Fabrication of high performance amine-rich magnetic composite fibers for the recovery of precious Pt(IV) from acidic solutions

Abstract

Magnetic nanoparticles (MNPs) possessing a high surface to volume ratio, copious chemically active sites, and ease of separation from aqueous solutions are emerging materials for water treatment. Further encapsulation of these nanoparticles (NPs) with polymeric materials may protect these NPs from direct contact with the aqueous environment and also enhance their sorption efficiency. In the case of polymer fibers, breakage will occur depending on environmental conditions, and the addition of MNPs in fibers will provide an opportunity for the complete recovery of fibers after the sorption process. In the present study, a mixed solution of amine-rich chitosan (CS) and polyethyleneimine (PEI) containing magnetic MnFe₂O₄ nanoparticles was utilized for fabricating versatile and robust magnetic polymer composite fibers (MPCFs) in a facile methodology. The effective fabrication of MPCFs was confirmed by using analytical techniques such as FTIR, XRD, VSM, FE-SEM, and TEM. Morphological characterization demonstrated that MnFe₂O₄ nanoparticles were well distributed in the composite fibers. Detailed batch sorption experiments revealed that MPCFs exhibited significant improvement in adsorption efficiency compared with bare MnFe₂O₄ nanoparticles. The MPCFs exhibited high adsorption capacity (371.35 ± 16.79 mg g⁻¹) and fast equilibrium (within 30 min). The Pt-loaded MPCFs were easily separated from aqueous solution under an external magnetic field. It can be concluded that MPCFs with amine-rich functional groups and magnetic properties are promising for Pt adsorption from aqueous solutions.

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Introduction

In recent years, the use of nanotechnology (NT) and nanoparticles (NPs) to resolve environmental difficulties has swiftly progressed, due to their unique size and physicochemical properties.¹ In particular, owing to their high surface-to-volume ratio and plentiful active adsorption sites, application of NPs for the treatment of polluted waters has garnered major studies.² However, the fate of engineered nanoparticles in aqueous environments has become a challenge because of the difficulties of separation after the treatment process. Thus it is essential to realize techniques that are appropriate to separate nanoparticles from the treated samples.

Toward this end, magnetic nanoparticles (MNPs) have attracted wide attention due to their magnetic field supported separation.^3,4 Magnetic separation allows the addition of a magnetic component which could advance the separation efficiencies of traditional water treatment techniques such as adsorption, catalytic, and membrane processes. Rapid, facile, and economic separation of magnetic carriers from the aqueous environment without centrifugation or filtration makes MNPs more promising than other nano adsorbents for industrial applications.

Although magnetic nanoparticles possess several advantages besides their easy separation, they have disadvantages such as agglomeration, sensitivity to oxidation, and ambiguous behaviors in aqueous environments.^5,6 Considering the unidentified health and ecological risks of MNPs, preparation of magnetic composites by means of various materials (carbon, polymers, and clays) has become a viable option.^7,8 Among the various hybrid materials of this type, magnetic polymer composites (MPCs), a unique new class of functional materials where magnetic nanoparticles are embedded in polymer matrixes, are attracting the attention of both academics and industry.^9–11 Hence, they became auspicious materials in place of traditional adsorbents owing to their potential advantages of high sorption efficiency, ease of separation, low toxicity, and ability for regeneration and re-use.^9,12 The polymer matrix not only protects the direct reactivity of magnetic nanoparticles with the aquatic environment but also provides large numbers of chelating functional groups.

Adsorbents have been constructed with different structures such as beads, fibers, powder, and mats,^13–16 however, fiber type adsorbents have shown high efficiency in water treatment.^17,18 Compared with other sorbent structure materials, fiber type sorbents have a larger specific surface area, high adsorption rates, and elution efficiency. Although fiber adsorbents display beneficial properties, several drawbacks limit their widespread application. Depending on the environmental conditions, breakage of fiber material is possible and resulting fiber debris in the aqueous solution causes subsequent secondary pollution.¹⁹ Thus full recovery of the fiber adsorbent after the treatment process is essential; this can be accomplished by inducing magnetic properties in the fiber adsorbents. Fiber sorbents prepared from natural polysaccharide materials are more suitable for water treatment applications, due to their environmentally friendly nature, biocompatibility, and biodegradability characteristics. Chitosan (CS), a linear cationic amino polysaccharide (2-amino-2 deoxy-D-glucose) having amino and hydroxyl groups, is the second most abundant biopolymer on the Earth and is of particular interested for fabricating magnetic composite materials.⁹ To further improve the chelating properties of CS, chemical modification with various chelating materials is required.²⁰ Polyethyleneimine (PEI), is a cationic polymer and well known for its metal chelating property. It has received tremendous attention as a versatile chemical to modify adsorbents due to the presence of an enormous number of amine groups, which enhances the sorption capacity.^12,21

