Preparation of magnetic manganese ferrite/graphene oxide nanocomposites as efficient adsorbents towards ibuprofen

Graphite (Gi) powder with particle size of 20 μm was purchased from Sigma-Aldrich (Germany). All analytical grade chemicals including H₂SO₄ (98 wt.%), H₃PO₄ (85 wt.%), HCl (36 wt.%), FeCl₃.6H₂O (99 wt.%), MnCl₂.4H₂O (99 wt.%), H₂O₂ (30 wt.%), and NaOH (99 wt.%) were purchased from Xilong Chemical, China. KMnO₄ and C₂H₅OH with purities of 99 wt.% were purchased from Vina Chemsol, Vietnam. Ibuprofen (C₁₂H₁₈O₂, 99 wt.%) was supplied by Institute of Drug Quality Control – Ho Chi Minh City, Vietnam.

GO was fabricated by improved Hummers' method [16]. MnFe₂O₄ particles and MnFe₂O₄/GO nanocomposites were synthesised by co-precipitation method [17]. The MnCl₂.4H₂O and FeCl₃.6H₂O were dissolved in GO suspension (5 g l⁻¹). Then, the black precipitates were formed by adding each drop of NaOH 2 M in solution to pH 11. The mixture was heated at 80 °C for 2 h under vigorous stirring. The black precipitates were washed with water and dried at 60 °C. The nanocomposites with different mass ratios of 30–60 wt.% MnFe₂O₄ were marked as MG30, MG40, MG50, and MG60, respectively.

The crystalline structure of materials was analysed on D8 Advance Bruker powder diffractometer with a Cu–K_α excitation source. The functional groups on the surface were studied by FTIR spectrum (Tensor 27, Bruker). The elemental composition was investigated by using scanning electron microscope coupled with energy-disperse X-ray analyser (SEM-EDX) (Jeol JMS 6490, Japan). The morphologies were identified by TEM technique (JEM-1400, Japan) at an accelerating voltage source of 200 kV. The BET specific surface area was measured through N₂ adsorption isotherm curve at 77.3 K and p_o = 765 mmHg (Nova 3200e, Quantachrome). The saturation magnetisation value was recorded using VSM (MicroSense Easy VSM v. 9.13 L).

The effects of MnFe₂O₄ content, time (15–600 min), pH (3–10), and concentration of ibuprofen (5–480 ppm) on the removal performances of the materials were studied. The conditions were conducted including 20 mg of adsorbent, 20 mL ibuprofen solution, shaking rate of 100 rpm, and room temperature. Finally, the adsorbent was recovered by magnet. The ibuprofen concentration was determined by UV-Vis spectroscopy (Horiba Dual FL, Japan). All experiments were triplicated to estimate the error. The adsorption capacity (q, mg g⁻¹) was calculated following equation (1).

$$\begin{eqnarray}&&q=\displaystyle \frac{({C}_{0}-{C}_{e})V}{m},\end{eqnarray} \tag{ 1 }$$

where, C_o and C_e (ppm) are the initial and equilibrium concentrations of ibuprofen, respectively; V (mL) is the volume of ibuprofen solution, and m (mg) is the mass of adsorbent.

Besides, the pH values at the potential of zero-point charge (pH_zpc) of materials were determined by using the pH drift method. The adsorption data were studied by pseudo-first-order and pseudo-second-order kinetic, Langmuir, and Freundlich isotherm models.

As shown in figure 1, the sharp peak of GO was displayed at 2θ = 11.30°. Gi was oxidised to create GO; and the oxygen-containing groups was inserted into Gi structure. As a result, there was an increase in the distance between Gi layers, which was the basis for Gi's separation of layers to form GO. In particular, the distance of crystalline phase of GO (0.80 nm) is higher than that of Gi (0.34 nm) [18]. Several peaks of MnFe₂O₄/GO are similar to the cubic spinel ferrite (JCPDS 10-0319) at 2θ = 27.5, 32.5, 46.2, 57.2, 68.4, and 76.5° corresponding to the (220), (311), (400), (511), (440), and (620) planes of MnFe₂O₄, respectively [19]. The diffraction peak of GO did not exist as shown in figure 1(b) due to the high diffraction intensities of MnFe₂O₄ NPs overwhelming the weak GO peak. The change in intensities of diffraction peaks in nanocomposites shows the effect of MnFe₂O₄:GO ratios on the formation of MnFe₂O₄ crystals. The sizes of MnFe₂O₄ particles calculated from (311) plane were 15.03, 17.21, 20.15 and 26.35 nm, corresponding to MG30, MG40, MG50, and MG60, respectively. The results revealed the role of GO effect on the creation of MnFe₂O₄ particles on the surface of the GO sheets.

