Heterogeneous photocatalysis of moxifloxacin in water: Chemical transformation and ecotoxicity

chemosphere thumb100 Due to their extensive use, continuous discharge and inefficient removal by wasterwater treatment plants, increasing levels of antibiotics are commonly detected in surface waters. Even though environmental levels are generally low, it has been shown that long-term exposure may adversely affect aquatic organisms. As such, there is an urgent need to improve classical water treatment techniques. Among other techniques, heterogeneous photocatalysis (HP) has proven its potential in degrading antibiotics from aqueous matrices. Yet the oxidative transformation of a compound does not necessarily result in the removal of its toxicity. It is therefore important to evaluate the ecotoxicity of a reaction solution after an HP treatment. In this study, the photocatalytic degradation products of moxifloxacin (MOX) were assessed by algal growth inhibition experiments using Pseudokirchneriella subcapitata. From our results, we concluded that the average growth inhibition by this commonly found antibiotic can be significantly reduced by heterogeneous photocatalysis. However, the fact that growth inhibition is still observed in treated solutions – from which MOX has been removed below its detection limit – also indicates that degradation products should not be neglected in toxicity assessment.

Scientific abstract

This work provides new insights on the impact of TiO2/UV catalyzed chemical transformation of moxifloxacin on ecotoxicity effects towards the green alga Pseudokirchneriella subcapitata. The moxifloxacin median effect concentration (EC-50 = 0.78 [0.56, 1.09] mg L-1), determined in accordance to the OECD 72-h growth inhibition test guideline, was 7 times lower than that of the older and widely used fluoroquinolone ciprofloxacin (EC-50 = 5.57 [4.86, 6.38] mg L-1). Applying heterogeneous photocatalysis as an advanced oxidation technique to degrade moxifloxacin in aqueous solution decreased the average growth inhibition from 72% to 14% after 150 min of treatment. No significant carbon mineralization was observed and liquid chromatography mass spectrometry analysis revealed the formation of 13 degradation products for which a chemical structure could be proposed based on accurate mass determination. Combined chemical and ecotoxicological analysis showed that as long as moxifloxacin is present in the reaction solution, it is the main compound affecting algal growth inhibition. However, also the contribution of the degradation products to the observed ecotoxicity cannot be neglected. Photocatalytically induced modifications of moxifloxacin mainly occur at the diazobicyclo-substituent as ring opening, oxidation into carbonyl groups, and hydroxylation. This results into the formation of  more hydrophilic compounds with a decreased biological activity compared with moxifloxacin. The change in lipophilicity, and possibly a modified acid–base speciation, most probably also affect the cell membrane permeation of the degradation products, which might be another factor explaining the observed lower residual ecotoxicity of the photocatalytically treated reaction solutions.

Full reference (link)

Van Doorslaer X, Haylamicheal ID, Dewulf J, Van Langenhove H, Janssen CR, Demeestere K. 2014. Heterogeneous photocatalysis of moxifloxacin in water: Chemical transformation and ecotoxicity. Chemosphere (in press)