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Article

Advancing Nanopulsed Plasma Bubbles for the Degradation of Organic Pollutants in Water: From Lab to Pilot Scale

by
Stauros Meropoulis
and
Christos A. Aggelopoulos
*
Laboratory of Cold Plasma and Advanced Techniques for Improving Environmental Systems, Institute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas (FORTH/ICE-HT), 26504 Patras, Greece
*
Author to whom correspondence should be addressed.
Technologies 2024, 12(10), 189; https://doi.org/10.3390/technologies12100189
Submission received: 22 August 2024 / Revised: 27 September 2024 / Accepted: 1 October 2024 / Published: 3 October 2024
(This article belongs to the Section Environmental Technology)
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Abstract

:
The transition from lab-scale studies to pilot-scale applications is a critical step in advancing water remediation technologies. While laboratory experiments provide valuable insights into the underlying mechanisms and method effectiveness, pilot-scale studies are essential for evaluating their practical feasibility and scalability. This progression addresses challenges related to operational conditions, effectiveness and energy requirements in real-world scenarios. In this study, the potential of nanopulsed plasma bubbles, when scaled up from a lab environment, was explored by investigating critical experimental parameters, such as plasma gas, pulse voltage, and pulse repetition rate, while also analyzing plasma-treated water composition. To validate the broad effectiveness of this method, various classes of highly toxic organic pollutants were examined in terms of pollutant degradation efficiency and energy requirements. The pilot-scale plasma bubble reactor generated a high concentration of short-lived reactive species with minimal production of long-lived species. Additionally, successful degradation of all pollutants was achieved in both lab- and pilot-scale setups, with even lower electrical energy-per-order (EEO) values at the pilot scale, 2–3 orders of magnitude lower compared to other advanced oxidation processes. This study aimed to bridge the gap between lab-scale plasma bubbles and upscaled systems, supporting the rapid, effective, and energy-efficient destruction of organic pollutants in water.

1. Introduction

Non-thermal plasma technology has emerged as one of the most promising methods for water purification, offering a sustainable and efficient solution to the escalating challenges of water pollution [1,2,3,4,5,6,7,8,9]. This cutting-edge technology generates reactive species capable of breaking down a wide range of organic contaminants, including persistent pollutants like pharmaceuticals, pesticides, dyes, antibiotics and PFAS that conventional methods often fail to eliminate [3,4,5,9,10,11,12,13,14,15]. The plasma process produces a variety of long- and short-lived reactive oxygen species (ROS) such as hydroxyl radicals, atomic oxygen, ozone and hydrogen peroxide, as well as reactive nitrogen species (RNS), including nitrate/nitrite anions and peroxynitrite [16,17]. These diverse species initiate chain reactions leading to the degradation of pollutants, supported by UV light and highly energetic electrons that engage in electron transfer reactions with organic molecules [13]. In this context, there is increasing interest in researching and developing optimized plasma remediation systems to enhance water quality and contribute to global efforts in water conservation and pollution control.
In recent years, significant efforts have been made to develop and explore various types of plasma reactors aimed at achieving more effective pollutant destruction, faster degradation rates, and greater energy efficiency [18,19,20,21]. Much of this research focuses on plasma-on-liquid systems, where species are generated by plasma–gas interactions and are subsequently transferred to the contaminated water through the plasma–liquid interface [18,22,23,24,25,26,27,28]. However, many reactors used for plasma-over-liquid processes encounter performance limitations due to the short-lived nature of many plasma reactive species and their limited penetration into the liquid phase [29,30]. As a result, a major challenge persists in achieving the complete degradation of pollutants due to the suboptimal utilization of UV radiation, electrons, and plasma reactive species.
In this context, a growing number of recent studies have highlighted the potential of plasma bubbles as a method to enhance the mass transfer of plasma species from the gas to the aqueous phase [31,32,33,34,35,36]. By generating plasma bubbles directly within the bulk liquid, both short-lived and long-lived reactive species can interact more effectively with pollutants, thereby addressing the limitations commonly observed in traditional gas–liquid systems. Moreover, direct comparisons between plasma bubbles and traditional gas–liquid plasma systems have underscored the importance of plasma bubbles [21]. For instance, dielectric barrier discharge (DBD) plasma bubbles, energized by high-voltage nanopulses, have shown significantly improved performance over gas–liquid DBD, achieving highly energy-efficient pollutant removal, with up to 82.1 g/kWh recorded for the degradation of methylene blue [21]. This study also revealed that the composition of plasma-activated water differed notably between the two systems due to distinct plasma-liquid interactions. Specifically, the concentration of short-lived ·OH was approximately twice as high in nanopulsed plasma bubbles compared to the gas–liquid system, whereas the concentrations of long-lived species (i.e., H2O2, NO3−/NO2− and O3) were higher in the gas–liquid system.
However, beyond the common challenges of scaling up remediation approaches, such as maintaining effective pollutant degradation and consistent energy efficiency, the complex interactions between plasma–bubble reactive species and organic pollutants add another layer of uncertainty regarding the successful implementation of this approach on a larger scale. It is, therefore, essential to demonstrate the effectiveness of this method in scaled-up systems to confirm its applicability under real-world conditions.
In this study, the potential of lab-scale nanopulsed plasma bubbles was explored at an upscaled level. A thorough investigation was conducted to examine various critical experimental parameters, such as plasma gas, pulse voltage, pulse repetition rate, gas flow rate, and water matrix that influence the process effectiveness and energy efficiency. To ensure the broad applicability of this remediation approach, different classes of organic pollutants (dyes, pharmaceuticals and antibiotics), all classified as highly toxic to the environment, were tested. Additionally, the composition of nanopulsed plasma bubble-activated water was assessed at a pilot scale to verify whether similar concentrations of reactive species are generated as in the lab-scale system [21]. A detailed comparison of pollutant degradation and process energy efficiency between the lab and pilot scales was also performed. This study provides valuable insights into the implementation and dynamics of utilizing nanopulsed plasma bubbles on a larger scale for the rapid, effective, and energy-efficient destruction of various types of organic pollutants.

