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Abstract

Magnetic nanoparticles are amongst the most promising adsorption materials for oil spill clean-up due to their high surface area, ease of functionalization with high oil affinity and facile separation after the cleaning process with an external magnetic field. In this work, we successfully synthesized magnetic cobalt ferrite nanoparticles (CoFe₂O₄ NPs) that were electrostatically stabilized and functionalized with various alkoxysilanes for effective oil adsorption and oil spill removal. Additionally, the adsorption capacity of CoFe₂O₄ NPs was determined, and the possibility of their reuse assessed. Prepared samples showed high oil adsorption capacities between 2.6 and 3.5 g of oil per g of nanoparticles and were successfully collected with an external magnet. Furthermore, the samples showed excellent properties after regeneration, as their adsorption capacity decreased by less than 3% after reuse. All the prepared samples were thoroughly characterized to better understand their behaviour and the differences in the use of various silanes were highlighted.

Keywords

Cobalt ferrite functionalized nanoparticles magnetic nanoparticles oil adsorbents alkoxysilanes

Introduction

Crude oil is an indispensable raw material for humanity and remains our crucial source of energy. Despite the preventative measures in the industry of oil drilling and transportation, quite some quantities are discharged in the ecosystem and approximately 0.1% of all oil is accidentally spilled into the oceans or the seas each year (Al-Majed et al., 2012, 2014; Bayat et al., 2005; Farias et al., 2015; Lin et al., 2012). In other words, out of 5 million tons of oil transported by sea, 500 tons end up getting spilled, which causes grave and irreparable damage to the marine and coastal environment (Annunciado et al., 2005; Liu et al., 2019). It is thus important to develop quick and efficient oil removal methods.

The ideal oil spill cleaning material must be hydrophobic, lipophilic (oleophilic), selective, stable, with high adsorption capacity and the ability to adsorb thick and thin layers of oil quickly and efficiently. Moreover, the material must be sustainable, environmentally friendly, and enabling immediate usage, whilst procedures for the synthesis of such material must be simple and economical (Chen et al., 2013; Gu et al., 2014). Since the material must be re-used, it needs to allow quick, simple and inexpensive separation from the contaminated water and subsequent regeneration (Tang and Lo, 2013; Zhang and Lei, 2012). Therefore, the development of an ideal oil cleaning agent remains a challenge and each oil spill removal requires an individual approach for which adsorption agents are considered to be the most economical, sustainable and efficient solution (Singh et al., 2014).

Researchers have developed various nanomaterials to clean oil spills such as aerogels, nanodispersants, nanocomposites, membranes, foams and nets, filters, carbon nanostructures, and micro- and nano-titanium oxide for photocatalytic degradation of oil (Kharisov et al., 2014). These materials have excellent adsorption capacity, but show poor regeneration yield. Also, they are not biodegradable and can, therefore, cause secondary pollution (Xu et al., 2012). Nanoparticles possess high surface area, are affordable, efficient and present an environmentally friendly solution for the removal of pollutants from water (Kharisov et al., 2014). The attempt to produce hydrophobic surfaces for oil spill removal led to studying of nanoparticles based on silicon dioxide, titanium dioxide, zinc oxide, alumina and magnetic nanoparticles based on iron oxide (magnetite and maghemite). The latter is of great interest in different fields of environmental protection, especially in adsorption of pollutants such as heavy metal ions and oil due to their superparamagnetic properties (Tang and Lo, 2013; Zhang and Lei, 2012), and ease of removal from the treated water by using the external magnetic field (Kharissova et al., 2015; Thinh et al., 2016).

Superparamagnetic iron oxide nanoparticles (SPIONs) can be synthesized by co-precipitation, microemulsion, thermal decomposition, hydrothermal or solvothermal synthesis, sonochemical synthesis, microwave-assisted synthesis, chemical evaporation method, incineration method, chemical reduction, laser pyrolysis and other methods (Faraji et al., 2010; Lu et al., 2007). Coprecipitation is simple and highly effective for the synthesis of SPIONs of controlled size, shape and chemical composition. Various iron oxides can be prepared using this method, such as magnetite (Fe₃O₄), maghemite (γ-Fe₂O₄), or cobalt ferrite (CoFe₂O₄).

Herein, the preparation of electrostatically stabilized and surface functionalized CoFe₂O₄ NPs and their successful use for adsorption of mineral oil from water is reported. Particles were functionalized using different alkoxysilanes – tetraethoxysilane (TEOS), TEOS in combination with methyltrimethoxysilane (MTMS), and TEOS with n-propyltrimethoxysilane (PTMS) in order to achieve hydrophobicity and oleophilicity via cross-linking reaction on the surface of pre-stabilized CoFe₂O₄ NPs. Hydrolysis and condensation of alkoxysilanes resulted in the formation of the SiO₂ coating. Prepared samples showed oil adsorption capacities between 2.6 and 3.5 g of oil per g of nanoparticles and were successfully regenerated, as the adsorption capacity decreased by less than 3%, showing great promise for potential applications in oil spill removal.

Materials and methods

Materials

Cobalt (II) chloride hexahydrate (CoCl₂·6H₂O), iron (III) chloride hexahydrate (FeCl₃·6H₂O), sodium hydroxide (NaOH, 99%), tetraethylorthosilicate (Si(OC₂H₅)₄ or TEOS, 99%), nitric acid (HNO₃, 65%), ammonium hydroxide (NH₄OH, 25%), n-propyltrimethoxysilane (C₃H₇Si(OCH₃)₃ or PTMS, 97%), acetone (C₃H₆O, 99.5%) were purchased from Sigma-Aldrich. Methyltrimethoxysilane (CH₃Si(OCH₃)₃ or MTMS, 98%) was purchased from Fluka. 2-Propanol (C₃H₈O, 99.5%) and ethanol (C₂H₅OH, 96% and 100%) were purchased from Gram-mol. All reagents and solvents were of analytical grade and used as obtained.

