## 3 Results and Discussion
### Comparison between SCCO\({}_{2}\) Regeneration and SCW Regeneration  
In order to make a strict comparison, SCW regeneration was first carried out at the only temperature within the SCCO\({}_{2}\) regeneration studies that was over the critical point of water, 400 \({}^{\circ}\)C. However, the process was unstable because this temperature was too close to the critical temperature of water, and in this region the physical properties of the fluid suffer strong alterations. The SCW regeneration assays were finally carried out at 425 \({}^{\circ}\)C. Figure 9 shows the effect of time and the number of adsorption-regeneration cycles on SCW regeneration at 300 bar and 425 \({}^{\circ}\)C with a SCW flow of 3 cm\({}^{3}\cdot\)min\({}^{-1}\). SCW regeneration also arose as a method able to completely recover, and even increase (RE values greater than 100%), the adsorption capacity of the original ACF. This increase was caused by the same phenomena than in the case of SCCO\({}_{2}\). SCW also opened a few pore entrances and modified the surface chemistry in such a way [34] that the regenerated ACF adsorbed a greater amount of phenol than the original ACF. The high efficiency of the method allowed preserving the adsorption capacity after three cycles, although a slight decrease in RE was observed when the ACF was regenerated during 5 min. When the regeneration step was lengthened to 15 min, RE values were higher. The effects caused by the two phenomena previously described were more remarkable for longer regeneration times, hence leading to a 5% increase in the adsorption capacity of the original ACF. We did not explore temperatures above 425 \({}^{\circ}\)C in SCW regeneration since at this temperature the regeneration was already complete and higher temperatures might cause structural damages in the ACFs. Consequently, SCW regeneration could be described as a method slightly more efficient than SCCO\({}_{2}\) regeneration (RE\({}_{\text{SCW,300 bar,425 \({}^{\circ}\)C,15 min}}\) = 104.6%; RE\({}_{\text{SCCO}_{2,100 bar,400 \({}^{\circ}\)C,15 min}}\) = 100.3%), although it must be noted that SCW pressure was thrice SCCO\({}_{2}\) pressure. Figure 9: Effect of time and the number of adsorption–regeneration cycles on RE for supercritical water (SCW) regeneration at 300 bar and 425 \({}^{\circ}\)C with a SCW flow of 3 cm\({}^{3}\cdot\)min\({}^{-1}\). SCW regeneration assays were also interesting from another point of view. Novel information about the regeneration mechanism could be obtained by analyzing the effluent aqueous stream proceeding from the regeneration reactor. The effluent was divided in three samples which were collected during different stages of the regeneration step: beginning stage, intermediate stage and final stage. Each sample was analyzed by UV-Vis spectrophotometry. Figure 10 shows the spectra for the three samples collected during a 7 min regeneration assay, as well as the spectrum for the original phenol solution. The samples were highly concentrated and they had to be diluted in order to fulfill the requirements of the Lambert-Beer law. The dilutions were as follows: 1:40 for the first sample, 1:20 for the second sample, and 1:4 for the third sample. The spectra for the samples collected during the first 2 min of the treatment, and between minutes 2 to 4, had a similar shape to that for the phenol solution. This similarity meant that during the first 4 min the phenol was removed in its original form (without any degradation). This fact pointed to the same regeneration pathways that governed SCCO\({}_{2}\) regeneration: thermal desorption and extraction. Taking into account the dilutions, the number of moles of phenol contained in these two samples was calculated. 93% of the adsorbed phenol was removed during the first 4 min. On the other hand and regardless of the evident lower signal, the spectrum for the sample collected during the final 3 min showed a significantly different shape from that for the phenol solution.