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Adsorption and kinetic behavior of Cu(II) ions from aqueous solution on DMSA functionalized magnetic nanoparticles
Physica B: Condensed Matter 571 (2019) 273–279
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Adsorption and kinetic behavior of Cu(II) ions from aqueous solution on DMSA functionalized magnetic nanoparticles
T
V.D. Chavana, V.P. Kothavaleb, S.C. Sahooc, P. Kollud, T.D. Dongalea, P.S. Patila,e, P.B. Patilf,* a
School of Nanoscience and Technology, Shivaji University Kolhapur, Maharashtra, 416004, India Department of Physics, Bhogawati Mahavidyalaya, Kurukali, Shivaji University, Kolhapur, Maharashtra, 416001, India c Department of Physics, Central University of Kerala, Kasaragod, Kerala, 671316, India d CASEST, School of Physics, University of Hyderabad, Gachibowli, Hyderabad, Telangana, 500046, India e Department of Physics, Shivaji University, Kolhapur, Maharashtra, 416004, India f Department of Physics, The New College, Shivaji University, Kolhapur, Maharashtra, 416012, India b
ARTICLE INFO
ABSTRACT
Keywords: Magnetic nanoparticles (MNPs) DMSA Functionalization Ligand-exchange Adsorption Ion removal
In this work, magnetic nanoparticles (MNPs) synthesized by the solvothermal method and functionalized by meso-2,3-dimercaptosuccinic acid (DMSA) by ligand-exchange protocol were used as nanoadsorbents to remove Cu(II) ions from aqueous solution. The magnetic nanoadsorbents were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometer (VSM). The functionalized MNPs were used for removal of Cu(II) by batch adsorption technique under different influencing parameters such as contact time, adsorbent dose, initial Cu(II) concentration, and pH. The adsorption behaviour of Cu(II) on MNP-DMSA follows Pseudo-first-order kinetic model. Removal efficiency was found to decrease from 98 to 64% by increasing the Cu(II) concentration in the solution from 50 to 300 ppm. The experimental data for the adsorption of Cu(II) were found to follow the Langmuir isotherm and the maximum adsorption capacity was 25.44 mg/g.
1. Introduction Rapid industrial development and urbanization are responsible for the increasing levels of ground-water pollution and due to this; the quality of drinking water has reduced significantly [1]. In recent years, water streams are polluting due to the discharge of toxic heavy metals from industries such as fertilizer, mining, metal plating, textile industries etc. Heavy metals are not decomposable and can accumulate in living organisms [2,3]. This can lead to various disorders and diseases. The most alarming toxic heavy metals are copper (Cu), mercury (Hg), chromium (Cr), arsenic (As), lead (Pb), cadmium (Cd), and nickel (Ni) [1,4–7]. Therefore it is of utmost important to remove these toxic heavy metal ions from contaminated wastewater. For the removal of toxic heavy metals from contaminated wastewater, various conventional methods are available. These methods include membrane filtration, microbial system, coagulation, electrochemical process, chemical precipitation, ion-exchange, adsorption, photocatalytic degradation etc. [8,9]. Among these methods, adsorption is the most effective option due to its ease of operation, low operating cost, high efficiency, and low fouling problems.
*
In last few decades, many types of adsorbents such as activated carbon [10], silica gel [11], zeolite [12], clay minerals [13], and magnetic nanoparticles (MNPs) [14] have been explored for heavy metal removal. Among these adsorbents, MNPs with high surface to volume ratio and good colloidal stability results in superior adsorption kinetics for various metal ions in aqueous solutions. Further unique magnetic properties of MNPs can be explored for easy and fast separation of nanoadsorbent from the aqueous solution. The combination of adsorption and magnetic separation holds the advantage due to operational flexibility, recovery of heavy metals, and reusability of adsorbent MNPs. Several factors need to be considered to achieve efficient removal while using MNPs as an adsorbent. For colloidal stability and easy magnetic separation, nanoparticles should be superparamagnetic with higher magnetization [15]. To improve the heavy metal uptake capacity by MNPs, they need to be surface modified to provide specific functional groups or reaction sites that can be selective and specific for ions uptake. MNPs can be functionalized with some compounds, such as humic acid [4], meso-2,3-dimercaptosuccinic acid (DMSA) [16], (3aminopropyl)triethoxysilane [17,18], and chitosan [19]. Different
Corresponding author. Department of Physics, The New College, Shivaji University, Kolhapur, Maharashtra, 416012, India. E-mail address: [email protected] (P.B. Patil).
https://doi.org/10.1016/j.physb.2019.07.026 Received 6 April 2019; Received in revised form 29 June 2019; Accepted 14 July 2019 Available online 15 July 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
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functionalization strategies can be employed to provide functional groups like –COOH, –NH2, –OH, –SH etc. for heavy metal adsorption [14]. These strategies can have a significant influence on the performance of the adsorbents. In this work, DMSA functionalized MNPs (MNP-DMSA) were employed as a nanoadsorbent for the removal of Cu(II) ions from aqueous solution. The adsorption and kinetic behavior of Cu(II) on MNP-DMSA were thoroughly investigated.
310 K with 100 Oe applied magnetic field. The visible spectrophotometer visiscan-167 of Systronics India was used to perform batch adsorption experiments. 2.5. Batch adsorption experiment
All the chemicals used were of analytical grade and used without further purification. Ferric acetyl-acetonate (Fe(acac)3) and meso-2,3dimercaptosuccinic acid (DMSA) were procured from Sigma Aldrich. Ethylene glycol (C2H6O2), oleic acid, oleylamine, dimethyl sulfoxide (DMSO), hexane, and ethanol were purchased from S.D. fine-chem India. Copper sulphate pentahydrate (CuSo4.5H2O) and ethylenediamine-tetraacetic acid (EDTA) was purchased from Fisher Scientific.