Synthesis of CoFe₂O₄ NPs

CoFe₂O₄ NPs were prepared by co-precipitation of an aqueous solution of CoCl₂·6H₂O and FeCl₃·6H₂O salts with molar ratio of Co²⁺/Fe³⁺ being 2:1 in an alkaline medium (NaOH) at pH value of 10.5. Typically, 75 mL of 1 M salts solution was added to 400 mL of 1 M NaOH. The reaction described below as equation (1) was carried out for 1 h at a temperature between 85°C and 90°C

{Co}^{2 +} + 2 {Fe}^{3 +} + 6 {OH}^{-} + 1 / 2 O_{2} \to {CoFe}_{2} O_{4} + 3 H_{2} O

(1)

Stabilization of CoFe₂O₄ NPs

The prepared CoFe₂O₄ NPs were saturated on a magnet. The residual liquid was flushed, and the nanoparticles were electrostatically stabilized with 50 mL of 2 M HNO₃ solution at room temperature with moderate agitation for 1 h. The particles were separated with the external magnet, the excess liquid was removed, and the sediment was treated with a fresh aqueous solution of Co²⁺/Fe³⁺ (1:2) described in previous paragraph for 1 h at 85°C. After saturating the particles on the magnet, the upper super-fluidic liquid phase was removed, and the particles were washed with 50 mL of 1 M HNO₃. In the last phase, the particles were saturated on a magnet, washed with acetone, centrifuged at 3500 r/min for 10 min and dispersed in 50 mL of distilled water.

Functionalization of stabilized CoFe₂O₄ NPs – Formation of core-shell structure

The dispersed CoFe₂O₄ NPs were surface functionalized with the selected methoxy silanes (MTMS and PTMS) in combination with TEOS or with TEOS alone according to the modified Stöber method. Table 1 shows the chemical and structural formula of the selected silanes.

Table 1.

Chemical and structural formula of the selected silanes.

Silane	Chemical formula	Structural formula
TEOS	Si(OC₂H₅)₄
MTMS	CH₃Si(OCH₃)₃
PTMS	CH₃CH₂CH₂Si(OCH₃)₃

MTMS: methyltrimethoxysilane; PTMS: n-propyltrimethoxysilane; TEOS: tetraethoxysilane.

Functionalization was carried out in a closed reaction vessel using 2-propanol, 25% NH₄OH as catalyst and water. The pH of the medium was adjusted to 10. The alcoholic mixture was stirred with 6 mL of an aqueous dispersion of magnetic nanoparticles with an average mass concentration of magnetic phase being 0.0914 g/mL and with a corresponding amount of TEOS precursors and selected alkoxysilane under the conditions shown in Table 2. The molar ratio between water and TEOS (R = [H₂O]/[TEOS]) was kept constant at R = 1006. Molar ratio of the two precursors (P = [TEOS]/[silane]) varied and was amounted to P values of 2; 1; 0.5; 0.25 as shown in Table 2. After a 24 h mixing at room temperature the content of the reaction vessel was centrifuged (5 min, 7500 r/min) and washed several times, with ethanol and distilled water. The product was dried at 120°C.

Table 2.

Composition of reaction mixture where molar ratio R between water and precursor TEOS was 1006.

Silane	P	Molar ratio
Silane	P	2-Propanol	Distilled water	Ammonium hydroxide	TEOS	Silane
PTMS	2	0.432	0.561	0.00548	0.00056	0.00028
	1	0.432	0.561	0.00548	0.00056	0.00056
	0.5	0.432	0.561	0.00500	0.00056	0.00112
	0.25	0.432	0.560	0.00547	0.00056	0.00223
MTMS	2	0.432	0.561	0.00548	0.00056	0.00028
	1	0.432	0.561	0.00548	0.00056	0.00056
	0.5	0.432	0.561	0.00500	0.00056	0.00112
	0.25	0.432	0.560	0.00547	0.00056	0.00223

MTMS: methyltrimethoxysilane; PTMS: n-propyltrimethoxysilane; TEOS: tetraethoxysilane.

Characterization

The phase composition of prepared samples of CoFe₂O₄ NPs was determined by X-ray powder diffraction (XRD) on Bruker D4 Endeavor apparatus with Topas software in the range of 20°–80° with 10° step. Observed diffraction peaks were also compared with standard JCPDS database (JCPDS card No. 22-1086). Particle size (d_XRD) and the lattice constant (a) of the cubic crystal structure were calculated.

Particle size, particle size distribution, morphology and core-shell structure of prepared NPs was observed by transmission electron microscopy (TEM) on Jeol TEM 2100 apparatus. The NP samples were placed on copper mesh and observed at 200 keV under different magnifications. TEM micrographs of nonfunctionalized CoFe₂O₄ NPs, as well as PTMS and MTMS functionalized CoFe₂O₄ NPs were obtained and compared.

The infrared spectra of the samples for determination of successful formation of functionalized silica coatings were measured between wavenumbers 4000 and 600 cm⁻¹ using an attenuated total reflection Fourier transformed infra-red (FT-IR) spectrophotometer model GX FT-IR. The spectra of functionalized samples, prepared with TEOS, a combination of TEOS with MTMS or PTMS at different molar ratios were compared with pure precursor spectra.

The specific surface area was determined by Brunauerr–Emmet–Teller (BET) method that predicts multi-layered or infinite adsorption of gases on the surface of the particles and no interactions between the layers of the adsorbent. Obtained BET isotherms provided information of adsorbed gas on dried NPs surface of a given mass. From the results of the specific surface area (SS_BET) the average diameter of NPs (D_BET) was calculated. The dependence of specific surface area and average diameter of functionalized NPs with different P molar ratios were also compared and discussed.

