastewater treatments.
2. MATERIALS AND METHODS
2.1 Reagents and chemicals The chemicals used in this study were all of analytical reagent grade and obtained from Sigma-Aldrich Co., Ltd. All working solutions of different heavy metals were prepared by diluting the stock solutions in appropriate proportions and the stock solutions were prepared by PbCl2, NiCl2, CuCl2 and ZnCl2 dissolved in deionized water, respectively. In all experiments, the pH values of the solutions were adjusted by 0.05 mmol/L HCl or 0.05 mmol/L NaOH. The glassware used was cleaned by soaking in 15% HNO3 before use in order to prevent metal contamination from laboratory glassware.
2.2 Preparation of the superparamagnetic nano-adsorbents Preparation of the bare Fe3O4: The Fe3O4 nanoparticles were prepared by a co-precipitation method. Firstly, 50 ml solution containing 0.05 mol/L FeCl3 and 0. 025 mol/L FeCl2 mixed with 10 ml 0.5 mol/L NaOH solution rapidly, then followed by 3 h incubation at 60°C with vigorous stirring. At last the bare Fe3O4 could be collected from suspension with a 0.6 T permanent magnet. Preparation of the Fe3O4-SH: The preparation of Fe3O4-SH was according to Bach’s method with some modifications (Bach et al., 2012). Briefly, 0.5 g bare Fe3O4 prepared was dispersed in 100 ml of 1:1 water/ethanol solution with 2.5 g MPTMS under reflux for 5 h at 90°C and pH of 12.0. Finally the thiolated magnetic nanoparticles were isolated using a 0.6 T permanent magnet from the suspension and washed repeatedly with deionized water to eliminate the residues on surface.
2.3 Equipment and characterization The wild wide-scan XPS spectra of the Fe3O4 and Fe3O4-SH nanoparticles were measured to obtain the elements on surface by an X-ray photoelectron spectrometer (PHI-5300, Perkin-Elmer, USA) with pass energy of 1147.5 eV and an Al-Kα excitation source. The FTIR spectra were recorded on a Fourier-transform infrared spectrometer (Nicoletis 10, ThermoScientific, USA) with samples using KBr pellets over the range of 400-4000 cm-1. Zeta potentials and agglomeration size distributions of the prepared nanoparticles were measured using a zeta potentiometer (Zetasizer Nano ZSP, Malvern, UK) at 25oC. The crystalline natures of the Fe3O4 and Fe3O4-SH
nanoparticles were judged by XRD patterns on an X-ray power diffractometer (D8, BRUKER, Germany) with Cu-Kα radiation (λ=0.154nm). Superparamagnetism and magnetic parameters of the nano-adsorbents were confirmed by a vibrating sample magnetometer (730T, Lakeshore, USA). The heavy metal concentrations in solutions were all detected dependent on an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) instrument (IRIS, ThermoElemental, USA). The pH values of working solutions were all determined using a pH meter (SevenEasy, Mettler, Switzerland). All experiments in this study were performed in triplicate and the variations among parallel experiments were less than 5%.
2.4 Adsorption and desorption experiments The adsorption performances of the Fe3O4-SH nanoparticles for heavy metal ions were all investigated by batch experiments. In brief, a certain amount of the Fe3O4-SH nanoparticles was added into 100 ml heavy metal solution at a given initial pH and then the flask was placed in a temperature-controlled shaker (Sky-200B, SuKun, China) at 175 rpm and 30oC. Meanwhile the supernatant was sampled at specified time intervals and the residual concentrations of the heavy metals were analyzed after the nanoparticles in samples were separated by a permanent magnet. The adsorption capacity q (mmol/g) and removal efficiency were determined according to Equation (1) and Equation (2).
q(0.L 1 ) 0c( hc
)m /
(c0 ch ) / c0 100%
(1) (2)
Where m (g) is the mass of Fe3O4-SH nano-adsorbents used, c0 (mmol/L) and ch (mmol/L) are the initial and residual heavy metal concentrations in solution. In the desorption experiments, the saturated Fe3O4-SH were mixed with a certain volume of 0.1 mol/L HCl as eluent in a 50 mL flask and incubated in the shaker at 175 rpm and 30oC. Then the eluent was sampled and detected the heavy metal concentrations at specified time intervals after the Fe3O4-SH nanoparticle were removed by the extern magnetic field, upon which the elution efficiency w was figured out according to Equation (3).
w V cw / (qs ms ) 100%
(3)
Where V (L) is the eluent volume, cw (mmol/L) is the heavy metal concentration in eluent, qs (mmol/g) represents the pre-saturated adsorption capacity of the Fe3O4-SH nanoparticles for the heavy metal, and ms (g) is the mass of saturated Fe3O4-SH nano-adsorbents.
2.5 Magnetic recovery of Fe 3 O 4 -SH In the magnetic recovery experiments, 0.1 g superparamagnetic nanoparticles were well dispersed in 30 ml deionized water at pH of 6.0. Then the suspension was under the extern field from a 0.6 T permanent magnet and sampled at given intervals. Following the pH values of samples obtained were adjusted to 1.0 via 1 mol/L HCl solution to dissolve the residual nanoparticles and the Fe ion concentrations were analyzed by ICP-AES to calculate the magnetic recovery efficiency r according to Equation (4):
r (cFe0 cFe1 ) / cFe0 100%
(4)
Where cFe 0 (g/L) and cFe1 (g/L) are the Fe concentrations in the supernatant before (as control) and after magnetic removal.
