521
Feasibility of combined electrochemical and biological treatment for tylosin removal
D. Belkheiri(a, d, e)*
, Y. Kadmi(a, b), F. Fourcade(a, b), F. Geneste(b, c), D. Floner(b, c), H. AïtAmar(d), A.
Amrane(a, b)
(a)Université de Rennes 1, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR6226, Laboratoire de Chimie et Ingénierie des Procédés (CIP), Rennes, France.
(b)Université Européenne de Bretagne, France
(c)Université Rennes 1, CNRS, UMR 6226, Equipe Catalyse et Organométalliques, Rennes, France
(d)Université des Sciences et de la Technologie Houari Boumediene, Laboratoire des Sciences du Génie des Procédés Industriels, Alger, Algérie
(e)Université ZIANE Achour de Djelfa, Faculté des sciences de l’ingénieur, Djelfa, Algérie
*Corresponding author. E-mail : dao17000@yahoo.fr
Received 15 Sept 2014, Revised 10 Oct 2014, Accepted 24 Oct. 2014
Abstract
The purpose of this work was to examine biodegradability improvement of tylosin - containing solutions after an electrochemical pre-treatment, since about 70% of the applied drugs are not metabolized and hence can be found in wastewaters. Cyclic voltammetry with nickel electrode revealed a significant electrochemical activity of tylosin, on nickel electrode. Electrochemical treatment was, therefore, performed in a home-made flow cell. Optimal conditions led to more than 95% conversion yield of tylosine, in oxidation, using alkaline media as supporting electrolyte. TOC analyses of the electrolyzed solution revealed that the level of mineralization remained low, underlying the interest of a combined electrochemical and biological treatment. The biodegradability, based on the BOD5 on COD ratio, increased from 0, for untreated tylosin, to 0.43 for electrolyzed tylosin at 0.55 V/ESC.
Keywords: Combined process; Electrochemical pre-treatment; Biodradability, Tylosin
1.
Introduction
Tylosin is an important group of antibiotics. It is commonly used in veterinary medicine. Only small portions of the administrated antibiotic, to the treated species, are metabolized in the body with most of the unchanged form of the drug being eliminated in faeces and urine. Between 30 and 90 % excreted unchanged into the waste system. The presence of tylosin residues and its potential to promote growth of resistant bacteria pose adverse health effects to humans.
Electrochemical oxidation shows a remarkable capacity to eliminate recalcitrant organic contaminants such as antibiotics [1]. Electrochemical oxidation for antibiotic degradation was mostly carried out on onerous anode materials such as Ti/RuO2, platinum, gold and diamond. In this work, electrochemical degradation of
522 tetracycline was performed in percolation system, using nickel /carbon felt anode in order to reduce its biological activity, without mineralizing it completely. The biodegradability of the different electrolysed tylosin solutions was checked. Some of them showed a significant predisposition to biological treatment which let to investigate the electrochemical degradation as a pre-treatment prior to the biodegradation. We report in this work that some electrooxidized products can be used as a substrate for a microbial culture with the objective of a total mineralization.
2.
Materials and Methods
2.1. Chemicals and reagentsTylosin tartrate, displayed in “figure.1”, was obtained from Sigma Aldrich. Acetonitrile (ACN) was HPLC grade from Fisher Scientific and formic acid (LC-MS grade, 98 %) was purchased from J.T. Baker.
Standards were prepared with ultra pure water. All other reagents were of analytical grade.
Figure 1. Chemical structure of tylosine.
2.2. Materials for the electrochemical pre-treatment
Electrochemical pre-treatment, in continuous system, was performed in a home-made flow cell “figure. 2”.
The working electrode was prepared by the electrodeposition of nickel on graphite felt material. It was separated from the two interconnected stainless steel counter-electrode compartments by cationic exchange membranes. The reference electrode (SCE) was positioned in the middle of the felt. The potential control was performed using an e-daq potentiostat linked to e-corder 401 converter. The tylosin concentration was 100 mg L-1. 0.1 M NaOH was used as supporting electrolyte. Tylosin solution percolated the porous electrode at various flow rates monitored by a peristaltic pump.