The adsorption behavior of Cu(II) on MNP-DMSA was performed by batch experiments at room temperature. To study the effect of contact time on Cu(II) adsorption behavior of MNP-DMSA, 75 mg MNP-DMSA was mixed in 25 mL of 200 ppm Cu(II) solution. The solution was shaken in an incubator shaker at 150 rpm to reach adsorption equilibrium. The magnetic adsorbents were then separated from aqueous solutions by using an external magnet. Similar batch experiments were carried out, to study the effect of adsorbent dose (range: 2–7 g/L), pH (range: 3–8) and initial Cu(II) concentration (range: 50–300 ppm). The pH of the solution was adjusted by 0.1 M HCl and 0.1 M NaOH. The concentration of Cu(II) before and after adsorption was analysed spectrophotometrically at 716 nm using 0.1 M EDTA as a complexing agent [23]. The removal efficiency (E) and the amount adsorbed (q) of Cu(II) were calculated by the following equations:
2.2. Synthesis of MNPs
E (%) =
2. Materials and methods 2.1. Chemicals
The MNPs were synthesized by the solvothermal method used by Gao et al. with slight modification [20]. Typically in this process, the solution of ethylene glycol (20 ml) was mixed with 4 mM oleic acid and 10 mM oleylamine and stirred to form a homogeneous solution. 1 mM Fe(acac)3 was added slowly in the reaction mixture and stirred vigorously for 20 min. The constant flow of Argon was passed into the solution to deairinate the solution. The prepared solution was then autoclaved at 200 °C for 30 min and then at 260 °C for 8 h. The product was washed several times with hexane and anhydrous ethanol before vacuum drying.
q=
(C0
C0
Ce Co
× 100
Ce ) V M
(1) (2)
where C0 and Ce are the initial and equilibrium concentration of Cu(II) solution (mg/L), V is the volume of Cu(II) solution (L) and M is the mass of the adsorbent used (g). 3. Results and discussion 3.1. Characterization of MNP-DMSA The phase formation and crystallinity of as-synthesized MNPs were studied by XRD. Fig. 1 shows the XRD patterns of MNPs, MNP-DMSA, and Cu adsorbed MNP-DMSA. All the peaks in the pattern correspond to inverse spinel structure of pure magnetite phase (JCPDS card no. 85–1435). No other phase impurity was observed indicating high phase purity of the sample. The average crystallite size of about 19 nm was calculated by Debye-Scherrer formula. XRD patterns of functionalized or Cu adsorbed MNPs does not show any change in crystallinity when compared to bare MNPs. The size and shape of the MNPs were investigated by TEM and Fig. 2 shows the TEM micrograph of MNPs.
2.3. Functionalization of MNPs by DMSA To functionalize MNPs by DMSA, a standard ligand-exchange protocol was used to replace oleic acid and oleylamine moieties by DMSA [21,22]. MNPs (50 mg) were washed and centrifuged several times by a mixture of ethanol and hexane (1:1 ratio). After eliminating the supernatant, coagulated particles were re-dispersed in the 20 ml of toluene. To this, a solution of DMSA (90 mg) in DMSO (5 ml) was added and sonicated. The solution was then incubated at room temperature for 48 h in a mechanical shaker. After the reaction, the translucent solvent was discarded and the black particles (MNP-DMSA) were collected by ethanol washing. Particles were then washed several times by centrifuging in ethanol. The MNP-DMSA were dispersed in the doubled distilled water, basified at pH 10 and dialyzed against doubled distilled water for 24 h. MNP-DMSA particles were re-dispersed in the aqueous solution of pH 7. Finally, functionalized MNPs were magnetically decanted from the solution and dried in vacuum. 2.4. Characterization X-ray diffraction (XRD) measurements were carried out by using Rigaku Miniflex 600 X-ray diffractometer with Cu-Kα radiation (1.5406 Å) in θ/2θ mode. Transmission electron microscope (TEM), Tecnai G2 S Twin at voltage 200 kV was used for particle size and microstructural investigations. The Brunauer Emmett-Teller (BET) analysis was performed using Quantachrome Instruments v11.03. Fourier transform infrared (FTIR) spectra of all the samples were collected on FT/IR-4600 Jasco spectrometer using KBr pellet technique in the 4000-400 cm−1 range. Magnetic measurements were done with Quantum Design's VersaLab physical property measurement system. Magnetization loops up to 30 kOe fields were measured at 300 K and 60 K. Thermomagnetic measurements were taken between 50 K and
Fig. 1. XRD pattern of (a) MNPs, (b) MNP-DMSA, and (c) Cu adsorbed MNPDMSA. 274
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Fig. 4. FTIR spectra of (a) MNPs, (b) MNP-DMSA, and (c) Cu adsorbed MNPDMSA.
(b), the stretching vibrations at, 2332-2355 cm−1 corresponding to thiol groups and 1745 cm−1 corresponding to C]O bond reveals the presence of DMSA on the MNPs [28]. The absence of oleic acid and oleylamine vibrations in the MNP-DMSA spectra confirms the successful ligand exchange of DMSA on the surface of MNPs. As expected Cu adsorbed MNP-DMSA spectra does not show any absorption corresponding to presence of Cu as it is not IR active. Absorption peaks corresponding to other functional groups are unchanged indicating the stability of the functional group decorated on the surface of magnetic nanoparticles. The room temperature magnetization curves of MNPs, and MNPDMSA are shown in Fig. 5 (a). It can be seen that the MNPs saturation magnetization (Ms) is 72 emu/g. After the functionalization the magnetization is reduced to 57 emu/g. In our previous work, we have observed similar reduction in magnetization after functionalization of MNPs by chitosan [19]. Low field magnetization curve (Fig. 5(b)) of MNPs at 300 K shows complete reversible nature without any hysteresis. The absence of coercivity and remanence at 300 K indicates the superparamagnetic state of MNPs. At 60 K temperature, hysteresis loop with 200 Oe coercivity was observed. This is due to the transition from superparamagnetic to blocked ferromagnetic state. Magnetization measurement as a function of temperature under zero field cooling (ZFC) and field cooling (FC) protocols of the MNP-DMSA is shown in Fig. 5(c). The measurements were carried between 50 and 310 K with 100 Oe measuring field. In ZFC curve the unblocked increase in the magnetization of MNPs from 50 K to 310 K was observed. The bifurcation of ZFC-FC curves at 310 K reveals that all the MNPs are unblocked at this temperature. ZFC-FC curves of bare MNPs (not shown here) are similar to that of MNP-DMSA.
Fig. 2. TEM micrograph and the inset shows SAED pattern of Fe3O4 MNPs.
Fig. 3. Nitrogen adsorption-desorption isotherm with inset showing BJH pore size distribution of (a) MNPs and (b) MNP-DMSA.