3. RESULTS AND DISCUSSION
3.1 Characterization In order to verify the thiol-groups (-SH) grafted on the Fe3O4 nanoparticle surface, multiple measurements, such as XPS, FTIR, Zeta potential, XRD, TEM and VSM, were applied to compare the physicochemical properties of the Fe3O4 before and after thiolation. Figure 1a gives the wide-scan XPS spectra of the bare Fe3O4 and the Fe3O4-SH nanoparticles. It could be obviously found that the characteristic peaks assigned for sulphur element (S2p at 163.6 eV) and silicon element (Si2p at 101.8 eV) appeared after modification, which unanimously interpreted that thiols had been anchored successfully onto the Fe3O4 surface. And the comparison of FTIR spectra is listed in Figure 1b, which could be witnessed that thiolation treatment had resulted in chemical changes on the Fe3O4 surface according to the varieties of characteristic bands. As shown, the bands at 1126, 1075 and 984 cm−1, assigned to the Si-O-H and Si-O-Si groups respectively, appeared on the spectrum of the Fe3O4-SH nanoparticles, indicating that the silane framework had been grafted successfully onto the bare Fe3O4 surface via Fe-O-Si bonds. And the adsorption band at 2578 cm−1, ascribed to the S-H stretching of the moieties grafted, confirmed that the Fe3O4-SH could be afforded by the condensation of MPTMS with the OH-groups on the bare Fe3O4 surface. In Figure
1c, the isoelectric points (pI) of the bare Fe3O4 and Fe3O4-SH nanoparticles are identified by the zeta potentials as a function of pH values in suspensions with concentration of 10 mg/L, where the pI value of nanoparticles increased to 8.5 from 6.9 due to the thiolation treatment. In addition, the XRD results in Figure 1d prove that thiolation didn’t bring about damages on the crystal structure of the Fe3O4 nanoparticles without peaks appearing or vanishing in the pattern after modification. a)
b)
c)
d)
Figure 1. (a) The wide-scan XPS spectra, (b) FTIR spectra, (c) pI values measurement, and (d) XRD patterns of the bare Fe3O4 and Fe3O4-SH nanoparticles at 25oC.
Agglomeration is an inevitable consequence for nanoparticles dispersed in aqueous solution, which is closely relative with the adsorption and recovery efficiencies of these nano-adsorbents (Lin et al., 2017a; Thommes and Cychosz, 2014). Figure 2 shows the size distributions and typical TEM images (illustrations) of the agglomerated particles of the bare Fe3O4 and Fe3O4-SH. The agglomeration could be observed in both of the TEM images while it was also discerned effortlessly that the Fe3O4-SH nanoparticles had better dispersion, which could be certified by the size distribution results correspondingly. The average size of nanoparticle agglomerations
decreased to 142.33 nm from 268.69 nm after the thiolation treatment, indicating that thiolation could enhance effectively the aqueous stability and dispersibility of the Fe3O4-based nanoparticles, contributing to the subsequent adsorption processes. a)
b)
Figure 2. The TEM image (illustrations) and size distribution of agglomerations of (a) the bare Fe3O4 and (b) Fe3O4-SH nanoparticles in deionized water with pH of 6.0 at 25 oC.
It is an enormous advantage for the Fe3O4-based nanoparticles that could be recovered easily using an extern magnetic field due to their intrinsic superparamagnetism, unclogging the application bottleneck of these common nano-adsorbents difficult to
separate with wastewater after use. In this study, the hysteresis loops of the bare Fe3O4 and Fe3O4-SH nanoparticles were also measured and compared to analyze the effect of thiolation on superparamagnetism. As shown in Figure 3a, it could be observed clearly that both of the hysteresis loops presents typical S-shaped curves on behalf of superparamagnetism. The results indicated that the thiolation treatment didn’t change the intrinsic magnetism of the Fe3O4 nanoparticles, ascertained also by the magnetic parameters listed in Table 1, where the coercive force (HC) and residual magnetism (MR) of the Fe3O4 and Fe3O4-SH were both close to zero. However, the saturation magnetization (Ms) decreased to 21.74 emu/g from 54.29 emu/g due to the damage of thiolation to the magnetic response of the bare Fe3O4 as well as the contribution of the non-magnetic outer layer (Gomez-Pastora et al., 2014; Hu). The fadedness of magnetic response was also embodied in the magnetic recovery courses listed in Figure 3b. It could be observed obviously that the recovery rate decreased markedly after thiolation though the Fe3O4-SH nanoparticles could still be accumulated and aggregated completely around the magnet with enough time. a)
b)
Figure 3. (a) The hysteresis loops and (b) magnetic recovery courses of the bare Fe3O4 and Fe3O4-SH nanoparticles at 25oC.
Table 1. The magnetic parameters of the Fe3O4 and Fe3O4-SH nanoparticles at 25oC. HC (Oe)
MR (emu/g)
MS (emu/g)
Bare Fe3O4
0.105
0.235
54.29
Fe3O4-SH
0.087
0.054
21.74
3.2 Effect of initial pH and ionic strength on adsorption In practice, the adsorption of heavy metal ions is commonly susceptible to the wastewater conditions. Hence, the adsorption tolerance to pH and ionic strength was investigated to compare the affinities of the Fe3O4-SH nano-adsorbents with Pb2+, Ni2+, Zn2+ and Cu2+, respectively. The removal percentage as a function of initial pH was examined with 0.1 g Fe3O4-SH and 1.0 mmol/L heavy metal under incubating time of 1 h at 30oC. It could be seen easily in Figure 4a that the magnitude of removal efficiencies of the heavy metal ions ranked in the sequence of Ni2+>Zn2+>Pb2+>Cu2+ at the fixed pH values, indicating the existence of the adsorption gaps for different heavy metal ions. And the removal efficacies all increased with the initial pH values
in a narrow pH range (2.5-6.0) to avoid chemical precipitations, indicating that pH played fairly positive effect on the adsorption. According to the isoelectric point result above, the Fe3O4-SH nanoparticles were ascertained to be positively charged at low pH, identifying with Pb2+, Ni2+, Zn2+ and Cu2+, which would hinder the adsorption dramatically due to the electrostatic repulsion and the competition between heavy metal ions and H+ for the adsorption sites on surface, which was consistent with the conclusions reported (Odio et al., 2016; Singh et al., 2011; Yantasee et al., 2007). Hence, the initial pH values were all set at 6.0 for fine adsorption performances in all following experiments. The ionic strength tolerance of the adsorption capacities for Pb2+, Ni2+, Zn2+ and Cu2+ was investigated separately with 0.1 g Fe3O4-SH to adsorb 1.0 mmol/L metal ion in 1 h. As shown in Figure 4b, the adsorption capacities of the Fe3O4-SH nano-adsorbents for various heavy metal ions all sharply fell along with the increase of NaCl concentration in the range of 0-10 mmol/L, indicating that the existence of background electrolyte could affect acutely the adsorption. While the downtrends were apparently assuaged as NaCl concentration exceeded 10 mmol/L in solution that there was just a relative handful of reduction on the adsorption capacities in the range of 10-50 mmol/L NaCl. According to the electric double layer theory, it was confirmed that the background electrolyte concentration could influence the thickness of the double layer resulting in the outer-sphere complexes susceptible to ionic-strength while no effect to inner-sphere complexes (Strawn and Sparks, 1999; Yoon et al., 2016). Therefore, it was induced that the outer-sphere complexes,
responsible for the rapid reduction stage, as well as the inner-sphere complexes had formed on the Fe3O4-SH surface during the adsorption. Meanwhile, by comparison of the adsorption capacities for various heavy metals with 50 mmol/L NaCl and without NaCl, it was confirmed that there was more Zn2+ adsorbed on the Fe3O4-SH surface dependent on the outer-sphere complexation and the tolerance of adsorption performances to ionic strength using the Fe3O4-SH was in the sequence of Pb2+> Ni2+> Cu2+>Zn2+. a)
b)
Figure 4. The effect of (a) initial pH and (b) ionic strength on the adsorption performances using the Fe3O4-SH nanoparticles at 30oC.