2.3 Analysis
2.3.1 Electrochemical analysis
Electrochemical analysis of tylosin was performed using a conventional three electrode-cell with a nickel electrode (7 mm2) as working electrode and a platinum wire as counter electrode. All the electrode potentials were measured with respect to a saturated calomel electrode (SCE) located near the working electrode.
Experiments were performed at ambient temperature under nitrogen atmosphere, to avoid dissolved oxygen.
Voltammograms were obtained by cyclic voltammetry (100 mVs−1) using an e-daq potenstiostat linked to e- corder 401 converter.
2.3.2 UPLC
The residual tylosine concentration was determined by UPLC (Liquid Chromatography Ultra-High Pressure) system involving a gradient pump Waters Acquity UPLC® H-Class and a PDA (photo diode array) UV
523 detector. A column C18 BEH (Bridged Ethylene Hybrid), 1.7µm (2.1 x 50mm) was used. A gradient elution was carried out with formic acid 0.1 % in ultra-pure water (solvent A) and Acetonitrile (solvent B) as follows: 90, 10 from 0 to 1 min, from 1 to 4.5 min. Eluent passes linearly from 90, 10% to 2, 98 %, maintained at 2, 98 % from 4.5 to 5 min, from 5 to 5.5 min, eluent passes from 2, 98 % to 90, 10 % where it is maintained from 5.5 to 10 min for solvents A and B respectively. The flow rate was 0.5 ml. Tylosin was detected at 286 nm.
Figure 2. Schematic diagram of the percolation cell: a: cationic membranes; b: saturated calomel electrode (SCE); c: working electrode (disc of graphite felt: 10 mm diameter, 10 mm thickness); d: auxiliary counter
electrodes
2.3.3. Total organic carbon (TOC)
TOC was measured by means of a Total Organic Analyzer Schimadzu (TOC-VCPH TOC-VCPN/TOC- VWP). HCl (2 N) and H3PO4 (25%) were considered for CO2 production. Organic carbon compounds are combusted and converted to CO2, which is detected by a non dispersive infrared detector (NDIR).
2.3.4 Chemical oxygen demand (COD) measurements
Chemical oxygen demand (COD) was measured by means of a Test Nanocolor® CSB 40 and 160 from Macherey-Nagel (Düren, Germany). The amount of oxygen required for the oxidation of the organic and mineral matter, at 148°C for 2 h, was quantified after oxidation with K2Cr2O7 at acidic pH and heating.
2.3.5 Biological oxygen demand (BOD5) measurements
BOD5 measurements were carried out in Oxitop IS6 (WTW, Alès, France). Activated sludge from a wastewater treatment plant was used to inoculate the flasks; the initial microbial concentration was 0.5 g L-1. The following mineral basis was used for all experiments (g L-1): MgSO4.7H2O, 22.5; CaCl2, 27.5; FeCl3, 0.15; NH4Cl, 2.0; Na2HPO4, 6.80; KH2PO4, 2.80.
3. Results and Discussion
3.1 Electrochemical behaviour of tylosine
The electroactivity of tylosine (4 g L-1) was studied in alkaline medium (NaOH 0.1 mol L−1) by cyclic voltammetry on nickel electrode. Alkaline conditions were considered for oxidation, since it is well-known that in these conditions, oxydydroxide NiOOH are formed, leading to electrocatalysis of organic compounds, mainly alcohols [2]. The voltammogram obtained during the anodic sweep displayed two peaks indicating an
524 oxidation of tylosin on the electrode at 0.4 and 0.6 V/SCE “figure.3”. An important oxidation wave appears from 0.4 V / ESV and coincides with the formation of nickel oxyhydroxide (NiOOH).
Figure 3. Current–potential curve obtained by cyclic voltammetry (100mVs−1) with a nickel electrode (S = 7 mm2), under nitrogen atmosphere and T = 298 K, of 0.1 g TC in 0.1 M NaOH.
3.2 UPLC results 3.2.1 Degradation ratio
The flow rate effect was examined at 0.55 V/ESC. A total tylosin removal was observed, for a flow rate of 1 mL min-1 (Table 1). Interestingly, for all the studied flow rates, the quasi-total degradation of tylosine was achieved.