Various magnetic nanoparticles have been utilized to fabricate magnetic polymer composites; iron oxides (such Fe₃O₄, and γ-Fe₂O₃ among others) are the most frequently used for this purpose. However, several disadvantages such as chelating properties with complex ions in aqueous environments, low oxidation stability, less magnetic responsiveness, and acid corrosion considerably inhibits their widespread usage.²² Recently, ferrites with spinel structure MFe₂O₄ (where M = Co, Cu, Ni, Mn, and Zn) nanoparticles have gained wide interest, due to their chemical stability, mechanical hardness, and high saturation magnetization characteristics.²³ Among spinel ferrites, MnFe₂O₄ nanoparticles, in particular, are of interest for fabricating various composites due to their high magnetic susceptibility and eco-friendly properties.^24,25 Thus in the present study, MnFe₂O₄ was used as a magnetic material to fabricate magnetic polymer fiber composites.

Platinum group elements (PGEs: platinum, palladium, ruthenium and so on) are extremely significant precious metal ions that have extensive industrial applications, owing to their definite physical and chemical characteristics. Among which, platinum (Pt), a vital and less abundant natural resource in the Earth's crust, is a strategic raw material for industries in several nations around the world.²⁶ Pt has been extensively used in various applications; after its usage, some of the Pt are released and bound in the environment.²⁷ The presence of Pt in aqueous environments causes various health associated problem and is becoming an important ecological issue.²⁸ Thus, the recovery of precious Pt from aqueous and waste solutions is of serious concern in order to satisfy the increasing demand for this metal and control the health effects associated with the presence of excess Pt in water. This has triggered extensive research on the development of novel high-efficiency adsorbents for Pt from aqueous and waste solutions.

In the present study, we have fabricated MPCFs by using CS, PEI, and magnetic MnFe₂O₄ particles in a facile approach. The study shows that (i) the magnetic properties of fabricated MPCFs can be useful for facile separation, (ii) the fiber morphology provides a large external surface area, and (iii) PEI and CS provides numerous amino groups useful as bindings sites for adsorption.

Experimental

Materials

Commercial grade PEI (average M_w: ∼750 000 by light scattering, concentration: 50 wt% in H₂O) and tripolyphosphate (85.0%) were purchased from Sigma-Aldrich (USA). CS (viscosity at 20 °C: 200–220 cP, deacetylation degree: 85.9%) was purchased from YBBio Co., Ltd. (Korea). Glutaraldehyde (GA) of 25 wt% and acetic acid (>99.7%) were obtained from Junsei Chemical Co., Ltd. (Japan). Manganese chloride tetrahydrate (MnCl₂·4H₂O, ≥99.0%) and iron chloride hexahydrate (FeCl₃·6H₂O, ≥98%) were purchased from Sigma-Aldrich. H₂PtCl₆·nH₂O (n = 5.5) was purchased from Kojima Chemicals Co., Ltd. (Japan). All of the other material used in this study were also of analytical grade and used without further treatment.

Fabrication of composite fibers

Preparation of MnFe₂O₄ nanoparticles. Magnetic jacobsite (MnFe₂O₄) nanoparticles were synthesized via the chemical coprecipitation process by combining the appropriate amount of Mn(II) and Fe(III) salts under alkaline conditions. The synthesis process is described by eqn (1)

MnCl₂ + 2FeCl₃ + 8NaOH → MnFe₂O₄(s) + 8NaCl + 4H₂O (1)

In a typical synthesis, 0.01 mol of MnCl₂·4H₂O and 0.02 mol of FeCl₃·6H₂O have been fully dissolved in deoxygenated distilled water under vigorous mechanical stirring at 670 rpm at a constant speed. Then the sodium hydroxide (2 M) solution was added dropwise while stirring until the pH value reached 11. The resultant mixture was then heated to 100 °C and kept at this temperature for 2 h. The obtained black precipitate was separated by an external magnetic field and was washed four times with double distilled water until free from Na⁺, OH⁻, and Cl⁻ ions. Finally, the product was washed once with ethanol and then freeze-dried to obtain pure jacobsite nanoparticles.²⁹