3.3 Adsorption kinetics and thermodynamics In order to quantitatively compare the adsorption abilities of the Fe3O4-SH nanoparticles for different heavy metal ions, the adsorption kinetics and thermodynamics for Pb2+, Ni2+, Zn2+ and Cu2+ were investigated in detail at 30oC. As shown, the time courses of the adsorption for 1.0 mmol/L heavy metal ion by 0.1 g Fe3O4-SH are listed in Figure 5. It could be observed that the adsorption processes underwent in analogous tracks for Pb2+, Ni2+, Zn2+ and Cu2+, where the first 10 min corresponded to a rapid adsorption stage and then the adsorption rates decreased with the lessened adsorption sites available, resulting in the Fe3O4-SH nanoparticles all saturated further in 50 min. These results indicated that the adsorption of heavy metal ions occurred on the Fe3O4-SH surface and rapidly proceeded towards saturation during the fast adsorption period. Thereafter, the residual sites on the Fe3O4-SH surface were competed for by adsorbates remaining in the solution, giving rise to low adsorption rates. Furthermore, in order to explore the adsorption processes, various adsorption kinetics models had been applied to fit above data including the first-order model, the pseudo-first-order model, the pseudo-second-order kinetic model, etc. And the results witnessed that the adsorption processes could all described well by the pseudo-second-order kinetic model quite well, which could be expressed as follows (Erdem et al., 2004):
qt t / (1/ v0 t / qe )
(5)
Where qe (mmol/g) and qt (mmol/g) are the adsorption capacities of heavy metal ions at equilibrium and the time of t (min), and v0 (mmol/min) represents the initial
adsorption rate. The pseudo-second-order kinetic model assumed the adsorption capacity of the adsorbent is determined by the adsorption sites at which electron sharing or electron transfer occurred between the adsorbent and adsorbate. Hence, the fine conformity between adsorption processes and the pseudo-second-order model, consistent with the high coefficients of determination (R2) summarized in Table 2, indicated that the adsorption for various heavy metal ions could be assigned to a charge transfer complex pattern, satisfying with the above conclusion of the ionic strength effect. Besides, the fitting v0 values manifested that the adsorption rates of the Fe3O4-SH for heavy metals followed the rank of Cu2+< Pb2+< Zn2+
Figure 5. Adsorption kinetics for heavy metal ions by the Fe3O4-SH nanoparticles at 30˚C.
Table 2. The pseudo-second-order model parameters for the adsorption at 30C. qe (mmol/g)
v0 (mmol/min)
R2
Pb2+
0.621
0.297
0.999
Ni2+
0.805
0.325
0.997
Zn2+
0.709
0.317
0.998
Cu2+
0.550
0.238
0.998
Besides, the adsorption thermodynamics for Pb2+, Ni2+, Zn2+ and Cu2+ were also examined and the data were fitted by the Langmuir model listed as Equation (6) (Hayes et al., 1988):
Qe
Qm a xb C e 1 bCe
(6)
Where Qe (mmol/g) is the adsorption capacity at equilibrium, Qmax (mmol/g) is the saturated adsorption capacity, Ce (mmol/L) is the equilibrium concentration of the heavy metal in solution, and b is the Langmuir equilibrium constant. As shown in Figure 6, it could be observed that the Fe3O4-SH nanoparticles performed the highest adsorption capacity for Ni2+ and the lowest adsorption capacity for Cu2+ at different equilibrium concentrations. The fitting parameters in Table 3 indicated the adsorption processes for Pb2+, Ni2+, Zn2+ and Cu2+ all obeyed to the Langmuir model well (R2>0.98), representing the monolayer adsorption. And the saturation capacities for different heavy metal ions followed the sequence of Ni2+>Zn2+>Pb2+>Cu2+, confirming that the Fe3O4-SH nanoparticles had the most excellent adsorption performance for Ni2+ while Cu2+ was hardest to remove. In general, the Fe3O4-SH nanoparticles demonstrated the gaps among the adsorption abilities for different heavy metal contaminants while the saturated adsorption
capacities for Pb2+, Ni2+, Zn2+ and Cu2+ were still superior to that of the common heavy metal adsorbents. For instance, the adsorbance for heavy metals of bio-char, chitosan composites, clays and so on are generally less than 0.50 mmol/g (Babel and Kurniawan, 2003; Meena et al., 2005; Wan Ngah et al., 2011). Therefore, the Fe3O4-SH nanoparticles have shown great application potential in the wastewater treatments involving heavy metals, which is consistent with the results of Yantasee and Shin (Shin and Jang, 2007; Yantasee et al., 2007).
