Sodium oxamate

Degradation of tetracycline at a boron-doped diamond anode: influence of initial pH, applied current intensity and electrolyte

C. I. Brinzila • N. Monteiro • M. J. Pacheco • L. Ciríaco • I. Siminiceanu • A. Lopes

Abstract

The anodic oxidation of tetracycline was performed in an up-flow reactor, operating in batch mode with recircu- lation, using as anode a boron-doped diamond electrode. The influence on the degradation rate of solution initial pH (2 to 12), applied current intensity (25 to 300 A m−2) and type of electrolyte (sodium sulphate or sodium chloride) were investigated. For the assays run at equal current density, with sodium sulphate as electrolyte, the solution’s initial pH of 2 presented the highest absorbance and chemical oxygen de- mand removals. Regarding the influence of current density, for equal charge passed, the organic load removal rate de- creased with the increase in applied current. When sodium sulphate was used as an electrolyte, high-performance liquid chromatography (HPLC) results have shown an almost com- plete removal of tetracycline after a 2-h assay. HPLC results have also shown the presence of oxamic acid as one of the intermediates of tetracycline anodic oxidation. The complete removal of tetracycline was much faster in the presence of chloride ions that promoted the complete degradation of this antibiotic in 30 min. However, in the presence of chloride ions, the tetracycline mineralization is slower, as observed by the lower organic carbon removal rate when compared to that of the tetracycline degradation in the presence of sulphate.

Keywords Anodic oxidation . Antibiotics degradation . BDD electrode . Electrochemical degradation . Tetracycline

