Self-assembly of ferria – nanocellulose composite fibres

An environmentally benign synthesis of a magnetically responsive carboxymethylated cellulose nanofibril-based material is reported. Applied experimental conditions lead to the in-situ formation of magnetite nanoparticles with primary particle sizes of 2.0 – 4.0 nm or secondary particles of 3.6 – 16.4 nm depending on whether nucleation occurred between individual carboxymethylated cellulose nanofibrils, or on exposed fibril surfaces. The increase in magnetite particle size on the cellulose fibril surfaces was attributed to Ostwald ripening, while the small particles formed within the carboxymethyl cellulose aggregates were presumably due to steric interactions. The magnetite nanoparticles were capable of coordinating to carboxymethylated cellulose nanofibrils to form large “ fibre-like ” assemblies. The confinement of small particles within aggregates of reductive cellulose molecules was most likely responsible for excellent conservation of magnetic characteristics on storage of this material. The possibility for using the material in drug delivery applications with release rate controlled by daylight illumination is presented.


Introduction
The UN 2030 agenda for sustainable development, highlights key areas where the development of sustainably produced materials is expected to play a crucial role (Transforming Our World: The 2030 Agenda for Sustainable Development | Department of Economic and Social Affairs, n. d.).It emphasizes the need for development of innovative solid materials for key applications, such as smart packaging, advanced adsorbents, wound healing and tissue engineering scaffolds using environmentally sustainable raw materials.Major focus today is therefore set on natural bio-based polymers.
Cellulose is the most abundant renewable polymer on the planet accounting for multiple teratons of annual biomass production (Klemm et al., 2005).Cellulose is found in all plant forms where it often forms the major constituent (e.g.cotton, wood).Historically and currently, these plant derived forms of cellulose have been used for everyday applications in the form of fabrics, pulp and paper and wood constructions (Hon, 1994).
One of the major challenges in the development and adaptation of cellulose-based materials is its relative inertness.In order to expand its usage beyond that of simple fibres or crystals, cellulose must be chemically modified to not only increase its solubility but also to diversify and increase the range of possible applications.The development of carboxymethyl cellulose (CMC) for example has led to its use in food as well as more technical applications such as protein purification (Hao et al., 2021;He et al., 2021) and coatings (Dimic-Misic et al., 2013;Souza et al., 2019).Properties of nanocellulose-derived materials are related to size, morphology and surface chemistry of the particles.The nanoparticles can be cellulose nanocrystals (CNC), cellulose nanofibres (CNF) or bacterial cellulose fibres (BCF) (Sacui et al., 2014).
A significant number of studies to date involving the use of natural biopolymers for advanced technical solutions have focussed on the incorporation of responsive metal oxide materials into, or onto a biopolymer matrix.By including the spinel-type iron oxides such as magnetite or iron(II)-deficient maghemite into biopolymer matrices, composite materials may be obtained with favourable magnetic and catalytic properties.These characteristics may be exploited in the development of materials suitable for applications as Magnetic Resonance Image contrasting agents (Abbasi Pour et al., 2017;Biliuta et al., 2017), antibacterial agents (Biliuta et al., 2017), in magneto-optical applications (Chen et al., 2020;Li et al., 2013), protein adsorption membranes (Wu et al., 2018), metal ion removal (Yu et al., 2012(Yu et al., , 2014)), electrochemical sensors (Khalilzadeh et al., 2020) and for medical applications (Abbasi Pour et al., 2017;Chaabane et al., 2020).
With magnetically responsive cellulose-iron oxide composite materials, work has focussed on the production of materials either by incorporating pre-synthesized iron oxide particles into a fibril-matrix or via the in-situ growth of particles onto pre-formed biopolymer fibrils/ fibres surfaces.Numerous techniques have been developed for production of nano-ferria in a broad range of sizes and morphologies.They include solvothermal synthesis starting from organic precursors or iron carbonyl, resulting in small uniform well-crystallized particles that are often rendered hydrophobic by the conditions of synthesis.An alternative approach is based on co-precipitation in aqueous media.Its drawback lies, however, in relatively appreciable solubility of ferria in polar aqueous media that can result in considerable size variation because of the Ostwald ripening (Thanh, 2012).The challenge in use of pre-formed particles lies in the difficulty of their uniform distribution.With the synthesis of composites, attention has generally been towards in-situ particle growth on a matrix.For efficient distribution of the inorganic content, the matrix should be activated via surface oxidation or esterification.The reactivity of cellulose resembles in this case that of graphene oxide with oxidized surface groups, for example by carboxylation (Dimic-Misic et al., 2019).To our knowledge, no reports have so far been made on cellulose based nanocomposite materials where magnetic iron oxides and nanocellulose self-assemble into large fibre-like structures.In earlier studies, focus was on the in-situ formation of magnetite nanoparticles on relatively large, unmodified, cellulose nanofibres, resulting in the formation of magnetite particle decorated nanofibres with metal oxide particles greater than 10 nm (Galland et al., 2013).
Our hypothesis was that producing magnetic iron oxide in-situ in the presence of highly functionalized nano cellulose would result in a dense self-assembled material with: 1) Potentially interesting morphologies; 2) Stable magnetic characteristics (through protection of magnetic particles within a dense composite through encapsulation in a reductive matrix); and 3) Capacity for visible light controlled release of adsorbed pharmaceuticals, exploiting photo magnetic properties of the obtained composite material.