Preparation of magnetic polymer composite fibers (MPCFs). Magnetic polymer composite fibers were fabricated as follows. First CS solution was prepared by dissolving 1.2 g of CS powder into 40 mL 2% (v/v) acetic acid and this solution was stirred for 24 h at room temperature. After stirring, it was filtered through a sieve (aperture: 250 μm) to obtain homogeneous CS solution. PEI (0.8 mL) was dispersed in 40 mL of the filtered CS solution with strong agitation (at 500 rpm) for 3 h at room temperature. Then, 0.1 g of MnFe₂O₄ nanoparticles were dispersed in one mL of distilled water under sonication. The resulting solution was added to the PEI–CS solution and was mechanically stirred overnight. After that, the mixture solution was spun through a stainless steel needle (internal diameter: 200 μm) into 1 L of 2% TPP solution and the fibers were kept for 2 h to coagulate. The resulting magnetic composite fibers were then cross-linked with 0.6% (v/v) of GA solution for 2 h in order to fix PEI onto the CS matrix and then 1 M H₂SO₄ was added to the cross-linking solution until pH 3.9 ± 0.3 was reached to enhance the strength of magnetic composite fibers. After 30 min, the solidified magnetic composite fibers were washed several times with distilled water to remove the presence of excess GA and sulfuric acid. Finally, the fibers were freeze-dried and used for the adsorption experiments. The fibers are labeled as MPCFs. The complete fabrication procedure used to manufacture MPCFs was schemed in Fig. 1.

Fig. 1 The schematic view of the MPCFs fabrication process.

Pt(IV) adsorption experiments

The adsorption characteristics of MnFe₂O₄ and MPCFs for Pt were studied at room temperature in batch adsorption experiments. The stock Pt(IV) solution was prepared by dissolving H₂PtCl₆·nH₂O (n = 5.5) in 0.1 M HCl solution to a concentration of 1000 mg L⁻¹. Desired standard solutions were obtained by diluting the stock solution. Kinetic experiments were conducted to determine the time to reach equilibrium as follows: 30 mg of MnFe₂O₄ and MPCFs were added into 50 mL Teflon tubes containing 30 mL of 100 ppm Pt(IV) solution and the samples were collected at specific time intervals from 0 to 240 min. To obtain the adsorption isotherm of Pt(IV), isothermal experiments were performed with different initial concentrations of Pt(IV) (0–1000 mg L⁻¹) using 30 mg of the adsorbents mass in a volume of 30 mL. All batch experiments were executed at least twice. The concentration of Pt(IV) before and after equilibrium was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES; ICPS-7500, Shimadzu, Japan). The pH of the solution was not controlled during the adsorption process, but it remained around pH 1.2. The amount of Pt(IV) adsorbed on adsorbents was calculated by using the following equation:where V is the sample volume in L, C_o and C_e represent the concentration of Pt(IV) (mg L⁻¹) before and after the adsorption process, respectively ‘m’ is the weight of the adsorbent (g), and q_e is the platinum uptake capacity at equilibrium (mg g⁻¹).

Instrumental information

The specific functional groups of the samples were analyzed by Fourier transform infrared spectrometer (FTIR, GX Perkin-Elmer, USA). The data was recorded by using samples as KBr pellets in wave number range of 4000–400 cm⁻¹. The magnetic properties of the samples were measured by using a vibrating sample magnetometer (VSM, Lakeshore 7404, USA). The morphologies and elemental information of the samples were characterized using a field emission scanning electron microscope (FE-SEM) coupled with an energy dispersive X-ray spectroscope (EDX) (SUPRA 40VP, Carl Zeiss, Germany). The chemical composition and crystalline nature of the samples were obtained by using X-ray diffraction (XRD) (XD2711N, Rigaku, Japan). Nanoparticles present in the products were analyzed by using standard and high-resolution transmission electron microscopy (TEM and HR-TEM) (JEM2010, JEOL, Japan).

Results and discussion

Physicochemical characterization

XRD patterns of the MnFe₂O₄ and MPCFs are shown in Fig. 2. Both materials showed characteristic peaks of MnFe₂O₄ corresponding to the lattice planes of (111), (220), (311), (222), (400), (422), (511), and (440). All the diffraction peaks corresponded with standard MnFe₂O₄ (JCPDS file no: 88-1965) for the reflection of manganese ferrite with spinel structure (space group: Fd3m 227).³⁰ Similar diffraction peaks for MnFe₂O₄ were reported earlier in the literature.³¹ The average primary crystal size of MnFe₂O₄ particles was about 7.65 nm, calculated using the Scherrer formula based on the strongest diffraction peak 311. The XRD patterns of MPCFs are similar to MnFe₂O₄; however, owing to its low MnFe₂O₄ proportion decreased peak intensity was observed. Also, a peak at 2θ = 20° was also seen in the XRD patterns of the MPCFs (Fig. 2), which can be allotted to the typical peak of CS.

Fig. 2 XRD patterns of MnFe₂O₄ and MPCFs.