2. Materials and Methods

2.1. Chemicals and Reagents

All chemicals and reagents used in this study were of analytical grade, purchased from Merck and used without further purification. Various classes of molecules were examined, including dyes (methylene blue, C16H18ClN3S (MB)), antibiotics (sulfamethoxazole, C10H11N3O3S (SMX)) and the angiotensin receptor blocker valsartan, C24H29N5O3 (VAL)). Aqueous solutions of each pollutant were prepared using tap water, and plasma experiments were conducted with tap water instead of deionized water to better reflect real-life conditions. Compressed dry air, oxygen and argon, supplied by Evoxa (Patra, Greece), were used as plasma feed gases.

2.2. Experimental Setup, Pilot-Scale Plasma Bubble Reactor, and Treatment Conditions

The experimental setup (Figure S1) consisted of a plasma bubble reactor (either lab- or pilot-scale), a gas feeding system, a nanosecond pulsed high-voltage (HV) power supply (NPG-18/3500), and an electrical characterization arrangement. Additional equipment for chemical analysis of pollutants and detection of plasma species in the aqueous phase was also included. For electrical characterization, a digital oscilloscope (Rigol MSO2302A, Suzhou, China) connected to voltage and current probes (Tektronix P6015A, Beaverton, OR, USA and Pearson electronics 2877, Palo Alto, CA, USA, respectively) was used. Details of the electrical measurements and the calculation of the instantaneous power and mean power dissipated in the plasma bubble reactor can be found in our previous work [37].
A pilot-scale plasma bubble reactor was constructed, tested, and optimized for the degradation of various classes of organic pollutants in water (Figure 1). The conceptual design was based on a previously reported lab-scale plasma bubble reactor capable of delivering plasma bubbles directly into the polluted water [14,38]. Specifically, the pilot-scale reactor utilized a coaxial dielectric barrier discharge (DBD) configuration positioned at the center of a cylindrical water reaction tank.
The reactor consists of a HV 316 stainless-steel rod (4 mm thick, 350 mm high) encased within an inner quartz tube (8 mm external diameter, 350 mm high), forming a coaxial DBD with an outer quartz tube (15 mm external diameter, 350 mm high). The working gas was injected into the space between the inner and outer tubes, with 10 uniformly distributed holes at the base of the outer tube (each hole size~400–500 μm), allowing for plasma bubbles to be directly injected into the contaminated water.
The polluted water solution, with a volume of 2.5 L, was contained within a quartz cylindrical reaction vessel (110 mm internal diameter, 340 mm high). A stainless-steel grid attached to the external surface of the reaction vessel served as the grounded electrode. Notably, unlike previously reported reactors that generate plasma discharges and/or bubbles within the aqueous phase [39], the design of this DBD-based reactor prevents unwanted contamination from particles or metal ions potentially released from the electrodes.
In all experiments, the initial concentration of pollutants in the water was set to 10 mg/L, with treatment times ranging from 2 to 20 min. In the lab-scale plasma bubble reactor, 70 mL of contaminated water was treated, while 2.5 L was treated in the pilot-scale reactor. The pilot-scale experiments were conducted using varying pulse voltages (19.4, 21.4, and 24.0 kV) and pulse frequencies (300, 400, and 600 Hz), with different plasma working gases (air, oxygen, and argon) and gas flow rates ranging from 5 to 15 L/min. The lab-scale experiments were carried out under optimized conditions reported in previous studies: a pulse voltage of 26.0 kV, pulse frequency of 200 Hz, and a gas flow rate of 3 L/min [21,38].