Introduction

In the last decades, antibiotics are being used in concentrates of animal feed, in order to prevent disease and promote growth of livestock. In the USA, it was estimated that more than 70 % of the antibiotics used are consumed by animals (Hileman 2001). Part of the ingested antibiotics goes directly to the environment, excreted in the urine, faeces and manure, as parent compounds or by-products. Leftover feed supplement- ed with antibiotics can also go directly to the soil and ground- water of the surrounding environment. The presence of anti- biotics in the environment can lead to hazardous events, since their presence in streams, lakes and water supplies may lead to the growth of multi-resistant bacteria in the animals them- selves, in humans and in wildlife (Chopra and Roberts 1995; Halling-Sørensen et al. 2002; Kümmerer 2009). Among vet- erinary antibiotics, tetracycline family is widely prescribed due to its characteristic as broad-spectrum antibiotic. Its favourable antimicrobial properties and the almost complete absence of side effects have led to their extensive use in the therapy of human and animal infections (Chopra and Roberts 2001). A study made in France, in 2006, by the Agence Française de Sécurité Sanitaire des Aliments, accounted for about 50 % of veterinary antibiotics sold there in 2004 which were from the tetracycline family (Yahiat et al. 2009). Due to its large use, residues of tetracycline were already detected in surface waters that received discharges from municipal waste- water treatment plants and agricultural drained (Hirsch et al. 1999, 2002). Residues of tetracyclines and their metabolites were also detected in eggs, milk, meat and animals based on different exposure methods, which have raised concerns in terms of food safety (Zurhelle et al. 2000; Kitazono et al. 2012).
The occurrence of tetracyclines, namely, minocycline, tet- racycline and its epimers, epitetracycline and doxycycline, in four hospital wastewater effluents and in the influents of municipal wastewater treatment plants (WWTPs) was also investigated (Pena et al. 2010). Minocycline and tetracycline were found in 41.7 % of the samples; epitetracycline and doxycycline were found in 25 and 8.3 % of the samples, respectively. The levels found ranged from 6 to 531.7 μg L−1 in hospital effluents, while their concentrations in WWTPs ranged from 95.8 to 915.3 μg L−1.
The tetracycline molecule is characterized by a fused four- ring structure, which is amphoteric due to the presence of the functional groups tricarbonyl methane, phenolic diketone and dimethyl ammonium, possessing also a carboxylamide func- tional group. The ionisable functional groups that exist as side chains are responsible for their pKa and for the net anionic, cationic or zwitterionic charge. According to different authors, tetracycline possesses three or four ionization equilibria and, respectively, four or five protonation states that, in the last case, can be represented by a cation, H4TC+, a zwitterion, H3TC, and the anions, H2TC−, HTC2−and TC3−, where TC stands for tetracycline (Stephens et al. 1956; Bhatt and Jee 1985; Duarte et al. 1999; Qiang and Adams 2004; Sassman and Lee 2005; Jin et al. 2007). According to Jin et al. (2007), the four ionization equilibria correspond to pKa at approxi- mate pH values of 3.2, 7.6, 9.6 and 12. However, there is still some ambiguity in the assignment of the various pKa values to the particular functional groups in the tetracycline molecule.
According to literature, the degradation of tetracycline is dependent on temperature and pH: in very strong acidic con- ditions, at pH below 2.0, occurs the formation of anhydro- tetracyclines; in acid solutions, at pH between 2.0 and 6.0, epimerization occurs, giving 4-epi-tetracyclines and in alka- line solutions, at pH higher than 7.5, tetracyclines cleave readily, giving origin to their respective isotetracyclines (Škrášková et al. 2013).
Among the various existing degradation techniques, special attention has been paid to the photolysis under solar irradiation for the oxidation of tetracycline, since it can happen in the natural aquatic environment (Andreozzi et al. 2003). According to literature, TC photolysis follows first-order kinetics, being dependent on TC concentration and solution pH. In fact, degra- dation rate decreased with initial concentration and was strongly enhanced at high pH values and in the presence of nitrates and dissolved organic matter (Jiao et al. 2008; Chen et al. 2008).
Besides photolysis, the degradation of tetracycline has also been studied by other oxidation techniques, most of them ad- vanced oxidation processes, where the presence of hydroxyl radical is the main responsible for the degradation process (Reyes et al. 2006; Bautitz and Nogueira 2007; Palominos et al. 2009; Liu et al. 2009; Zhang et al. 2009; Miyata et al. 2011; Wang et al. 2011a, 2011b, 2011c; Yahiat et al. 2011; Kitazono et al. 2012; Brinzila et al. 2012; Wu et al. 2012). In these processes, the efficiency depends essentially on the adsorption mode and the concentration of hydroxyl radicals. It seems that, with an increase in the pH, higher quantities of hydroxyl radicals are formed and, besides that, the TC molecule is negatively charged, which enhances the attraction to the electrophilic hydroxyl radical, leading to higher degradation rates in alkaline conditions (Jiao et al. 2008). Similar conclusions were obtained by different techniques: in the photocatalytic degradation with TiO2, it was found that degradation rate varied with pH in the order of pH 9> pH 6>pH 12>pH 3 (Wang et al. 2011a); in ozonation process, it was found that TC removal rate increased significantly with increasing pH before the pH reached 7.8, but thereafter, the influence of pH became insignificant and the degradation rate of tetracycline increased with the increasing generation rate of hydroxyl radicals (Wang et al. 2011b). Electrochemical process- es are also very promising techniques for the degradation of organic molecules, namely, pharmaceutical compounds (Brillas et al. 2009; Sirés and Brillas 2012; Oturan et al. 2013). In general, these technologies are considered clean processes, since they can operate at low temperature and, in general, without the addition of chemicals. In particular, anodic oxidation performed with adequate anode material, like boron-doped diamond (BDD), can promote the formation of hydroxyl radicals, enabling the oxidation of the organic matter by direct and indirect oxidation reactions (Panizza and Cerisola 2009).
In a previous work (Brinzila et al. 2012), our research group has studied the influence of flow rate and initial TC concentration on anodic degradation of TC at a BDD anode. The objective of this work was to assess the influence of initial pH, presence of chloride and applied current intensity on the removal rates of TC and organic load from solution.

