3.1 Adsorbent characterization
3.1.1 Surface area and pore volume analysis
The BET analysis was applied in order to estimate the surface area (SA) and pore volume (PV) of the normal and loaded activated carbon utilized as adsorbents in the removal of phenolic compounds. As listed in Table 5, it was observed that after the loading process of KOH over AC, the PV and SA of AC decrease. This demeanor is due to the occupation of KOH in some areas within the AC [17, 18].
Table 5 BET results for adsorbents
Sample | pore volume (cm3/gm) | specific surface area (m2/gm) | pore size (nm) |
AC | 0 .487 | 804.014 | 2 .423 |
10% KOH/AC | 0 .397 | 612.645 | 2 .562 |
3.1.2 X-ray diffraction (XRD)
The XRD patterns of the normal and loaded activated carbon are shown in Fig. 2. As illustrate in Fig. 2, It can be notice that the pattern of AC exhibited an amorphous halo at 2θ = 26.4°, which indicates to the reflection of the plane (002), a familiar characteristic of noncrystalline structures such as AC [19, 20]. After the loading of KOH, several peaks appeared at various positions (2θ = 24°, 29.8°, 34°, 39.2°, 40.4°) refers to the potassium hydroxide phase [21, 22].
3.1.3 Scanning Electron Microscopy (SEM)
The surface nature of the normal and loaded AC is displayed by utilizing the SEM test. As shown in Fig. 3, the SEM images denote several micropores located on the AC surface. The micropores presented on the surface of AC provide high surface area of the adsorbent. Figure 4 displays the SEM images of the loaded AC (10% KOH/AC). The SEM images of the loaded AC (10% KOH/AC) appear that the micropores on the surface of AC were blocked by the molecules of KOH, which emphasizes that molecules of KOH were substantially adsorbed onto the surface of AC.
3.2 Results of Adsorption process
3.2.1 Influence of 10% KOH loaded over the AC on phenol removal efficiency
The influence of the loading of KOH over activated carbon on the adsorption process of phenolic compound from wastewater in FBAC was studied under various operating condition. It was observed that the adsorption efficiency was enhanced by loading of KOH. This is due to the creation of functional groups (hydroxyl groups) on the surface of activated carbon, which are improved the adsorption characteristics of the AC [23, 24].
3.2.2 Influence of temperature on phenol removal efficiency
The influence of reaction temperature on phenol adsorption efficiency was studied by varying temperature from 25 to 75°C. From Figs. 6 and 7, it was noticed that the adsorption efficiency improved with increasing of reaction temperature above room temperature. This might be owing to an increase in the pores number on the surface of adsorbent. The high temperature increases the kinetic energy of phenol molecules and decreases the thickness of outer adsorbent surface. As a result phenol molecules are readily adsorbed on the surface of adsorbent [25].
3.2.3 Influence of feed flow rate on phenol removal efficiency
The influence of the feed flow rate on the efficiency of adsorption process was investigated with various feed flow rates (1, 2, 3 and 4 mL/sec). Figures 8 and 9 illustrate that the removal of phenolic compounds improved when the feed flow rate was reduced from 4 mL/s to 1 mL/s. This phenomenon is attributed to that with high feed flow rate, the residence time of the phenolic compounds in the FBAC is not long enough. Moreover, the contact time between the phenolic compounds and the adsorbent layer is so fast, leading to decrease in the efficiency of adsorption.
3.2.4 Influence of bed height on phenol removal efficiency
The effect of the height of adsorbent bed on efficiency of adsorption process of phenolic compounds was detected at four various adsorbent bed heights 2.5 cm, 5 cm, 7.5 cm and 10 cm. as illustrated in Figs. 10 and 11, the removal of phenolic compounds increased when the bed height was increased. A raise in the adsorbent bed height increases the presence of adsorption sites, which results an enhancing the contact between the adsorbent and feed, and leads to an increase in the surface area available for adsorption.
According to the results of the adsorption process, high adsorption efficiency was achieved by using continuous process and in the presence of loaded AC (10% KOH/AC) and the removal of phenolic compounds are improved in comparison with related previous study [17].
3.3 Deactivation and regeneration of the adsorbents
3.3.1 Deactivation study
In this work, the adsorbents efficiency was evaluated after six adsorption cycles at the best conditions (temperature = 75°C, adsorbent bed height = 10 cm and feed flow rate = 1 mL/sec). The phenol removal efficiencies for the adsorbents after each cycle are illustrated in Fig. 12. As shown in Figure, The adsorbents attained a peripheral reduction in overall phenol elimination after six cycles. This behavior indicates the high stability of the adsorbents under best operating conditions. The slight reduction in the efficiency of adsorbent may be owing to the loss of some active adsorption sites during the process of recovery.
3.3.2 Regeneration study
Several solvents were utilized to evaluate the solvents performance in regeneration process of the spent adsorbents (after six cycles) in terms of phenolic compounds adsorption. In order to regenerate the used adsorbents, methanol, ethanol and iso-octane are employed as regeneration solvents. Figure 13 summarizes the adsorption efficiencies of the adsorbents after the regeneration process by using various solvents. As shown in Figure, the regeneration performance of the used solvents of adsorbents reduces as follows: iso-octane > ethanol > methanol. So, by utilizing iso-octane as a regeneration solvent, normal and loaded AC can be excellently regenerate.
3.4 The Mechanism for ODS Reaction
The suggested mechanism for ODS process depend on the prepared catalyst with H2O2 is illustrated in Fig. 7. The desulfurization reactions is carried out based to the following steps. In the first step, the DBT molecules are absorbed into the pore channel of catalyst (12% ZnO/γ-Al2O3) during adsorption process. After that, the oxygen released from the oxidizing agent (H2O2) oxidizes the DBT molecules to corresponding sulfoxides (DBTO) through the second step. During the third step, the formed sulfoxides (DBTO) are oxidized to yield sulfones (DBTO2), which are more polar than DBT.