2.3. Plasma Reactive Species in Liquid Phase and Measurement of Temperature

The major short- and long-lived plasma-induced reactive species within the liquid phase were analyzed. Specifically, concentrations of H2O2, nitrate ions (NO3−), nitrite ions (NO2−), and O3 were accurately quantified using a QUANTOFIX® Relax unit (Macherey-Nagel, GmbH, Düren, Germnay) and Hanna multiparameter photometers (HI83399-2), respectively, along with certified and calibrated reagent kits. Hydroxyl radicals (·OH) were detected via photoluminescence spectroscopy (Hitachi F2500, Tokyo, Japan), employing terephthalic acid (TA) as a trapping reagent. The reaction between ·OH and TA yields 2-hydroxyterephthalic acid (HTA), which emits strong fluorescence with a peak at 425 nm when excited with UV-A light at 310 nm [40]. The pH of the solution at each stage was measured by the appropriate strips using the QUANTOFIX® Relax unit. A thermocouple inserted into the treated water samples was used to measure the temperature using a digital multimeter (Mastech MS8209, Charlotte, NC, USA). The initial temperature of the water samples was ~21 °C, increasing slightly to ~23 °C after 20 min of treatment in the pilot-scale plasma bubble reactor (Figure S2). Therefore, the temperature remained at room temperature during reactor operation, consistent with previously published results using HV nanopulses to drive the plasma process [41].

2.4. Chemical Analysis of Water Samples

The degradation efficiency of methylene blue (MB) was assessed using UV-Vis spectroscopy (Shimadzu, UV-1900, Kyoto, Japan) by monitoring its characteristic absorption peak at 664 nm. The concentrations of SMX and VAL were determined using high-performance liquid chromatography (HPLC, Shimadzu LC-2050C, Kyoto, Japan). For SMX, a C18 column (4.6 mm × 50 mm, 3 μm particle size) and a UV detector were used, while for VAL, an Agilent Eclipse XDB-C18 column (4.6 × 150 mm, 5 μm) and a UV detector were used. For SMX, following the method of Liang et al. [42], the column temperature was set at 30 °C, the detection wavelength to 271 nm, and the pressure to 100 bar. The flow rate was maintained at 1.0 mL/min, with an injection volume of 20 μL. The mobile phase was isocratic, composed of 0.2 mol/L NaH2PO4 solution and methanol in a 70%/30% (v/v) ratio. For VAL, an isocratic method was used, with the mobile phase consisting of 46% acetonitrile (solvent A), 44% water adjusted to pH = 3.5 with phosphoric acid (solvent B), and 10% methanol (solvent C). The column temperature was maintained at 50 °C with a flow rate of 1.0 mL/min. The absorbance of Valsartan was measured at 230 nm.
The pollutant degradation efficiency, D % , was calculated using the following equation:
D % = C 0 − C f C 0 × 100
where C 0 and C f are the pollutant concentrations in water before and after plasma bubble treatment, respectively.
The pollutant degradation rate was described by a pseudo-first order reaction with respect to the pollutant concentration. Therefore, the experimental data were fitted to the following equation:
l n C 0 C f = k a p p t
where k a p p (min−1) is the apparent rate constant.
A normalized volume-based kinetic constant ( k ′ ), in order to elucidate the degradation kinetics, taking into account the differences of the under-treatment water volume for each reactor was calculated:
k ′ = k a p p V
where k ′ is the normalized volume-based apparent rate constant (mL min−1), k a p p is the apparent rate constant (min−1), and V the under-treatment water volume for each reactor (mL).
The electrical energy per order (EEO), defined as the electrical energy in kWh required to reduce a contaminant’s concentration by one order of magnitude in 1 m3 of contaminated water, was calculated using the following equation:
E E O = P t V log ( C 0 C f )
where P is the mean power dissipated in the plasma bubble reactor, V is the volume of the treated solution, C 0 is the pollutant concentration before plasma treatment, and C f is the concentration after treatment time t .
The reproducibility of the results was quite satisfactory being conducted in triplicate with negligible standard deviation.