Materials and methods

The tetracycline used in this study was the alkali form (C22H24N2O8.xH2O) purchased from Sigma-Aldrich (purity 99 %), and it was used without further purification.
The cyclic voltammetric measurements were performed in a potentiostat/galvanostat VoltaLab PGZ 301, in a one com- partment cell, with a 3-mm2 BDD electrode as working elec- trode, a platinum plate, with identical area, as the counter electrode and a commercial Ag/AgCl (KCl sat) as reference electrode. Cyclic voltammetries were recorded in 5-g L−1 Na2SO4 aqueous solutions with tetracycline concentration of 200 mg L−1, at a scan rate of 10 mV s−1.
Tetracycline electrodegradation experiments were conducted in batch mode, with recirculation, in an up-flow electro- chemical cell, composed by a BDD anode (Diachem®) of 20 cm2 geometric area, obtained from Adamant Technologies, and a stainless steel cathode, with identical area. The gap between electrodes was 1 cm, and the volume of the cell was 20 cm3. The recirculation of the solution was enabled by a pump, Concessus, little giant, 2md, that allowed the use of different flow rates, varying from 75 to 100 L h−1. A GW, Lab DC, model GPS-3030D (0∼30 V, 0∼3 A), was used as power supply. The assays were conducted at room temperature, and the processed volume of solution was 200 mL, with TC initial concentration of 100 mg L−1, using as support electrolyte anhydrous sodium sulphate (Merck, 99.5 %) or sodium chlo- ride (Merck, 99.5 %), with a concentration of 5 g L−1. Assays were run up to 4 h and collected samples were monitored by the following parameters, determined according to stan- dard procedures (Eaton et al. 2005): total organic carbon (TOC) and total nitrogen (TN), measured in a Shimadzu TOC-V CPH analyser combined with a TNM-1 unit; chemical oxygen demand (COD), performed using the dichromate closed reflux titrimetric method; total Kjeldahl (TKN) and ammonia nitrogen (N-NH3), using a Kjeldatherm block- digestion-system and a Vapodest 20s distillation system, both from Gerhardt. UV-visible absorption spectra were also per- formed, with measurements made between 200 and 800 nm, using a Shimatzu UV-1800 spectrophotometer.
High-performance liquid chromatography (HPLC) was per- formed using a Merck-Hitachi LaChrom Elite HPLC system, equipped with a diode array detector L-2455, a Column Owen L-2300 and a pump L-2130. A RP-18 reversed phase Purospher® STAR (Merk) column (250 mm×4 mm, I.D. 5 μm) was used. For the tetracycline, the elution was performed isocratically with an oxalic acid aqueous solution (0.01 M) and acetonitrile, 70:30 (v/v), mixture at a flow rate of 1 mL min−1 and 40°C, and the selected wavelength was 360 nm. For the oxamic acid, the mobile phase was a phosphate buffer of pH 3 (10 mM) and methanol, 90:10 (v/v), mixture, analysis was performed at a flow rate of 0.5 mL min−1, 30°C and the photodiode array detector set to a wavelength of 210 nm. The acetonitrile, methanol, phosphoric acid and oxalic acid were HPLC grade and supplied by Sigma-Aldrich. All the solutions for HPLC were prepared with ultrapure water obtained with Milli-Q® equipment (water type II). Measurements of pH were carried out with a Mettler-Toledo pH meter and pH adjustment were made with H2SO4 and NaOH concentrated solutions. Conductivity was measured using a con- ductivity meter Mettler Toledo (SevenEasy S30K).

Results and discussion Influence of initial pH Voltammetric study

The cyclic voltammograms run at different initial pH for sodium sulphate aqueous solutions, 5 g L−1, without and with tetracycline, 200 mg L−1, at a scan rate of 10 mV s−1 and are depicted in Fig. 1. For the electrolyte solutions with initial pH between 2 and 10, the anodic peak appearing at 1.7 V vs. Ag/AgCl for the solution with initial natural pH, characteristic of the oxidation of sulphate ion to persulphate at pH 7 (Pourbaix 1963), changes its intensity with pH. The intensity of this peak is higher for natural pH, closer to neutral pH, decreasing in intensity to lower or higher pH values.
Only at very alkaline pH values, the voltammogram suffers a variation, and the presence of a new peak at lower potential is detected. This new peak must be due to the oxidation of the OH− that, at pH 12, presents a remarkable concentration. In the voltammograms containing also tetracycline, the anodic peaks observed present higher intensity. This means that tet- racycline can also be oxidized in this potential region, and this is true for all the protonated, neutral or deprotonated forms of tetracycline, since it happens for all initial pH tested. Although this situation could be of interest when tetracycline conversion is the objective, if the complete mineralization of tetracycline is the target, an applied current density high enough to guar- antee an overpotential that allows the formation of hydroxyl radicals should be used.