Figure 6. Adsorption thermodynamics of heavy metals by the Fe3O4-SH nanoparticles at 30oC. Table 3. Langmuir model parameters for the adsorption thermodynamics. Qmax (mmol/g)
b
R2
Pb2+
0.631
55.19
0.995
Ni2+
0.860
157.38
0.974
Zn2+
0.738
119.31
0.978
Cu2+
0.572
67.84
0.990
3.4 Competitive adsorption The composition of industrial wastewaters is always complicated including various heavy metal contaminations. Therefore, it is necessary to investigate the adsorption selectivity to different contaminants and the interference between each other, which is the basis for the application of the Fe3O4-SH nano-adsorbents in complicated wastewater treatments. The adsorption courses were studied in mixed solution with different doses of nano-adsorbents to remove Pb2+, Ni2+, Zn2+ and Cu2+, whose initial concentrations were all 1.0 mmol/L. It can be seen in Figure 7 that Pb2+ was always prior to remove in the mixed solution whatever the dose of the Fe3O4-SH was, indicating the strong affinity between Pb2+ and SH-groups, corresponding to the ionic strength effect results above. In the case of 0.1 g Fe3O4-SH, as shown in Figure 7a, the concentration of Pb2+ decreased to 0.49 mmol/L at adsorption equilibrium, while little of Ni2+, Zn2+ and Cu2+ were removed from solution. As the dose of Fe3O4-SH increased to 0.25 g in Figure 7b, Pb2+ could be removed thoroughly in 50 min. Meanwhile Ni2+ could also be removed, whose residual concentrations decreased to 0.194 mmol/L at adsorption equilibrium in 100 min. With a further increasing dose of adsorbents to 0.50 g in Figure 7c, Pb2+ could be removed completely in less than 30 min and Ni2+ could not be detected in solution in 75 min meanwhile. Simultaneously, the concentration of Cu2+ began to decline obviously in the solution while little Zn2+ was removed after the rapid adsorption stage of Pb2+ and Ni2+. At last, as 1.0 g Fe3O4-SH added in Figure 7d, all heavy metals could be removed entirely in 75 min. The results above indicated that the increase of the nano-adsorbents dose would cut
down the time of adsorption equilibrium effectively. At the same time, the adsorption selectivity followed the sequence of Pb2+>Ni2+>Cu2+>Zn2+. a)
b)
c)
d)
Figure 7. The residual concentrations of heavy metals ions as a function of time under doses of (a) 0.10 g, (b) 0.25 g, (c) 0.50 g and (d) 1.0 g of the Fe3O4-SH nanoparticles in the mixed solution.
3.5 Desorption Desorption was significantly influenced by the affinity between heavy metal ions and the SH-groups on the Fe3O4-SH surface. In this study, the elution courses of various metal ions from saturated nano-adsorbents surface in single systems and the mixed system were both investigated. It could be judged that the time needed for elution
equilibrium of Pb2+, Ni2+, Zn2+ and Cu2+ were discriminating from the individual elution percentage as a function of time, which was listed in Figure 8a. The experiments were conducted with 10 ml eluent mixed with 0.10 g saturated Fe3O4-SH nanoparticles at 30oC. As shown in Figure 8a, the desorption rates followed the sequence of Pb2+
a)
b)
Figure 8. (a) Desorption kinetics in single systems and (b) desorption efficiency as a function of eluent volume in mixed system at 30oC.
4. CONCLUSION
In this study, the Fe3O4-SH nano-adsorbents exhibited excellent adsorption performances for Pb2+, Ni2+, Zn2+ and Cu2+ with rapid adsorption rates and great adsorption capacities. Furthermore, it was notable that the adsorption selectivity of the Fe3O4-SH for different heavy metal ions was hierarchical and followed the sequence of Pb2+>Ni2+>Cu2+>Zn2+ according to the adsorption and desorption experiments. The
present work has determined the adsorption competition and interference among different heavy metal ions using the Fe3O4-SH nanoparticles as adsorbents, which is full of significance to provide a valid reference for the environmental application of Fe3O4-SH nanoparticles in industrial wastewater treatment in future.
ACKNOWLEDGEMENTS
This work was supported by China Postdoctoral Science Foundation (2017M611476) and the Fundamental Research Funds for the Central Universities (WB1714038).
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S0263-8762(17)30405-7 http://dx.doi.org/doi:10.1016/j.cherd.2017.07.027 CHERD 2768
To appear in: Received date: Revised date: Accepted date:
18-5-2017 14-7-2017 17-7-2017
Please cite this article as: Lin, Sen, Liu, Lili, Yang, Yong, Zhang, Wei, Lian, Cheng, Lin, Kuangfei, Comparison of the adsorption preference using superparamagnetic Fe3O4SH nanoparticles to remove aqueous heavy metal contaminants.Chemical Engineering Research and Design http://dx.doi.org/10.1016/j.cherd.2017.07.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
TITLE Comparison of the adsorption preference using superparamagnetic Fe3O4-SH nanoparticles to remove aqueous heavy metal contaminants AUTHORS AND ADDRESSES Sen Lin1, Lili Liu1, Yong Yang2, Wei Zhang1, Cheng Lian3*, Kuangfei Lin1* 1 State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, East China University of Science and Technology, Shanghai, 200237, China 2 College of Engineering, Peking University, Beijing, 100084, China 3 School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China Email: [email protected]; [email protected] *Corresponding authors
Graphical abstract
Highlights: 1. Fe3O4-SH has excellent adsorption performances for heavy metals. 2. The effect of thiolation on magnetic recoverability has been confirmed. 3. The adsorption selectivity and interference among heavy metals were confirmed.
ABSTRACT
In this study, thiolated magnetic iron oxide nanoparticles (Fe3O4-SH) were prepared and characterized in detail, which exhibited benign superparamagnetism and magnetic recoverability. And the adsorption and desorption processes for various heavy metals, including Pb2+, Ni2+, Zn2+ and Cu2+, had been investigated comprehensively both in single and mixed systems using the Fe3O4-SH nanoparticles as adsorbents. The results showed that heavy metal ions could be removed rapidly depending on the complexation with the SH-groups on the Fe3O4-SH surface. Besides, the adsorption was all sensitive to pH and ionic strength while the nano-adsorbents had the highest adsorption capacity and rate for Ni2+ according to the adsorption kinetics and thermodynamics. In addition, the corresponding competitive adsorption processes and desorption experiments reflected that Fe3O4-SH had the strongest affinity and adsorption selectivity for Pb2+, followed by Ni2+ and finally by Cu2+ and Zn2+. Generally, the present study might provide a serviceable direction for the future efforts towards the application of Fe3O4-SH nano-adsorbents in industrial wastewater treatments involving multiple heavy metal contaminants.