3. Results

3.1. Electrical Characterization of Pilot- and Lab-Scale Plasma Bubble Reactors Energized by HV Nanopulses

The typical voltage and current nanopulses for the lab-scale and pilot-scale plasma bubble systems, operating under air plasma gas, are shown in Figure 2. The pulse patterns for both systems were similar, consisting of a series of pulses: initially, a high-amplitude pulse with a short rise time (~4 ns) was observed, followed by pulses of smaller amplitude [43]. The peak discharge current was notably higher for the pilot-scale reactor (~90 A) compared to the lab-scale system (~45 A). Despite the very high instantaneous power peaks (~2.2 MW for the pilot-scale and ~1.2 MW for the lab-scale plasma bubbles), the average power remained quite low due to the very low duty cycle, measuring ~5.5 W for the pilot-scale reactor and ~0.6 W for the lab-scale reactor.
The instantaneous voltage and current waveforms for both the pilot-scale and lab-scale plasma bubble reactors operating under argon plasma gas were also measured and are presented in Figure S3. The peak discharge current under argon plasma was higher than that under air plasma for both the lab-scale and pilot-scale reactors, resulting in greater power dissipation in the reactors. In particular, the current was ~110 A for the pilot-scale reactor and ~55 A for the lab-scale reactor, resulting in a power of ~7.0 W for the pilot-scale reactor and ~1.2 W for the lab-scale reactor.

3.2. Species Formation and Water Physicochemical Properties in the Pilot-Scale Plasma Bubble Reactor

The significance of both short-lived and long-lived reactive oxygen and nitrogen species (RONS) for the degradation of organic pollutants, especially ⋅OH, H2O2, and O3, is undeniable, regardless of the plasma reactor configuration, whether through discharge generation at the gas–liquid interface or as plasma bubbles within the bulk liquid. Interestingly, the concentrations of these reactive species vary significantly depending on the reactor configuration due to the differing plasma–liquid interactions in the two systems [21]. Specifically, as previously analyzed in detail, the concentration of long-lived species is much higher in gas–liquid systems compared to plasma bubble systems, whereas the opposite is observed for short-lived species, which are the primary driving force for pollutant degradation in the plasma bubble approach [21]. Our previous work on water composition treated by nanopulsed plasma bubbles at the lab-scale revealed that the concentration of nitrogen-related species (NO3−/NO2−) was negligible [21]. Additionally, the concentrations of the H2O2 and O3 species in the plasma bubble reactor were relatively lower compared to those in existing gas–liquid systems, while the concentration of short-lived ⋅OH was higher in the plasma bubbles. In the current plasma bubble reactor, the plasma gas is pre-activated before it comes into contact with the aqueous phase via the underwater bubble, resulting in significantly lower concentrations of long-lived species and minor pH changes. This occurs due to the reduced electric field strength inside the underwater plasma bubble compared to the DBD in the center of the plasma bubble reactor [14].
It is therefore crucial to investigate whether the reactive species composition observed in lab-scale plasma bubble treatment holds true following upscaling to the pilot reactor level. Interestingly, the trend in the pilot reactor closely mirrors that detected in lab-scale nanopulsed plasma bubbles. Specifically, the concentrations of O3 and H2O2 increased with treatment time, though they remained low compared to gas–liquid systems. In the pilot nanopulsed plasma bubbles system driven by O2, the O3 concentration rapidly reached 0.20 mg/L after 6 min, whereas the air-driven system achieved a corresponding value of 0.13 mg/L after 10 min of treatment (Figure 3a). These values closely align with those observed at the lab scale [21].
Similarly, the H2O2 concentration (Figure 3b) was higher under an O2 atmosphere than under air (i.e., 0.68 under O2 and 0.38 under air, after 6 min of treatment), consistent with lab-scale observations [21]. In contrast to the low concentrations of long-lived species, the pilot reactor exhibited a high concentration of the short-lived ⋅OH, similar to the lab-scale nanopulsed plasma bubbles [21]. The ⋅OH concentration increased rapidly at short treatment times under O2, reaching ~25 mg/L after 10 min, and gradually increased under an air atmosphere, reaching ~20 mg/L after 20 min of treatment (Figure 3c).
Furthermore, the concentration of NO3−/NO2− was negligible, consistent with findings from the lab-scale system [21]. This resulted in the pH remaining stable (~7) throughout the treatment (Figure 3d), underscoring the potential for reusing the treated water for various purposes, such as drinking water or irrigation. Another reason for the stable pH of plasma-treated water can be attributed to the presence of carbonate compounds in tap water, such as carbonic acid (H2CO3), bicarbonates (HCO3−), and carbonates (CO32−). As reported [44], tap water is resistant to plasma-induced acidification due to the increased buffering capacity of CO32− and HCO3−, which reduces NO3− by scavenging hydronium ions (H+) that formed during the process.
The similarity between the lab-scale and pilot-scale nanopulsed plasma bubbles systems, with no significant deviations in water composition or reactive species concentrations, is highly promising for the broader application of plasma bubbles at an industrial scale.