3.2.2 Chromatograms
Several oxidation products, more and less polar than tylosine, appeared at 2.40 ; 2.51 ; 2.59 ; 2.74 ; 2.90;
3.01 ; 3.09 et 3.15 et 3.32 minutes, for different flow rates. Their nature seems to be independent of the contact time with the electrode; since they have the same retention time “figure.4”.
The maximum UV absorption of tylosin is obtained for λ = 286 nm. The tylosin oxidation products are better detected at λ = 360 nm “figure.5” than at 286 nm “figure.4”, which indicates that changes happened in the chemical structure of tylosin molecule.
Figure 4. Tylosin chromatograms before (black) and after oxidation at Ni/GF electrode at 0.55 V/ESC, for different flow rates (blue, green, light blue, indigo, brown, resp. for : 1, 2, 3, 4 et 5 mL min-1), λ = 286 nm.
C18 column.
525 3.3 TOC measurements
Even if tylosine was completely degraded after oxidation, the mineralization level remained low; it did not exceed 29% (Table.2), in the studied flow rate range. It should be noted that the electrochemical process was carried out in order to, selectively, degrade the target compound to obtain by products which were expected to be biologically
Table 1. Effect of the percolation flow rate on Tylosin degradation.
Flow rate
mL min-1 1 2 3 4 5
Elim. Ratio % 99.47 97.12 96.78 98.06 92.26
assimilated by microorganisms from activated sludge. Therefore, such electrochemical pretreatment seems to be relevant for the tylosin molecule (Tyl), owing to the important amount of residual organic carbon available for a subsequent biological treatment.
Figure 5. Tylosin chromatograms before (black) and after oxidation at Ni/GF electrode at 0.55 V/ESC, for different flow rates (blue, green, light blue, indigo, brown, resp. for: 1, 2, 3, 4 et 5 mL min-1), λ = 360 nm.
C18 column.
Table 2. Oxidation, mineralization and biodegradability of oxidized tylosin on Ni/GF electrode in alkaline medium (NaOH 0.1 M) at 0.55 V/ESC.
Flow rate (mL min-1) Tyl 1 2 3
TOC (mg L-1) 56.31 40.22 39.02 45.17
COD (mg O2.L-1) 143 60 63 58
BOD5/COD 0.00 0.42 0.39 0.43
COD/TOC 2.54 1.50 1.61 1.28
AOS -6.15 -1.96 -2.45 -1.13
526 3.4 COD measurements
Generally COD decrease involves a chemical oxidation of the target molecule and therefore a modification of its chemical structure that could lead to a decrease of its toxicity while low mineralization is desired to ensure sufficient residual organic carbon for a subsequent biological treatment [3].
Consequently, a favourable trend is a decrease of the ratio COD/TOC [4] or an increase of the average oxidation state (AOS) [5].
AOS = 4(TOC-COD)/TOC
With TOC and COD expressed in molar carbon per liter and molar oxygen per liter respectively. The COD on TOC ratio decreased from 2.54, for the untreated solution to 1.28, for the electrolyzed solution, while the AOS parameter increased from -6.15 to -1.13 (Table.2). Such evolution of these two parameters is, therefore, advantageous for electrolysis prior to a biological treatment.
3.5 BOD5 measurements
Biodegradability was checked by the determination of the BOD5 on COD ratio, since for values above 0.4, the effluent can be considered as biodegradable [5]. The favorable trend was confirmed, since after electrolysis the BOD5 on COD ratio was above the biodegradability threshold for 1 and 3 mL min-1 and close to this threshold for 2 mL min-1 (Table 2), showing the biodegradability of the by-products from tylosin oxidation.
4. Conclusion
Owing to the electroactivity of tylosin, confirmed by cyclic voltammetry, the biodegradability of oxidized tylosin was checked. Tylosin was electrolyzed in a flow cell, using nickel covered carbon felt as working electrode, at different flow rates values. UPLC analysis showed an almost total degradation of the target molecule. Mineralization remained low ensuring a significant amount of carbon for a possible subsequent biological treatment; The BOD5/COD ratio increased from 0 to 0.43 a biodegradable solution, obtained for untreated and oxidized tylosin at 0.55 V/SCE, respectively.
References
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