Materials
Carboxymethylated nanocellulose fibrils (C-CNF, derived from wood, Degree of Substitution (DS) of cellulose surface hydroxyl groups 0.098) was prepared at RISE Bioeconomy and Health according to the method of Wågberg et al. (2008) as a hydrogel with solid concentration 2.26% by weight.DS is the (average) number of substituent groups attached per base unit (in the case of condensation polymers) or per monomeric unit (in the case of addition polymers).The term has been mainly used in cellulose chemistry.The DS value indicates that approximately 10% of all hydroxyl groups have been carboxymethylated (Wågberg et al., 2008).After carboxymethylation of a softwood sulphite dissolving pulp (Domsjö Dissolving Plus), the cellulose material was passed through a homogeniser (Microfluidizer M-110EH, Microfluidics Corp., USA) at 1700 bar with two serial chambers of 200-and 100 μm, respectively.The carboxylate content of the nanocellulose was determined via conductometric titration.The C-CNF applied here differs in its characteristics from TEMPO oxidized nano cellulose in that it has a combination of both carboxyl-and carboxymethyl surface functional groups, while TEMPO-CNF has aldehyde and carboxyl groups (Aaen et al., 2019).In addition, the crystal structure is a slightly different between the two grades.Furthermore, the mechanical treatment to delaminate the pulp fibres into fibrils induces variations and commonly the C-CNF has more residual fibre fragments, unless extra cleaning and separation steps are performed.
Iron(II) sulphate heptahydrate (pro analysis), anhydrous iron(III) chloride, and ammonia (25% based on NH 3 ) were obtained from Sigma-Aldrich, Sweden AB.All chemicals were used without further purification.Water was purified using a Millipore system and purged with nitrogen for several hours prior to use.Ammonia solution was prepared using nitrogen purged water and stored under nitrogen.

Characterization
Samples were characterized using a Bruker Dimension FastScan Atomic Force Microscope (AFM) with a Nanoscope V controller in Sca-nAsyst mode using a Fastscan-B AFM probe (silicon tip, f 0 : 400 kHz, k:4 N/m, tip radius: 5 nm nominally) and a scan rate of 1-3 Hz.Samples were prepared on freshly exfoliated mica.Data was processed using Gwyddion 2.56 with align rows-median to remove skipping lines.
Scanning Electron Microscopy (SEM) observations were conducted using a Hitachi FlexSEM 1000 at an acceleration voltage of 5 kV, spot size 20, and 5 mm working distance.Samples were prepared on Cu foil from aqueous suspensions.
Powder X-ray Diffraction (PXRD) data was obtained using lyophilized samples on a Bruker APEX-II diffractometer equipped with an AXS Smart APEX CCD Area Detector and graphite-monochromated Mo-K α (λ = 0.71073 Å) radiation source.Data was processed with the EVA-12 software package.
Fourier Transform Infrared (FTIR) analysis was done on a Perkin Elmer Spectrum 100 FT-IR Spectrometer using KBr pellets.Thermogravimetric Analysis (TGA) was done using a Perkin Elmer Pyris 1 TGA at a heating rate of 5 • C/min.
Magnetic measurements were performed using a Lake Shore Cryotronics Series 7400 vibrating sample magnetometer (VSM).Measurements were performed at 300 K (26,85 • C) in the magnetic field range ±10 kOe with the magnetic moments normalized using the weight of iron oxide solid phase.