MNPs have a roughly spherical shape with an average particle size 23 nm. The selected area electron diffraction (SAED) pattern of MNPs is shown in the inset of Fig. 2. The SAED pattern corroborates with the XRD analysis. Fig. 3 (a) and (b) shows the nitrogen adsorption-desorption isotherms of the bare MNPs and MNP-DMSA respectively measured by the BET technique. The inset shows pore radius distribution of the corresponding samples. For the bare MNPs average pore radius calculated by the Barrett-Joyner-Halenda (BJH) method was 5.7 nm and the BET surface area was 29.64 m2/g. This confirms the mesoporous structure of the MNPs. After the surface modification of the MNPs by DMSA, the average pore radius was increased to 7.8 nm and BET surface area was increased to 46.24 m2/g. The functionalization of DMSA on the MNPs was investigated by the FTIR spectroscopy. Fig. 4 shows the different bending and stretching vibrations for (a) MNPs, MNP-DMSA (b), and (c) Cu adsorbed MNPDMSA respectively. The sharp peak at 573 cm−1 in all the spectra corresponds to Fe–O stretching vibrations in Fe3O4. The broad peak at 3400 cm−1 is due to stretching vibrations of the –OH groups. In MNPs spectra, the bands at 2918 cm−1 and 2842 cm−1 are asymmetric CH2 and symmetric CH2 vibration bands respectively of oleic acid [24]. The chemisorption of oleic acid on the MNPs as carboxylate can be seen by the broad peak of single C–O stretching at 1050 cm−1 [25]. Moreover, the band in the region 1330-1470 cm−1 corresponds to NH2 bending mode of oleylamine [26]. The pure bands of oleic acid and oleylamine are not present in the spectra, which indicate the chemisorption of oleic acid and oleylamine on the MNPs [25,27]. In the spectra of MNP-DMSA
3.2. Adsorption studies 3.2.1. Effect of contact time Contact time is an important parameter in the wastewater treatment process. Fig. 6 shows the effect of contact time on the removal efficiency. It can be seen that the removal efficiency rapidly increased with contact time and reached 32% at 105 min. After this, the adsorption curve reached equilibrium. Initially, removal of Cu(II) ions was fast since the adsorbent sites were vacant and free from internal diffusion resistance. 3.2.2. Effect of adsorbent dose To study the effect of adsorbent dose, 2–10 g/L MNP-DMSA was 275
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Fig. 5. (a) Magnetization curves of MNPs (black) and MNP-DMSA (red) measured at room. temperature, (b) Low field magnetization curves of MNPs measured at 300 K (red) and 60 K(black), and (c) Zero field cooled (ZFC) and field cooled (FC) curves of MNP-DMSA.
availability of adsorption sites was increased. The maximum removal efficiency was found to be 79% for 8 g/L of adsorbent dose. Further increase in adsorbent dose, did not affect removal efficiency and the adsorption curve reached equilibrium. Therefore the adsorbent dose of 8 g/L was used for further experiments. 3.2.3. Effect of initial ion concentration The initial concentration has a remarkable effect on the removal of Cu(II) from aqueous solution. The percentage removal efficiency of Cu (II) was decreased with the increase in the initial concentration (Fig. 8). As the amount of adsorbent dose is fixed, the total available adsorption sites are limited. This leads to the decrease in removal efficiency with increasing initial concentration. However as can be seen from Fig. 8, the adsorption capacity of the Cu(II) ions was gradually increased with increasing initial Cu(II) concentration [29]. 3.2.4. Effect of pH The influence of the pH of aqueous solution on the adsorption of Cu (II) on MNP-DMSA is shown in Fig. 9. It is clear that with the increase in solution pH values from 2 to 6, the removal efficiency was increased from 75% to 99% and then decreased to 96% for pH 7. Above pH 7, the precipitate formation was started in Cu(II) solution. With the increase in the solution pH from 2 to 6, the effect of protonation becomes weaker, leading to increasing of the number of negative groups. The removal efficiency was increased at higher solution pH via electrostatic
Fig. 6. Effect of contact time on removal efficiency of Cu(II) by MNP-DMSA nanoadsorbents. (Concentration of Cu(II) = 200 ppm, Adsorbent dose 3 g/L).
Fig. 7. Effect of adsorbent dose on removal efficiency of Cu(II) by MNP-DMSA. nanoadsorbents (Contact Time = 105 min, Concentration of Cu (II) = 200 ppm).
added to 10 mL aqueous solution of Cu(II). It can be seen from Fig. 7 that the removal efficiency of Cu(II) was increased from 23 to 79% with increasing adsorbent dose. With the increase in MNP-DMSA, the
Fig. 8. Effect of initial concentration on removal efficiency of Cu(II) by MNPDMSA. nanoadsorbents (Contact Time = 105 min, adsorbent dose = 8 g/L). 276
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are listed in Table 1. For the first order kinetic model, the experimental and calculated values of qe are nearly equal. Also R2 for first-order kinetic model is 0.9888 which is higher than the R2 of second-order kinetic model. This suggests that the adsorption of Cu (II) follows the pseudo-first-order kinetic and reaction is more inclined towards physisorption. 3.4. Adsorption isotherms The Langmuir and Freundlich adsorption isotherm models were applied to experimental data to explore the surface properties and mechanism of adsorption. The Langmuir isotherm model can be expressed as [32].
Ce C 1 = e + qe qm KL qm
where Ce is the equilibrium concentration of Cu(II) (mg/L), qe is the adsorption capacity (mg/g), qm is the maximum capacity of adsorbent (mg/g) and KL is the Langmuir adsorption constant (L/mg). To determine the value of qm and KL, the experimental data were plotted as Ce/qe versus Ce as shown in Fig. 11 (a). The Freundlich isotherm model can be expressed as [33].
