Ozonation and advanced oxidation by the peroxone process of ciprofloxacin in water

Abstract

A bubble reactor was used for ozonation of the antibiotic ciprofloxacin. Effects of process parameters ozone inlet concentration, ciprofloxacin concentration, temperature, pH and H₂O₂ concentration were tested. Desethylene ciprofloxacin was identified, based on HPLC–MS analysis, as one of the degradation products. Formation of desethylene ciprofloxacin was highly dependent on pH, with the highest concentration measured at pH 10. Radical scavengers t-butanol and parachlorobenzoic acid were added in order to gain mechanistic understanding. Radical species other than hydroxyl radicals were suggested to occur at acidic pH which can explain fast ciprofloxacin ozonation at pH 3.

Introduction

The antibiotic ciprofloxacin is the most widely prescribed quinolone in Europe [1]. It is a second generation quinolone. They are designed to expand the activity against gram-negative bacteria but have only limited gram-positive activity.

Because pharmaceuticals are made to interfere with biological systems, even low concentrations can be harmful after prolonged exposure. Pharmaceuticals can exhibit toxic effects on cells, organs, organisms and populations. Specifically for antibiotics, the increased use and exposure during the last decades can increase bacterial resistance against them, favored by low concentrations [2]. Moreover, exposure to one compound can lead to resistance against a whole class of antibiotics.

Ciprofloxacin was found in wastewater treatment plant (WWTP) influents typically at concentrations of 313–568 ng l⁻¹ [3], [4] and up to 124.5 μg l⁻¹ [5] in raw sewage hospital water. Because of its high wastewater–sludge partition coefficient (log k_d ∼ 4), 79–92% of the initial concentration is removed in a WWTP. However, Kümmerer et al. [6] found that ciprofloxacin was not biodegradable in closed bottle tests. Because of the low biodegradability, ciprofloxacin could still be found in WWTP effluents by Golet et al. [3], [4] at concentrations of 61–106 ng l⁻¹. Concentrations up to 514 ng l⁻¹ and 5.6 μg⁻¹ were detected by other authors [7], [8]. Advanced oxidation processes (AOPs), which generate hydroxyl radicals, are a promising tool for removal of persistent pharmaceutical compounds. Ozonation can also be seen as an AOP because ozone decomposes into hydroxyl radicals at higher pH.

Until now, only few authors described degradation of quinolones by AOPs while none of them identified degradation products. Balcioğlu and Ötker [9] described ozonation of enrofloxacin mixed with 10% organic and inorganic additives, next to ozonation of ceftriaxone sodium and penicillin VK. The authors studied the influence of pH, initial COD, H₂O₂ and biodegradability although they did not measure enrofloxacin itself but BOD, COD, TOC and UV₂₅₄ values next to O₃ and H₂O₂ concentrations. In a second article, the authors also investigated the influence of ozonation on enrofloxacin adsorption on zeolite [10].

Andreozzi et al. [11] investigated the toxicity of a mixture of pharmaceuticals containing the quinolone ofloxacin on algae and invertebrates. Recently, Dodd et al. [12] determined the second order rate constants of 14 pharmaceuticals, including ciprofloxacin, with ozone and hydroxyl radicals. Other radical species were not taken into account.

In this research, ozonation of ciprofloxacin is investigated. By our knowledge, no parameter study has been carried out so far for this compound. The effect of ciprofloxacin concentration, pH and H₂O₂ concentration is studied. Focus lays on ozone consumption and on the compound degradation itself, in contrast with the enrofloxacin parameter study of Balcioğlu and Ötker [9]. Next, influence of temperature and ozone concentration is tested as well. In order to gain more insight in the degradation mechanism, hydroxyl radical scavengers t-butanol and parachlorobenzoic acid (pCBA) were added. Finally, a first degradation product was identified.