Synthesis
22.089 g of 2.26 wt% C-CNF (499 mg solid C-CNF) was transferred to a 250 mL round bottom flask equipped with a Teflon coated stirring bean and nitrogen inlet.C-CNF suspended in 200 mL nitrogen purged ultrapure water was added and vigorously stirred for 30 min.Then, mg FeCl 3 (2.02mmol) and 328.5 mg (1.18 mmol) FeSO 4 ⋅7H 2 O was added, forming a 1.7:1 stoichiometry between Fe 3+ /Fe 2+ and concentration of 9.10 mM Fe 3+ /5.33 mM Fe 2+ respectively.The pH was adjusted to pH 5 (according to litmus), using 0.5 mM HCl and the suspension allowed to stir for 30 min at room temperature under a constant flow of bubbling nitrogen.Then, 1.5 M NH 4 OH was added dropwise using a syringe until pH 9 (according to litmus).Stirring was stopped and the particles allowed to settle for 5 min, before decanting into 50 mL falcon tubes and washing 4× with N 2 -purged ultrapure water and collection via centrifugation (10 min, 5000 RPM).

Adsorption experiments
An aliquot equating to ca. 15 mg composite material was removed and mixed with ca. 3 mg tetracycline, placed in an aluminium-wrapped falcon tube and diluted to a final volume of 40 mL.This was placed on an orbital shaker and allowed to shake for several days.Periodically, aliquots were taken and the composite collected via a strong magnet.The supernatant was measured using UV-VIS at 357 nm.Experiments were repeated in triplicate for reproducibility.

Desorption experiments
An aliquot equating to ca. 15 mg composite material was taken and mixed with ca. 3 mg tetracycline in a 50 mL pointed flask fitted with a stirring bar and diluted to 30 mL.The flask was heated overnight at 50 • C in a darkened fume hood with the setup wrapped in aluminium to avoid incident light.After stirring overnight, the product was cooled to room temperature and divided equally into two aluminium wrapped falcon tubes.The composite material was collected using a strong magnet, and the supernatant removed and diluted to 17 mL with 0.02 M citrate buffer (pH 6.0).One of the aluminium jackets was removed and both samples placed on an orbital shaker under a daylight lamp.Aliquots were periodically removed, the composite collected, and the tetracycline content in the supernatant determined via UV-VIS at 357 nm.Experiments were repeated in triplicate for reproducibility.

Production of iron oxide in the presence of carboxymethylated nanocellulose fibrils
Magnetite is an easily produced magnetic metal oxide with an inverse spinel structured metal oxide consisting of iron(III) and iron(II) in a 1:2 stoichiometry which may be produced by co-precipitation in the presence of ammonia in the following reaction: Dissolution of Fe(II) and Fe(III) salts in the presence of C-CNFs followed by the slow addition of ammonia lead to the formation of a slight orange hued suspension, with the suspension darkening to a reddishbrown and then black as the pH increased.When the final pH was achieved, a black precipitate was present which slowly turned reddishbrown over time in the presence of ambient air due to the oxidation of Fe 2+ to Fe 3+ .Thermogravimetric analysis (SI Fig. S3) of the resulting sample showed a thermal decomposition temperature of 256-257 • C with a residual mass of 31.8 wt%.

XRD patterns of nanocomposite material
To determine which iron oxide phase is formed during the coprecipitation reaction of iron(II/III) in the presence of C-CNF, the powder X-ray pattern was measured and compared with certified  patterns for both magnetite and the iron(II) deficient maghemite (Fig. 1A and SI Fig. S2).It was proven that the primary phase of the iron oxide formed in the presence of C-CNFs nanofibrils was magnetite, maghemite or a mixture of the two oxides.However, with the current setup it was impossible to differentiate between the two iron oxides.
Scherrer analysis of the crystallite size was made applying the formula τ = Kλ / (β ⋅ cosƟ) and showed that the average size of freshly produced pure ferria under the applied conditions was 3.7 nm, while for the composite it was 3.4 nm.This shows that composite formation contributed to the preservation of smaller particles.

