*3.3. SEM Analysis*

The morphology of the fresh, spent and regenerated activated-carbon glass beads was studied by the SEM technique. The micrographs obtained for the adsorbent beads are presented in Figure 7. As shown in Figure 7a, the activated carbon possessed a cellular and honeycomb-like structure, despite the process undergone in the preparation of the adsorbent using the glass beads, epoxy resin and activated carbon. The thin porous sheet-like structure of the adsorbent was homogenously distributed, which elevated the pore density of the activated carbon beads. These numerous pores hosted the necessary active sites for phenol adsorption [38].

**Figure 7.** SEM micrographs of the activated-carbon-coated glass beads: (**a**) before adsorption; (**b**) after adsorption; and (**c**) after desorption.

The SEM image for the spent adsorbent (Figure 7b) showed the successful adsorption of the phenol molecules on the activated-carbon-coated glass beads. The densified nature of the adsorbent with complete coverage of the porous honeycomb structure confirmed the phenol binding to the adsorbent surface. The micrograph also showed a uniform coverage of the active sites by the phenol molecules, which indicated the energetically homogenous nature of the activated-carbon glass beads [39].

Figure 7c depicts the regenerated surface of the activated-carbon-coated glass beads. The quasi-porous nature of the regenerated adsorbent clearly showed that the ethanol medium efficiently desorbed the phenol molecules from the pores of the adsorbent, allowing the reuse of the adsorbent in the LSCFB.

#### *3.4. FTIR Analysis*

The surface chemistry of the activated-carbon glass beads at different stages of the experiment was analyzed using the FTIR technique. Figure 8 and Table 4 contain the results for the pristine adsorbent. The characteristic stretch observed at 3626.17 cm−<sup>1</sup> was due to the –OH band of the adsorbed water whereas the peaks at 3095.75 cm−<sup>1</sup> and 3066.82 cm−<sup>1</sup> were due to the –OH vibrations of the carboxylic

acids group present in the activated carbon. The spectra showed the presence of peaks in the fingerprint region, which corresponded to the presence of SiO2 and C–C bonds, thus confirming the presence of glass beads and activated carbon, respectively. The epoxy resin (C21H25ClO) showed a strong C–Cl presence in the infrared region between 800 and 600 cm<sup>−</sup>1. The large number of teeth-like structures was attributed to the occurrence of vibrations of the C–C bonds when subjected to infra-red radiation. The absence of other peaks throughout the entire wavelength region confirmed that the adsorbent contained only the aforementioned compounds [40].

**Figure 8.** FTIR analysis of the adsorbent: (**a**) before adsorption; (**b**) after adsorption; and (**c**) after desorption.



FTIR spectra for the phenol-adsorbed activated-carbon glass beads are presented in Figure 8 and Table 4. The results indicated the presence of phenol at a stretch between 1410 and 1310 cm−<sup>1</sup> and at an appropriate value of 1388.75 cm<sup>−</sup>1. Secondly, the prevalence of teeth-like structures between 2500 and 2000 cm−<sup>1</sup> showed strong C = C functionality due to the presence of an aromatic ring. Moreover, the existence of a benzene ring in phenol adsorbed by the adsorbent can be confirmed by the peak at 821.68 cm−1. Given the fact that the solutions taken were dilute, peaks were not obtained between 3200 and 3100 cm−1, which would have indicated an alcohol-water solution in the system. The red shift in the –OH band from 3095.75 cm−<sup>1</sup> (pertaining to fresh adsorbent) to 3093.82 cm−<sup>1</sup> indicated the

interactions of the carboxylic groups present in the adsorbent for phenol adsorption. Additionally, the peaks obtained in the fingerprint region adhere to those previously obtained, thus proving that no other compounds were formed during adsorption [41].

The results for surface functionality of the ethanol-washed adsorbent beads are presented in Figure 8 and Table 4. The presence of a peak at 794.67 cm−<sup>1</sup> indicated the weak presence of an alcohol group, which was possibly ethanol. Secondly, the broad stretch between 1600 and 1400−<sup>1</sup> with a peak at 1537.27 cm−<sup>1</sup> showed a weak aromatic character, which indicated the reduced aromatic tendencies as compared to Figure 8b. This confirmed the successful desorption of phenol in the regenerated adsorbent beads; also, the restoration of the characteristic bands at positions 3098.87 cm−<sup>1</sup> and 3074.81 cm−<sup>1</sup> confirmed the successful regeneration of the adsorbent beads. The presence of no other peaks highlighted that no other side compounds were formed that led to any changes in the system and both adsorption and desorption occurred successfully. Additionally, the fingerprint region remained the same as before.