Degradation study

In order to choose the optimum wavelength to follow the degradation tests by UV-visible (UV-Vis) spectrophotome- try, spectra with TC in aqueous sodium sulphate solutions at different pH were recorded and are presented in Fig. 2 (initial solutions, 0 h). The spectra of TC initial solution show two maxima near 276 and 360 nm, and, as it was already discussed in literature (Chen et al. 2008), pH has a clear influence in this absorption spectrum due to the various possible protonation states of this molecule. This influence is expressed as successive increases in the wave- length of the absorption bands with pH and also in the variation of the relative absorbance of the two main bands, already mentioned.
Figure 2 also includes the UV-Vis spectra for the samples collected during the anodic oxidation assays of the TC’s solutions, performed at different initial pH. For all the tested solutions, there is a gradual decrease in absorbance at all wavelength that is more marked for the solution with initial pH of 2. Apparently, the fully protonated form of TC is more easily degraded, since there is a greater absorbance decay with time for the assay run at pH of 2 and, for this experiment, after a 0.5-h assay only the 268.5-nm band still remains. This observation is also expressed in Fig. 3a, b, being the absor- bance decay, of both bands, between 0 and 2 h more pro- nounced for the assay run at initial pH of 2. However, in this assay, the intermediate products still present after 2 h, which are responsible for the absorbance at 268.5 nm, seem to be more resistant to posterior degradation. COD removal rate, as well as tetracycline concentration determined by HPLC (Fig. 3c, e), are also dependent on the initial pH, and the best results have been obtained again for the assay with initial pH of 2. However, if this assay is excluded, the COD removal rate increases with pH.
For pH below 2, the degradation leads to the formation of anhydro-tetracyclines (Škrášková et al. 2013), which are then more easily oxidized. However, for alkaline conditions, the direct anodic oxidation is favoured, as observed in Fig. 1. Re- garding TOC removal rate (Fig. 3d), it does not show any particular behaviour pattern, being the lowest value obtained 85 %, at a 4-h assay, in the experiment run at initial pH of 5.6 (natural). This different behaviour with initial pH regarding COD and TOC evolutions in time lead to different mineralization indexes, measured as ΔTOC/ΔCOD (Table 1), that increased with initial pH up to 8.6 and then decreased for higher initial pH. In a previous work (Brinzila et al. 2012) related to the electrochemical degradation of TC at a BDD anode, it was studied the evolution of different nitrogen forms during the assays, since TC molecule contains nitrogen. It was observed that the total Kjeldahl nitrogen, which includes organic and ammonium forms of nitrogen, was always higher than ammo- nium nitrogen, meaning that organic compounds containing nitrogen were still present, even when TC was no longer detected by HPLC. In this work, the presence of oxamic acid, a nitrogen-containing organic compound, was detected by HPLC, being its evolution during a degradation assay, run at natural pH, presented in Fig. 3e inset. The increase in oxamic acid concentration during the assay means that it must be one oxygen formed is reduced to hydroperoxide ion, decreasing the efficiency of the process (Brillas et al. 2009). In this case, the of the main intermediates, prior to the final mineralization of the tetracyline molecule.
The variation of pH during the different assays is depicted in Fig. 3f, and for assays run at initial pH of 2 and 4, there is almost no variation of pH during the anodic oxidation. For initial pHs between 5.6 and 10, there is an initial decrease in pH, followed by a poste- rior increase for values higher than 7, and for the highest initial pH, a slight decrease is observed.