Fig. 9. Effect of pH on removal efficiency of Cu(II) by MNP-DMSA nanoadsorbents. (Contact Time = 105 min, Adsorbent dose = 8 g/L and Concentration of Cu(II) = 50 ppm).
adsorption between negatively charged surface and Cu(II). A similar increase in removal efficiency with increasing pH was observed to Huang et al. [29].
log q e = log KF +
To study the adsorption behavior of Cu(II) ions on MNP-DMSA, kinetic experiments were carried out. The pseudo-first-order and pseudo-second-order kinetic models were used to analyze the adsorption kinetics of Cu(II). The pseudo-first-order kinetic model was expressed as [30].
qt ) = log(qe )
k1 t 2.303
(3)
Pseudo-second-order kinetic model was expressed as [31].
t 1 1 = + t qt qe k2 qe2
1 log Ce n
(6)
where qe is the adsorption capacity (mg/g) and Ce is the equilibrium concentration of Cu(II) (mg/L). KF and n are Freundlich constant related to adsorption capacity (L/g) and heterogeneity factor respectively. The value of KF and n were calculated from the intercept and slope of log qe versus log Ce graph shown in Fig. 11 (b). The Langmuir and Freundlich isotherm constants are summarized in Table 1. From R2 it can be seen that the Langmuir isotherm model (R2 = 0.99) fits better to the experimental data than the Freundlich model (R2 = 0.9843). Also, the maximum absorption capacity calculated by the Langmuir model (25.44 mg/g) fits well with the experimental maximum adsorption capacity (24 mg/g). Thus it can be concluded that the monolayer Langmuir adsorption isotherm is more suitable to explain the adsorption behavior between MNP-DMSA and Cu(II) and adsorption takes place on the homogeneous surface. Obtained Langmuir adsorption capacity (25.44 mg/g) was comparable to the previous results for magnetic nanoadsorbents such as, γ -Fe2O3 (19.4 mg/g) [34], δ-FeOOH-coated γ -Fe2O3 (25.8 mg/g) [34], chitosan-bound Fe3O4 (21.5 mg/g) [35], and carboxymethyl chitosan–Fe3O4 (20.4 mg/g) [36] hexadiamine functionalized Fe3O4 (25.77 mg/g) [2]. Moreover, the obtained maximum capacity with MNP-DMSA is higher than reported values 12.43 mg/g and 10.41 mg/g for amino-functionalized polyacrylic acid coated Fe3O4 and silica coated Fe3O4 respectively [37,38].
3.3. Adsorption kinetics
log(qe
(5)
(4)
where qe and qt (mg/g) are the adsorption capacity at equilibrium and time t (min), respectively, k1 (min−1) and k2 (g/mg/min) are the rate constants for pseudo-first-order and pseudo-second-order adsorption models respectively. The plot of log (qe-qt) against t is shown in Fig. 10 (a). The slope and intercept of the graph are used to determine the values of k1 and qe respectively. The plot of t/qt against t is shown in Fig. 10 (b). The slope and intercept of the graph are used to determine the values of the values of qe and k2 respectively. The values of kinetic parameters obtained from Eq. (3) and Eq. (4)
4. Conclusions DMSA functionalized MNPs were prepared, characterized and employed as a magnetic nanoadsorbent for the removal of toxic heavy metal Cu(II) from aqueous solution. XRD and TEM studies of MNPs showed pure magnetite phase with 23 nm average particle size. Magnetic measurements revealed that MNPs are in the superparamagnetic state at room temperature. As prepared MNPs synthesized by solvothermal method were hydrophobic due to the coating of oleic acid and oleylamine surfactants. The ligand-exchange protocol was successfully employed to replace the hydrophobic oleic acid and oleylamine moieties from the surface of MNPs by DMSA. The effective ligand-exchange of DMSA on the surface of MNPs was confirmed by FTIR. The kinetic studies indicated that the adsorption reaction follows the pseudo-first-order kinetic model and adsorption was a physical process. The Adsorption isotherm study showed that the Langmuir model fits better than the Freundlich model with the experimental data,
Fig. 10. (a) Pseudo-first-order kinetic model and, (b) Pseudo-second-order kinetic model for. the adsorption of Cu(II) on MNP-DMSA nanoadsorbents. 277
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Table 1 Parameters of kinetic models and adsorption isotherm models for the adsorption of Cu(II) onto MNP-DMSA nanoadsorbents. Parameters of kinetic models for adsorption of Cu(II)
Parameters of adsorption isotherm model for the adsorption between of Cu(II) 2
Kinetic Model
qe (Exp.) (mg/ g)
qe (Cal.) (mg/ g)
Rate constants
R
Pseudo-first-order Pseudo-secondorder
24 24
23.48 43.47
K1 = 0.01893/min K2 = 0.00020 g/mg/min
0.9888 0.9316
Langmuir model qm(mg/g) 25.44
KL(L/mg) 0.1043
Freundlich model R2 0.99
KF (L/mg) 6.77
n 3.6812
R2 0.9843
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Fig. 11. (a) Langmuir isotherm model and, (b) Freundlich isotherm model for Cu(II) adsorption on MNP-DMSA nanoadsorbents.