Magnetic nature of adsorbents used for oil spills removal facilitates the process of their removal from water media after the clean-up via the use of the external magnetic field. These particles are afterwards regenerated and can be re-used in the adsorption process. Achieving the sufficient magnetic response of adsorption materials is therefore crucial in ensuring the quality removal of oils from the water media. Magnetic properties of nonfunctionalized and functionalized CoFe₂O₄ NPs, such as saturation magnetization (M_s), remanence of magnetization (M_r) and coercivity (H_c) were measured with the LakeShore 7400 Series VSM magnetometer. Above listed magnetic properties were measured as a function of a magnetic field with strength H at room temperature and were presented as magnetic hysteresis.

To determine the hydrophilicity or hydrophobicity of the prepared NPs surfaces, the water contact angle measurements were conducted on goniometer Dataphysics OCA35. Measurements of the static contact angle were carried out by Zisman model by spreading a uniform amount of the NPs sample onto the carrier and applying a drop of water of a certain volume to the surface of the samples using a metal tube on the gauge. By image analysis, the changes in height, width, volume and diameter of the drop in a certain time state were monitored (Černe et al., 2008).

Oil adsorption capacity and evaluation of oil removal efficiency of prepared NPs

The oil adsorption capacity of functionalized magnetic CoFe₂O₄ NPs was evaluated by used motor oil as testing material. Used motor oil was chosen because it is already intensely coloured to ease the visual observation without the need to add colourings that could negatively affect the adsorption. Due to its less volatile nature than crude oil, petrol or diesel fuel, it is also safer to handle and finally, it represents an actual environmental pollutant.

The oil adsorption capacity was determined gravimetrically in two parallel tests. Oil removal testing is described below and depicted in Figure 1. The functionalized magnetic NPs (m₁ = 50 mg) were added to the pre-weighed petri dish containing distilled water and used motor oil (m₂) as seen in Figure 1(a) and (b). Oil volume of 0.25 mL was constant. After a certain time of adsorption, the material with adsorbed oil was collected with an external magnet placed in a tube (Figure 1(c)). Remaining oil (Figure 1(d)) was weighted together with the petri dish and water (m₃). A tube was placed in a vial filled with ethanol and the magnet was removed (Figure 1(e)). Then, the vial was placed in an ultrasonic bath for 5 min in order to remove the NPs with adsorbed oil from the surface of the tube (Figure 1(f)). A vial containing ethanol, NPs and oil was placed over a permanent magnet (Figure 1(g) and (h)). After 30 min, the upper phase (ethanol with desorbed oil) was transferred to a separate vial. The NPs were washed twice with ethanol and dried for 2 h at 120°C (Figure 1(i)). Dry samples were weighted in order to evaluate the regenerated NPs.

Figure 1.

Procedure of oil removal from water using functionalized magnetic NPs.

The adsorption capacity of each prepared material was calculated according to equation (2) (Chen et al., 2013; de Souza et al., 2010; Ge et al., 2014; Liu et al., 2014; Zhu et al., 2010)

Q = (m_{2} - m_{3}) / m_{1}

(2)

Two samples with the highest oil adsorption capacity results were also evaluated for regeneration, reuse and its effectiveness.

Results and discussion

X-ray powder diffraction of CoFe₂O₄ NPs

During the co-precipitation reaction in alkali media the Co²⁺ and Fe³⁺ ions firstly precipitate in the form of hydroxides (Cornell and Schwertmann, 2003) and secondly, oxidize to CoFe₂O₄ NPs. XRD pattern of prepared CoFe₂O₄ NPs is shown in Figure 2. Diffraction peaks at (220), (311), (222), (400), (422), (511), and (440) were compared with the standard data for CoFe₂O₄ (JCPDS card No. 22-1086). The observed peaks are characteristic for cobalt ferrite and belong to cubic spinel crystal structure.

Figure 2.

X-ray powder diffraction pattern of CoFe₂O₄ NPs.

The average particle size (d_XRD) was calculated based on the width of the most intensive diffraction peak at (311) and with the help of Debye-Scherrer equation (3)

d_{XRD} = \frac{K λ}{β \cos θ_{B}}

(3)where constant K depends on the relative orientation of the scattering vector to the external shape of the crystallite, λ is the x-ray wavelength, β is the full-width at half-maximum (Θ) of the diffraction peak and θ_B is the Bragg angle (Holzwarth and Gibson, 2011; Muniz et al., 2016). The calculated d_XRD of CoFe₂O₄ NPs was 11.4 nm.

Using the Bragg equation (equation (4)) and the position of the individual diffraction peaks and Miller indices (h, k, l), lattice constant (a) of the cubic crystal structure was calculated according to equation (5)

2 d \sin θ = n λ

(4)

d = \frac{a}{\sqrt{h^{2} + k^{2} + k^{2}}}

(5)where λ is the wavelength of the moving electrons or X-ray beam, d is the spacing of the crystal planes in the atomic lattice,

θ

is the angle between the incident ray and the scattering planes and n is an integer (Thornton and Rex, 2013).

Obtained a was 8.36 Å, which coincides with the JCPDS database. We can conclude that the CoFe₂O₄ NP with an average size of 11.4 nm consists of an average of 14 cubic basic crystal cells.

Transmission electron microscopy of nonfunctionalized and functionalized CoFe₂O₄ NPs

A TEM micrograph of nonfunctionalized CoFe₂O₄ NPs (Figure 3(a)) shows their spherical morphology and narrow size distribution of 11.5 ± 1.9 nm. The estimated particle size coincides with the calculated D_XRD results. The electron diffraction pattern of CoFe₂O₄ (Figure 3(b)) confirms the formation of a spinel crystal structure with a high degree of crystallinity of the material, which is consistent with the HRTEM image of CoFe₂O₄ (Figure 3(c)).

