Loading of Iron (II, III) Oxide Nanoparticles in Cryogels Based on Microfibrillar Cellulose for Heavy Metal Ion Separation

Cryogels based on microfibrillar cellulose (MFC) and reinforced with chitosan to endow water resistance were loaded with magnetite nanoparticles (MNPs) and characterized by TEM, XRD, and TGA. *e MNP-loaded cryogels were tested for heavy metal ion removal from aqueous matrices. *e adsorption capacity under equilibrium conditions for Cr(VI), Pd(II), Cd(II), and Zn(II) was measured to be 2755, 2155, 3015, and 4100mg/g, respectively. *e results indicate the potential of the introduced bicomponent cryogels for nanoparticle loading, leading to a remarkably high metal ion sorption capacity.


Introduction
Magnetic nanoparticles have been a topic of interest in a wide range of applications, including catalysis [1,2], biotechnology/biomedicine [3], and environmental remediation [4,5]. In most of the envisaged applications, the particles perform best when their sizes are below a critical value, typically around 10-20 nm. In such a case, the nanoparticles are a single magnetic domain and show superparamagnetic behavior at temperatures above the socalled blocking temperature [6].
Among the wide variety of magnetic materials that can be prepared in the form of nanoparticles, iron oxides have been considered the most intensive one while being safe. In addition, iron oxide (magnetite) nanoparticles can be easily prepared through coprecipitation of iron salts in alkaline aqueous solutions in the presence of stabilizers [7]. Such coprecipitation is also a convenient way to synthesize iron oxides (either Fe 3 O 4 or c-Fe 2 O 3 ) from aqueous Fe 2+ /Fe 3+ salt solutions by the addition of a base under the inert atmosphere at room or elevated temperatures. Using proper stabilizing agents with carboxylate or hydroxyl carboxylate groups, it is possible to produce monodispersed iron oxide magnetic nanoparticles. Moreover, surface complexes readily form between Fe 2+ /Fe 3+ and the stabilizing agent, undergoing nucleation and crystal growth, favoring the formation of small units, and preventing aggregation.
Micro/nanofibrillar cellulose (MFC) is isolated from natural cellulose fibers by mechanical action after optional enzymatic or chemical pretreatment. MFC's high aspect ratio, biocompatibility, and abundant active hydroxyl and carboxyl groups make it attractive for applications as a host for particles [8,9], such as silica [10], silver [11], magnetic [12], and drug [13] nanoparticles. e respective preparation can be carried out in the slurry form, which implies a relatively large usage of water to remove loosely bound reactants and particles. Moreover, since associated reactions occur in the system, with typical high surface areas, large reactant amounts are consumed.
An alternative typical MFC reparation, MFC cryogel, has proven particularly useful in applications where biocompatibility and biodegradability are needed. e cryogels are porous materials of interconnected nanostructures made from precursor hydrogels after replacing the liquid by the gas phase [14]. e unique properties associated with cryogels have led to a wide range of applications such as in catalysts [15] and as a reaction support [16], as super thermal and sound insulators [17,19], and as electronics, filters, and storage media, for example, gases in fuel cells [20]. For nanoparticle stabilization and size control, MFC porous materials are very effective as hosts [21,22]. In the reduction reaction of metal ions, the small pores have been used as sites for nucleation and growth, thus restricting the particle size. Moreover, the resultant hybrid materials have some advantages if given functionalities of the matrix are transferred to the nanoparticles [14]. Here, we propose MFC cryogels for direct use as templates for the preparation of magnetic nanoparticles.
We firstly report a facile method to prepare magnetic nanoparticles and their loading in MFC cryogels followed by a demonstration of their application. e wet strength of the cryogels was improved with the addition of chitosan, and magnetic nanoparticles were successfully synthesized in such a bicomponent system. e hybrid cryogels were shown to be effective in heavy metal ion separation, including Cr(VI), Pb(II), Cd(II), and Zn(II).

MFC Preparation.
Fully bleached eucalyptus fibers were processed with a microgrinder (x6 passes) at a concentration of 2.3%, producing an MFC suspension, which was kept under refrigeration until further use. e acidic group content of MFC was measured by conductive titration using 0.01 M NaOH, which was added dropwise until the conductivity of the system displayed no further drop. e amount of NaOH consumed was used to calculate the acidic group content [23], calculated to be 70 μmol/g.