The FTIR spectra of MnFe₂O₄ and MPCFs are shown in Fig. 3. MnFe₂O₄ exhibited two absorption bands at 3410 cm⁻¹ and 1630 cm⁻¹, corresponding, respectively, to the stretching and bending vibrations of –OH groups of adsorbed water.³² FTIR spectrum of MnFe₂O₄ exhibited absorption peak at 573 cm⁻¹; this could be assigned to the Fe–O stretching vibration of manganese ferrite.³³ In the FTIR spectrum of MPCFs, the broad band observed at 3450–3200 cm⁻¹, corresponds to the absorption of ν(O–H) and ν(N–H) groups.³² The peaks observed at 2932 and 2872 cm⁻¹ were due to the ν(C–H) asymmetric and symmetric stretching vibrations.³⁴ The strong peaks observed at 2932 and 2872 cm⁻¹ confirm the presence of high amount of CH₂ in MPCFs.³⁵ The bands appearing at 1659 cm⁻¹ were attributed to the imine group.³⁶ The peaks at 1376 and 1450 cm⁻¹ correspond to the –C–O stretching vibration of the primary alcoholic group in chitosan.^37,38 The peak observed at 1070 cm⁻¹ may be attributed to the ν(C=O) of the amide group.³⁹ Further, the peak at 560 cm⁻¹ corresponds to the Fe–O vibration. These results further confirmed the successful fabrication of MPCFs composite.

Fig. 3 FTIR spectra of MnFe₂O₄ and MPCFs.

The morphologies of the MnFe₂O₄ and MPCFs were characterized by using FE-SEM combined with EDX mapping and also by TEM. The FE-SEM image shown in Fig. 4 demonstrates that MnFe₂O₄ particles exhibited spherical shape structures and that the particles are slightly agglomerated. To further investigate the chemical composition of MnFe₂O₄, EDX analysis, and elemental mapping were performed. The EDX spectrum of MnFe₂O₄, shown in Fig. S1† confirms the presence of Mn, Fe, and O and validates the purity of the jacobsite; the results are in good agreement with the FTIR and XRD analysis. Elemental distribution on the jacobsite was further characterized by elemental mapping, which shows different color images corresponding to Mn, Fe, and O-enriched areas of the jacobsite (Fig. 4). It is notable that the Mn and Fe were finely distributed on the surface of jacobsite. To further investigate the nanosized particles of jacobsite TEM measurement was made; it was observed that small aggregates were formed due to the mutual magnetic interactions of MnFe₂O₄ nanoparticles (Fig. 4). The selected-area electron diffraction (SAED) pattern was further recorded and is shown in Fig. 4.

Fig. 4 SEM (a), TEM and SAED (insert figure) (b), and elemental mapping Mn (c), Fe (d), and O (e) images of MnFe₂O₄.

Further FE-SEM analysis was used to observe the surface morphology and the distribution of MnFe₂O₄ particles within the composite fibers (MPCFs). Fig. 5 shows the FE-SEM image of the composite fiber, which exhibited homogeneous morphology with the smooth exterior surface. However, a high magnification of composite fibers showed the presence of spherical shaped jacobsite agglomerate particles on the surface and also embedded in the internal composite fiber structure. Further SEM images of fiber periphery were taken and observed porous structure with interconnected porous channels with clusters of MnFe₂O₄ particles attached to them. This porous morphology enhances the sorption efficiency and sorption rate. EDX elemental mapping confirmed the composition of MPCFs, which shows the existence of Mn, Fe, P, O, N, S, and C (Fig. S2†). TEM measurements were further used to examine the MnFe₂O₄ nanoparticles in the composite fibers and are shown in Fig. 5. According to the TEM analysis, the MPCFs contains well separated MnFe₂O₄ particles wrapped by CS–PEI polymer with particles of various sizes dispersed in the polymer shell. HR-TEM image of MPCFs showed one individual MnFe₂O₄ particle clearly surrounded by the CS–PEI polymer shell.

Fig. 5 SEM (a and b) and TEM (c and d) of MPCFs.

The specific magnetization curves of both MnFe₂O₄ and composite samples were obtained at room temperature with the applied magnetic field sweeping from −15 to 15 kOe using VSM. The magnetization curves of MnFe₂O₄ are shown in Fig. S3† with a good specific saturation magnetization (Ms) value of 27.05 emu g⁻¹. The magnetic hysteresis loop of the MPCFs is shown in Fig. 6, and the saturation magnetization of the MPCFs was estimated to be 1.42 emu g⁻¹. The M_s value of the composites was lower than that of MnFe₂O₄ particles, due to the existence of non-magnetic PEI–CS. The decrease in saturation magnetization of composite materials was previously reported in various studies.^40–42 For example, in a recent study Ag–CoFe₂O₄–GO nanoparticles exhibited 0.37 emu g⁻¹, although a lower saturation magnetization was observed the composite was rapidly separated from the aqueous solution after adsorption by using magnetic assistance.⁴³ The magnetic separability experiment of the composite showed (Fig. 6 insert photograph) that the adsorbent could be separated easily from solution in a short period by applying an external magnetic field. The rapid and easy separation of MPCFs from the aqueous solution after the sorption process was necessary for not only recycling the adsorbent moreover avoiding subsequent sludge generation and ecological hazards. These results suggest that MPCFs could be potentially used as a magnetic adsorbent to remove contaminants.