Figure 3.

(a) TEM image of CoFe₂O₄ NPs, (b) diffraction image of CoFe₂O₄ NPs, (c) HRTEM image of CoFe₂O₄ NPs, (d) TEM image of SiO₂@CoFe₂O₄ NPs and CoFe₂O₄ NPs functionalized with TEOS in combination with silane PTMS at molar ratios P = 1 (e), P = 0.5 (f) and silane MTMS at molar ratios P = 1 (g) and P = 0.5 (h).

Figure 3(d) shows functionalized CoFe₂O₄ NPs coated with a layer of silicon dioxide (SiO₂@CoFe₂O₄), which contributed to an increase in the particle size to the average value of 13.1 nm. Since organic coatings can be sensitive to ionizing radiation present in conventional TEM microscopes and are known to have poor electronegative contrast of the image, SiO₂ coatings around NPs are poorly seen (Egerton et al., 2004; Gontard et al., 2014). The average thickness of the CoFe₂O₄ surface coating of the NPs prepared in combination with the selected alkoxysilanes, estimated based on TEM images, occupies values between 1.5 and 6 nm. Figure 3(e) to (h) shows TEM images of SiO₂@CoFe₂O₄ in combination with silanes (P = 1 and P = 0.5).

FT-IR spectroscopy

FT-IR spectra of pure TEOS and TEOS functionalized magnetic NPs are shown in Figure 4. Typical precursor peaks are between 2975 and 2880 cm⁻¹ and represent C–H stretching in the ester groups (Yong et al., 2014). The peaks between 1443 and 1296 cm⁻¹ can be attributed to the asymmetric bending or wagging of C–H bonds. Peaks at 1168 and 968 cm⁻¹ belong to C–H rocking. The ethoxy group bound to the silicon atom (Si–OCH₂CH₃) is characterized by peaks around 1100 cm⁻¹ (Viart and Rehspringer, 1996;Ypenburg and Gerding, 1972). SiO₂@CoFe₂O₄ NPs show absorption peaks at 3388 cm⁻¹, which is characteristic for the O–H bond, whilst the peaks at 1100 and 1050 cm⁻¹ correspond to Si–O–Si bonds (Heidari et al., 2009). The absence of a peak at 2975 cm⁻¹, reduction of the 968 cm⁻¹ peak and its shift to 962 cm⁻¹ confirm the complete conversion of TEOS ester groups to silanols, the complete hydrolysis (Bauman, 2010) and successful functionalization of CoFe₂O₄ NPs.

Figure 4.

FT-IR spectra of pure precursor TEOS and SiO₂@CoFe₂O₄ NPs.

Figure 5 shows the FT-IR spectra of functionalized samples, prepared with precursor TEOS with MTMS (Figure 5(a)), and TEOS with PTMS (Figure 5(b)) at molar ratios P = 2, 1, 0.5 and 0.25. FT-IR spectra of pure precursors (TEOS, MTMS, PTMS) are shown for comparison.

Figure 5.

FT-IR spectra of (a) MTMS@SiO₂@CoFe₂O₄ and (b) PTMS@SiO₂@CoFe₂O₄NPs.

In Figure 5(a), FT-IR spectra of precursor MTMS, CoFe₂O₄ NPs functionalized with TEOS and MTMS in different molar ratios (P) are depicted. In the spectra of MTMS, a peak around 2944 cm⁻¹ can be attributed to the asymmetric C–H stretching, whilst the pronounced narrow peak at 2841 cm⁻¹ is characteristic for the Si–O–CH₃ symmetric stretching (Sibeaud et al., 2016). The two C–H bending vibration peaks were observed at 1466 and 1409 cm⁻¹ (Kumar Trivedi, 2015). The peak at 1268 cm⁻¹ is typical for the bending of Si–CH₃ group (Jamil Ikhmayies et al., 2016). Peaks at 1190 and 1076 cm⁻¹ were attributed to the Si–O–CH₃ group. The peak at 835 cm⁻¹ is typical of the Si–CH₃ group. At a wavenumber of 788 cm⁻¹, the tip is attributed to the longitudinal oscillation of the Si–C bond (Wasyluk et al., 2010). The peak at 738 cm⁻¹ can be attributed to the bending of the C–H bond. In the samples of MTMS@SiO₂@CoFe₂O₄NPs, peaks are observed between the wavenumber of 3402 and 3386 cm⁻¹, and between 1638 and 1630 cm⁻¹, which present the O–H bond in water adsorbed on the surface of the material. The absorption peaks between 2980 and 2877 cm⁻¹ present the asymmetrical C–H stretching (Viart and Rehspringer, 1996; Ypenburg and Gerding, 1972). In all four samples of functionalized NPs, the peak present between 1277 and 1271 cm⁻¹ indicates Si–CH₃ bond, whilst peaks at 1045 and 1028 cm⁻¹ indicate Si–O–Si bond. Smaller peaks at wavenumbers between 900 and 790 cm⁻¹ were attributed to Si–C bonds and a more pronounced peak at a wavenumber of about 780 cm⁻¹ was attributed to the Si–CH₃ group (Lee Smith, 1960; Montgomery, 1997). We can, therefore, conclude that the functionalization of MTMS@SiO₂@CoFe₂O₄ NPs was successful.