Chitosan and MFC Cryogel
Synthesis. Chitosan (MW 10253) was purchased from Aldrich (degree of deacetylation ∼85%). Chitosan solutions were prepared by dissolving chitosan (2% by weight) in 1% (by volume) aqueous acetic acid solution [24]. e solution was sonicated for 30 min before use. e MFC suspension was mixed with chitosan at different weight loading ratios, 100/0, 90/10, and 80/20 (MFC/chitosan) as a 0.5% suspension in DI water. Highshear mixing (5 min) was used for this purpose, followed by magnetic bar stirring (3 h) and sonication for another 30 min. en, the mixture was centrifuged (1.0 × 10 4 rpm, 0.5 h) to remove excess water. A hydrogel with 3% solid content was thus formed and was frozen with liquid nitrogen. Following this, it was placed in a freeze-dryer (Labconco) for at least 48 h. After water sublimation, the obtained cryogel was characterized (density, strength, and BET surface area). e wet strength was determined qualitatively by immersion and stirring in water for a given time.
e BET area of the MFC cryogels was measured before and after MNP loading, after 4 h degassing at 105°C (Gemini VII Series Surface Area Analyzer, Micromeritics Instrument Corporation).

Nanoparticle Loading.
e in situ loading of magnetic nanoparticles (MNPs) in the cryogels was carried out through the coprecipitation of iron salts of ferrous chloride (Fe 2+ , 0.05 M) and ferric chloride (Fe 3+ , 0.1 M) by adding alkaline aqueous solution (0.2 N NaOH) at 60°C. Firstly, the iron solution was prepared by mixing a given volume of 0.05 M Fe 2+ and 0.1 M Fe 3+ solutions. e cryogel was loaded with the iron solution dropwise and left undisturbed for 30 min. e iron solution was saturated in the porous cryogel. After air-drying (∼1 h), a partially dehydrated matrix was obtained. It was then immersed in 0.2 N NaOH solution at 60°C for 1 h so that the coprecipitation was completed as indicated by no changes in color. e hybrid material was rinsed with Milli-Q water for 1 h to remove water-soluble substances and loosely bound iron particles. Finally, the hybrid material was frozen in liquid nitrogen and freeze-dried. e obtained, magnetic MFC cryogel is thereafter referred to as the MMFC cryogel.

Cryogel Morphology.
e porous structures of the MFC cryogels were examined using a field-emission scanning electron microscope (FESEM) at an accelerating voltage of 20 kV (a JEOL 6400F high-resolution cold field-emission SEM). e MMFC was characterized using a VPSEM (variable-pressure scanning electron microscope (Hitachi S3200N)) with an energy dispersive X-ray spectrometer.
A JEOL 2000FX transmission electron microscope (TEM) operating at 20.0 kV was further utilized for MMFC characterization. For this purpose, a specimen was prepared by cutting the sample into thin slices, using a diamond saw, then cutting 3 mm diameter disks from the slice, thinning the disk on a grinding wheel, dimpling the thinned disk, and then ion milling it to achieve electron transparency.

ermal Behavior.
In order to determine the thermal decomposition temperature of the hybrid cryogels, a thermogravimetric analysis (TGA) apparatus was used (Perki-nElmer TGA Q500), operating with a heating rate of 10°C/ min to 500°C in a nitrogen atmosphere. In order to obtain the MNP loading in the cryogel, TGA was performed under oxygen in the T range between 500 and 575°C. e isothermal time at 575°C was 25 min.

X-Ray
Diffraction. MMFC samples were subjected to XRD analysis following 2 theta signals at both small and wide angles to identify iron crystallites. For this purpose, a PANalytical Empyrean X-ray diffractometer was used. e scan range was in 5-40 for small angles, and 10-80 for wide angles.