Key words: Fe3O4-SH; ionic strength; competitive adsorption; desorption; adsorption selectivity; magnetic recovery
1. INTRODUCTION
With the growth spurts of industries worldwide, including metal plating facilities, mining operations, tanneries, batteries, pesticides, etc., a mass of wastewater containing heavy metal contaminants is directly or indirectly discharged into the environment increasingly every year, which has severely threatened human health and ecological security (Hasselriis and Licata, 1996; Rani et al., 2014; Yadav, 2010). Unlike organic contaminants, heavy metals are not biodegradable and tend to accumulate in living organisms. So far, a variety of heavy metals have been confirmed to be teratogenetic, mutagenic and carcinogenic, whose severe harm on human health has regularly reviewed by international organizations such as the WHO, etc. (Babich and Stotzky, 1985; Jarup, 2003; Sharara et al., 1998; Tubafard et al., 2008). Although these adverse health effects have been known for a long time, the exposure to heavy metal pollution continues and even keeps increasing in some areas nowadays (Li et al., 2014; Sures, 2004; Tchounwou et al., 2012; Wang et al., 2013). Hence, it is still a great challenge to remove heavy metal contaminants from these industrial wastewater with safe and efficient technologies. Hitherto, various technologies have been developed to treat aqueous heavy metal contaminants, such as chemical precipitation, electrolysis, ion exchange, adsorption, membrane separation, bio-concentration, etc. (Fu and Wang, 2011; Fu et al., 2012; Fu et al., 2014; Hashim et al., 2011; Wan Ngah
and Hanafiah, 2008). In these technologies, adsorption has been widely used due to its high efficiency and economy as well as the convenience of subsequent processing (Areco et al., 2012; Bailey et al., 1999; Ge et al., 2012; Ihsanullah et al., 2016; Lin et al., 2012b; Qu, 2008; Uddin, 2017). Up to now, the superparamagnetic Fe3O4 nanoparticles have attract great attention due to the high adsorption capacities and the magnetic recoverability, which overcomes the instinctive hardness of nano-adsorbents to separate and regenerate from wastewater after use (Lin et al., 2017b; Lin et al., 2012a). However, the bare Fe3O4 is easily oxidized and prone to agglomeration in wastewater, so the effective surface modifications are essential to protect the magnetic nano-adsorbents (Gomez-Pastora et al., 2014; Lin et al., 2017c; Singh et al., 2011). Therein, thiolation has been a fairly common functionalization to enhance the stability and efficacy of the bare Fe3O4 nanoparticles as heavy metal adsorbents in wastewater. Nowadays, vast studies have focused on the adsorption performances of the Fe3O4-SH nanoparticles for different heavy contaminants (Bach et al., 2012; Jung et al., 2011; Odio et al., 2016; Villa et al., 2016; Yantasee et al., 2007). While the researches on concrete adsorption processes using the Fe3O4-SH nanoparticles as adsorbents, referring to the properties of competition and interference among different heavy metal ions in mixed system, are still far from sufficient. However, the exploration is of great significance for the practical application of the Fe3O4-SH nanoparticles in industrial wastewater treatments (Gomez-Pastora et al., 2014). Therefore, in our study the adsorption competition and interference among different heavy metal ions were investigated extensively as well as the adsorption performances. The work started
with the preparation and characterization of the Fe3O4-SH nanoparticles by grafting ammonium hydroxide (3-mercaptopropyl)-trimethoxysilane (MPTMS) onto the bare Fe3O4 surface. Following, the adsorption performances of the prepared Fe3O4-SH for various typical heavy metals ions were examined comprehensively, including Pb2+, Ni2+, Zn2+ and Cu2+, representing the typical heavy metal contaminants. Meanwhile, the adsorption selectivity and competition among these heavy metals were also surveyed in detail dependent on the adsorption and desorption experiments in single and coexisting systems, which is essential for offering valid guidance for the application of the Fe3O4-SH nanoparticles in wastewater treatments.
2. MATERIALS AND METHODS
2.1 Reagents and chemicals The chemicals used in this study were all of analytical reagent grade and obtained from Sigma-Aldrich Co., Ltd. All working solutions of different heavy metals were prepared by diluting the stock solutions in appropriate proportions and the stock solutions were prepared by PbCl2, NiCl2, CuCl2 and ZnCl2 dissolved in deionized water, respectively. In all experiments, the pH values of the solutions were adjusted by 0.05 mmol/L HCl or 0.05 mmol/L NaOH. The glassware used was cleaned by soaking in 15% HNO3 before use in order to prevent metal contamination from laboratory glassware.
2.2 Preparation of the superparamagnetic nano-adsorbents Preparation of the bare Fe3O4: The Fe3O4 nanoparticles were prepared by a co-precipitation method. Firstly, 50 ml solution containing 0.05 mol/L FeCl3 and 0. 025 mol/L FeCl2 mixed with 10 ml 0.5 mol/L NaOH solution rapidly, then followed by 3 h incubation at 60°C with vigorous stirring. At last the bare Fe3O4 could be collected from suspension with a 0.6 T permanent magnet. Preparation of the Fe3O4-SH: The preparation of Fe3O4-SH was according to Bach’s method with some modifications (Bach et al., 2012). Briefly, 0.5 g bare Fe3O4 prepared was dispersed in 100 ml of 1:1 water/ethanol solution with 2.5 g MPTMS under reflux for 5 h at 90°C and pH of 12.0. Finally the thiolated magnetic nanoparticles were isolated using a 0.6 T permanent magnet from the suspension and washed repeatedly with deionized water to eliminate the residues on surface.
2.3 Equipment and characterization The wild wide-scan XPS spectra of the Fe3O4 and Fe3O4-SH nanoparticles were measured to obtain the elements on surface by an X-ray photoelectron spectrometer (PHI-5300, Perkin-Elmer, USA) with pass energy of 1147.5 eV and an Al-Kα excitation source. The FTIR spectra were recorded on a Fourier-transform infrared spectrometer (Nicoletis 10, ThermoScientific, USA) with samples using KBr pellets over the range of 400-4000 cm-1. Zeta potentials and agglomeration size distributions of the prepared nanoparticles were measured using a zeta potentiometer (Zetasizer Nano ZSP, Malvern, UK) at 25oC. The crystalline natures of the Fe3O4 and Fe3O4-SH
nanoparticles were judged by XRD patterns on an X-ray power diffractometer (D8, BRUKER, Germany) with Cu-Kα radiation (λ=0.154nm). Superparamagnetism and magnetic parameters of the nano-adsorbents were confirmed by a vibrating sample magnetometer (730T, Lakeshore, USA). The heavy metal concentrations in solutions were all detected dependent on an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) instrument (IRIS, ThermoElemental, USA). The pH values of working solutions were all determined using a pH meter (SevenEasy, Mettler, Switzerland). All experiments in this study were performed in triplicate and the variations among parallel experiments were less than 5%.
