Ozone-Based Regeneration of Granular Zeolites Loaded with Organic Micropollutants

Abstract

The removal of organic micropollutants (OMPs) in the aquatic environment is crucial to avoid health hazards. Zeolites have been confirmed as a selective adsorbent and can effectively remove target OMPs. To achieve sustainable application of adsorbents, regeneration of zeolites is required.
The objective of this study was to investigate the regeneration performance of dried OMP-loaded granular zeolites through gaseous ozonation process, and the regeneration feasibility in long-term adsorption-regeneration processes. Three types of zeolites (MOR, MFI and BEA) were applied for target OMP (benzotriazole, methyl-benzotriazole, carbamazepine, diclofenac hydrochlorothiazide, sulfamethoxazole, metoprolol, sotalol, trimethoprim, propranolol, and clarithromycin) removal. A sequential process coupling zeolite adsorption and oxidation by gaseous ozone was established in batch mode. To assess the ozone effect on OMP degradation and zeolite itself, ozone bubbling tests and adsorption isotherm experiments were executed as pre-experiments. The relative adsorption capacity obtained through regeneration was used to demonstrate regeneration performance. Operating conditions, adsorption duration and regeneration duration were determined and applied. Ultimately the regeneration performance in long-term adsorption-regeneration processes was investigated.
Experimental results showed that all target OMPs were not resistant to ozonation in the water phase. Gaseous ozone was showed no influence on the adsorption capacities of zeolite granules. 120 hours and 500 mgL-1 zeolite granules were applied in OMP-loading adsorption experiments. Zeolites always showed high adsorption capacities of metoprolol, trimethoprim and sotalol, which regeneration effect was not evidenced. 60 minutes of ozonation was effective and sufficient for regenerating low and medium adsorption OMPs, except for carbamazepine. The regeneration of carbamazepine probably required a longer regeneration duration. In four cycles of adsorption-regeneration experiments, regeneration of sulfamethoxazole could be achieved after four rounds of ozonation. Regarding carbamazepine, diclofenac, benzotriazole, and methyl-benzotriazole, the regeneration performance were significantly reduced after the first cycle of regeneration. The ozonation duration is supposed to be extended above 60 min in long-term regeneration experiments. Intermediates were potentially responsible for the reduction of regeneration performance in ozonation and adsorption processes. Particularly, the effect of intermediates accumulation might be the main factor that hampered the regeneration performance of low and medium adsorption OMPs in long-term operation.