Figure 5(b) shows that PTMS absorbs infrared waves between 2957 and 2872 cm⁻¹, which was ascribed to the C–H stretching (Mansur et al., 2002). A pronounced narrow peak at 2840 cm⁻¹ is characteristic of the Si–O–CH₃ group (Bellamy, 1975; Montgomery, 1997; Sibeaud et al., 2016). The peak at 1215 cm⁻¹ was attributed to the Si–CH₂CH₂CH₃ group, whilst the peaks at 1190 and 1080 cm⁻¹ can be attributed to the Si–O–CH₃ group. The Si–O bond was characterized by the peak at 1003 cm⁻¹, Si–C bond by the peaks between 896 and 774 cm⁻¹, and –CH₂– bond by the peak at 752 cm⁻¹ (Coates, 2000; Lee Smith, 1960; Montgomery, 1997). The O–H bond from the water adsorbed onto the surface of the material can be seen in the FT-IR spectra of NPs functionalized with TEOS and PTMS (peaks between 3401 and 3382 cm⁻¹, and between 1632 and 1628 cm⁻¹). Peaks at around 1220 cm⁻¹ in samples P = 2, P = 0.5 and P = 0.25 indicate the presence of the CH₂CH₂CH₃ group. The formation of the silica coating was further confirmed by the presence of the Si–O–Si bond (peaks between 1051 and 1034 cm⁻¹) and Si–C bond (less pronounced peaks between 910 and 778 cm⁻¹). The presented FT-IR spectra thus confirmed successful functionalization of PTMS@SiO₂@CoFe₂O₄ NPs.

Specific surface area and calculated diameter of CoFe₂O₄ NPs and functionalized CoFe₂O₄ NPs using BET method

The results of specific surface area measurements of the nanoparticle samples are shown in Table 3.

Table 3.

Specific surface area of CoFe₂O₄ NPs prepared in combination of TEOS with alkoxysilanes in various molar ratios.

Silane	Specific surface area of nanoparticles with molar ratio P [TEOS]/[silane] (m²g⁻¹)
Silane	2	1	0.5	0.25
PTMS	95.05 ± 0.54	95.97 ± 0.46	95.89 ± 0.63	38.29 ± 0.34
MTMS	57.12 ± 0.33	115.16 ± 0.42	100.04 ± 0.50	55.42 ± 0.18
SiO₂@CoFe₂O₄	91.23 ± 0.64
Nonfunctionalized CoFe₂O₄ NPs	82.50 ± 0.18

NPs: nanoparticles; MTMS: methyltrimethoxysilane; PTMS: n-propyltrimethoxysilane; TEOS: tetraethoxysilane.

From the data of the NPs specific surface area determined by the BET method, the average diameter of nanoparticles (D_BET) was calculated. These results are presented in Table 4.

Table 4.

Average diameter of CoFe₂O₄ NPs prepared in combination of TEOS with alkoxysilanes in various molar ratios.

Silane	D_BET of nanoparticles with molar ratio P [TEOS]/[silane] (nm)
Silane	2	1	0.5	0.25
PTMS	12.9	12.8	12.8	32.0
MTMS	21.4	10.6	12.2	23.1
SiO₂@CoFe₂O₄	13.4
Nonfunctionalized CoFe₂O₄ NPs	14.8

MTMS: methyltrimethoxysilane; PTMS: n-propyltrimethoxysilane; TEOS: tetraethoxysilane.

The particle size of CoFe₂O₄ NPs determined with the measurements of the specific surface area takes a slightly higher value (14.8 nm) than the particle size determined by X-ray powder diffractometry (11.4 nm) and electron microscopy (11.5 nm), which can be attributed to the agglomeration of the particles.

Calculated D_BET values are in the range between 10.6 and 32.0 nm. The smallest particles are functionalized with MTMS at P = 1 and the largest are prepared using PTMS at P = 0.25.

We expected that the specific surface area of the particles will decrease with the increasing length of the aliphatic chain from methyl (MTMS) to propyl (PTMS), and that consequently the size of the particles will increase. As the length of the aliphatic chain of the precursor increases, the interactions between the alkyl parts in the structure begin to dominate, leading to an increase in steric obstruction between silane molecules. Compared to long aliphatic chains, short aliphatic chains allow molecules of silanes a less obstructed approximation to the surface, which is reflected in a greater proportion of the surface coverage of the particles, a lower specific surface and a larger particle size. In the case of selected silanes, the specific surface follows the findings and, on average, decreases from silane with the short chain (MTMS) to the longer chain silane (PTMS; Figure 6) for the molar ratios P = 1 and P = 0.5, whilst at values P = 2 and P = 0.25, the specific surface first rises from the MTMS silane and then drops in the case of PTMS silane use. The reason for this is in the poor coverage of the surface of the particles with the MTMS silane which is attributed to the small amount of the silane additive in the case of P = 2 and the pronounced saturation of the surface of the particles with the MTMS silane at P = 0.25 (due to the increased molarity), which is reflected in reduction of the specific surface and in increased particle size (Figure 6(a)). Other authors have come to similar conclusions in their works (Sewvandi et al., 2014).

Figure 6.

Dependence of specific surface area and diameter on molar ratio P for samples (a) MTMS@SiO₂@CoFe₂O₄ and (b) PTMS@SiO₂@CoFe₂O₄.

Depending on the amount of silane addition, we found that the largest diameter (D_BET) have NPs functionalized with the maximum amount of added single methoxysilane (P = 0.25). With the molar ratio of silanes P = 0.25, the largest D_BET was measured for PTMS@SiO₂@CoFe₂O₄ NPs, which have in their structure the longest aliphatic part – the propyl chain. For NPs of PTMS@SiO₂@CoFe₂O₄, the specific surface area of NPs at molar ratios P = 2, P = 1 and P = 0.5 remained on average approximately the same (from 95 to 96 m² g⁻¹), whilst in the ratio P = 0.25, the specific surface area was significantly reduced to a value of 38.3 m² g⁻¹. The latter can be attributed to the length of the aliphatic chains and the attractive interactions between them. From the comparison of the samples, we found that the specific surface varied the most with the MTMS@SiO₂@CoFe₂O₄ NPs, and was the smallest for the samples with the ratio P = 2 (57.1 m² g⁻¹) and P = 0.25 (55.4 m² g⁻¹), whilst for the P = 1 and P = 0.5 higher values (115.2 and 100.0 m² g⁻¹, respectfully) were taken. The dependence of the specific surface area and size of the functionalized NPs on the molarity ratio P of TEOS and MTMS or PTMS silanes is shown in Figure 6.

