SCIENTIFIC REPORT

Project PN-III-P3-3.1-PM-RO-FR-2019-0165, Contract no.: 18 BM/2019

“Efficient removal of antibiotic pollutants in water by non-thermal plasma

coupled with other advanced oxidation processes”

July - November 2019

1. Introduction

The project scope is to develop an efficient method for the degradation of pharmaceutical compounds in water, based on non-thermal plasma coupled with other advanced oxidation processes (AOPs), such as ozonation and heterogeneous catalysis. The project intends to combine experimental research and numerical simulations in order to achieve deeper insight into the degradation of organic pollutants in water by non-thermal plasma. The studies focus on antibiotics, since these compounds are hardly biodegradable and difficult to destroy by conventional treatment techniques and thus these highly persistent compounds are of greatest concern regarding water contamination.

The dramatic increase in the consumption of various medicines during the last years is due to several reasons, such as the population growth, the inverting age structure and the appearance of new target age groups. The discovery of new drugs or of new uses for the existing ones also contribute to the increased pharmaceuticals consumption. This leads to the high risk of water contamination with these compounds. Harmful effects on aquatic and terrestrial organisms as well as potential effects on human health made pharmaceutical pollutants to be regarded as a major concern, especially keeping in mind the low biodegradability in wastewater treatment plants observed for many of these chemicals [1]. Antibiotics represent one of the most dangerous classes of pharmaceuticals from the viewpoint of water contamination. The large-scale use in human and veterinary medicine leads to the continuous introduction of these compounds in the environment and explains the observed bioaccumulation and the pseudo-persistent character [2]. Moreover, antibiotics can produce resistance in bacterial populations [3], causing serious difficulties in the treatment of various pathologies.

Conventional water treatment techniques proved rather inefficient with respect to antibiotics removal, especially in case of compounds presenting high toxicity towards the microorganisms used in biological systems [2]. Therefore, research focused more and more on alternative removal methods, generally known as advanced oxidation processes (AOPs), such as ozonation, Fenton and photo-Fenton processes, photocatalysis etc. Although ozonation proved effective for antibiotics removal, it achieves very low mineralization and the resulting degradation products are extremely toxic, as indicated by ecotoxicological tests of the treated water [4]. Photo-Fenton oxidation appears as a more promising method from the point of view of mineralization and biodegradability of solutions containing antibiotics [3,5]. The governing principle of AOPs is based on the formation of powerful oxidizing species, especially hydroxyl radicals (OH), which react to the organic contaminants and degrade them.

Non-thermal plasma also generates a large variety of oxidizers, including OH, and as an advantage over the other AOPs, plasma produces the oxidizers in-situ, without requiring the introduction of external additives [6]. Recently, the plasma removal of several antibiotics has been reported [7-10], but the degradation products are quite resistant and therefore the mineralization degree was relatively low. The present work will be focused on enhancing mineralization and increasing the degradation efficiency of antibiotics by combining non-thermal plasma treatment with ozonation and heterogeneous catalysis.

In the first step of the project a detailed characterization of the electrical discharge was proposed, as well as the optimization of experimental conditions. A pulsed corona discharge above water has been used, since this configuration already proved efficient for the degradation of various organic contaminants in water [11-14]. The effect of discharge characteristics, particularly the pulse duration, on the removal of antibiotics from water has been investigated. The influence of the gaseous atmosphere (oxygen, air) and of solution properties (conductivity, pH, tap water vs. distilled water) has also been studied. The evolution of the major degradation products has been followed. Preliminary experiments comparing the results obtained by plasma alone and by plasma-ozonation have been carried out.

2. Experimental set-up

2.1.Plasma reactor design

The experiments were carried out using a pulsed corona discharge above water at atmospheric pressure and room temperature. Oxygen or air were flown through the plasma reactor with a flow rate of 300 mL/min. The plasma reactor (Fig. 1) is a parallelepipedal vessel containing an aluminum ground electrode placed on the bottom (Al tape) and the high voltage electrode consisting in an array of 20 copper wires (100 mm diameter, 6 mm distance between adjacent wires) placed above the aqueous solution. The distance between the wires and the ground electrode was approximately 10 mm and the height of solution layer was 5 mm, so the discharge gap was around 5 mm. The discharge is filamentary, it starts at the wires and propagates towards the surface of the liquid.