Fourier Transform Infrared spectroscopy (FTIR)
In an attempt to distinguish between the magnetite and maghemite iron oxides, the product was examined using FTIR.It should be noted however, that infrared spectroscopic results of magnetite/maghemite mixtures are not absolute due to a strong overlap of the most characteristics bands of iron oxides (Ellid et al., 2003).The iron oxide produced via in-situ precipitation of iron(III) chloride and iron(II) sulphate by ammonia is shown in Fig. 1B.The product obtained exhibits strong absorption bands at 627 and 576 cm − 1 , with a minor adsorption band in the region of 446 cm − 1 .Ishii et al. (1972) assigned the IR band at 565 cm − 1 to the ν 1 (F 1u ) vibration mode in magnetite, while a small shift to a higher wave number may be attributed to sub-stoichiometric magnetite (Ellid et al., 2003).The peak at 627 cm − 1 can be assigned to the Fe -O vibration in the iron(II) deficient maghemite, which has formed due to oxidation (Klotz et al., 1999).The additional peaks at 1128, 1043, 975 cm − 1 may be attributed to the presence of bound sulphate groups present in the sample (Musić et al., 2000).Finally, the peak observed at 1624 cm − 1 and the broad peak at 3400 cm − 1 can be attributed to moisture.
The produced iron oxide-C-CNF composite material exhibited similar absorption bands in the range 580-620 cm − 1 with a smaller, less wellpronounced peak at 665 cm − 1 , which is slightly shifted, compared to the synthesized magnetite sample.This may be attributed to the magnetite formed in the sample, being coordinated to carboxylate groups present in C-CNF with some maghemite having formed due to oxidation in air.Additional absorption bands appear at 2890,1597,1426,1373,1318,1200,1160,1110,1060, 1022, 897 cm − 1 which are primarily attributed to the various absorption bands present within C-CNF.

Magnetic characterization
Fig. 1C shows the magnetization versus magnetic field for the powder and liquid samples.The measured magnetizations at an applied field of 10 kOe are 63 emu/g and 53 emu/g for the powder and liquid samples, respectively.Both values are somewhat smaller than expected for magnetite and maghemite, which may be explained by spin disorder and spin canting for surface spins in iron oxide nanoparticles.Moreover, as expected, the nanoparticles exhibit superparamagnetic behaviour at 300 K (26,85 • C) and hence zero remanence and coercivity.A plausible explanation for the minor drop in magnetization of the sample dispersed in ethanol is that partial oxidation has occurred with time, but only to rather low extent.The samples were stored for 10 months before measurements and thus revealed considerable stability against oxidation.Bulk magnetic properties are demonstrated in SI Fig. S4 and in a Supplementary video.The major volume of ferria was kept in the form of non-aggregated primary particles bound within the formed selfassembly fibres.This precludes both diffusion of oxygen and release of ions from the coordination-saturated surface of the particles.As a result no apparent oxidation occurs on storage.

Atomic force microscopy
To examine the nanoscale structures formed when iron oxide is precipitated in the presence of C-CNFs, the sample was examined using AFM where it was apparent that the reaction is capable of producing a composite consisting of "fibre-like" structures (Fig. 2).Observations along the axis of these fibre-like structures showed random increases in surface height, likely attributed to the presence of iron oxide formed during the co-precipitation reaction.
The fibre-like structures observed had an average width of 18.55 ± 1.66 nm with lengths ranging from 70 nm to nearly a micrometer in longitudinal direction.However, the measured widths do not take tip convolution into account, which increases the observed widths compared to actual widths.By decreasing the measuring area to 200 × 200 nm (Fig. 2C, E), spherical to ellipsoidal particles were observed which appear surrounded by a C-CNF network.
The presence of multiple spherical to ellipsoidal particles across the longitudinal direction of the fibre-like structures (Fig. 2C, E) indicate that the iron oxide particles formed during the precipitation of iron(II) and iron(III) interact with the C-CNFs to form a composite material that self-assembled into fibre-like domains.This interesting property that should be exploited in future applications.However, it was impossible to determine from measured AFM data whether the particles were distributed on the actual surface of the network or within the C-CNF network itself without more detailed examination.

