## 3 Results and Discussion
### SCCO\({}_{2}\) Regeneration  
#### 3.2.1 Effect of Temperature  
SCO\({}_{2}\) flows of 3 cm\({}^{3}\cdot\)min\({}^{-1}\) at 100 bar were circulated through exhausted samples of ACF during 15 min to study the effect of temperature on the regeneration. Four temperatures were assayed, 200, 250, 300, and 400 \({}^{\circ}\)C. The samples were exposed to two consecutive adsorption-regeneration cycles at each temperature. Figure 2 shows the phenol breakthrough curves for the original ACF and the sample regenerated at 250 \({}^{\circ}\)C after the first and the second cycle. The curve for the sample regenerated once almost overlapped the curve for the original ACF, suggesting that, under these regeneration conditions, the original adsorption rate and capacity of the ACF were essentially preserved after one cycle. However, the curve for the sample regenerated twice was displaced towards the left side. This trend pointed that the ACF was able to adsorb a smaller amount of phenol after the second regeneration. This fact was confirmed when the REs were calculated, Figure 3.  
The ACF recovered 97.8% of its original adsorption capacity when it was regenerated once at 250 \({}^{\circ}\)C. The recovery of the original adsorption capacity was reduced to 93.5% when the regenerated ACF was saturated again, and regenerated at this temperature. Temperature has two opposite effects on SCCO\({}_{2}\) regeneration, which are related to the mechanistic pathways involved in the process [11; 19]. SCCO\({}_{2}\) can remove a retained pollutant through two pathways: thermal desorption and extraction. Desorption phenomenon implies the supply of an amount of thermal energy (heating of the adsorbent) large enough to break the interactions established between the adsorbate and the adsorbent; in this way, regeneration is improved when high temperatures are used. On the other hand, the increase of the regeneration temperature diminishes the density of the supercritical fluid. A lower density means the reductions of the solubility of phenol in SCCO\({}_{2}\) and the ability of the supercritical fluid to extract the pollutant. Considering this, regeneration is worsened when high temperatures are used. Figure 2: Phenol breakthrough curves for the original activated carbon fiber (ACF) and the sample regenerated with SCCO\({}_{2}\) at 250 \({}^{\circ}\)C after the first and the second adsorption–regeneration cycle. Despite the meaningful reduction in the density of SCCO\({}_{2}\) (\(\rho_{100\text{bar, }200\,^{\circ}\text{C}}=0.122\) g\(\cdot\)cm\({}^{-3}\); \(\rho_{100\text{bar, }400\,^{\circ}\text{C}}=0.079\) g\(\cdot\)cm\({}^{-3}\)), Figure 3 shows that the thermal effect prevailed under the conditions studied herein. RE progressively increased when SCCO\({}_{2}\) was heated from 200 to 300 \({}^{\circ}\)C; an overheating up to 400 \({}^{\circ}\)C did not lead to an increase in RE. Consequently, 400 \({}^{\circ}\)C was the highest temperature explored in SCCO\({}_{2}\) regeneration since under these conditions regeneration was already complete and higher temperatures might cause structural damages in the ACF. The RE at 200 \({}^{\circ}\)C hardly exceeded 80%. Studying how phenol was adsorbed on the ACF gave more information about why the regeneration was not complete. The nature of the adsorption of phenol on the ACF used herein was studied by TPD using pressurized water as the carrier fluid, Figure 4.  
The thermogram showed two peaks. The first peak appeared at temperature between 100 and 250 \({}^{\circ}\)C, in the liquid water region. The second peak appeared above 350 \({}^{\circ}\)C, in the SCW region. Similar results have been obtained when the nature of the adsorption of phenol on other carbon materials was studied [23], even when the characterization was made with the classical TPD with inert gas as the carrier [6; 31]. The two peaks were associated to different interactions between phenol and the ACF. The first peak was removed at low temperature because that fraction established a weak physical interaction with the adsorbent.