2.4 Adsorption and desorption experiments The adsorption performances of the Fe3O4-SH nanoparticles for heavy metal ions were all investigated by batch experiments. In brief, a certain amount of the Fe3O4-SH nanoparticles was added into 100 ml heavy metal solution at a given initial pH and then the flask was placed in a temperature-controlled shaker (Sky-200B, SuKun, China) at 175 rpm and 30oC. Meanwhile the supernatant was sampled at specified time intervals and the residual concentrations of the heavy metals were analyzed after the nanoparticles in samples were separated by a permanent magnet. The adsorption capacity q (mmol/g) and removal efficiency were determined according to Equation (1) and Equation (2).
q(0.L 1 ) 0c( hc
)m /
(c0 ch ) / c0 100%
(1) (2)
Where m (g) is the mass of Fe3O4-SH nano-adsorbents used, c0 (mmol/L) and ch (mmol/L) are the initial and residual heavy metal concentrations in solution. In the desorption experiments, the saturated Fe3O4-SH were mixed with a certain volume of 0.1 mol/L HCl as eluent in a 50 mL flask and incubated in the shaker at 175 rpm and 30oC. Then the eluent was sampled and detected the heavy metal concentrations at specified time intervals after the Fe3O4-SH nanoparticle were removed by the extern magnetic field, upon which the elution efficiency w was figured out according to Equation (3).
w V cw / (qs ms ) 100%
(3)
Where V (L) is the eluent volume, cw (mmol/L) is the heavy metal concentration in eluent, qs (mmol/g) represents the pre-saturated adsorption capacity of the Fe3O4-SH nanoparticles for the heavy metal, and ms (g) is the mass of saturated Fe3O4-SH nano-adsorbents.
2.5 Magnetic recovery of Fe 3 O 4 -SH In the magnetic recovery experiments, 0.1 g superparamagnetic nanoparticles were well dispersed in 30 ml deionized water at pH of 6.0. Then the suspension was under the extern field from a 0.6 T permanent magnet and sampled at given intervals. Following the pH values of samples obtained were adjusted to 1.0 via 1 mol/L HCl solution to dissolve the residual nanoparticles and the Fe ion concentrations were analyzed by ICP-AES to calculate the magnetic recovery efficiency r according to Equation (4):
r (cFe0 cFe1 ) / cFe0 100%
(4)
Where cFe 0 (g/L) and cFe1 (g/L) are the Fe concentrations in the supernatant before (as control) and after magnetic removal.
3. RESULTS AND DISCUSSION
3.1 Characterization In order to verify the thiol-groups (-SH) grafted on the Fe3O4 nanoparticle surface, multiple measurements, such as XPS, FTIR, Zeta potential, XRD, TEM and VSM, were applied to compare the physicochemical properties of the Fe3O4 before and after thiolation. Figure 1a gives the wide-scan XPS spectra of the bare Fe3O4 and the Fe3O4-SH nanoparticles. It could be obviously found that the characteristic peaks assigned for sulphur element (S2p at 163.6 eV) and silicon element (Si2p at 101.8 eV) appeared after modification, which unanimously interpreted that thiols had been anchored successfully onto the Fe3O4 surface. And the comparison of FTIR spectra is listed in Figure 1b, which could be witnessed that thiolation treatment had resulted in chemical changes on the Fe3O4 surface according to the varieties of characteristic bands. As shown, the bands at 1126, 1075 and 984 cm−1, assigned to the Si-O-H and Si-O-Si groups respectively, appeared on the spectrum of the Fe3O4-SH nanoparticles, indicating that the silane framework had been grafted successfully onto the bare Fe3O4 surface via Fe-O-Si bonds. And the adsorption band at 2578 cm−1, ascribed to the S-H stretching of the moieties grafted, confirmed that the Fe3O4-SH could be afforded by the condensation of MPTMS with the OH-groups on the bare Fe3O4 surface. In Figure
1c, the isoelectric points (pI) of the bare Fe3O4 and Fe3O4-SH nanoparticles are identified by the zeta potentials as a function of pH values in suspensions with concentration of 10 mg/L, where the pI value of nanoparticles increased to 8.5 from 6.9 due to the thiolation treatment. In addition, the XRD results in Figure 1d prove that thiolation didn’t bring about damages on the crystal structure of the Fe3O4 nanoparticles without peaks appearing or vanishing in the pattern after modification. a)
b)
c)
d)
Figure 1. (a) The wide-scan XPS spectra, (b) FTIR spectra, (c) pI values measurement, and (d) XRD patterns of the bare Fe3O4 and Fe3O4-SH nanoparticles at 25oC.
Agglomeration is an inevitable consequence for nanoparticles dispersed in aqueous solution, which is closely relative with the adsorption and recovery efficiencies of these nano-adsorbents (Lin et al., 2017a; Thommes and Cychosz, 2014). Figure 2 shows the size distributions and typical TEM images (illustrations) of the agglomerated particles of the bare Fe3O4 and Fe3O4-SH. The agglomeration could be observed in both of the TEM images while it was also discerned effortlessly that the Fe3O4-SH nanoparticles had better dispersion, which could be certified by the size distribution results correspondingly. The average size of nanoparticle agglomerations
decreased to 142.33 nm from 268.69 nm after the thiolation treatment, indicating that thiolation could enhance effectively the aqueous stability and dispersibility of the Fe3O4-based nanoparticles, contributing to the subsequent adsorption processes. a)
b)
Figure 2. The TEM image (illustrations) and size distribution of agglomerations of (a) the bare Fe3O4 and (b) Fe3O4-SH nanoparticles in deionized water with pH of 6.0 at 25 oC.
It is an enormous advantage for the Fe3O4-based nanoparticles that could be recovered easily using an extern magnetic field due to their intrinsic superparamagnetism, unclogging the application bottleneck of these common nano-adsorbents difficult to
separate with wastewater after use. In this study, the hysteresis loops of the bare Fe3O4 and Fe3O4-SH nanoparticles were also measured and compared to analyze the effect of thiolation on superparamagnetism. As shown in Figure 3a, it could be observed clearly that both of the hysteresis loops presents typical S-shaped curves on behalf of superparamagnetism. The results indicated that the thiolation treatment didn’t change the intrinsic magnetism of the Fe3O4 nanoparticles, ascertained also by the magnetic parameters listed in Table 1, where the coercive force (HC) and residual magnetism (MR) of the Fe3O4 and Fe3O4-SH were both close to zero. However, the saturation magnetization (Ms) decreased to 21.74 emu/g from 54.29 emu/g due to the damage of thiolation to the magnetic response of the bare Fe3O4 as well as the contribution of the non-magnetic outer layer (Gomez-Pastora et al., 2014; Hu). The fadedness of magnetic response was also embodied in the magnetic recovery courses listed in Figure 3b. It could be observed obviously that the recovery rate decreased markedly after thiolation though the Fe3O4-SH nanoparticles could still be accumulated and aggregated completely around the magnet with enough time. a)
b)
Figure 3. (a) The hysteresis loops and (b) magnetic recovery courses of the bare Fe3O4 and Fe3O4-SH nanoparticles at 25oC.