According to the results, it can be concluded that the length of the aliphatic chain affects the specific surface of functionalized NPs (SS_BET) at lower mass ratios P. The specific surface decreases with the prolongation of the alkyl chain. We note that the amount of added TEOS affects the specific surface of functionalized NPs, which increases with the increased TEOS amount. The exceptions are NPs that are functionalized with a smaller amount of TEOS. We assume that in such samples, most likely due to the thin silicate coating with short functional chains, attractive forces still prevail amongst the NPs. Therefore, the nanoparticles begin to aggregate, resulting in a reduced surface area and increased size. Such an effect can most likely be attributed to observed phenomena of the MTMS@SiO₂@CoFe₂O₄ NPs sample at P = 2.

VSM characterization

Figure 7(a) shows the hysteresis curve of the nonfunctionalized CoFe₂O₄ NPs sample. CoFe₂O₄ NPs reach a saturated magnetic response of 52.00 emu g⁻¹ at the magnetic field strength H = 10000 Oe. With the demagnetization of the sample, the value of the magnetic response decreases and, at the intensity of the field H = 0 emu g⁻¹, the remanent magnetization M_r is about 10.17 emu g⁻¹. These results show that nonfunctionalized particles still have a considerable part of the magnetic properties after the removal of the external magnetic field. The coercivity H_c is 269.69 G.

Figure 7.

Magnetic properties of (a) CoFe₂O₄ and (b) SiO₂@CoFe₂O₄ NPs.

Figure 7(b) shows the hysteresis curve of surface-treated CoFe₂O₄ NPs sample (SiO₂@CoFe₂O₄). The specific magnetization of this sample is lower compared to the nonfunctionalized CoFe₂O₄ NPs sample. That can be contributed to the non-magnetic contribution of the amorphous phase of silicon dioxide (SiO₂) covering the surface of CoFe₂O₄ NPs.

It is therefore expected that the further functionalization of SiO₂@CoFe₂O₄ NPs with the molecules of selected alkoxysilanes will further reduce the specific magnetization of the samples due to the increased contribution of non-magnetic alkoxysilanes to the total magnetization.

The magnetic properties of materials are largely influenced by their intrinsic properties and particle size, which is determined by the high ratio between surface and volume. The particle size affects both the value of the specific magnetization (M_s), as well as the coercivity (H_c) and the remanence (M_r). Various authors reported that specific magnetization increases with increasing particle size to a certain maximum value and vice versa the values of M_s, H_c and M_r decrease with the decreasing particle size. Ferromagnetism occurs when the diameter of the particles is above a certain value, called critical diameter. Only below these critical particles’ diameter values, the nanoparticles become superparamagnetic. The coercivity of magnetic NPs also increases with increased size until critical diameter value is reached, after which the coercivity is decreasing. Another crucial property of NPs that influences their magnetic characteristics is their surface. Surface atoms of magnetic nanoparticles namely lack outer neighbouring atoms and thus have disordered spins. Decreasing the particles’ size increases the number of disordered spins and consequently decreases the specific magnetization, coercivity and remanence. To overcome this phenomenon adequate surface functionalization can be employed (Goldman, 2006; Gubin, 2009; Kolhatkar et al., 2013; Maaz et al., 2007; Spaldin, 2011).

Determining the crucial diameter of synthesized magnetic NPs is of utmost importance due to its influence on magnetic properties and to understand the response of prepared nanoparticles to the external magnet. For the CoFe₂O₄ NPs, the reported values of critical diameter vary from 23 nm (Maaz et al., 2007) up to 70 nm or more (Berkowitz and Schuele, 1959). With the presumption that the prepared CoFe₂O₄ NPs have a spherical shape, we used equation (6) to determine the critical diameter

D_{c} = \frac{9 \times σ_{W}}{2 π M_{s}^{2}}

(6)where σ_w represents the energy of the domain wall that can be calculated according to equation (7)

σ_{W} = {(\frac{2 k_{B} T_{C} |K_{1}|}{a})}^{\frac{1}{2}}

(7)where D_C (m) is critical diameter of nanoparticle, M_S (emu g⁻¹) is specific magnetization, k_B (J K⁻¹) is Boltzmann constant, T_C (K) is Curie temperature, K₁ (erg cm⁻³) is constant of magneto-crystalline anisotropy and a (m) is lattice constant.

With the literature data (Mathew and Juang, 2007; Rondinone et al., 2000) and obtained experimental values critical diameter for prepared nonfunctionalized nanoparticles is 512 nm. Because the prepared nonfunctionalized nanoparticles have much lower values than calculated critical diameter confirmed by BET and TEM analysis, we concluded that all prepared functionalized nanoparticles are single-domain particles.

Figure 8 shows the magnetic properties of sample SiO₂@CoFe₂O₄ NPs prepared in combination with alkoxysilanes MTMS and PTMS in molar ratio P = 1.

Figure 8.

Magnetic properties of (a) PTMS@SiO₂@CoFe₂O₄, (b) MTMS@SiO₂@CoFe₂O₄ samples at molar ratio P = 1.