Fig. 1. Experimental set-up used for plasma degradation of antibiotics [15]. The valve is on position V1 for plasma alone and on position V2 for plasma-ozonation experiments

2.2.Electrical circuit

The high voltage pulses were generated using two different electrical circuits.

(1) by using a capacitor of 6 nF charged at high voltage and discharged by a solid-state thyristor switch (Behlke HTS 240-800-SCR) triggered by a variable frequency pulse generator (ThurlbyThandar Instruments (TTI) TGP-110, 10 MHz) – Fig. 2(a).

(2) by using a coaxial cable of 10 m length and a spark-gap switch operated in self-breakdown mode to short-circuit the pulse forming line – Fig. 2(b).

(a) (b)

Fig. 2. Electrical circuits used for the generation of high voltage pulses:

(a) capacitor-based circuit (C = 6 nF); (b) pulse-forming line circuit (C = 1.5 nF, R = … W).

HV – d.c. high voltage power supply, SG – switch, Z – plasma reactor

Negative polarity was used in all experiments. The voltage amplitude was 17-18 kV in both cases. In most experiments the pulse repetition rate was maintained at 25 Hz, with one exception where the pulse forming line circuit was used at 125 Hz, which will be detailed in the results section.

The discharge voltage and current were measured using probes (Tektronix P6015—voltage, Tektronix TCP0150—current) and were monitored by a digital oscilloscope (Tektronix DPO 2024). The energy per pulse was determined from the integration of the current–voltage product over the pulse duration. The average power dissipated in the discharge was calculated by multiplying the energy per pulse by the frequency.

2.3.Solution treatment configurations

The solution to be treated (total volume 330 mL) is continuously circulated between the plasma reactor and the solution reservoir (ozonation reactor) by a peristaltic pump, with a flow rate of 160 mL/min. Most experiments were performed in plasma alone configuration (valve on position V1), with the effluent gas from the discharge vented immediately after the plasma reactor. For comparison, an experiment was carried out in the plasma-ozonation configuration (valve on position V2), with the effluent gas bubbled through the solution contained in the ozonation reactor.

2.4.Solution analysis

Several antibiotics have been selected for the studies, according to various criteria, such as the usage / consumption, the frequency of occurrence in the environment, the biodegradability, the toxicity, the solubility in water etc. The chosen antibiotics are listed in Table 1, together with their chemical formula and chemical structure, as well as other relevant information. In the following section the results obtained with amoxicillin will be presented in detail.

Solutions containing the investigated antibiotics were prepared in distilled water in most experiments. For comparison, some experiments were performed in tap water, and this information will be mentioned where appropriate. The solution conductivity was adjusted to the desired level with Na₂SO₄. Two values of the conductivity were selected: 200 mS/cm and 500 mS/cm.

The initial pH of solutions containing amoxicillin was 6.5. Experiments were also performed in acidic pH, adjusted to 3.6 by H₂SO₄, and in basic pH, adjusted to 10 by NaOH.

Table 1. Antibiotics selected for the experimental investigations

Antibiotic	Class	Chemical structure, formula, Molecular mass	Solubility	EU watchlist
Amoxicillin	penicillin	C16H19N3O5S, 365.40 g/mol	3.4 g/L @ 25 ^oC	yes
Ciprofloxacin	quinolone	C17H18FN3O3, 331.346 g/mol	29.8 g/L	yes
Trimethoprim		C14H18N4O3, 290.32 g/mol	610 mg/L @ 37 ^oC	no
Sulfamethoxazole	sulfonamide	C10H11N3O3S, 253.279 g/mol	400 mg/L @ 25 ^oC	no
Doxycycline	tetracycline	C22H24N2O8, 444.43 g/mol	640 mg/L	no