implying the monolayer adsorption between nanoadsorbent and Cu(II) ions. The 99% removal efficiency was observed with 8 g/L adsorbent dose of MNP-DMSA for 50 ppm Cu(II) concentration at pH 6. MNPDMSA nanoadsorbent could be regarded as a potential candidate for removing Cu(II) from aqueous solution. Conflicts of interest None. References [1] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, J. Environ. Manag. 92 (2011) 407–418, https://doi.org/10.1016/j.jenvman.2010.11. 011. [2] Y.M. Hao, C. Man, Z.B. Hu, Effective removal of Cu (II) ions from aqueous solution by amino-functionalized magnetic nanoparticles, J. Hazard Mater. 184 (2010) 392–399, https://doi.org/10.1016/j.jhazmat.2010.08.048. [3] M.J.K. Ahmed, M. Ahmaruzzaman, A review on potential usage of industrial waste materials for binding heavy metal ions from aqueous solutions, J. Water Process Eng. 10 (2016) 39–47, https://doi.org/10.1016/j.jwpe.2016.01.014. [4] J.F. Liu, Z.S. Zhao, G. Bin Jiang, Coating Fe3O4magnetic nanoparticles with humic acid for high efficient removal of heavy metals in water, Environ. Sci. Technol. 42 (2008) 6949–6954, https://doi.org/10.1021/es800924c. [5] J.Z., X.C. Xiangtao Wang, Yifei Guo, Li Yang, Meihua han, nanomaterials as sorbents to remove heavy metal ions in wastewater treatment, J. Environ. Anal. Toxicol. 74 (2012) 476–481, https://doi.org/10.4172/2161-0525.1000154. [6] R. Nouri, E. Tahmasebi, A. Morsali, Capability of magnetic functional metal-organic nanocapsules for removal of mercury(II) ions, Mater. Chem. Phys. 198 (2017) 310–316, https://doi.org/10.1016/j.matchemphys.2017.06.018. [7] M. Hashemzadeh, A. Nilchi, A.H. Hassani, R. Saberi, Synthesis of novel surfacemodified hematite nanoparticles for the removal of cobalt-60 radiocations from aqueous solution, Int. J. Environ. Sci. Technol. 16 (2019) 775–792, https://doi.org/ 10.1007/s13762-018-1656-4. [8] A. Ahmadpour, M. Tahmasbi, T.R. Bastami, J.A. Besharati, Rapid removal of cobalt ion from aqueous solutions by almond green hull, J. Hazard Mater. 166 (2009) 925–930, https://doi.org/10.1016/j.jhazmat.2008.11.103. [9] A. Bhatnagar, A.K. Minocha, M. Sillanpää, Adsorptive removal of cobalt from aqueous solution by utilizing lemon peel as biosorbent, Biochem. Eng. J. 48 (2010) 181–186, https://doi.org/10.1016/j.bej.2009.10.005. [10] F. Di Natale, A. Erto, A. Lancia, Desorption of arsenic from exhaust activated carbons used for water purification, J. Hazard Mater. 260 (2013) 451–458, https://doi. org/10.1016/j.jhazmat.2013.05.055. [11] F. Cao, P. Yin, X. Liu, C. Liu, R. Qu, Mercury adsorption from fuel ethanol onto phosphonated silica gel prepared by heterogenous method, Renew. Energy 71 (2014) 61–68, https://doi.org/10.1016/j.renene.2014.05.028. [12] K. Rida, S. Bouraoui, S. Hadnine, Adsorption of methylene blue from aqueous solution by kaolin and zeolite, Appl. Clay Sci. 83–84 (2013) 99–105, https://doi.org/
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Adsorption and kinetic behavior of Cu(II) ions from aqueous solution on DMSA functionalized magnetic nanoparticles
T
V.D. Chavana, V.P. Kothavaleb, S.C. Sahooc, P. Kollud, T.D. Dongalea, P.S. Patila,e, P.B. Patilf,* a
School of Nanoscience and Technology, Shivaji University Kolhapur, Maharashtra, 416004, India Department of Physics, Bhogawati Mahavidyalaya, Kurukali, Shivaji University, Kolhapur, Maharashtra, 416001, India c Department of Physics, Central University of Kerala, Kasaragod, Kerala, 671316, India d CASEST, School of Physics, University of Hyderabad, Gachibowli, Hyderabad, Telangana, 500046, India e Department of Physics, Shivaji University, Kolhapur, Maharashtra, 416004, India f Department of Physics, The New College, Shivaji University, Kolhapur, Maharashtra, 416012, India b
ARTICLE INFO
ABSTRACT
Keywords: Magnetic nanoparticles (MNPs) DMSA Functionalization Ligand-exchange Adsorption Ion removal
In this work, magnetic nanoparticles (MNPs) synthesized by the solvothermal method and functionalized by meso-2,3-dimercaptosuccinic acid (DMSA) by ligand-exchange protocol were used as nanoadsorbents to remove Cu(II) ions from aqueous solution. The magnetic nanoadsorbents were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometer (VSM). The functionalized MNPs were used for removal of Cu(II) by batch adsorption technique under different influencing parameters such as contact time, adsorbent dose, initial Cu(II) concentration, and pH. The adsorption behaviour of Cu(II) on MNP-DMSA follows Pseudo-first-order kinetic model. Removal efficiency was found to decrease from 98 to 64% by increasing the Cu(II) concentration in the solution from 50 to 300 ppm. The experimental data for the adsorption of Cu(II) were found to follow the Langmuir isotherm and the maximum adsorption capacity was 25.44 mg/g.
1. Introduction Rapid industrial development and urbanization are responsible for the increasing levels of ground-water pollution and due to this; the quality of drinking water has reduced significantly [1]. In recent years, water streams are polluting due to the discharge of toxic heavy metals from industries such as fertilizer, mining, metal plating, textile industries etc. Heavy metals are not decomposable and can accumulate in living organisms [2,3]. This can lead to various disorders and diseases. The most alarming toxic heavy metals are copper (Cu), mercury (Hg), chromium (Cr), arsenic (As), lead (Pb), cadmium (Cd), and nickel (Ni) [1,4–7]. Therefore it is of utmost important to remove these toxic heavy metal ions from contaminated wastewater. For the removal of toxic heavy metals from contaminated wastewater, various conventional methods are available. These methods include membrane filtration, microbial system, coagulation, electrochemical process, chemical precipitation, ion-exchange, adsorption, photocatalytic degradation etc. [8,9]. Among these methods, adsorption is the most effective option due to its ease of operation, low operating cost, high efficiency, and low fouling problems.
*
In last few decades, many types of adsorbents such as activated carbon [10], silica gel [11], zeolite [12], clay minerals [13], and magnetic nanoparticles (MNPs) [14] have been explored for heavy metal removal. Among these adsorbents, MNPs with high surface to volume ratio and good colloidal stability results in superior adsorption kinetics for various metal ions in aqueous solutions. Further unique magnetic properties of MNPs can be explored for easy and fast separation of nanoadsorbent from the aqueous solution. The combination of adsorption and magnetic separation holds the advantage due to operational flexibility, recovery of heavy metals, and reusability of adsorbent MNPs. Several factors need to be considered to achieve efficient removal while using MNPs as an adsorbent. For colloidal stability and easy magnetic separation, nanoparticles should be superparamagnetic with higher magnetization [15]. To improve the heavy metal uptake capacity by MNPs, they need to be surface modified to provide specific functional groups or reaction sites that can be selective and specific for ions uptake. MNPs can be functionalized with some compounds, such as humic acid [4], meso-2,3-dimercaptosuccinic acid (DMSA) [16], (3aminopropyl)triethoxysilane [17,18], and chitosan [19]. Different
Corresponding author. Department of Physics, The New College, Shivaji University, Kolhapur, Maharashtra, 416012, India. E-mail address: [email protected] (P.B. Patil).
https://doi.org/10.1016/j.physb.2019.07.026 Received 6 April 2019; Received in revised form 29 June 2019; Accepted 14 July 2019 Available online 15 July 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
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functionalization strategies can be employed to provide functional groups like –COOH, –NH2, –OH, –SH etc. for heavy metal adsorption [14]. These strategies can have a significant influence on the performance of the adsorbents. In this work, DMSA functionalized MNPs (MNP-DMSA) were employed as a nanoadsorbent for the removal of Cu(II) ions from aqueous solution. The adsorption and kinetic behavior of Cu(II) on MNP-DMSA were thoroughly investigated.