Table 1. The magnetic parameters of the Fe3O4 and Fe3O4-SH nanoparticles at 25oC. HC (Oe)
MR (emu/g)
MS (emu/g)
Bare Fe3O4
0.105
0.235
54.29
Fe3O4-SH
0.087
0.054
21.74
3.2 Effect of initial pH and ionic strength on adsorption In practice, the adsorption of heavy metal ions is commonly susceptible to the wastewater conditions. Hence, the adsorption tolerance to pH and ionic strength was investigated to compare the affinities of the Fe3O4-SH nano-adsorbents with Pb2+, Ni2+, Zn2+ and Cu2+, respectively. The removal percentage as a function of initial pH was examined with 0.1 g Fe3O4-SH and 1.0 mmol/L heavy metal under incubating time of 1 h at 30oC. It could be seen easily in Figure 4a that the magnitude of removal efficiencies of the heavy metal ions ranked in the sequence of Ni2+>Zn2+>Pb2+>Cu2+ at the fixed pH values, indicating the existence of the adsorption gaps for different heavy metal ions. And the removal efficacies all increased with the initial pH values
in a narrow pH range (2.5-6.0) to avoid chemical precipitations, indicating that pH played fairly positive effect on the adsorption. According to the isoelectric point result above, the Fe3O4-SH nanoparticles were ascertained to be positively charged at low pH, identifying with Pb2+, Ni2+, Zn2+ and Cu2+, which would hinder the adsorption dramatically due to the electrostatic repulsion and the competition between heavy metal ions and H+ for the adsorption sites on surface, which was consistent with the conclusions reported (Odio et al., 2016; Singh et al., 2011; Yantasee et al., 2007). Hence, the initial pH values were all set at 6.0 for fine adsorption performances in all following experiments. The ionic strength tolerance of the adsorption capacities for Pb2+, Ni2+, Zn2+ and Cu2+ was investigated separately with 0.1 g Fe3O4-SH to adsorb 1.0 mmol/L metal ion in 1 h. As shown in Figure 4b, the adsorption capacities of the Fe3O4-SH nano-adsorbents for various heavy metal ions all sharply fell along with the increase of NaCl concentration in the range of 0-10 mmol/L, indicating that the existence of background electrolyte could affect acutely the adsorption. While the downtrends were apparently assuaged as NaCl concentration exceeded 10 mmol/L in solution that there was just a relative handful of reduction on the adsorption capacities in the range of 10-50 mmol/L NaCl. According to the electric double layer theory, it was confirmed that the background electrolyte concentration could influence the thickness of the double layer resulting in the outer-sphere complexes susceptible to ionic-strength while no effect to inner-sphere complexes (Strawn and Sparks, 1999; Yoon et al., 2016). Therefore, it was induced that the outer-sphere complexes,
responsible for the rapid reduction stage, as well as the inner-sphere complexes had formed on the Fe3O4-SH surface during the adsorption. Meanwhile, by comparison of the adsorption capacities for various heavy metals with 50 mmol/L NaCl and without NaCl, it was confirmed that there was more Zn2+ adsorbed on the Fe3O4-SH surface dependent on the outer-sphere complexation and the tolerance of adsorption performances to ionic strength using the Fe3O4-SH was in the sequence of Pb2+> Ni2+> Cu2+>Zn2+. a)
b)
Figure 4. The effect of (a) initial pH and (b) ionic strength on the adsorption performances using the Fe3O4-SH nanoparticles at 30oC.
3.3 Adsorption kinetics and thermodynamics In order to quantitatively compare the adsorption abilities of the Fe3O4-SH nanoparticles for different heavy metal ions, the adsorption kinetics and thermodynamics for Pb2+, Ni2+, Zn2+ and Cu2+ were investigated in detail at 30oC. As shown, the time courses of the adsorption for 1.0 mmol/L heavy metal ion by 0.1 g Fe3O4-SH are listed in Figure 5. It could be observed that the adsorption processes underwent in analogous tracks for Pb2+, Ni2+, Zn2+ and Cu2+, where the first 10 min corresponded to a rapid adsorption stage and then the adsorption rates decreased with the lessened adsorption sites available, resulting in the Fe3O4-SH nanoparticles all saturated further in 50 min. These results indicated that the adsorption of heavy metal ions occurred on the Fe3O4-SH surface and rapidly proceeded towards saturation during the fast adsorption period. Thereafter, the residual sites on the Fe3O4-SH surface were competed for by adsorbates remaining in the solution, giving rise to low adsorption rates. Furthermore, in order to explore the adsorption processes, various adsorption kinetics models had been applied to fit above data including the first-order model, the pseudo-first-order model, the pseudo-second-order kinetic model, etc. And the results witnessed that the adsorption processes could all described well by the pseudo-second-order kinetic model quite well, which could be expressed as follows (Erdem et al., 2004):
qt t / (1/ v0 t / qe )
(5)
Where qe (mmol/g) and qt (mmol/g) are the adsorption capacities of heavy metal ions at equilibrium and the time of t (min), and v0 (mmol/min) represents the initial
adsorption rate. The pseudo-second-order kinetic model assumed the adsorption capacity of the adsorbent is determined by the adsorption sites at which electron sharing or electron transfer occurred between the adsorbent and adsorbate. Hence, the fine conformity between adsorption processes and the pseudo-second-order model, consistent with the high coefficients of determination (R2) summarized in Table 2, indicated that the adsorption for various heavy metal ions could be assigned to a charge transfer complex pattern, satisfying with the above conclusion of the ionic strength effect. Besides, the fitting v0 values manifested that the adsorption rates of the Fe3O4-SH for heavy metals followed the rank of Cu2+< Pb2+< Zn2+
Figure 5. Adsorption kinetics for heavy metal ions by the Fe3O4-SH nanoparticles at 30˚C.
