Dispersion and distribution of the CNTs in the polymer solution
Figure 4 shows the results of the sedimentation rate test for the prepared SA-CNT samples. Comparison of the images clearly show that for the suspension containing CNT-NH2, the stability conditions are disturbed after 4 h, and the CNTs are precipitated as large clusters. This phenomenon occurred for the CNT-OH and CNT-COOH samples after 16 h and 8 h, respectively. These observations imply that the CNT-COOH and CNT-OH samples present relatively more stability than the CNT-NH2 sample in aqueous suspension. The similar results have been reported in other works, but in different settling rate for CNTs functionalized with carboxyl and hydroxyl groups [40, 43, 44].
Figure 5 shows the CA-CNT adsorbents produced by the stable and unstable suspension of CNT. As can be seen in the figure, the dark spots (white arrows) indicate the presence of CNT clusters demonstrating the poor dispersion of the CNT in the adsorbent matrix.
The size of beads produced by electrical and mechanical spraying
Figure 6 depicts the beads’ size distributions of the CAMS and CAES adsorbents, which are produced in similar conditions. As shown, the bead diameter spans for the samples produced by electrical and mechanical spraying are 180–580 µm and 1900–2140 µm, respectively. This indicates that the size mode of beads produced by electrospray is smaller as compared to those beads produced by mechanical spray. These results are in agreement with other works representing the effect of electric field on the size reduction of sprayed droplets [45–48].
As mentioned before, the freeze-drying route was utilized to evaluate the CNTs’ distribution in the matrix of each adsorbent. Figure 7 illustrates the bead images before and after the freeze-drying. Besides, the SEM micrographs from the outer surface of the CA-CNTMS and CA-CNTES beads are also shown in this figure. As can be seen from SEM images, the electrospray and freeze-drying of CA-CNTES has led to more shrinkage with wrinkled surface, and smaller size of the beads as compared to CA-CNTMS.
Figure 8 shows the SEM images from the outer surfaces of the CA-CNTMS, CA-CNTES, CA-CNTM/MS and CA-CNTM/ES freeze-dried beads. The preliminary observation of the surfaces morphology indicates that the surface grooves are directional especially for CA-CNTES (Figs. 8c and 8d) as compared to CA-CNTMS (Fig. 8a). Furthermore, it seems that the traces of CNTs are seen on the surface of CA-CNTMS sample in which the nano-tubes are most probable to be uniformly distributed in the alginate structure (Fig. 8a). For others there is no sign of CNTs on the surface, and therefore, those might be embedded more inside the absorbent beads. In general, it is obvious from the surface morphology of beads that the magnetic and electric fields have affected the structural characteristics of the produced beads.
To investigate the effect of the electrical and the magnetic fields on the arrangement and the orientation of the CNTs in the polymeric matrix, SEM images were taken from the fractured surfaces of the CA-CNTES, and CA-CNTM/ES samples. As shown in Figs. 9a and 9b it can see that the CNTs have been distributed well in the polymer matrix.
For the CA-CNTES sample, it looks that the CNTs have no specified orientation, and therefore, the electric field has not been able to orientate the CNTs (Fig. 9a). This would be attributed to insufficient force applied on CNTs for their alignment within the viscous calcium alginate suspension as well as a very short retention time of suspension flow under the electrospray field. For the CA-CNTM/ES sample, however, the CNTs have been partially oriented demonstrating the ability of the magnetic field to orient the CNTs (Fig. 9b) under the specific processing conditions. The similar result for the magnetic field was also reported in the work of Shi et al. .
MB adsorption onto the beads
As shown in Fig. 10, MB adsorption onto the six fabricated adsorbent beads has been evaluated at different time steps. As seen, the adsorption of the MB is generally increased with the CA-CNTs beads, as compared to the bare CA sample. It is interesting to note that for the samples free from CNTs (i.e. CAMS and CAES), the MB adsorption is almost same at the early stages, while after about 60 min, it becomes higher for the CAMS sample as compared to CAES one. In the presence of CNTs the adsorption trends get closer between CA-CNTMS and CA-CNTES. However, as the effects of magnetic and electric fields, the MB elimination rate trend for CA-CNTM/ES beads is much better, and it achieves the highest adsorption efficiency among five other samples.
SA droplets can acquire the shell and core gel type characteristics when carboxylate groups of the alginate are cross-linked by divalent cations such a calcium. This called the “egg-box” model for the gelation mechanism. The name “egg-box” is used because one divalent cation Ca+ 2 interacts with four –COOH groups [50, 51]. The latter phenomenon, so-called external gelation, may lead to formation of mechanically stable gel network of CA shell around the SA solution core [50–54]. Figure 11 illustrates schematically the egg-box model and the cross sectional views of a produced SA-CA core-shell beads.
The MB species in the bulk of aqueous solution, surrounding the beads, are initially adsorbed onto the CA shell, followed by their molecular diffusions through the porous shell and then being transferred into SA core solution . The same mechanism would also prevail for the CA-CNTs beads. This mechanism has been well detailed in the drug release from calcium alginate beads [54, 56, 57].
As depicted in Fig. 10, the adsorption of MB for all embedded CNT samples initially exhibited a fast uptake. Wang et al, reported that the uptake process of MB for MWCNT is very fast and then showed a steady trend after the equilibration point . It can be concluded that at the early stages of the adsorption the physical interaction of the beads’ surface with MB prevails, and then the migration process would proceed through shell intward the core .
To compare more precisely the combined effect of mechanical, magnetically, and electrical spraying, the adsorption trend of the MB species as a function of contact time for the first hour are shown in Figs. 12 and 13, respectively. As seen in Fig. 12, the presence of CNTs and their orientation by magnetic field (M) enhances the adsorption rate in the absence of electric field and under mechanical spray (MS), where the beads are larger. The adsorption efficiency of these three samples have been calculated in 60 min, so that the CA-CNTM/MS sample adsorbs about 30% of the MB. However, this value is about 23% for CA-CNTMS and 19% for CAMS. It is noted that absorbance values of the samples are significant too (p-value < 0.05).
For the samples that electrospray route is employed (Fig. 13), there is a significant difference between the absorption rates of the CA-CNTES and CA-CNTM/ES samples, as compared to the neat ones (p-value < 0.05). In fact, as an effect of electric field in producing smaller beads, the adsorption mechanism were mainly controlled by the diffusive transport of the MB compounds within the pore network of the nanocomposites and the CNT’s sites . Calculating the adsorption efficiency of the MB onto the CA-CNES and CA-CNTM/ES beads indicates that in the first hour, about 65% of the MB is adsorbed by the CA-CNTM/ES bead, and it is approximately 22% and 12% for the CA-CNTES and neat beads, respectively. It must be noted that in 60 min, the absorbance values of the CA-CNTES and CA-CNTM samples are not significant (p-value < 0.05).
After 1 hour, the slopes of the adsorption are decreased in all samples. The CA-CNTM/ES bead could remove about 88% of the MB concentration after 16h, and this value is about 67% for the same sample fabricated by the mechanical spraying. In this study, the equilibrium time for the CA-CNTs absorbent is longer than has been reported in other works [58–60]. This could be due to differences in pore tortuosity of the nanocomposites.
These observations confirm that the carbon nanotubes increase the efficiency of the MB adsorption, and their orientation in a polymeric structure can enhance the capability of the adsorption. The similar results have been reported in other experimental works [61, 62].