## 3 Results and Discussion  
### Preliminary Studies: Effect of Supercritical Treatments on the Textural Properties of the ACF  
Supercritical fluids can be used to prepare ACFs [28; 29]. In this application, supercritical fluids react with a raw carbonized fiber at temperatures above 700 \({}^{\circ}\)C. The fiber is gasified and its porosity is increased, hence becoming an ACF. Consequently, when the regeneration procedure of an ACF is performed with supercritical fluids, the textural properties of the ACF may be changed, which would play an important role in the recovery of the adsorption capacity. Although the temperatures at which the gasification of similar fibers begins have been previously probed to be higher than the regeneration temperatures used herein [28; 29], the mere exposure of the ACF to CO\({}_{2}\) and water above their critical points may cause certain changes in its porosity.  
A few experiments were made to assess the possible changes in the textural properties of the ACF when it was exposed to the supercritical fluids. The original ACF was exposed to a SCCO\({}_{2}\) flow at 100 bar and two different temperatures, 250 \({}^{\circ}\)C (SCCO\({}_{2}\).250) and 700 \({}^{\circ}\)C (SCCO\({}_{2}\).700), during 180 min; and to a SCW flow at 650 \({}^{\circ}\)C and 300 bar during 180 min (SCW650). Table 1 shows the results obtained.  
The original ACF was essentially microporous, as reflected by its high S\({}_{\text{BET}}\) and micropore volumes. Although SCCO\({}_{2}\) at 250 \({}^{\circ}\)C did not gasify the raw material, it was able to unblock a few pore entrances. This unblocking led the surface area to slightly increase since some micropores become accessible. When SCCO\({}_{2}\) was heated up to 700 \({}^{\circ}\)C, gasification began and some mass losses were registered. The textural parameters point that some narrow micropores detected by CO\({}_{2}\) adsorption were being widened and turned into micropores with a larger diameter, as reflected by the increase in V\({}_{0}\)(N\({}_{2}\)) and S\({}_{\text{BET}}\). A similar phenomenon was observed when the original ACF was exposed to SCW at 650 \({}^{\circ}\)C.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Sample** & **S\({}_{\text{BET}}\) (m\({}^{2}\).g\({}^{-1}\))** & **V\({}_{0}\)(N\({}_{2}\)) (cm\({}^{3}\).g\({}^{-1}\))** & **V\({}_{\text{meso}}\) (cm\({}^{3}\).g\({}^{-1}\))** & **V\({}_{0}\)(CO\({}_{2}\)) (cm\({}^{3}\).g\({}^{-1}\))** & **Mass Losses (\%)** \\ \hline Original & 1030 & 0.511 & 0.008 & 0.397 & - \\ SCCO\({}_{2}\).250 & 1112 & 0.524 & 0.002 & 0.393 & Negligible \\ SCCO\({}_{2}\).700 & 1300 & 0.540 & 0.001 & 0.384 & 8 \\ SCW/650 & 1438 & 0.590 & 0.001 & 0.341 & 15 \\ \hline \end{tabular}
\end{table}
Table 1: Changes in the textural properties of the original activated carbon fiber (ACF) when it is exposed to supercritical fluids.  
SCW is a more reactive gasification agent than SCCO\({}_{2}\) and because of that both the mass losses and the increases in V\({}_{0}\)(N\({}_{2}\)) and S\({}_{\text{BET}}\) were higher than in the previous case, although the temperature was lower.  
The results of Table 1 may encourage regenerating the ACF at high temperatures: phenol would be removed more efficiently and the adsorption capacity of the regenerated material would be enlarged thanks to the increase in S\({}_{\text{BET}}\). However, this approach is not advisable because of the induced mass losses. The regeneration method must be applicable in several consecutive cycles. If the ACF was regenerated several consecutive times, it would progressively lose a meaningful amount of mass, what may cause a collapse of the material [30].