Table 2. The pseudo-second-order model parameters for the adsorption at 30C. qe (mmol/g)
v0 (mmol/min)
R2
Pb2+
0.621
0.297
0.999
Ni2+
0.805
0.325
0.997
Zn2+
0.709
0.317
0.998
Cu2+
0.550
0.238
0.998
Besides, the adsorption thermodynamics for Pb2+, Ni2+, Zn2+ and Cu2+ were also examined and the data were fitted by the Langmuir model listed as Equation (6) (Hayes et al., 1988):
Qe
Qm a xb C e 1 bCe
(6)
Where Qe (mmol/g) is the adsorption capacity at equilibrium, Qmax (mmol/g) is the saturated adsorption capacity, Ce (mmol/L) is the equilibrium concentration of the heavy metal in solution, and b is the Langmuir equilibrium constant. As shown in Figure 6, it could be observed that the Fe3O4-SH nanoparticles performed the highest adsorption capacity for Ni2+ and the lowest adsorption capacity for Cu2+ at different equilibrium concentrations. The fitting parameters in Table 3 indicated the adsorption processes for Pb2+, Ni2+, Zn2+ and Cu2+ all obeyed to the Langmuir model well (R2>0.98), representing the monolayer adsorption. And the saturation capacities for different heavy metal ions followed the sequence of Ni2+>Zn2+>Pb2+>Cu2+, confirming that the Fe3O4-SH nanoparticles had the most excellent adsorption performance for Ni2+ while Cu2+ was hardest to remove. In general, the Fe3O4-SH nanoparticles demonstrated the gaps among the adsorption abilities for different heavy metal contaminants while the saturated adsorption
capacities for Pb2+, Ni2+, Zn2+ and Cu2+ were still superior to that of the common heavy metal adsorbents. For instance, the adsorbance for heavy metals of bio-char, chitosan composites, clays and so on are generally less than 0.50 mmol/g (Babel and Kurniawan, 2003; Meena et al., 2005; Wan Ngah et al., 2011). Therefore, the Fe3O4-SH nanoparticles have shown great application potential in the wastewater treatments involving heavy metals, which is consistent with the results of Yantasee and Shin (Shin and Jang, 2007; Yantasee et al., 2007).
Figure 6. Adsorption thermodynamics of heavy metals by the Fe3O4-SH nanoparticles at 30oC. Table 3. Langmuir model parameters for the adsorption thermodynamics. Qmax (mmol/g)
b
R2
Pb2+
0.631
55.19
0.995
Ni2+
0.860
157.38
0.974
Zn2+
0.738
119.31
0.978
Cu2+
0.572
67.84
0.990
3.4 Competitive adsorption The composition of industrial wastewaters is always complicated including various heavy metal contaminations. Therefore, it is necessary to investigate the adsorption selectivity to different contaminants and the interference between each other, which is the basis for the application of the Fe3O4-SH nano-adsorbents in complicated wastewater treatments. The adsorption courses were studied in mixed solution with different doses of nano-adsorbents to remove Pb2+, Ni2+, Zn2+ and Cu2+, whose initial concentrations were all 1.0 mmol/L. It can be seen in Figure 7 that Pb2+ was always prior to remove in the mixed solution whatever the dose of the Fe3O4-SH was, indicating the strong affinity between Pb2+ and SH-groups, corresponding to the ionic strength effect results above. In the case of 0.1 g Fe3O4-SH, as shown in Figure 7a, the concentration of Pb2+ decreased to 0.49 mmol/L at adsorption equilibrium, while little of Ni2+, Zn2+ and Cu2+ were removed from solution. As the dose of Fe3O4-SH increased to 0.25 g in Figure 7b, Pb2+ could be removed thoroughly in 50 min. Meanwhile Ni2+ could also be removed, whose residual concentrations decreased to 0.194 mmol/L at adsorption equilibrium in 100 min. With a further increasing dose of adsorbents to 0.50 g in Figure 7c, Pb2+ could be removed completely in less than 30 min and Ni2+ could not be detected in solution in 75 min meanwhile. Simultaneously, the concentration of Cu2+ began to decline obviously in the solution while little Zn2+ was removed after the rapid adsorption stage of Pb2+ and Ni2+. At last, as 1.0 g Fe3O4-SH added in Figure 7d, all heavy metals could be removed entirely in 75 min. The results above indicated that the increase of the nano-adsorbents dose would cut
down the time of adsorption equilibrium effectively. At the same time, the adsorption selectivity followed the sequence of Pb2+>Ni2+>Cu2+>Zn2+. a)
b)
c)
d)
Figure 7. The residual concentrations of heavy metals ions as a function of time under doses of (a) 0.10 g, (b) 0.25 g, (c) 0.50 g and (d) 1.0 g of the Fe3O4-SH nanoparticles in the mixed solution.
3.5 Desorption Desorption was significantly influenced by the affinity between heavy metal ions and the SH-groups on the Fe3O4-SH surface. In this study, the elution courses of various metal ions from saturated nano-adsorbents surface in single systems and the mixed system were both investigated. It could be judged that the time needed for elution
equilibrium of Pb2+, Ni2+, Zn2+ and Cu2+ were discriminating from the individual elution percentage as a function of time, which was listed in Figure 8a. The experiments were conducted with 10 ml eluent mixed with 0.10 g saturated Fe3O4-SH nanoparticles at 30oC. As shown in Figure 8a, the desorption rates followed the sequence of Pb2+
a)
b)
Figure 8. (a) Desorption kinetics in single systems and (b) desorption efficiency as a function of eluent volume in mixed system at 30oC.
4. CONCLUSION
In this study, the Fe3O4-SH nano-adsorbents exhibited excellent adsorption performances for Pb2+, Ni2+, Zn2+ and Cu2+ with rapid adsorption rates and great adsorption capacities. Furthermore, it was notable that the adsorption selectivity of the Fe3O4-SH for different heavy metal ions was hierarchical and followed the sequence of Pb2+>Ni2+>Cu2+>Zn2+ according to the adsorption and desorption experiments. The
present work has determined the adsorption competition and interference among different heavy metal ions using the Fe3O4-SH nanoparticles as adsorbents, which is full of significance to provide a valid reference for the environmental application of Fe3O4-SH nanoparticles in industrial wastewater treatment in future.
ACKNOWLEDGEMENTS
This work was supported by China Postdoctoral Science Foundation (2017M611476) and the Fundamental Research Funds for the Central Universities (WB1714038).
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