Heavy Metal Ion Separation.
A stock solution of Cr(VI) (100 mg/L) was prepared by dissolving a certain amount of K 2 Cr 2 O 7 (from Sigma-Aldrich) in 100 mL water obtained from a Millipore ™ purification unit. A series of standard metal solutions were prepared by diluting the stock metal solution. A typical adsorption experiment was conducted as follows: 5 mg cryogel was added to a 100 mL Erlenmeyer flask containing 25 mL 100 mg/L Cr(VI) solution and magnetically stirred at 120 rpm for the predetermined time. e pH of the working solution was maintained at a specified value using 0.01 M NaOH and HCl solutions (both from Sigma-Aldrich). e concentrations of Cr(VI) during adsorption tests were analyzed by spectrophotometry (Agilent 8453 UV-Vis dissolution testing system, Agilent, USA). Pb(II), Cd(II), and Zn(II) solutions were prepared following similar protocols using Cd(NO 3 ) 2 , Pb(NO 3 ) 2 , and Zn(NO 3 ) 2 ·6H 2 O (Sigma-Aldrich). e uptake capacity of MFC cryogel adsorption Q e (mg/g) was defined according to the following equation: where C 0 is the metal ion concentration in the initial solution, C e is the equilibrium concentration (mg/L), V is the volume of metal ion solution (L), and m is the mass of the absorbent material (g).

Physical Properties of MMFC Cryogels.
e density of the neat MFC cryogel was 0.0258, 0.0505, and 0.0807 g/cm 3 , depending on the concentration of the hydrogel precursor of 2, 5, and 8%, respectively. e MFC density was 0.0390 and 0.0395 g/cm 3 upon chitosan loadings of 10 and 20%, respectively, indicating the densification of the system in the presence of the cationic polymer. e MFC and MMFC surface area was measured after 4 h degassing at 105°C (Table 1). e MFC cryogel with 10% CH loading displayed a higher BET area than that with 20% CH, i.e., chitosan acted as a binder for MFC. In the absence of chitosan, the MFC cryogels were redispersed in water after immersion under magnetic stirring at 610 rpm for 4 h. e cryogels loaded with 10% and 20% chitosan remained intact after immersion, under the same conditions (Figure 1), indicating the reinforcing effect of chitosan.

MFC and MMFC Morphology.
e presence of chitosan produced a denser MFC structure ( Figure 2). In the absence of chitosan, the cryogel structure was open, with pores being easily observed (Figure 2(a)). At low chitosan addition (Figure 2(b)), thin "walls" formed between the pores. With the increasing chitosan content to 20% (Figure 2(c)), the thickness of such walls increased.
is is because of the characteristic properties of chitosan and its primary amino groups. Besides hydrogen bonding, the reaction between amino and carboxyl groups took place. is reaction has been applied in several cellulose/chitosan hybrid preparations [25][26][27]. Such chitosan-reinforced MFC cryogels were stable in water.
After MNP loading, the MMFC cryogels exhibited a similar SEM morphology to that before loading. A denser cryogel was relatively tighter, and less capillaries were present, which reduced fluid penetration, resulting in the generation of lower amounts of MNPs. e MNP loading in the MMFC cryogels, as determined by TGA at 575°C in the oxygen atmosphere following 25 min isothermal time, corresponded to 16 and 18%; for MFC cryogels, the chitosan concentration was 10 and 20% (note that an MFC film sample and filter paper yielded 2.8 and 1.8%, respectively).
MMFC samples were subjected to SEM at high magnification ( Figure 3). Iron particles were observed on the surface of the fibrils. is was also observed for two other types of cellulose substrates, after the same coprecipitation method, namely, a filter paper and MFC films (see Figure 4). ese two substrates showed scattered, monodispersed metal particles but low loadings ( Table 2). MFC cryogels comprised abundant, open pores and large BET surface area, offering a good opportunity as a template for iron nanoparticle loading. Based on the homogeneous particle distribution observed in the SEM images in Figure 3, the MFC cryogel with 20% CH was selected for particle observation under TEM (Figure 4). Both the filter paper and MFC film samples loaded with particles were also observed, as a reference. From the TEM images, the particles were determined to be monodispersed and their size was in the nanoscale. MMFC retained more iron than both the paper and MFC films; the iron particles in such a case were larger and clustered.