The degradation of amoxicillin was assessed using high performance liquid chromatography (HPLC), employing a Rigol L-3000 system equipped with a reverse phase C18 column (250 x 4,6 mm, particle size 5 mm). The mobile phase consisted of acetonitrile (7%) and water with 0.1 % formic acid (93%), at a flow rate of 2 mL/min. The oven temperature was set to 27 ^oC. Quantification of amoxicillin was done using a diode array detector (DAD) set at a wavelength of 210 nm, while the 230 nm was used for the specificity check. The peak UV-Vis spectrum was registered between 210 and 410 nm.

3. Results

3.1.Electrical characteristics of the discharge

Voltage and current waveforms of the pulses used in the experiments are shown in Fig. 3 for the solution containing amoxicillin (100 mg/L) of initial conductivity 500 mS/cm and pH 6.5. As previously mentioned, the amplitude of the voltage was set to 17-18 kV.

Fig. 3. Voltage and current waveforms of the discharge pulses obtained with

the capacitor-based circuit (left) and with the pulse forming line circuit (right)

(initial concentration of amoxicillin solution 100 mg/L, initial conductivity 500 mS/cm, initial pH 6.5)

The voltage pulses obtained by discharging the capacitor have a typical shape (Fig. 3(a)), a rise time of approximately 55 ns (determined by the switch characteristics) and a duration (FWHM – full width at half maximum) of approximately 300 ns (influenced by the solution conductivity). The amplitude of the current pulses was 325 A. The instantaneous power (I × V) during the discharge pulse reached 4 MW. The energy per pulse (obtained by integrating the instantaneous power over the entire pulse duration) was 1.1 J. The pulse repetition rate for these experiments was 25 Hz, and therefore the average power dissipated in the discharge was 27.5 W.

The waveforms obtained using the pulse forming line based on the coaxial cable have shapes as shown in Fig. 3(b). The rise time of the voltage pulses was 13.5 ns and the duration (FWHM) was 106 ns. The pulse width depends on the length of the cable, being twice the time needed for the wave to travel along the cable. Thus, for a 10 m cable, the pulse width is around 100 ns, and is independent on solution conductivity. The current amplitude was 155 A. The following values were calculated from the current and voltage waveforms: instantaneous power 2.3 MW, energy per pulse 207 mJ and average power 5.2 W.

3.2.Influence of the pulse duration on the degradation of amoxicillin

Fig. 4 shows the results obtained with pulse durations of 106 and 300 ns with respect to the degradation of amoxicillin (initial concentration 100 mg/L) in aqueous solutions of 500 mS/cm conductivity and pH 6.5. The experiments were performed for a pulse repetition rate of 25 Hz.

a b

Fig. 4. Degradation of amoxicillin (initial concentration 100 mg/L) for pulse widths of 106 and 300 ns

as a function of: (a) – treatment time; (b) – input energy.

(initial solution conductivity 500 mS/cm, initial pH 6.5)

The amoxicillin concentration in the solutions exposed to plasma decreased exponentially with treatment time (Fig. 4(a)). After 60 min treatment, 73% degradation of the antibiotic was obtained using the shorter pulses, while in case of the 300 ns pulses the removal was 89%. The faster degradation achieved with the longer discharge pulses is due to the higher input power, i.e. 27 W, as compared to only 5-6 W for the discharge operated with 106 ns pulses. For a more appropriate comparison, the data were plotted as a function of input energy (Fig. 4(b)), defined as the total amount of energy introduced in the plasma during water treatment (i.e. the average power multiplied by the treatment time). Obviously, the shorter discharge pulses are more efficient for the degradation of contaminants, since the same values of removal are obtained for lower input energy in case of the 106 ns pulses.