Influence of current density

The results obtained for the assays run at different applied current density, keeping constant the other experimental var- iables, are presented in Fig. 4. Results from absorbance, COD and TOC measurements are typical from a process controlled by diffusion, where charge efficiency decreases with the in- crease in current density. However, for the experiments run at 200 and 300 A m−2 applied current densities, the current efficiencies are almost identical, particularly in the results concerning absorbance measurements. One possible explana- tion for this fact is that for very high current densities, there is an excess in hydroxyl radicals formed at the anode that are not being used in the degradation of the organic matter still present.
All the assays whose results are presented in Fig. 4 were run for 4 h, and the last point of each curve in Fig. 4c, d corresponds to the result at 4 h. We can observe that final COD removal rate slightly decreases with current density and final TOC removal rate slightly increases with current density. This means that the ratio ΔTOC/ΔCOD increases with current density, as can be observed in Table 1. This fact must be due to the indirect oxidation process, since higher hydroxyl radi- cals concentration leads to a less specific attack to the pollut- ant and by-products and, consequently, to higher mineraliza- tion degree. Regarding pH variation during the assays (inset of Fig. 4b), there is an initial decrease followed by a posterior increase that augments with current density.

Influence of the electrolyte

In order to study the effect of the background electrolyte upon TC oxidation rate, degradation assays were performed with sodium sulphate and sodium chloride and the obtained results are presented in Fig. 5 and Table 1. In the experiments performed with sodium sulphate, there is a regular decay of absorbance with time for all spectrum, as it was already referred before. On the other hand, with sodium chloride, after 5 min, there is a high decrease in absorbance at 358.5 nm and only a small shift to lower wavelength for the peak at 276.5 nm. This means that during these initial minutes, the structure of most of the TC molecules in solution is destroyed, forming by-products that absorb at about 270 nm. After that, at 10 min, there is no particular absorbance peak, which must be consistent with a complex mixture of by- products. At 0.5 h, there is the development of a new band at 290 nm, mainly due to the formation of hypochlorite, and its intensity increases up to 2 h and then starts to decrease. As to COD, TOC and TC concentration, determined by HPLC, it can be observed in Fig. 5d that in the case of the electrolyse with sulphate, the COD and TOC decays are similar, pointing to a high degree of mineralization. In contrast, when chloride is the supporting electrolyte, TOC removal rate is much lower than COD removal rate, as can be seen by the lower values of the ΔTOC/ΔCOD ratios presented in Table 1. The presence of chloride leads to lower mineralization rate, probably due to the extra indirect oxidation of TC molecules by the hypochlo- rite in the solution’s bulk. This later specie must also be responsible for the higher reduction rate in TC concentration when chloride is the electrolyte. In fact, according to Aquino et al. (2012), the presence of chloride in the supporting elec- trolyte promotes a mediated oxidation in the solution’s bulk, by electrogenerated oxidants species, like hypochlorous acid or hypochlorite ions, that are not powerful enough to miner- alize TC molecules, but they simply oxidizes them. On the other hand, the degradation process in sulphate medium oc- curs with no significant formation of intermediates, since the presence of sulphate ions may lead to the formation of peroxosulphate mediators that act near the anode surface and not in the solution’s bulk, promoting the mineralization of TC molecules easily, simultaneously with hydroxyl radicals (Aquino et al. 2012).

Conclusions

The influence of the initial pH of the solution on COD, TOC and absorbance decays during TC oxidation, using sodium sulphate as electrolyte, was assessed, and it was observed that the highest COD and absorbance removal rates were attained at initial pH of 2. TOC decay was only slightly influenced by the initial pH of the solution. However, if a mineralization ratio is defined as ΔTOC/ΔCOD, then initial pH that most favours TC mineralization is the solutions’ natural pH of 5.6. Regarding the influence of current density on the organic load removal, at TC’s concentration under study, an increase in current density leads to a decrease in the current efficiency of the process, as expected, since pollutant concentration is low, but it increases the ratio ΔTOC/ΔCOD, probably be- cause of the high efficiency of the hydroxyl radicals formed, whose concentration increases with current density. Concerning the influence of the electrolyte, the presence of the chloride ion increases TC degradation rate, as pointed out by HPLC results, and COD removal rate, but it drastically decreases TOC removal rate and, consequently, the minerali- zation index. HPLC results have also shown the presence of oxamic acid as one of the intermediates of tetracycline anodic oxidation.

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