XRD
Analysis. An X-ray diffractometer was used to characterize the microstructural and crystallographic features of the iron particles in MMFC (see the XRD pattern at a small angle in Figure 5(a)). MMFC displayed an intense signal at 2θ � 35.28°, corresponding to pure Fe 3 O 4 crystals (Fe 3 O 4 is identified at 35°), confirming their presence. Wideangle scanning showed similar results to that at small angles ( Figure 5(b)). e observed broadness and lower intensity of the profiles indicate a lower degree of crystallinity of Fe 3 O 4 particles imbedded in the cellulose matrix.

ermal Gravimetric Analysis.
In view of the importance of thermal stability in many MMFC applications, we examined the thermal decomposition of the MMFC cryogels by thermogravimetry in the nitrogen atmosphere until 500°C ( Figure 6). All TGA curves showed small weight losses below 150°C, from evaporation of the adsorbed moisture. Under nitrogen, the decomposition behavior of MFC was nearly the same at different chitosan loadings. e TGA for MMFC indicated a decomposition temperature from 264 to 318°C. MNPs weakened the substrate thermal stability. e isothermal run (25 min at 575°C) was used to determine the residual mass (MNP amount) in MMFC (TGA under oxygen was run in the 500-575°C range; Table 2).

Heavy Metal Ion Separation.
MFC cryogels were expected to result in partial degradation in acidic solutions at long contact time and high temperature. If the pH of the adsorption solution is lowered, the metal ion separation is maximized. e metal adsorption experiments were conducted under the near-neutral condition at pH 6. Different contact time and initial concentrations were tested for four metal ions: Cr(VI), Pb(II), Cd(II), and Zn(II). Figure 7 shows the removal rate as a function of the contact time and initial ion concentration.

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In Figure 7(a), the four ion types showed a similar trend. During the first 20 min contact time, the removal rate reached a maximum adsorption. At longer contact times, the removal rate remained the same. e MMFC could quickly absorb the metal ions. Cr(VI), Pb(II), Cd(II), and Zn(II) presented different removal extent (Figures 7 and 8). Cd(II) and Zn(II) adsorbed more readily than Cr(VI) and Pb(II). e most effective separation corresponded to Cd(II), for a    Advances in Polymer Technology removal >95% (Cr(VI) and Pb(II) showed removal % close to 52%). e initial concentration is an important factor in metal ion separation. e 50-1000 mg/L concentration range was adsorbed by MMFC at 60 min contact time, which is long enough for adsorption. e removal % is shown in Figure 7(b). MMFC maintained a high adsorption capacity for all initial concentration, with linear profiles except for Pd 2+ and Cd(II). e Pd 2+ removal rate was lowered, from 57% to 43%, and Cd(II) lowered from 95% to 55%. e results indicate that MMFC is a suitable material to remove heavy metal ions from aqueous solution.
In order to express MMFC adsorption capacity, the equilibrium absorption amount Q e was calculated using equation (1). Figure 8 shows the results for different initial concentrations of the respective metal ion. Four near linear profiles are presented and indicate a high capacity for metal ion removal. e adsorbents were not saturated in solution. Cr(VI), Pd 2+ , Cd(II), and Zn(II) equilibrium removal amounts were 2755, 2155, 3015, and 4100 mg/g, respectively.
Using cellulose nanocrystals (CNCs) and Fe 2 O 3 composite as a Pd 2+ absorbent, the maximum capacity was measured to be 3.6 mg/g, but 3.3 mg/g for Fe 2 O 3 CNC [31].  Figure 4: TEM images of the MMFC cryogel and filter paper/MFC film with MNPs (note that the filter paper was treated three times for iron coprecipitation).    Aldehyde-functionalized NFC cryogels showed an improved Pb(II) adsorption of 156 mg/g [32]. Chitosan-based magnetic hydrogel beads reached 171 mg/g Pd 2+ adsorption capacity with Fe 2 O 3 and NFC addition, but only 118 mg/g with no addition [33]. Yu et al. [34] reported that carboxylation of CNC with succinic anhydride resulted in a sorbent with a very high maximum adsorption uptake for Pb(II) (458 mg/g) and Cd(II) (335 mg/g). A Cd(II) adsorption of 3.2 mg/g was reported for CNC and Fe 2 O 3 composite adsorbents [31]. Grafting carboxylic groups to chitosan improved the Zn(II) removal from 168 mg/g to 290 mg/g [35]. Amino-functionalized microfibrillated cellulose with the maximum uptake capacity of 388 mg/g was measured for Cd(II) [36]. According to the recent review of nanocellulosebased materials for water purification [37], the MFC/NFC absorbent exhibited a high uptake amount, as high as that of MMFC used in this study.
ese values for MFC cryogels as metal ion separators indicate an excellent potential for nanoparticle-loaded adsorbents from aqueous solution.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare no conflicts of interest.