In order to compare the results for the same average power, a series of experiments were performed with the shorter pulses at higher frequency. Thus, the power dissipated in the discharge is 27 W in both cases, i.e. 106 ns pulses at 115 Hz and 300 ns pulses at 25 Hz. The results regarding amoxicillin degradation are shown in Fig. 5.

a b

Fig. 5. Degradation of amoxicillin (initial concentration 100 mg/L) for pulse widths of 106 and 300 ns, at the same average power (27 W) obtained by changing the pulse frequency (25 Hz / 115 Hz).

(a) concentration as a function of treatment time; (b) logarithmic representation as a function of input energy. Initial solution conductivity 500 mS/cm, initial pH 6.5

Using shorter pulses with higher repetition rate appears beneficial for the plasma removal of antibiotic contaminants in water, leading to faster degradation of the compounds (Fig. 5(a)). The entire amount of amoxicillin was eliminated during 60 min plasma treatment. The logarithmic representation of the data (Fig. 5(b)) shows a non-linear behavior in case of shorter discharge pulses, specifically the acceleration of the degradation for long treatment time.

The efficiency of the treatment method can be evaluated by calculating the energy yield, i.e. the amount of pollutant removed per unit of energy introduced in the process. A comparison from this point of view of the data presented so far is shown in Fig. 6. The 300 ns pulses exhibit the lowest efficiency: at 50% conversion, the energy yield was 1.9 g/kWh. Operating the discharge at the same power, but with shorter pulses (106 ns) at higher frequency enhances the efficiency, in this case the energy yield at 50% removal being 3.4 g/kWh. This effect is related to the formation of reactive oxygen species (ROS) in the plasma and should be correlated to the plasma characteristics and to the generation and consumption mechanisms of these ROS [15].

Fig. 6. Energy yield for the plasma degradation of amoxicillin in water as a function of the removal rate. Comparison between different discharge pulse widths.

(initial concentration 100 mg/L, initial conductivity 500 mS/cm, initial pH 6.5)

Low power operation with short discharge pulses at low frequency appears to be the most efficient method for the degradation. At 50% amoxicillin removal the energy yield was 5.3 g/kWh, almost three times larger than the value obtained with the longer pulses. However, the higher efficiency comes at the cost of slower degradation: as previously mentioned, only 73% of the initial amoxicillin was removed after 60 min treatment at low power.

3.3.Influence of the solution conductivity

The previous data obtained for pulses of 106 ns at 25 Hz frequency and 500 mS/cm initial solution conductivity are compared to results obtained for 200 mS/cm in order to illustrate the effect of solution conductivity on the degradation (Fig. 7). The data are plotted as a function of input energy to account for the slight difference in discharge power between the experiments (5.7–5.9 W). A logarithmic representation is also shown (Fig. 7(b)) for a clearer picture of the degradation behavior.

Faster removal of the target compound was observed for lower solution conductivity. After 60 min plasma treatment 88% of the initial amoxicillin from the 200 mS/cm solution was degraded, as compared to 73% in case of the higher conductivity. Higher solution conductivity (higher Na₂SO₄ concentration) could affect the oxidative degradation of the target pollutant by: (i) diminishing the amoxicillin solubility in the aqueous solution; (ii) more efficient interaction of sulfate ions with hydroxyl radicals. Sulfate anions compete with the organic pollutants for the hydroxyl radicals [16]:

HO‧ + SO₄^2- → SO₄^-‧ + OH^-

However, the resulted sulfate radical anions are strong oxidizer species. In the degradation of amoxicillin, they have comparative reaction rate (2.9 ‧10⁹ – 3.9 ‧10⁹ M^-1s^-1) [17,18] with the hydroxyl radicals (3.9 ‧ 10⁹ – 6.9 ‧ 10⁹ M^-1s^-1) [18,19].

