Biotic and Abiotic Degradation and Removal Processes of Chiral Pharmaceuticals in the Environment

Removal of pharmaceuticals in surface waters can be due to photo-transformation, biodegradation, hydrolysis and partition to sediment (Liu et al. 2009). Pharmaceuticals are generally highly polar compounds since they are produced in a way which promotes their transport and excretion in the organism. Thus, their removal in Waste Water Treatment Plants (WWTP) is mostly restricted to biodegradation and to abiotic processes as oxidation and sorption. Air-stripping and photo-transformation are also abiotic processes but normally are insignificant in the removal of pharmaceuticals in WWTP. Sipma et al. (2010) reviewed the comparison of Conventional Activated Sludge systems and Membrane Bioreactors, where the removal efficiencies of many pharmaceuticals were higher in Membrane Bioreactors. These compounds include racemates of chiral pharmaceuticals like b-blockers (atenolol, metoprolol, propranolol), antidepressants (fluoxetine, paroxetine) and NSAIDs (ibuprofen, indomethacin, ketoprofen, mefenamic acid, naproxen, propyphenazone).

Air-stripping can be potentiated using Membrane Bioreactors in the case of some pharmaceuticals with high Henry air/water coefficient (Sipma et al. 2010). However, regarding photo-degradation it can occur at a higher extent in WWTP than in Membrane Bioreactors since photo-transformation requires the exposition of the wastewater to sunlight which occurs in a moderate way at the superficial layer of wastewater and in the secondary clarifiers; conversely the Membrane Bioreactors do not have a second clarifier and have a high biomass concentration which difficult the sunlight penetration. Concerning sorption, pharmaceuticals normally have low sorption constants due to their high polarity leading to a slight contribution to the global removal in WWTP, justifying their low efficiency during the primary treatment. In the case of the few pharmaceuticals with a higher sorption constant, Membrane Bioreactors may remove them more efficiently due to the absence of suspended solids in the final effluent (Sipma et al. 2010) . So, oxidation and biodegradation are the treatments with more promising results. However, oxidation has some disadvantages like process costs and may lead to the production of metabolites more resistant to biodegradation (Sipma et al. 2010).

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