Zoom In Zoom Out Reset image size

As shown in figure 2, FTIR spectra indicated several bands in GO at 3450, 1720, 1640, 1225, and 1080 cm⁻¹ assigned to stretching vibrations of –OH, –C=O, –C=C, –C–OH, –C–O, respectively. This result confirms the oxidation and exfoliation of Gi to create GO. For MnFe₂O₄/GO nanocomposites, the reduction of the intensities of functional groups indicates that the Mn²⁺ and Fe³⁺ ions were linked to –OH, –C=O, and –COOH groups as electronegative positions of GO. The MnFe₂O₄ NPs are formed at the sites on the GO surface [20]. The elemental analysis of MG50 was confirmed by EDX analysis and shown as table 1. The results indicate that the weight percentage of MnFe₂O₄ in MG50 is 58.49 wt.%, which is higher than the mass ratio of MnFe₂O₄ synthesis (50 wt.%) as calculated initially. This is because during the synthesis of a small amount of reduced oxygen-containing functional groups, the ratio of the GO mass in the material sample decreases. This indicates that Mn²⁺ and Fe³⁺ are successfully linked to GO surface creating NPs.

Zoom In Zoom Out Reset image size

The results revealed the effective combination of GO and MnFe₂O₄ in nanocomposites, which is consistent with TEM images (figure 3). The MnFe₂O₄ particles exhibited a nanocluster-shaped morphology, which presented the agglomeration phenomena. For MnFe₂O₄/GO, the particles were anchored on the surface of GO with the size gradually increasing from 15 to 25 nm. The uniform distribution of particles on the GO accelerates the active sites and surface area of nanocomposite as shown in table 2.

Zoom In Zoom Out Reset image size

These above data showed that the effect of GO substrate helps disperse the formed particles to diminish its size and prevent agglomeration. The oxygen containing groups of GO sheets make the product well-dispersed in aqueous and high chemical activity. These functionalised groups have been negatively charged as anchoring sites for linking the Fe³⁺ and Mn²⁺ cations through electrostatic interactions. The MnFe₂O₄ NPs were formed and deposited on GO sheets via co-precipitation reaction as the following equations (2)–(6). The synthesis process of MnFe₂O₄/GO was presented in figure 4.

$$\begin{eqnarray}&&{{\rm{FeCl}}}_{3}+6{{\rm{H}}}_{2}{\rm{O}}\rightleftarrows {\rm{Fe}}{({{\rm{H}}}_{2}{\rm{O}})}_{6}^{3+}+3{{\rm{Cl}}}^{-},\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{{\rm{MnCl}}}_{2}+6{{\rm{H}}}_{2}{\rm{O}}\rightleftarrows {\rm{Mn}}{({{\rm{H}}}_{2}{\rm{O}})}_{6}^{2+}+2{{\rm{Cl}}}^{-},\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{\rm{M}}{({{\rm{H}}}_{2}{\rm{O}})}_{6}^{3+}+3{\rm{GO}}-{\rm{OH}}\rightleftarrows {\rm{M}}{({{\rm{H}}}_{2}{\rm{O}})}_{6}^{3+}{({\rm{O}}-{\rm{GO}})}_{3},\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{\rm{M}}{({{\rm{H}}}_{2}{\rm{O}})}_{6}^{3+}+3{\rm{GO}}-{\rm{COOH}}\rightleftarrows {\rm{M}}{({{\rm{H}}}_{2}{\rm{O}})}_{6}^{3+}{({\rm{OOC}}-{\rm{GO}})}_{3},\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&\begin{array}{l}{{\rm{MnCl}}}_{2}+2{{\rm{FeCl}}}_{3}+8{\rm{NaOH}}+{\rm{GO}}\rightleftarrows {{\rm{MnFe}}}_{2}{{\rm{O}}}_{4}/{\rm{GO}}\\ \,+8{\rm{NaCl}}+4{{\rm{H}}}_{2}{\rm{O}}{\rm{.}}\end{array}\end{eqnarray} \tag{ 6 }$$

Zoom In Zoom Out Reset image size

When further increasing the amount of MnFe₂O₄, the particles are effortless to link together, reducing the number of active sites and adsorption abilities of materials [24]. Figure 5 shows that adsorption capacities decreased with an increment of MnFe₂O₄:GO mass ratios. The uptake of MG30 and MG40 was higher than of others materials. However, the lower amount of magnetic oxide makes it difficult to recover by using an external magnetic field [25].

