Abstract
The properties of supercritical CO2 (SCCO2) and supercritical water (SCW) turn them into fluids with a great ability to remove organic adsorbates retained on solids. These properties were used herein to regenerate an activated carbon fiber (ACF) saturated with a pollutant usually contained in wastewater and drinking water, phenol. Severe regeneration conditions, up to 225 bar and 400 °C, had to be employed in SCCO2 regeneration to break the strong interaction established between phenol and the ACF. Under suitable conditions (regeneration temperature, time, and pressure, and flow of SCCO2) the adsorption capacity of the exhausted ACF was completely recovered, and even slightly increased. Most of the retained phenol was removed by thermal desorption, but the extra percentage removed by extraction allowed SCCO2 regeneration to be significantly more efficient than the classical thermal regeneration methods. SCCO2 regeneration and SCW regeneration were also compared for the first time. The use of SCW slightly improved regeneration, although SCW pressure was thrice SCCO2 pressure. The pathways that controlled SCW regeneration were also investigated.
Highlights
The critical point marks a borderline above which the physical properties of a fluid suffer severe changes
When the regeneration procedure of an activated carbon fiber (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
The temperatures at which the gasification of similar fibers begins have been previously probed to be higher than the regeneration temperatures used [28,29], the mere exposure of the ACF to CO2 and water above their critical points may cause certain changes in its porosity
Summary
The critical point marks a borderline above which the physical properties of a fluid suffer severe changes. They have larger surface areas because their pores are more uniform and less branched than the pores of ACs, their adsorption capacity is greater [25,26] It is an expensive material so an effective regeneration method is essential to guarantee the economical viability of the purification method based on the adsorption of pollutants on ACFs. In order to meet this requirement, the regeneration conditions were hardened in this investigation, reaching pressure and temperature up to 225 bar and 400 ◦ C in SCCO2 regeneration and 300 bar and 425 ◦ C in SCW regeneration. Taking into account the scarce information regarding SCW regeneration, this work reports the comparison between SCCO2 regeneration and SCW regeneration of an ACF exhausted with phenol for the first time
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