It is known that the specific magnetization M_s of samples decreases with the length of the silane alkyl chain, since the length of the silane chain and the silicate coating contribute their share to the mass of NPs, and M_s is defined per unit mass. Because alkyl chains of here used precursors are shorter (1 or 3 C-atoms), we did not expect very pronounced M_s drops.

The magnetic properties of prepared NPs are influenced by the NPs size (D_BET). Smaller NPs show less coercivity (H_c), lower specific magnetization (M_s) and lower remanent magnetization (M_r; Table 5), and vice versa, which is also in line with expectations, since we are in the size range where NPs are single dose. To compare the influence of the chain length on M_s we calculated the M_s/D_BET factor, which is increasing with the reduction of the aliphatic chain. At the same particle diameter, lower specific magnetization is expected for samples of PTMS@SiO₂@CoFe₂O₄ than for samples of MTMS@SiO₂@CoFe₂O₄. This can be attributed to the influence of aliphatic chains or specifically their contribution to the mass of the coating of a functionalized nanoparticle. Likewise, coatings on NPs can be different, since different silane molecules are incorporated. The values of coercivity and remanent magnetization are similar in all compared samples but higher than nonfunctionalized CoFe₂O₄ NPs. For oil spill cleaning, it is more desirable for the magnetic material to mostly behave as the soft magnet (it has the lowest H_c and M_r), since we do not want the NPs to show a magnetic response after removal of the external magnetic field. These measurements confirm that here reported NPs are suitable for use as adsorbents because they are satisfactorily magnetically responsive.

Table 5.

Comparison of the average particles size (D_BET) and their magnetic properties of CoFe₂O₄, SiO₂@CoFe₂O₄ NPs and silane functionalized NPs at ratio P = 1.

NPs	D_BET (nm)	Specific magnetization(M_s) (emu g⁻¹)	Coercivity(Hc) (G)	Remanent magnetization(M_r) (emu g⁻¹)	M_s/D_BET
CoFe₂O₄		52.00	269.69	10.17
SiO₂@CoFe₂O₄		50.28	311.18	12.35
PTMS@SiO₂@CoFe₂O₄	12.8	49.95	360.74	12.98	3.9
MTMS@SiO₂@CoFe₂O₄	10.6	49.32	355.36	12.38	4.7

NPs: nanoparticles; MTMS: methyltrimethoxysilane; PTMS: n-propyltrimethoxysilane.

Water contact angle measurements

Measurements of water contact angle for nonfunctionalized NPs showed that water droplet immediately wetted the surface of CoFe₂O₄ NPs firmly pressed on the adhesive substrate, which gives the value of contact angle of 0°. According to the literature of hydrophilic/hydrophobic theory the contact angles below 90° represent hydrophilic surfaces and contact angles above 90° represent hydrophobic surfaces (Arkles, 2011). SiO₂-coated CoFe₂O₄ NPs have also shown hydrophilic surface, although the water droplet took 4 s to wet the surface.

PTMS functionalized magnetic NPs showed somewhat less hydrophilic character for molar ratios of P = 2 and P = 1 where the measured contact angle was 3°. The molar ratio of P = 0.5 was wetted by the water droplet in 2 s. Hydrophobic properties were observed for functionalized sample with molar ratio P = 0.25, where the water droplet had a contact angle of 95° also shown in Figure 9(d).

Figure 9.

Water contact images of PTMS functionalized magnetic NPs with molar ratio (a) P = 2, (b) P = 1, (c) P = 0.5 and (d) P = 0.25.

MTMS functionalized magnetic NPs had for all molar ratios hydrophilic character, but the time until the water droplet fully wetted the surface was longer than observed for nonfunctionalized NPs, showing that the functionalization with MTMS lowers their hydrophilic character.

According to the results, the alkyl chain length influences the hydrophilicity. With increased chain length, the hydrophilicity decreases and the hydrophobicity increases. Methyl chains present in MTMS give more hydrophilic character to the functionalized magnetic NPs, whereas the propyl chains of PTMS give the NPs a hydrophobic character, as was observed for the sample PTMS@SiO₂@CoFe₂O₄ with molar ratio P = 0.25.

Evaluation of oil removal capacity, oil removal efficiency and recovery of prepared magnetic NPs adsorbents

According to the procedure described in ‘Materials and methods’ section the obtained adsorption capacities of PTMS and MTMS functionalized magnetic NPs are summarized in Table 6.

Table 6.

Adsorption capacity values obtained for PTMS and MTMS functionalized magnetic NPs.

	Average adsorption capacity of used motor oil (g of oil per 1 g NPs)
Molar ratio	PTMS@SiO₂@CoFe₂O₄	MTMS@SiO₂@CoFe₂O₄
P = 2	2.9 ± 0.1	3.0 ± 0.1
P = 1	3.6 ± 0.1	3.3 ± 0.1
P = 0.5	2.9 ± 0.1	3.2 ± 0.1
P = 0.25	2.8 ± 0.2	2.6 ± 0.2

MTMS: methyltrimethoxysilane; PTMS: n-propyltrimethoxysilane.

The results show very good used motor oil adsorption capacity for prepared functionalized magnetic NPs with values between 2.6 and 3.6 g of oil for 1 g of NPs, which is roughly three times the value of their own weight. Both silanes used for functionalization of magnetic NPs have the best adsorption capacity when their molar ratio is P = 1, which coincides with the highest surface area value obtained with BET measurements. It is a widely accepted and repeatedly confirmed fact that higher specific surface area would also have the highest adsorption rate.

Higher adsorption capacity of PTMS functionalized NPs can be attributed to longer aliphatic chain of propyl, compared to shorter methyl chains of MTMS. Higher C atom presence in longer aliphatic chains presumably have more Van der Waals attractive forces on the NPs surface that can interact with oil molecules, giving the particles higher adsorption capabilities. Measured water contact angles also showed that PTMS functionalized NPs had slightly more hydrophobic character than MTMS functionalized NPs. Hydrophobicity is also an important property of oil spill adsorbent material, so that it adsorbs mostly oil and not the water.