Electron microscopy
To supplement the AFM data, samples were prepared for SEM and TEM as described in the method section (Fig. 3).SEM confirmed that the fibre-like structures were not completely homogeneous showing aggregates along the surface of the individual fibre networks (Fig. 3A).Energy-dispersive X-ray analysis across the aggregates showed iron and oxygen, indicating the metal oxide was distributed homogeneously within the aggregate structure (Fig. 3A).Cross sections of these aggregates shown with TEM further suggest a homogeneous structure (Fig. 5).
Examination of the composite by TEM without negative staining (Fig. 4, SI Fig. S1) showed the iron-C-CNF network was composed of single fibrils having a cross section of 3-4.5 nm or double fibrils in the range of 5.5-8 nm (SI Fig. S1) and as well as strongly scattering elements in the order of 5.5-8 nm, distributed along the lateral dimensions of the network.Under normal circumstances, cellulose-based samples require staining with uranyl acetate or similar heavy metal stains to be visible.However, in the present case, the composite fibre structure was visible due to the presence of iron oxide nanoparticles bound to individual carboxymethyl cellulose fibrils.
Based on the presence of iron oxide in both the aggregates and fibrillike structures within the sample, it would seem feasible that initially the metal salts hydrolyse to form hydrated species, which then coordinate with partially deprotonated C-CNFs.The addition of base leads to further deprotonation of the C-CNFs and production of iron hydroxide species that nucleate to form magnetite at high pH (Seisenbaeva and Kessler, 2014), with the surface remaining iron coordinated to the carboxymethylated cellulose nanofibrils.However, this does not explain the formation of both the "fibre-like" assemblies and larger aggregates.Using electron microscopy and AFM, our investigation revealed three distinct composite structures.This included self-assembled fibril structures containing surface-bound iron oxide nanoparticles, larger C-CNF aggregates with iron oxide nanoparticles in the range of a few nanometers, and large iron oxide particles that formed on the surface of fibril aggregates, the latter stimulated by Ostwald ripening.The larger particles may result from both aggregation and Ostwald ripening, although the larger crystal domain size for the composite indicates domination of the Ostwald ripening phenomenon.It is assumed however, that the source of C-CNFs will play a significant role in the formation of the observed structures.In this study, the C-CNFs were derived from wood and had uniform particle sizes in the range of 3-4.5 nm for single fibrils according to TEM.
To determine whether the iron oxides are precipitated along the outer regions of the large C-CNFs network, or were incorporated within the bulk of the aggregates, samples were embedded, sectioned and examined by TEM to provide additional information on the ultrastructural nature of the composite material.
As shown in Figs.3-5 and S1, Fig. S1, the iron oxides were present in three major forms, including, iron oxide aggregates, inclusion of iron oxides within the C-CNF aggregate structure, and adsorption onto the C-CNF fibril surfaces.While the surface adsorbed iron oxide nanoparticles grew into large sizes due to Ostwald ripening (i.e.ca.5,5-8 nm), the iron oxide nanoparticles encapsulated within the C-CNF structure remained in the order of 1,5-2,5 nm.This observation is in good agreement with calculations of the average size of crystalline domain applying the Scherrer formula.It indicates that carboxymethylated cellulose nanofibrils not only allow for the formation of C-CNF-iron oxide network clusters, it is also effective in retaining the iron oxide particle size range to a few nanometers, so long as the particles are encapsulated by     cellulose fibrils and not present on the surface where Ostwald ripening can occur.Ostwald ripening otherwise sometimes called isothermal distillation is a phenomenon associated with minimization of the surface energy in a precipitate, which results in dissolution of smaller particles and simultaneous growth of the larger ones in the system (Voorhees, 1985).Whether the formation of these large composite aggregates is caused by the formation of small iron oxide particles after bonding of ionic iron to C-CNF or by the intercalation of the iron oxide particles within the C-CNF after metal oxide formation remains unknown.TEM suggests the iron oxides associate with the outer regions of the nanosized C-CNFs, following the orientation of the individual cellulose fibrils (Figs. 5,S,S1).The statistic distribution of sizes for single-domain particles according to TEM (see Fig. 5) shows that it is clearly smaller inside the fibres 2.0-4.0 nm compared to that on their surface 3.6-16.4nm, indicating that it was in the first hand Ostwald ripening that produced the larger particles on the surface.

