Mineralization of the antibiotic chloramphenicol by solar photoelectro-Fenton: From stirred tank reactor to solar pre-pilot plant
Graphical abstract
Introduction
Recently, there exists an increasing attention on pharmaceuticals as potential bioactive chemicals in the aquatic environment. Many pharmaceutical drugs have been detected at relatively low contents up to μg L−1 level in soils, surface waters, ground waters and even drinking waters [1], [2], [3]. Antibiotics are the most commonly drugs found in the aquatic environment because their antimicrobial nature prevents effective removal in sewage treatment plants [4], [5], [6]. The occurrence and fate of antibiotics as well as their metabolites in water streams is recognized as one of the emerging issues in environmental chemistry [1], [3], [7], [8]. Several authors reported that these pollutants can produce multi-resistant strains of microorganisms, can affect the endocrine systems of fishes and invertebrates, and are toxic on small invertebrates and algae [9], [10], [11], [12]. Powerful oxidation treatments then need to be developed for the removal of antibiotics from waters and wastewaters to avoid their potential adverse health effects on humans and animals.
Several electrochemical advanced oxidation processes (EAOPs) are being currently developed for water prevention [12], [13], [14], [15], [16], [17]. EAOPs are based on the in situ generation of hydroxyl radical (•OH), which is the second strongest oxidant known after fluorine since it has so high standard reduction potential (E°(•OH/H2O) = 2.80 V/SHE) that can non-selectively react with organic pollutants up to their mineralization to CO2, water and inorganic ions. The most potent EAOPs use both, heterogeneous and homogeneous •OH formed at the anode and in the solution bulk, respectively, as oxidizing agents. Mediated electro-oxidation with homogeneous •OH is usually achieved by decomposition of H2O2 generated from the two-electron cathodic reduction of injected O2 [12], [16]:O2 + 2H+ + 2e− → H2O2
Good efficiencies for H2O2 generation from reaction (1) have been reported for carbonaceous cathodes such as carbon sponge [18], carbon nanotubes-polytetrafluoroethylene (PTFE) [19], [20], carbon-felt [18], [21], [22], [23], [24], [25], graphite-felt [26], boron-doped diamond (BDD) [27] and carbon-PTFE gas (O2 or air) diffusion electrodes [21], [22], [28], [29], [30], [31], [32].
In our laboratory, we have checked the good oxidation ability of EAOPs like anodic oxidation with electrogenerated H2O2 (AO-H2O2) [28], [32], electro-Fenton (EF) [21], [22], [28], [29], [30], [31], UVA photoelectro-Fenton (PEF) [28], [29], [30] and solar photoelectro-Fenton (SPEF) [29], [31] to destroy several antibiotics and other drugs in acidic solutions using small stirred tank reactors equipped with either a BDD or Pt anode and a gas-diffusion cathode. Our interest is to show that these EAOPs can be useful for the treatment of wastewaters contaminated with antibiotics. To do this and in view of the large variety of these drugs, it is necessary to know the degradative characteristics of more compounds. In this way, the scaling-up of EAOPs to a pre-pilot plant also needs to be assessed in order to demonstrate their possible viability at industrial level.
This paper aims to investigate the mineralization of the antibiotic chloramphenicol (2,2-dicloro-N-[1,3-dihydroxy-1-(4-nitrophenyl)propan-2-yl] acetamide) in acidic medium by AO-H2O2, EF, PEF and SPEF in order to clarify: (i) the role of generated •OH in the degradative processes using stirred Pt/air-diffusion and BDD/air-diffusion tank reactors of 100 mL under comparable conditions, (ii) the photolytic action of UVA and solar radiation in these systems, (iii) the effect of experimental parameters on substrate decay and mineralization rate, (iv) the products formed and their evolution to propose a general reaction pathway for chloramphenicol mineralization and (v) the viability of SPEF in a 10 L pre-pilot plant with a Pt/air-diffusion filter-press reactor coupled to a solar compound parabolic collectors (CPCs) photoreactor. Note that chloramphenicol is a broad-spectrum antibiotic, which is effective against a wide variety of Gram-positive and Gram-negative bacteria, including most anaerobic organisms. While in the developed countries it is currently prescribed only to treat bacterial conjunctivitis, chloramphenicol is widely used in developing countries. For this reason, it has been detected worldwide in ground waters, lakes, rivers and influents and effluents of sewage treatment plants [3], [6], [7], [8], [33], [34], [35], [36]. However, less is known about the degradation of chloramphenicol from waters and its oxidation products formed from •OH attack have not been identified yet. It has been reported that it can be removed by UVC radiation [37], TiO2/UV photocatalysis [38] and ozonation [39]. The electrochemical reduction of its nitro group to hydroxylamine or amine at a graphite cathode has also been described by means of cyclic voltammetry [40].
Section snippets
Chemicals
Chloramphenicol (98% purity) was of reagent grade from Sigma–Aldrich. Carboxylic acids were of reagent grade from Panreac and Avocado. Anhydrous sodium sulfate and heptahydrated ferrous sulfate were of analytical grade from Fluka. Solutions treated in the stirred tank reactor were prepared with ultrapure water obtained from a Millipore Milli-Q system (resistivity > 18 MΩ cm at 25 °C). Solutions of 10 L to be degraded in the solar pre-pilot plant were prepared with deionized water. All solutions were
Comparative mineralization of chloramphenicol by EAOPs in a stirred tank reactor
The relative oxidation ability of AO-H2O2, EF, PEF and SPEF to destroy chloramphenicol was assessed by electrolyzing 100 mL of a synthetic solution with 245 mg L−1 drug (100 mg L−1 of DOC) and 0.05 M Na2SO4 at pH 3.0 using stirred Pt/air-diffusion and BDD/air-diffusion tank reactors at 33.3 mA cm−2 and 35 °C. The pH of 3.0 was chosen because it was found optimal for the treatment of other aromatics by these EAOPs [12], [28], [29], [30], [31] and treatments were made at 35 °C since this is the maximum
Conclusions
It has been demonstrated that SPEF with BDD is the most potent EAOP for chloramphenicol mineralization at pH 3.0 using a 100 mL stirred BDD/air-diffusion tank reactor. Total mineralization of antibiotic solutions with 0.05 M Na2SO4 and 0.5 mM Fe2+ at pH 3.0 can be rapidly attained by the combined action of BDD(•OH), •OH and sunlight. PEF with BDD was less powerful due to the lower intensity of UVA radiation to photolyze generated Fe(III)-carboxylate species. EF and AO-H2O2 with BDD led to partial
Acknowledgments
The authors gratefully acknowledge the financial support from MICINN (Ministerio de Ciencia e Innovación, Spain) under the Project CTQ2010-16164/BQU, co-financed with FEDER funds. S. Garcia-Segura thanks the grant awarded from MEC (Ministerio de Educación y Ciencia, Spain) and E.B. Cavalcanti acknowledges the financial support from CAPES/MEC/Brazil and Universidade Tiradentes/UNIT.
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