310 K with 100 Oe applied magnetic field. The visible spectrophotometer visiscan-167 of Systronics India was used to perform batch adsorption experiments. 2.5. Batch adsorption experiment
All the chemicals used were of analytical grade and used without further purification. Ferric acetyl-acetonate (Fe(acac)3) and meso-2,3dimercaptosuccinic acid (DMSA) were procured from Sigma Aldrich. Ethylene glycol (C2H6O2), oleic acid, oleylamine, dimethyl sulfoxide (DMSO), hexane, and ethanol were purchased from S.D. fine-chem India. Copper sulphate pentahydrate (CuSo4.5H2O) and ethylenediamine-tetraacetic acid (EDTA) was purchased from Fisher Scientific.
The adsorption behavior of Cu(II) on MNP-DMSA was performed by batch experiments at room temperature. To study the effect of contact time on Cu(II) adsorption behavior of MNP-DMSA, 75 mg MNP-DMSA was mixed in 25 mL of 200 ppm Cu(II) solution. The solution was shaken in an incubator shaker at 150 rpm to reach adsorption equilibrium. The magnetic adsorbents were then separated from aqueous solutions by using an external magnet. Similar batch experiments were carried out, to study the effect of adsorbent dose (range: 2–7 g/L), pH (range: 3–8) and initial Cu(II) concentration (range: 50–300 ppm). The pH of the solution was adjusted by 0.1 M HCl and 0.1 M NaOH. The concentration of Cu(II) before and after adsorption was analysed spectrophotometrically at 716 nm using 0.1 M EDTA as a complexing agent [23]. The removal efficiency (E) and the amount adsorbed (q) of Cu(II) were calculated by the following equations:
2.2. Synthesis of MNPs
E (%) =
2. Materials and methods 2.1. Chemicals
The MNPs were synthesized by the solvothermal method used by Gao et al. with slight modification [20]. Typically in this process, the solution of ethylene glycol (20 ml) was mixed with 4 mM oleic acid and 10 mM oleylamine and stirred to form a homogeneous solution. 1 mM Fe(acac)3 was added slowly in the reaction mixture and stirred vigorously for 20 min. The constant flow of Argon was passed into the solution to deairinate the solution. The prepared solution was then autoclaved at 200 °C for 30 min and then at 260 °C for 8 h. The product was washed several times with hexane and anhydrous ethanol before vacuum drying.
q=
(C0
C0
Ce Co
× 100
Ce ) V M
(1) (2)
where C0 and Ce are the initial and equilibrium concentration of Cu(II) solution (mg/L), V is the volume of Cu(II) solution (L) and M is the mass of the adsorbent used (g). 3. Results and discussion 3.1. Characterization of MNP-DMSA The phase formation and crystallinity of as-synthesized MNPs were studied by XRD. Fig. 1 shows the XRD patterns of MNPs, MNP-DMSA, and Cu adsorbed MNP-DMSA. All the peaks in the pattern correspond to inverse spinel structure of pure magnetite phase (JCPDS card no. 85–1435). No other phase impurity was observed indicating high phase purity of the sample. The average crystallite size of about 19 nm was calculated by Debye-Scherrer formula. XRD patterns of functionalized or Cu adsorbed MNPs does not show any change in crystallinity when compared to bare MNPs. The size and shape of the MNPs were investigated by TEM and Fig. 2 shows the TEM micrograph of MNPs.
2.3. Functionalization of MNPs by DMSA To functionalize MNPs by DMSA, a standard ligand-exchange protocol was used to replace oleic acid and oleylamine moieties by DMSA [21,22]. MNPs (50 mg) were washed and centrifuged several times by a mixture of ethanol and hexane (1:1 ratio). After eliminating the supernatant, coagulated particles were re-dispersed in the 20 ml of toluene. To this, a solution of DMSA (90 mg) in DMSO (5 ml) was added and sonicated. The solution was then incubated at room temperature for 48 h in a mechanical shaker. After the reaction, the translucent solvent was discarded and the black particles (MNP-DMSA) were collected by ethanol washing. Particles were then washed several times by centrifuging in ethanol. The MNP-DMSA were dispersed in the doubled distilled water, basified at pH 10 and dialyzed against doubled distilled water for 24 h. MNP-DMSA particles were re-dispersed in the aqueous solution of pH 7. Finally, functionalized MNPs were magnetically decanted from the solution and dried in vacuum. 2.4. Characterization X-ray diffraction (XRD) measurements were carried out by using Rigaku Miniflex 600 X-ray diffractometer with Cu-Kα radiation (1.5406 Å) in θ/2θ mode. Transmission electron microscope (TEM), Tecnai G2 S Twin at voltage 200 kV was used for particle size and microstructural investigations. The Brunauer Emmett-Teller (BET) analysis was performed using Quantachrome Instruments v11.03. Fourier transform infrared (FTIR) spectra of all the samples were collected on FT/IR-4600 Jasco spectrometer using KBr pellet technique in the 4000-400 cm−1 range. Magnetic measurements were done with Quantum Design's VersaLab physical property measurement system. Magnetization loops up to 30 kOe fields were measured at 300 K and 60 K. Thermomagnetic measurements were taken between 50 K and
Fig. 1. XRD pattern of (a) MNPs, (b) MNP-DMSA, and (c) Cu adsorbed MNPDMSA. 274
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Fig. 4. FTIR spectra of (a) MNPs, (b) MNP-DMSA, and (c) Cu adsorbed MNPDMSA.
