## 3 Results and Discussion
### SCCO\({}_{2}\) Regeneration
#### 3.2.1 Effect of Temperature
This fraction is known as physisorbed phenol. The second peak was related to a fraction of phenol that needed more energy to be removed. In that case, a strong chemical interaction was established between the adsorbent and the adsorbate. This fraction is known as chemisorbed phenol. The removal of this fraction is the main challenge to achieve an efficient regeneration. Figure 4: Desorption thermogram of phenol from the ACF using pressurized water as the carrier fluid. Figure 3: Effect of temperature and the number of adsorption–regeneration cycles on RE (regeneration efficiency). It seems probable that, under relatively mild conditions, 200 \({}^{\circ}\)C, the strongly retained chemisorbed phenol was not completely removed. The textural characterization of the regenerated ACFs allowed delving into this phenomenon. Figure 5 shows the N\({}_{2}\) adsorption-desorption isotherms for the ACFs regenerated at the lowest and the highest temperatures investigated, as well as for the original ACF. Due to the incomplete removal of phenol at 200 \({}^{\circ}\)C, the surface area available for the subsequent adsorption was reduced (recovery of S\({}_{\text{BET}}\) = 73%), which explained why the regenerated ACF did not fully recover its adsorption capacity. Further accumulation of phenol happened after a second adsorption step, thus leading the ACF regenerated twice at 200 \({}^{\circ}\)C to lose about 25% of its original adsorption capacity. The ACFs treated at 300 and 400 \({}^{\circ}\)C adsorbed a greater amount of phenol after being regenerated than in their original form, as shown by the RE values over 100%. The high efficiency of the method under these conditions was preserved even when the ACF was exposed to two consecutive adsorption-regeneration cycles: the ACFs regenerated twice had the same adsorption capacity than the original ACF. The slight increase in the adsorption capacity regarding the original ACF can be partially explained by the results of Table 1. Under these conditions, SCCO\({}_{2}\) removed the retained phenol and unblocked the entrance of a few pores, but Figure 5 shows that the N\({}_{2}\) isotherm for the ACF regenerated at 400 \({}^{\circ}\)C was below the isotherm for the original ACF. Namely, with the treatment at 400 \({}^{\circ}\)C only 90% of the original S\({}_{\text{BET}}\) was recovered. However, another phenomenon was occurring during the regeneration that may also be contributing to that increase. The surface chemistry of the adsorbent also plays a key role in the adsorption phenomenon. The adsorption of phenol and its derivates is hindered by the presence of oxygen-containing surface groups [32; 33]. On the other hand, it has been probed that the surface chemistry of carbon materials is modified when they are exposed to supercritical fluids. The exposure of this ACF to a SCCO\({}_{2}\) flow at 100 bar and 250 \({}^{\circ}\)C removes about 60% of the carboxylic acid surface groups of the material, and this percentage increases with temperature [34]. Consequently, it is possible that the surface chemistry of the regenerated ACFs was more suitable for phenol adsorption than that of the original ACF. The combination of the probed notable recovery of the textural properties and a possible suitable modification of the surface chemistry may clarify why RE reached values above 100%. There is not an unanimous conclusion about the effect of temperature on supercritical regeneration. Some authors affirm that density is the controlling parameter at pressures just above the critical pressure, and regeneration gets worse with increasing temperatures [7; 14; 15]. It is also stated that as the fluid is compressed, viscosity becomes progressively more influential and the negative effect of temperature is then mitigated [7; 9; 14; 15].