The solution conductivity shows a slight increase during plasma treatment, reaching 521 mS/cm and 245 mS/cm, respectively, at the end of 60 min plasma exposure.

ab

Fig. 7. Degradation of amoxicillin (initial concentration 100 mg/L) as a function of input energy for solution conductivities of 200 and 500 mS/cm (initial pH 6.5, pulse width 106 ns, frequency 25 Hz)

3.4. Influence of solution pH on the degradation of amoxicillin

The effect of solution pH has been considered as well. Besides the previously mentioned results obtained for pH 6.5, the degradation of amoxicillin in solutions of pH 3.6 and 10 have been investigated. The data plotted as a function of input energy as well as the logarithmic representation are shown in Fig. 8.

The solution pH decreases during plasma treatment due to the formation of acidic degradation products. For the solution of initial pH 6.5, the final pH value, after 60 min plasma exposure was 4.54. For the basic solution, the pH decreased to 6.73. On the contrary, the acidic solution showed a slight pH increase as a result of plasma exposure, the final value being 4.02.

Slightly faster degradation of the target compound was observed in the acidic solution as compared to the one of pH 6.5. In case of the basic solution, the removal is considerably higher. After 60 min plasma treatment. 94.3% of the initial amoxicillin was eliminated in the solution of initial pH 10, as compared to 73% degradation in the solution of pH 6.5. The main reason of the higher removal of the target compound at basic pH could be the increased rates of hydrogen peroxide and ozone decomposition with formation of highly reactive hydroxyl radicals [20,21]. Increased solubility of amoxicillin at basic pH (dissociation constants: pK₁ = 2.7 and pK₂ = 7.2 – 7.4) [22] can also have a contribution to his faster removal rate.

Fig. 8. Degradation of amoxicillin (initial concentration 100 mg/L) as a function of input energy for various solution pH: 3.6, 6.5, 10 (initial conductivity 500 mS/cm, pulse width 106 ns, frequency 25 Hz)

3.5. Comparison tap water vs. distilled water

Tap water is a useful model for the application of plasma treatment to wastewater and drinking water purification, and therefore a comparison was made between the previously mentioned data, obtained with distilled water, and results obtained for solutions prepared in tap water (Fig. 9). The conductivity of tap water was 306 mS/cm and the pH was 7.75. When adding amoxicillin into tap water the solution pH became 6.84, while the conductivity was adjusted to 500 mS/cm with Na₂SO₄. After 60 min plasma treatment the final pH value was 7.35 and a slight increase in conductivity to 517 mS/cm was observed. The carbonates present in tap water buffer the solution, so the pH does not decrease during plasma treatment due to acids formation.

Although the carbonate salts in tap water quench OH radicals [23] and thus should negatively influence the degradation of organic compounds, the elimination of amoxicillin was actually faster as compared to distilled water. After 60 min plasma treatment the amoxicillin removal from tap water reached 88.2%, as compared to only 73% in case of distilled water. This result can be explained by the buffering effect of bicarbonate, which appears to dominate over adverse effects due to the OH-scavenging character [24]. Obviously, this finding represents an advantage in view of possible applications for water remediation.

a b

Fig. 9. Degradation of amoxicillin (100 mg/L) in solutions prepared in distilled water and tap water as a function of input energy (initial conductivity 500 mS/cm, pulse width 106 ns, frequency 25 Hz)

3.6. Influence of the gaseous atmosphere

A comparison between the experiments performed with plasma generated in oxygen and in air has been made and the results regarding amoxicillin degradation are shown in Fig. 10.

The pH drop in case of the air plasma is more important than in solution treated in oxygen: the final solution pH (after 60 min plasma treatment) was 3.71 for air, as compared to 4.54 for oxygen. This is due to the accumulation in the aqueous solution of nitrous and nitric acids, as a result of reactive dissolution of nitrogen oxides generated in the air plasma system.

ab

Fig. 10. Degradation of amoxicillin (initial concentration 100 mg/L) by plasma generated in air and oxygen as a function of input energy (distilled water, initial conductivity 500 mS/cm, initial pH 6.5, pulse width 106 ns, frequency 25 Hz)

The influence of the gaseous atmosphere on the removal of the target antibiotics appears to be rather small under these conditions: 77.8% degradation in air as compared to 73% in oxygen after 60 min plasma treatment. This is not a significant difference, so it can be said that both gases performed equally well. This is another important result from the viewpoint of practical application of plasma to the water remediation process.