(b), the stretching vibrations at, 2332-2355 cm−1 corresponding to thiol groups and 1745 cm−1 corresponding to C]O bond reveals the presence of DMSA on the MNPs [28]. The absence of oleic acid and oleylamine vibrations in the MNP-DMSA spectra confirms the successful ligand exchange of DMSA on the surface of MNPs. As expected Cu adsorbed MNP-DMSA spectra does not show any absorption corresponding to presence of Cu as it is not IR active. Absorption peaks corresponding to other functional groups are unchanged indicating the stability of the functional group decorated on the surface of magnetic nanoparticles. The room temperature magnetization curves of MNPs, and MNPDMSA are shown in Fig. 5 (a). It can be seen that the MNPs saturation magnetization (Ms) is 72 emu/g. After the functionalization the magnetization is reduced to 57 emu/g. In our previous work, we have observed similar reduction in magnetization after functionalization of MNPs by chitosan [19]. Low field magnetization curve (Fig. 5(b)) of MNPs at 300 K shows complete reversible nature without any hysteresis. The absence of coercivity and remanence at 300 K indicates the superparamagnetic state of MNPs. At 60 K temperature, hysteresis loop with 200 Oe coercivity was observed. This is due to the transition from superparamagnetic to blocked ferromagnetic state. Magnetization measurement as a function of temperature under zero field cooling (ZFC) and field cooling (FC) protocols of the MNP-DMSA is shown in Fig. 5(c). The measurements were carried between 50 and 310 K with 100 Oe measuring field. In ZFC curve the unblocked increase in the magnetization of MNPs from 50 K to 310 K was observed. The bifurcation of ZFC-FC curves at 310 K reveals that all the MNPs are unblocked at this temperature. ZFC-FC curves of bare MNPs (not shown here) are similar to that of MNP-DMSA.
Fig. 2. TEM micrograph and the inset shows SAED pattern of Fe3O4 MNPs.
Fig. 3. Nitrogen adsorption-desorption isotherm with inset showing BJH pore size distribution of (a) MNPs and (b) MNP-DMSA.
MNPs have a roughly spherical shape with an average particle size 23 nm. The selected area electron diffraction (SAED) pattern of MNPs is shown in the inset of Fig. 2. The SAED pattern corroborates with the XRD analysis. Fig. 3 (a) and (b) shows the nitrogen adsorption-desorption isotherms of the bare MNPs and MNP-DMSA respectively measured by the BET technique. The inset shows pore radius distribution of the corresponding samples. For the bare MNPs average pore radius calculated by the Barrett-Joyner-Halenda (BJH) method was 5.7 nm and the BET surface area was 29.64 m2/g. This confirms the mesoporous structure of the MNPs. After the surface modification of the MNPs by DMSA, the average pore radius was increased to 7.8 nm and BET surface area was increased to 46.24 m2/g. The functionalization of DMSA on the MNPs was investigated by the FTIR spectroscopy. Fig. 4 shows the different bending and stretching vibrations for (a) MNPs, MNP-DMSA (b), and (c) Cu adsorbed MNPDMSA respectively. The sharp peak at 573 cm−1 in all the spectra corresponds to Fe–O stretching vibrations in Fe3O4. The broad peak at 3400 cm−1 is due to stretching vibrations of the –OH groups. In MNPs spectra, the bands at 2918 cm−1 and 2842 cm−1 are asymmetric CH2 and symmetric CH2 vibration bands respectively of oleic acid [24]. The chemisorption of oleic acid on the MNPs as carboxylate can be seen by the broad peak of single C–O stretching at 1050 cm−1 [25]. Moreover, the band in the region 1330-1470 cm−1 corresponds to NH2 bending mode of oleylamine [26]. The pure bands of oleic acid and oleylamine are not present in the spectra, which indicate the chemisorption of oleic acid and oleylamine on the MNPs [25,27]. In the spectra of MNP-DMSA
3.2. Adsorption studies 3.2.1. Effect of contact time Contact time is an important parameter in the wastewater treatment process. Fig. 6 shows the effect of contact time on the removal efficiency. It can be seen that the removal efficiency rapidly increased with contact time and reached 32% at 105 min. After this, the adsorption curve reached equilibrium. Initially, removal of Cu(II) ions was fast since the adsorbent sites were vacant and free from internal diffusion resistance. 3.2.2. Effect of adsorbent dose To study the effect of adsorbent dose, 2–10 g/L MNP-DMSA was 275
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Fig. 5. (a) Magnetization curves of MNPs (black) and MNP-DMSA (red) measured at room. temperature, (b) Low field magnetization curves of MNPs measured at 300 K (red) and 60 K(black), and (c) Zero field cooled (ZFC) and field cooled (FC) curves of MNP-DMSA.
availability of adsorption sites was increased. The maximum removal efficiency was found to be 79% for 8 g/L of adsorbent dose. Further increase in adsorbent dose, did not affect removal efficiency and the adsorption curve reached equilibrium. Therefore the adsorbent dose of 8 g/L was used for further experiments. 3.2.3. Effect of initial ion concentration The initial concentration has a remarkable effect on the removal of Cu(II) from aqueous solution. The percentage removal efficiency of Cu (II) was decreased with the increase in the initial concentration (Fig. 8). As the amount of adsorbent dose is fixed, the total available adsorption sites are limited. This leads to the decrease in removal efficiency with increasing initial concentration. However as can be seen from Fig. 8, the adsorption capacity of the Cu(II) ions was gradually increased with increasing initial Cu(II) concentration [29]. 3.2.4. Effect of pH The influence of the pH of aqueous solution on the adsorption of Cu (II) on MNP-DMSA is shown in Fig. 9. It is clear that with the increase in solution pH values from 2 to 6, the removal efficiency was increased from 75% to 99% and then decreased to 96% for pH 7. Above pH 7, the precipitate formation was started in Cu(II) solution. With the increase in the solution pH from 2 to 6, the effect of protonation becomes weaker, leading to increasing of the number of negative groups. The removal efficiency was increased at higher solution pH via electrostatic
Fig. 6. Effect of contact time on removal efficiency of Cu(II) by MNP-DMSA nanoadsorbents. (Concentration of Cu(II) = 200 ppm, Adsorbent dose 3 g/L).