Zoom In Zoom Out Reset image size

The magnetic properties of MnFe₂O₄/GO nanocomposites enhanced with an increase in the MnFe₂O₄ content. The hysteresis loops were examined as shown in figure 6(a) with the curve of S-like shape. The saturation magnetisation (M_s) values increased with an increase in the MnFe₂O₄ content from 15.93 emu g⁻¹ (MG40) to 20.51 emu g⁻¹ (MG50) whereas the M_s value of MnFe₂O₄ was 53.18 emu g⁻¹ as shown in table 3. The results are lower than other materials, indicating the reduced size of magnetic particles and non-magnetic ratios in sample [26]. The MnFe₂O₄ particles were covered by GO sheets in nanocomposites, which also affect the M_s value. MG40 and MG50 were easily separated and recycled by applying an external magnetic field as shown in figure 6(b). The MG50 had a higher percentage of magnetic material, which made easier to recover after adsorption. Thus, MG50 has been recommended as an adsorbent to investigate the effect of factors on ibuprofen removal performance.

Zoom In Zoom Out Reset image size

Figure 7(a) showed that the uptake rapidly enhanced with adsorption time and reached the equilibrium after 180 min. Subsequently, the active sites in the MG50 structure were gradually saturated, which leaded to a decrease in the adsorption capacity. The effective time of MG50 for ibuprofen was 180 min. Moreover, the data fitted well with the pseudo-second-order model with the correlation coefficient (R²) of 0.9922 as shown in figure 7(d). Therefore, the adsorption kinetic was mainly affected by the chemical interaction between MG50 adsorbent surface and ibuprofen.

Zoom In Zoom Out Reset image size

Figure 7(b) shows that when increasing pH, the concentration of H⁺ in the solution decreased, limiting the competition process between H⁺ ions and ibuprofen for adsorbent sites, raising the adsorption capacity of MG50 for ibuprofen [6]. The adsorption capacity increased in the range of pH 4 – 6 and reached the highest at pH 6. This was explained that when pK_{a ibuprofen} = 4.33 < pH < pH_pzc 5.50, the ibuprofen with positively charged surfaces and the negatively charged oxygen-containing groups on the surface of MG50 (–O⁻, –COO⁻) created ibuprofen –COOH₂ ⁺, increasing electrostatic interaction and hydrogen bonding between nanocomposite and ibuprofen. However, the adsorption capacity decreased when the pH was higher than 6, due to repulsion interaction between the nanocomposite surface and the ibuprofen group converted into negatively charged anions which create, and reduce the adsorption capacity. The suitable pH for adsorption was determined at pH 6 [2, 11].

Figure 7(c) shows that the uptake correlated with the initial concentration of ibuprofen. The curve went up, then levelled off horizontally, the amount of adsorbed substance increased and then occupied sites in MG50 and obtained saturation state. The adsorption data followed the Langmuir model with a correlation coefficient of nearly 1 (R² = 0.9857) (figure 7(e)).

And, the maximum adsorption capacity q_max was 156.25 mg g⁻¹, which is higher than other materials in table 4. This was explained due to an advance in the number of adsorption sites to link with ibuprofen when decorating MnFe₂O₄ on GO [15, 27]

The change of adsorbent after the adsorption process was investigated by using SEM-EDX analysis and FTIR spectra. The SEM image shows that the surface of adsorbent has changed insignificantly after adsorption, indicating the reusable potential of the material in the adsorption field as shown in figure 8. The elemental compositions of MG50 before and after adsorption are shown in table 5. The significant increase of C and O contents after adsorption was higher than before adsorption, which can be explained that ibuprofen was successfully adsorbed onto the surface. Figure 9 shows the FTIR spectra of MG50 before and after ibuprofen adsorption. The intensity of –OH, –C=O, C=C, and –C–O peaks was significant changed, the acidic protons of oxygen-containing groups existing on MG50 surface interacted with ibuprofen due to the appearance of the characteristic peaks of on the spectrum.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

In particular, the ibuprofen removal ability of MnFe₂O₄/GO was attributed to the electrostatic interaction and hydrogen bonding between the oxygen-containing groups (–OH, –COOH) on the MnFe₂O₄/GO surface and –OH group in ibuprofen structure. Simultaneously, π-π interaction between the carbon ring of GO and ibuprofen also increases the adsorption capacity [17]. The donor - acceptor mechanism between the oxygen containing groups of GO sheets and the π aromatic ring of ibuprofen was attended in the adsorption process [2].