Fig. 6 Magnetic hysteresis curves of MPCFs. Insert photograph shows the platinum solution before (left) and after (right) addition of MPCFs and application of magnetic field.

Adsorption isotherm

In order to design efficient practical adsorption systems, it is important to investigate the equilibrium data by using diverse sorption isotherm models. Adsorption behavior of Pt(IV) on MNPs and MPCFs has been studied by carrying sorption isotherm experiments at room temperature by varying the initial concentration of Pt(IV) from 0–1000 mg L⁻¹. The contact time of 24 h was used to ensure the adsorption equilibrium. Non-linear regression of two-well known isotherm models, Langmuir and Freundlich, were used to investigate the sorbate–sorbent interactions; the resulting isotherm curves are shown in Fig. 7. The calculated isotherm parameters and their correlation coefficients along with their non-linear equation used for modeling are presented in Table 1. The amount of Pt(IV) adsorbed by the MPCFs was increased gradually and finally reached a plateau with an increase in equilibrium Pt(IV) concentration. The generated curves confirm that the adsorption data was better described by the Langmuir model the entire concentration range. Furthermore, a high correlation coefficient (R² > 0.97) was observed for the Langmuir isotherm model, while the correlation coefficient of Freundlich isotherm was low (0.877). These results further confirm that adsorption data was better fitted to the Langmuir model than the Freundlich model, suggesting that the Pt(IV) adsorbed form monolayer coverage on the MPCFs. The maximum adsorption capacity (q_max) calculated from the non-linear simulation was found to be 371.35 ± 16.79 mg g⁻¹. To evaluate the affinity between platinum and MPCFs the dimensionless constant generally known as separation factor R_L was calculated, which induced from Langmuir equation.where b is the Langmuir adsorption constant and C_o is the initial Pt(IV) concentration. Based on the equation, the values of R_L were calculated and found that 0 < R_L < 1, suggesting favorable adsorption of platinum onto MPCFs.⁴⁴ The adsorption capacity of jacobsite was rather lower than MPCFs; in addition, a higher b value of the MPCFs compared to jacobsite confirms the strong bonding of Pt(IV). The magnitude of K_F and n values of the Freundlich isotherm model suggests the high adsorptive capacity and favorable adsorption. The n value of Freundlich model was more than 1 (5.47 for MPCFs), signifying that Pt(IV) adsorption on MPCFs was a favorable adsorption process. These results suggest that the presence of MnFe₂O₄ in the composite is accountable for the magnetic separability, while PEI–CS are the main cause of the high adsorption capacity.

Fig. 7 Adsorption isotherm of Pt onto MnFe₂O₄ and MPCFs.

Table 1 Adsorption isotherm parameters for platinum adsorption on MnFe₂O₄ and MPCFs

Isotherm	Expression formula	Parameters	MCPFs	MnFe2O4
Langmuir		qmax (mg g^−1)	371.35 ± 16.79	10.80 ± 0.13
		b (L mg^−1)	0.251 ± 0.055	0.018 ± 0.001
		R^2	0.975	1.000
Freundlich	qe = KFCe^1/n	KF (mg g^−1) (L g^−1)^1/n	122.4 ± 34.1	2.0 ± 0.4
		n	5.47 ± 1.50	3.84 ± 0.57
		R^2	0.877	0.987

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Adsorption kinetics

Equilibrium contact time is one of the vital parameters for industrial wastewater treatment applications. In order to assess the rate of Pt(IV) recovery from the acidic solution by the MnFe₂O₄ and MPCFs, sorption kinetic tests were performed. The adsorption capacity of both sorbents for Pt(IV) as a function of contact time was studied. The sorption kinetic experiments were conducted for 240 min to make sure the complete equilibrium was attained. As shown in Fig. 8, both sorbents exhibited rapid Pt(IV) uptake with short equilibrium time; 10 min for MnFe₂O₄ and 30 min for MPCFs. The extreme rapid adsorption process may be due to the instantaneous monolayer adsorption of Pt(IV) on the surface of adsorbents, which is in good agreement with the Langmuir isotherm results. Although rapid equilibrium was observed, MPCFs exhibited significant higher adsorption efficiency than MnFe₂O₄. The relatively rapid and efficient adsorption process was because of the abundant active amino sites on the MPCFs; preferentially rapid Pt(IV) transportation from solution onto the surface of adsorbent was due to large electrostatic attractions. In order to better understand the kinetic behavior of both adsorbents, pseudo-first order (PFO)⁴⁵ and pseudo-second order (PSO)^46,47 rate models were employed to interpret the experimental adsorption data. From these two models, the kinetic parameters, including the kinetic constants, correlation coefficient, and q_e values were obtained by non-linear regression analysis and are summarized in Table 2. The validity of the models was examined by correlation coefficient values besides the experimental and calculated data. In the case of MPCFs, the calculated q_e values were close to the experimental q_e values of both PFO and PSO models. In addition, high correlation coefficient values were observed for the both models.