Fig. 7. Effect of adsorbent dose on removal efficiency of Cu(II) by MNP-DMSA. nanoadsorbents (Contact Time = 105 min, Concentration of Cu (II) = 200 ppm).
added to 10 mL aqueous solution of Cu(II). It can be seen from Fig. 7 that the removal efficiency of Cu(II) was increased from 23 to 79% with increasing adsorbent dose. With the increase in MNP-DMSA, the
Fig. 8. Effect of initial concentration on removal efficiency of Cu(II) by MNPDMSA. nanoadsorbents (Contact Time = 105 min, adsorbent dose = 8 g/L). 276
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are listed in Table 1. For the first order kinetic model, the experimental and calculated values of qe are nearly equal. Also R2 for first-order kinetic model is 0.9888 which is higher than the R2 of second-order kinetic model. This suggests that the adsorption of Cu (II) follows the pseudo-first-order kinetic and reaction is more inclined towards physisorption. 3.4. Adsorption isotherms The Langmuir and Freundlich adsorption isotherm models were applied to experimental data to explore the surface properties and mechanism of adsorption. The Langmuir isotherm model can be expressed as [32].
Ce C 1 = e + qe qm KL qm
where Ce is the equilibrium concentration of Cu(II) (mg/L), qe is the adsorption capacity (mg/g), qm is the maximum capacity of adsorbent (mg/g) and KL is the Langmuir adsorption constant (L/mg). To determine the value of qm and KL, the experimental data were plotted as Ce/qe versus Ce as shown in Fig. 11 (a). The Freundlich isotherm model can be expressed as [33].
Fig. 9. Effect of pH on removal efficiency of Cu(II) by MNP-DMSA nanoadsorbents. (Contact Time = 105 min, Adsorbent dose = 8 g/L and Concentration of Cu(II) = 50 ppm).
adsorption between negatively charged surface and Cu(II). A similar increase in removal efficiency with increasing pH was observed to Huang et al. [29].
log q e = log KF +
To study the adsorption behavior of Cu(II) ions on MNP-DMSA, kinetic experiments were carried out. The pseudo-first-order and pseudo-second-order kinetic models were used to analyze the adsorption kinetics of Cu(II). The pseudo-first-order kinetic model was expressed as [30].
qt ) = log(qe )
k1 t 2.303
(3)
Pseudo-second-order kinetic model was expressed as [31].
t 1 1 = + t qt qe k2 qe2
1 log Ce n
(6)
where qe is the adsorption capacity (mg/g) and Ce is the equilibrium concentration of Cu(II) (mg/L). KF and n are Freundlich constant related to adsorption capacity (L/g) and heterogeneity factor respectively. The value of KF and n were calculated from the intercept and slope of log qe versus log Ce graph shown in Fig. 11 (b). The Langmuir and Freundlich isotherm constants are summarized in Table 1. From R2 it can be seen that the Langmuir isotherm model (R2 = 0.99) fits better to the experimental data than the Freundlich model (R2 = 0.9843). Also, the maximum absorption capacity calculated by the Langmuir model (25.44 mg/g) fits well with the experimental maximum adsorption capacity (24 mg/g). Thus it can be concluded that the monolayer Langmuir adsorption isotherm is more suitable to explain the adsorption behavior between MNP-DMSA and Cu(II) and adsorption takes place on the homogeneous surface. Obtained Langmuir adsorption capacity (25.44 mg/g) was comparable to the previous results for magnetic nanoadsorbents such as, γ -Fe2O3 (19.4 mg/g) [34], δ-FeOOH-coated γ -Fe2O3 (25.8 mg/g) [34], chitosan-bound Fe3O4 (21.5 mg/g) [35], and carboxymethyl chitosan–Fe3O4 (20.4 mg/g) [36] hexadiamine functionalized Fe3O4 (25.77 mg/g) [2]. Moreover, the obtained maximum capacity with MNP-DMSA is higher than reported values 12.43 mg/g and 10.41 mg/g for amino-functionalized polyacrylic acid coated Fe3O4 and silica coated Fe3O4 respectively [37,38].
3.3. Adsorption kinetics
log(qe
(5)
(4)
where qe and qt (mg/g) are the adsorption capacity at equilibrium and time t (min), respectively, k1 (min−1) and k2 (g/mg/min) are the rate constants for pseudo-first-order and pseudo-second-order adsorption models respectively. The plot of log (qe-qt) against t is shown in Fig. 10 (a). The slope and intercept of the graph are used to determine the values of k1 and qe respectively. The plot of t/qt against t is shown in Fig. 10 (b). The slope and intercept of the graph are used to determine the values of the values of qe and k2 respectively. The values of kinetic parameters obtained from Eq. (3) and Eq. (4)
4. Conclusions DMSA functionalized MNPs were prepared, characterized and employed as a magnetic nanoadsorbent for the removal of toxic heavy metal Cu(II) from aqueous solution. XRD and TEM studies of MNPs showed pure magnetite phase with 23 nm average particle size. Magnetic measurements revealed that MNPs are in the superparamagnetic state at room temperature. As prepared MNPs synthesized by solvothermal method were hydrophobic due to the coating of oleic acid and oleylamine surfactants. The ligand-exchange protocol was successfully employed to replace the hydrophobic oleic acid and oleylamine moieties from the surface of MNPs by DMSA. The effective ligand-exchange of DMSA on the surface of MNPs was confirmed by FTIR. The kinetic studies indicated that the adsorption reaction follows the pseudo-first-order kinetic model and adsorption was a physical process. The Adsorption isotherm study showed that the Langmuir model fits better than the Freundlich model with the experimental data,
Fig. 10. (a) Pseudo-first-order kinetic model and, (b) Pseudo-second-order kinetic model for. the adsorption of Cu(II) on MNP-DMSA nanoadsorbents. 277
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Table 1 Parameters of kinetic models and adsorption isotherm models for the adsorption of Cu(II) onto MNP-DMSA nanoadsorbents. Parameters of kinetic models for adsorption of Cu(II)
Parameters of adsorption isotherm model for the adsorption between of Cu(II) 2
Kinetic Model
qe (Exp.) (mg/ g)
qe (Cal.) (mg/ g)
Rate constants
R
Pseudo-first-order Pseudo-secondorder
24 24
23.48 43.47
K1 = 0.01893/min K2 = 0.00020 g/mg/min
0.9888 0.9316
Langmuir model qm(mg/g) 25.44
KL(L/mg) 0.1043
Freundlich model R2 0.99
KF (L/mg) 6.77
n 3.6812
R2 0.9843
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Fig. 11. (a) Langmuir isotherm model and, (b) Freundlich isotherm model for Cu(II) adsorption on MNP-DMSA nanoadsorbents.
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