Regeneration of prepared oil adsorbent magnetic NPs was conducted with facile ethanol rinsing in the ultrasound bath for 5 min in three repetitions, described in other work (Chen et al., 2013; Zhu et al., 2010). Regenerated NPs were used again for removal of oil from water and evaluation of specific adsorption. PTMS (P = 1) functionalized magnetic NPs had adsorption capacity of 3.6 g of oil per 1 g NPs at first adsorption and 3.5 g of oil per 1 g NPs at subsequent adsorption, whilst MTMS functionalized NPs had adsorption capacity of 3.3 g of oil per 1 g NPs at first adsorption and also at reuse, which proved that prepared NPs have superb regeneration rate and that facile regeneration procedure is appropriate.

For the adsorption capacities comparison with other published work, similar researches were chosen in terms of using functionalized magnetic NPs for adsorption of oil types – new and used lubricating oils, such as motor oil. The adsorption capacity of here reported functionalized magnetic NPs were amongst the highest with the comparable research shown in Table 7 and their almost unchanged adsorption capacities after regeneration are the highest amongst reported, thus showing a great potential for their application in real case scenarios.

Table 7.

Comparison of adsorption capacity of lubricating oils using functionalized magnetic NPs and their regeneration.

Magnetic NPs	Coating or functionalization	Oil type	Adsorption capacity(g of oil per 1 g NPs)	Adsorption capacity percentage after regeneration	Reference
Fe₃O₄	Polystyrene	Lubricating oil	3.0	Determined by CA^a	Chen et al. (2013)
Fe₃O₄	Aminopropyl-trimethoxysilane and palm acid	Lubricating oil	3.5	77%	Rozi et al. (2018)
Fe₃O₄	Yeast	Used motor oil	2.8	Not reported	Debs et al. (2019)
CoFe₂O₄	Propyltrimethoxy-silane	Used motor oil	3.6	97%	This work
CoFe₂O₄	Methyltrimethoxy-silane	Used motor oil	3.3	100%	This work

NPs: nanoparticles.

^aThe regeneration and reuse of NPs was evaluated by water contact angle measurements.

Conclusions

In the presented research work, we have successfully synthesized monodisperse CoFe₂O₄ magnetic NPs with spinel crystal structure with diameters of 11.5 ± 1.9 nm and electrostatically stabilized them to obtain a stable colloidal solution. The magnetic NPs were functionalized with TEOS in combination with two different chain length silanes (PTMS and MTMS) in various molar ratios. Extensive characterization of nonfunctionalized and functionalized magnetic NPs was conducted in order to understand their behaviour when used as oil adsorbents. Silicon layer increased the particles size from 11.4 to 13.1 nm. The specific surface area that is crucial for good adsorbent material was determined with BET method for all prepared NPs and was the highest for MTMS and PTMS functionalized NPs at molar ratio P = 1. Influence of the length of aliphatic chain and the amount of added alkoxysilanes was discussed, where shorter aliphatic chains contribute to greater surface coverage due to attractive interactions which results in lower specific surface area and larger particle size at lower molar ratios. The saturation and coverage of the NPs surface with alkoxysilanes was determined to be the main factor for obtained variations in size and specific surface area properties. Vibrating-sample magnetometer (VSM) measurements showed good magnetic properties of nonfunctionalized CoFe₂O₄ NPs, which had slightly decreased specific magnetization after their functionalization with alkoxysilanes but were still responsive to an external magnet and thus acceptable for final oil adsorption studies. Higher hydrophobicity, also important for better oil adsorption was observed for longer aliphatic chain alkoxysilane PTMS. All functionalized magnetic NPs showed good adsorption capacities for used motor oil adsorption, ranging from 2.6 to 3.6 g of oil per 1 g NPs. MTMS and PTMS functionalized magnetic NPs with the highest specific surface area and highest experimental oil adsorption capacities also showed that facile regeneration procedure of rinsing in ethanol was proven very successful because the regenerated nanoparticles had almost unchanged adsorption capacity at reuse.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by a Grant for Young Researches (No. 1000–16-0552) from the Slovenian Research Agency (ARRS).

ORCID iD

Ajra Hadela

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Novel reusable functionalized magnetic cobalt ferrite nanoparticles as oil adsorbents

Abstract

Keywords

Introduction

Materials and methods

Materials

Synthesis of CoFe2O4 NPs

Stabilization of CoFe2O4 NPs

Functionalization of stabilized CoFe2O4 NPs – Formation of core-shell structure

Characterization

Oil adsorption capacity and evaluation of oil removal efficiency of prepared NPs

Results and discussion

X-ray powder diffraction of CoFe2O4 NPs

Transmission electron microscopy of nonfunctionalized and functionalized CoFe2O4 NPs

FT-IR spectroscopy

Specific surface area and calculated diameter of CoFe2O4 NPs and functionalized CoFe2O4 NPs using BET method

VSM characterization

Water contact angle measurements

Evaluation of oil removal capacity, oil removal efficiency and recovery of prepared magnetic NPs adsorbents

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

ORCID iD

References

Synthesis of CoFe₂O₄ NPs

Stabilization of CoFe₂O₄ NPs

Functionalization of stabilized CoFe₂O₄ NPs – Formation of core-shell structure

X-ray powder diffraction of CoFe₂O₄ NPs

Transmission electron microscopy of nonfunctionalized and functionalized CoFe₂O₄ NPs

Specific surface area and calculated diameter of CoFe₂O₄ NPs and functionalized CoFe₂O₄ NPs using BET method