Fig. 8 Adsorption kinetics of Pt onto MnFe₂O₄ and MPCFs.

Table 2 Kinetic model parameters for platinum adsorption onto the MnFe₂O₄ and MPCFs

Model	Expression formula	Parameters	MCPFs	MnFe2O4
PFO	qt = q1(1 − exp(−k1t))	qe,exp (mg g^−1)	97.5	11.3
		q1 (mg g^−1)	97.41 ± 0.05	9.25 ± 0.30
		k1 (L min^−1)	0.311 ± 0.003	0.215 ± 0.084
		R^2	1.000	0.929
PSO		q2 (mg g^−1)	97.78 ± 0.16	9.43 ± 0.33
		k2 (g mg^−1 min^−1)	0.022 ± 0.002	0.058 ± 0.041
		R^2	1.000	0.938

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Performance evaluation

Platinum adsorption has been studied by means of diverse adsorbent materials. The maximum adsorption capacity of MPCFs was compared with those of other platinum adsorbents reported in the literature, to evaluate its sorption performance. Langmuir maximum adsorption capacity (q_max) values were used for the comparison and are summarized in Table 3. Materials of various components with a variety of shapes were used for platinum recovery, whereas magnetic materials were rarely reported in the literature. A high adsorption capacity 371.35 ± 16.79 mg g⁻¹ was observed for MPCFs compared with the available reports; this high sorption efficiency might be attributed to a large number of amino groups of CS and PEI besides its fibrous structure. In addition, the comparison also revealed that composite fibers exhibited more rapid equilibrium (within 30 min) than individual adsorbents. Although a direct comparison between the MPCFs and other reported adsorbents is not pertinent, owing to the various experiments condition used in each study. However, in view of magnetic properties, facile synthesis in large scale with high sorption efficiency with rapid kinetics, as fabricated MPCFs adsorbent in the present study, was promising for treating platinum-containing wastewaters compared to the reported adsorbents.

Table 3 Comparative evaluation of maximum platinum adsorption capacities of different adsorbents

Adsorbent	Adsorbent type	Equilibrium time (min)	qmax (mg g^−1)	Reference
GMCCR	Resin	300	122.5	51
Thiourea-modified chitosan	Microspheres	300	130	52
LMCCR	Resin	120	129	53
EMCN	Particles	<60	171	54
BTICF	Fiber membrane	60	41.7	55
PS-ATD	Resin	900	222	56
MnFe2O4	Particles	10	10.8	Present study
MPCFs	Fiber	30	371	Present study

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Adsorption mechanism

According to the reports, electrostatic attraction between the chloride anionic platinum and positively charged amino group sorbents was the principal adsorption mechanism. The adsorption mechanism of Pt(IV) onto MPCFs was investigated by using elemental mapping and FTIR analysis. A detailed chemical analysis was carried using EDX mapping to study the Pt distribution on MPCFs. The Pt elemental mapping in Fig. 9 shows that Pt signals are found throughout the composite fiber. The EDX spectrum of the Pt-loaded MPCFs shown in ESI Fig. S4† further confirmed the presence of Pt element. From the FTIR spectrum, it was observed that after sorption the peak observed around 3420 cm⁻¹ attributed to the amine groups was shifted to 3436 cm⁻¹ and the peak intensity was decreased, this may be due to the binding of Pt with amine groups of MPCFs (Fig. S5†). The results confirm the successful binding of anionic chloride Pt via electrostatic interaction with MPCFs. Several other researchers observed similar mechanisms that involved between the anionic metal ions and amine group containing adsorbents.^{17,35,48–50}

Fig. 9 Pt elemental mapping on MPCFs after adsorption.

