Regeneration of Adsorbents Using Heterogeneous Photocatalytic Oxidation

The overall objective of this study is to investigate the technical feasibility of removal and destruction of organic contaminants in water using adsorption and photocatalytic oxidation. The process consists of two consecutive operational steps: (1) removal of organic contaminants using fixed-bed adsorption; and (2) regeneration of the spent adsorbent using photocatalytic regeneration. To facilitate the regeneration, some of the adsorbents were impregnated with photocatalysts, which would allow them to act as both an adsorbent for capturing the organics and a photocatalyst for destroying the organics during regeneration. Laboratory-scale adsorption processes were conducted using rapid small-scale column tests (RSSCTs) for the loading of F-400 GAC (60X80) and Ambersorb 563 (60X80) with trichloroethene (TCE) and para-dichlorobenzene (p-DCB), respectively. The spent adsorbents were regenerated in a plate photoreactor and the organic-free water was used. For the impregnated F-400 carbon, catalyst loading was observed to have little impact on the adsorption capacity for TCE. The Pt-TiO ₂ impregnated F-400 carbon loaded with TCE (1,000 μg/L) could be regenerated using artificial light with a regeneration time identical to the adsorption time. About 30 loss in capacity was observed during the four adsorption/regeneration cycles. For Pt-TiO ₂ impregnated Ambersorb 563, the capacity loss from the first regeneration was about 40 and no further loss was observed for the remaining adsorption/regeneration cycles. Based on these studies, the regeneration process was found to be limited by the reaction rate at the beginning of the regeneration cycle and then desorption of adsorbates from the interior adsorbent surface limited the rate. This was especially true for p-DCB, which is very strongly adsorbed. Upon further testing, it was observed that increasing the temperature significantly improved the regeneration rate. The laboratory studies confirmed that both F-400 and Ambersorb 563 could be regenerated using photocatalytic oxidation; however, a low photoefficiency was observed as compared to photocatalysis alone because the desorption of the organics from the interior of the adsorbent was very slow even at elevated temperatures.