3.7. Comparison between plasma alone and plasma-ozonation

Preliminary results achieved in the plasma-ozonation configuration are presented in Fig. 11 and compared with the data obtained by plasma alone. Here only a description of the results is presented, while a through analysis makes the object of further research that will be conducted in the next step of the project.

a b

Fig. 11. Degradation of amoxicillin (100 mg/L) by plasma alone and plasma-ozonation as a function of input energy (distilled water, oxygen plasma, initial conductivity 500 mS/cm, initial pH 6.5, pulse width 106 ns, frequency 25 Hz)

Plasma-ozonation greatly enhances degradation, as clearly shown in the above figure. Amoxicillin is almost completely degraded after 40 min treatment. After only 5 min plasma-ozonation, more than 50% of the initial amoxicillin was removed. The reaction rate constant was almost four times larger for plasma-ozonation than for plasma alone. The deviation from linearity observed in the logarithmic representation of the data (Fig. 11(b)) will need further study.

With respect to the efficiency of the degradation process, the energy yield was 35.6 g/kWh for plasma-ozonation, as compared to 5.3 g/kWh for plasma alone. The observed degradation products are also eliminated in a large extent, indicating a high degree of mineralization.

3.8. Participation and tasks of the Romanian researchers

Dr. M. Nistor and Dr. D. Dobrin have scientific tasks mainly related to the discharge electrical characterization: measurements of discharge current and voltage under various experimental conditions and with various pulse generation systems, calculation of energy and power etc.

Dr. C. Bradu and F. Bilea are mainly in charge with the chemical characterization of plasma-treated solutions: measurements of pH and conductivity, chromatographic analysis, identification of degradation products, total organic carbon analysis etc. It is worth mentioning that F. Bilea recently started his PhD studies in the area of Analytical Chemistry, on a topic concerning the plasma degradation of pharmaceutical compounds, therefore very close to the project objective.

Dr. M. Magureanu supervises the team activity and participates in all the experimental activities.

3.9. Common activities of the partners

Dr. Monica Magureanu and Dr. Corina Bradu have visited GREMI during the period 20-26 October 2019. They visited the various research groups of the GREMI laboratory and discussed on the main topics of interest. During this visit each of them presented a seminar:

Dr. C. Bradu: “Chimie de l’eau a Bucarest”

Dr. M. Magureanu: “Degradation of organic pollutants in water by non-thermal plasma” – web‑seminar broadcasted by Réseaux des Plasmas Froids (http://plasmasfroids.cnrs.fr/spip.php?article365)

The kick-off meeting of the project was also organized during this visit. The participants shared ideas regarding the experimental research activities and their correlation with the theoretical simulations, discussed the selection criteria of the target compounds and decided on the antibiotics to be treated by plasma, and finally discussed about the long-term visits of the two PhD students (Noussaiba Korichi and Florin Bilea) that will take place in 2020. Discussions about common publication(s) that will include the results of the Romanian team (presented above) and the results of the French group have also taken place.

Dr. Hervé Rabat and Dr. Olivier Aubry have visited NILPRP during the period 10-15 November. During this visit, each of them presented a seminar:

Dr. H. Rabat “From an overview of the rreaserch at the GREMI laboratory to a focus on the different researches in the Arc Team”

Dr. O. Aubry “Pharmaceutical molecules degradation in water by non-thermal plasma and heterogeneous catalysis coupling”

The French researchers visited several research groups from the Plasma Physics and Nuclear Fusion Laboratory: the Plasma Chemistry and Advanced Functional Materials Group, the Plasma Surface Engineering Group and the X-ray Imaging Laboratory: Microtomography and Microfluorescence. They have also visited the PROTMED research center of the University of Bucharest (Research Center for Environmental Protection and Waste Management).

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