Conclusions

In summary, MnFe₂O₄ nanoparticles and MPCFs have been fabricated and systematic characterization of the materials was carried out. SEM and TEM observations showed that the composite fibers have a smooth surface and good dispersion of the magnetic nanoparticles in composite fibers. XRD pattern confirms the spinel structure of MnFe₂O₄. Pt(IV) recovery from acidic solution was studied in batch sorption experiments using both adsorbents. The isothermal adsorption of Pt(IV) was better described by the Langmuir isotherm model than the Freundlich model. The maximum monolayer adsorption capacity q_max based on Langmuir adsorption isotherm model was found to be 371.35 ± 16.79 mg Pt per g. Therefore, the magnetic property, high adsorption capacity, and rapid adsorption kinetics make the composite fibers promising candidates for Pt(IV) recovery from acidic solutions.

Acknowledgements

The authors would like to acknowledge the financial support by the Korean Government through NRF-2014R1A2A1A09007378 grant.

References

1. F. Brandl, N. Bertrand, E. M. Lima and R. Langer, Nat. Commun., 2015, 6, 7765.
2. S. J. Tesh and T. B. Scott, Adv. Mater., 2014, 26, 6056–6068.
3. J. Gómez-Pastora, E. Bringas and I. Ortiz, Chem. Eng. J., 2014, 256, 187–204.
4. Y. Teng, L. X. Song, W. Liu, L. Zhao, J. Xia, Q. S. Wang, M. M. Ruan, Z. Yang and Y. X. Qian, Dalton Trans., 2016, 45, 9704–9711.
5. A. H. Lu, E. L. Salabas and F. Schuth, Angew. Chemie., 2007, 46, 1222–1244.
6. S. C. N. Tang and I. M. C. Lo, Water Res., 2013, 47, 2613–2632.
7. Q. Liu, J. Zhu, L. Tan, X. Jing, J. Liu, D. Song, H. Zhang, R. Li, G. A. Emelchenko and J. Wang, Dalton Trans., 2016, 45, 9166–9173.
8. L. H. Reddy, J. L. Arias, J. Nicolas and P. Couvreur, Chem. Rev., 2012, 112, 5818–5878.
9. D. H. K. Reddy and S.-M. Lee, Adv. Colloid Interface Sci., 2013, 201–202, 68–93.
10. O. Philippova, A. Barabanova, V. Molchanov and A. Khokhlov, Eur. Polym. J., 2011, 47, 542–559.
11. F. Llanes, C. Diaz, H. Ryan and R. H. Marchessault, Int. J. Polym. Mater., 2002, 51, 537–545.
12. S. Kalia, S. Kango, A. Kumar, Y. Haldorai, B. Kumari and R. Kumar, Colloid Polym. Sci., 2014, 292, 2025–2052.
13. Q. Liu, L.-B. Zhong, Q.-B. Zhao, C. Frear and Y.-M. Zheng, ACS Appl. Mater. Interfaces, 2015, 7, 14573–14583.
14. S. Nasirimoghaddam, S. Zeinali and S. Sabbaghi, J. Ind. Eng. Chem., 2015, 27, 79–87.
15. S. B. Hammouda, N. Adhoum and L. Monser, J. Hazard. Mater., 2015, 294, 128–136.
16. S. Wang, C. Wang, B. Zhang, Z. Sun, Z. Li, X. Jiang and X. Bai, Mater. Lett., 2010, 64, 9–11.
17. S. W. Won, I. S. Kwak and Y.-S. Yun, Bioresour. Technol., 2014, 160, 93–97.
18. S. W. Won, S. Kim, P. Kotte, A. Lim and Y.-S. Yun, J. Hazard. Mater., 2013, 263, 391–397.
19. Z. Jiang, L. D. Tijing, A. Amarjargal, C. H. Park, K.-J. An, H. K. Shon and C. S. Kim, Composites, Part B, 2015, 77, 311–318.
20. D. H. K. Reddy and S.-M. Lee, J. Appl. Polym. Sci., 2013, 130, 4542–4550.
21. C. Yin, M. Aroua and W. Daud, Water, Air, Soil Pollut., 2008, 192, 337–348.
22. M. Bagheri, M. A. Bahrevar and A. Beitollahi, Ceram. Int., 2015, 41, 11618–11624.
23. W.-H. Xu, L. Wang, J. Wang, G.-P. Sheng, J.-H. Liu, H.-Q. Yu and X.-J. Huang, CrystEngComm, 2013, 15, 7895–7903.