Drug adsorption and desorption
Fe 3 O 4 NPs are of interest not only for their magnetic properties but also for their optical properties, as they are known to display photothermal conversion which may be exploited for therapy and drug delivery (Estelrich & Busquets, 2018;Johnson et al., 2018;Sadat et al., 2014;Wang et al., 2014).In order to examine the potential of the composite material as a drug delivery vehicle, we tested the adsorption and desorption of tetracycline, a broad-spectrum antibiotic.After 72 hour contact time, tetracycline showed a maximum drug adsorption of 62 μg/mg (79%) at room temperature with up to 27 μg/mg (35%) within the first 240 min indicating that the initial adsorption is fast and thereafter slows down (Fig. 6A).
To test the viability of the composite for drug delivery and the influence daylight plays on the release of tetracycline, samples were prepared in batches and split equally.One part was exposed to a daylight lamp during desorption, and the other kept dark by wrapping in foil.Citrate buffer was added and release of tetracycline followed by UV-VIS (Fig. 6B).Initial drug release was relatively slow with an approximate 11% release in the absence of light and 20% release in light after 3 h.This increases to 33 and 85% with-and without light respectively, after 2 days indicating that the release desorption rate of tetracycline was strongly influenced by daylight.

Conclusions
In this work, we demonstrated the synthesis of a magnetically responsive composite material based on carboxymethyl cellulose and insitu synthesized magnetite, that self-assembled into fibre-like nanostructures which were characterized by AFM, SEM, TEM, FTIR, TGA and PXRD.The material displayed stable magnetic characteristics on storage, both in solid state and in solution.In addition, the novel material was studied in solution state as a potential drug vehicle for the delivery of tetracycline.Thus, the main hypothesis of this work was proved valid.
Carboxymethylated cellulose nanofibrils derived from wood were successfully decorated with iron oxide particles in an in situ process so that magnetic iron oxide particles were of relative uniform size and assembled into larger composite structures together with cellulose.These structures could be divided into three broad categories: i) Large C-CNF aggregates where the iron oxide nanoparticle size was small with growth limited by the C-CNF structure; ii) large iron oxide particles that form on the surface of the fibre aggregates, where particle growth is stimulated by Ostwald ripening, and iii) cellulose-iron oxides forming long fibre networks comprising iron oxide and cellulose with longitudinal dimensions far exceeding that of the initial components.For the fibre-like networks, it is highly likely that the morphology and pH response of both the metal oxide and C-CNF play a crucial role in its formation.Variations in cellulose source and synthetic conditions may have significant influence on the overall structures formed.The phase of iron oxide synthesized in this method is the magnetically responsive magnetite, which will oxidize to iron(II)-deficient maghemite with time in the presence of oxygen.This method is simple and cost-effective, which can lead to the development of further magnetically relevant materials.However, the synthesis of the "fibre-like" structures remains difficult with subtle changes in synthetic conditions having a profound effect on the structures obtained.
Electronic supporting information includes additional details on TEM, XRD and TGA studies, and demonstration of magnetic properties of obtained materials (as photo and video evidence).Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbpol.2022.119560.

CRediT authorship contribution statement
TB has performed all the synthetic work and adsorption and desorption experiments and wrote the draft of the manuscript, GD has performed the TEM characterization and contributed to formulation and  language editing of the manuscript, DH and PS have performed the magnetic measurements and helped with their interpretation, VK contributed with XRD measurements, HG and KH provided the C-CNF material and helped with interpretation of data, GS contributed with the project idea, TGA, FTIR, AFM and ESEM measurements and performed the final editing of the manuscript.All authors participated actively in discussion of results and contributed to editing of the manuscript.

Declaration of competing interest
The authors declare that they have no affiliations with or involvement in any organization or entity with any financial interest or nonfinancial interest in the subject matter or material discussed in this manuscript.

Fig. 3 .
Fig. 3. A) SEM-EDS image of a large aggregate showing the presence and distribution of carbon, oxygen and iron, attributed to carboxymethylated cellulose nanofibrils and iron oxide respectively.B) SEM image of the iron oxide -C-CNF.C) TEM image without staining of iron oxide-C-CNF.

Fig. 4 .
Fig. 4. TEM Images of iron-oxide carboxymethyl cellulose composites without staining and at varying magnification.

Fig. 5 .
Fig. 5. TEM images of a cross-sectioned resin embedded iron oxide -C-CNFs.Large aggregates are visible on the surface (inset shows the particle size distribution in nm) and open structure of the iron-cellulose composite, while smaller particles appear aligned with individual fibrils (inset shows the particle size distribution in nm).

Fig. 6 .
Fig. 6.Tetracycline adsorption and desorption.A) Tetracycline adsorption in mg/g and B) tetracycline desorption in mg/g as a function of time.