24. R. H. Vignesh, K. V. Sankar, S. Amaresh, Y. S. Lee and R. K. Selvan, Sens. Actuators, B, 2015, 220, 50–58.
25. L. Yang, Y. Zhang, X. Liu, X. Jiang, Z. Zhang, T. Zhang and L. Zhang, Chem. Eng. J., 2014, 246, 88–96.
26. F. Reith, S. G. Campbell, A. S. Ball, A. Pring and G. Southam, Earth-Sci. Rev., 2014, 131, 1–21.
27. A. Turner, M. Crussell, G. E. Millward, A. Cobelo-Garcia and A. S. Fisher, Environ. Sci. Technol., 2006, 40, 1524–1531.
28. L. Bencs, K. Ravindra and R. Van Grieken, in Encyclopedia of Environmental Health, ed. J. O. Nriagu, Elsevier, Burlington, 2011, pp. 580–595,  DOI:10.1016/b978-0-444-52272-6.00602-4.
29. C. Chinnasamy, A. Yang, S. Yoon, K. Hsu, M. Shultz, E. Carpenter, S. Mukerjee, C. Vittoria and V. Harris, J. Appl. Phys., 2007, 101, 09M509.
30. L. Shao, Z. Ren, G. Zhang and L. Chen, Mater. Chem. Phys., 2012, 135, 16–24.
31. S. Kumar, R. R. Nair, P. B. Pillai, S. N. Gupta, M. A. Iyengar and A. K. Sood, ACS Appl. Mater. Interfaces, 2014, 6, 17426–17436.
32. S. K. Das and A. K. Guha, Colloids Surf., B, 2007, 60, 46–54.
33. X. Hou, J. Feng, Y. Ren, Z. Fan and M. Zhang, Colloids Surf., A, 2010, 363, 1–7.
34. T. L. Tan, L. L. Wang, J. Zhang, D. D. Johnson and K. W. Bai, ACS Catal., 2015, 5, 2376–2383.
35. B. Chen, X. Zhao, Y. Liu, B. Xu and X. Pan, RSC Adv., 2015, 5, 1398–1405.
36. M. M. Beppu, R. S. Vieira, C. G. Aimoli and C. C. Santana, J. Membr. Sci., 2007, 301, 126–130.
37. X. Zhang, H. Niu, Y. Pan, Y. Shi and Y. Cai, Anal. Chem., 2010, 82, 2363–2371.
38. D. H. Kim, S. H. Lee, K. H. Im, K. N. Kim, K. M. Kim, I. B. Shim, M. H. Lee and Y. K. Lee, Curr. Appl. Phys., 2006, 6(1), e242–e246.
39. W. Jiang, W. Wang, B. Pan, Q. Zhang, W. Zhang and L. Lv, ACS Appl. Mater. Interfaces, 2014, 6, 3421–3426.
40. L. Ai, H. Huang, Z. Chen, X. Wei and J. Jiang, Chem. Eng. J., 2010, 156, 243–249.
41. G. Zhang, J. Qu, H. Liu, A. T. Cooper and R. Wu, Chemosphere, 2007, 68, 1058–1066.
42. M. Abdel Salam, M. A. Gabal and A. Y. Obaid, Synth. Met., 2012, 161, 2651–2658.
43. S. Ma, S. Zhan, Y. Jia and Q. Zhou, ACS Appl. Mater. Interfaces, 2015, 7, 10576–10586.
44. K. R. Hall, L. C. Eagleton, A. Acrivos and T. Vermeulen, Ind. Eng. Chem. Fundam., 1966, 5, 212–223.
45. S. Lagergren, Handlingar, 1898, 24, 1–39.
46. Y. S. Ho and G. McKay, Process Biochem., 1999, 34, 451–465.
47. Y.-S. Ho, Water Res., 2006, 40, 119–125.
48. O. E. Fayemi, A. S. Ogunlaja, P. F. M. Kempgens, E. Antunes, N. Torto, T. Nyokong and Z. R. Tshentu, Miner. Eng., 2013, 53, 256–265.
49. B. Liu and Y. Huang, J. Mater. Chem., 2011, 21, 17413–17418.
50. S. W. Won, J. Park, J. Mao and Y.-S. Yun, Bioresour. Technol., 2011, 102, 3888–3893.
51. A. Ramesh, H. Hasegawa, W. Sugimoto, T. Maki and K. Ueda, Bioresour. Technol., 2008, 99, 3801–3809.
52. L. Zhou, J. Liu and Z. Liu, J. Hazard. Mater., 2009, 172, 439–446.
53. K. Fujiwara, A. Ramesh, T. Maki, H. Hasegawa and K. Ueda, J. Hazard. Mater., 2007, 146, 39–50.
54. L. Zhou, J. Xu, X. Liang and Z. Liu, J. Hazard. Mater., 2010, 182, 518–524.
55. H.-w. Ma, X.-p. Liao, X. Liu and B. Shi, J. Membr. Sci., 2006, 278, 373–380.
56. C. Xiong, Y. Zheng, Y. Feng, C. Yao, C. Ma, X. Zheng and J. Jiang, J. Mater. Chem. A, 2014, 2, 5379–5386.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16190a

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