3.3. Effect of Pulse Voltage and Pulse Repetition Rate on MB Degradation Efficiency

The effects of pulse voltage and pulse frequency, key plasma operational parameters, were evaluated based on the degradation efficiency of methylene blue (MB). It is evident that MB degradation efficiency increased with a higher pulse voltage, regardless of treatment duration (Figure 4a). A rapid increase in MB degradation efficiency was observed at all pulse voltages, achieving 56.7%, 69.0%, and 80.6% at pulse voltages of 19.4 kV, 21.4 kV, and 24.0 kV, respectively, after just 4 min of treatment. The corresponding apparent rate constants were 0.17, 0.29, and 0.49 min−1, respectively (Figure 4b). However, maximum degradation efficiency was reached at different treatment times depending on the pulse voltage: >99% after 10 min at 24.0 kV after 15 min at 21.4 kV, and 96.3% after 20 min at 19.4 kV, which, while high, was less effective. Generally, it is well known that a higher applied voltage generates a stronger electric field and supplies greater electric power to the plasma reactor, leading to more intense UV light and an increased production of plasma reactive species [45,46].
The effect of pulse frequency on MB degradation is depicted in Figure 3c. Although a pulse repetition rate of 300 Hz was insufficient to achieve satisfactory degradation efficiency, resulting in only 47.2% after 20 min of treatment with the pilot nanopulsed plasma bubbles reactor, a much higher degradation efficiency was observed at higher pulse repetition rates. In particular, both 400 Hz and 600 Hz led to a substantial enhancement in MB degradation. After 8 min of treatment, complete pollutant degradation (>99%) was achieved at pulse frequencies of 400 and 600 Hz, whereas the degradation efficiency at 300 Hz was only 29.5%. The corresponding apparent rate constants were 0.55, 0.49, and 0.31 min−1 at 600, 400, and 300 Hz, respectively (Figure 4d). Similar to the pulse voltage, an increased pulse repetition rate leads to a higher number of discharges per unit time and, consequently, greater electric power supplied to the system. In fact, at a constant peak voltage applied to the reactor, the discharge power increased with higher pulse repetition rates. This results in the enhanced production of plasma reactive species and more efficient degradation of organic pollutants in water [47]. However, it should be noted that beyond the beneficial increase in energy from higher pulse frequencies, a very high pulse repetition rate is primarily associated with reactor heating rather than further optimization of the process, which limits improvements in degradation efficiency as the pulse repetition rate increases. This explains why increasing the pulse frequency from 400 to 600 Hz did not result in a significant increase in overall MB degradation efficiency. Based on these findings, the optimal pulse voltage and pulse frequency were determined to be 24.0 kV and 400 Hz, which were the values used in subsequent experiments.

3.4. Effect of Plasma Gas and Its Flow Rate on MB Degradation Efficiency

The efficiency of plasma-based remediation techniques is significantly influenced by both the type of feed gas and the gas flow rate. The choice of feed gas determines the plasma chemistry, which in turn affects the composition of reactive species in plasma-treated water. In this study, air, O2, and Ar atmospheres were examined to indirectly identify the key reactive species generated by the plasma. Previous research on lab-scale plasma bubbles demonstrated that O2 and air plasmas were highly effective in degrading MB [21]. This enhanced performance of O2 and air plasmas can be attributed to the dominant role and higher oxidation potential of reactive oxygen species (ROS). Consistent with these findings, the pilot-scale experiments also showed high degradation efficiency for O2 and air-driven systems (Figure 5a). Even with short treatment times, significant degradation was observed with O2 and air plasmas (93.0 and 80.7%, respectively, after 4 min of treatment), while Ar plasma achieved only 6.4% degradation, which remained insufficient even after 20 min (14%). The rapid kinetics of MB degradation with O2 and air plasma bubbles are clearly illustrated (Figure S4), with corresponding apparent rate constants of 0.66 and 0.49 min−1. Complete MB degradation was achieved within 6 min and 10 min of plasma treatment with O2 and air, respectively. These results underscore the crucial role of ROS in the degradation mechanism, while argon ions and hydrated electrons produced by Ar plasma play a minimal role in the process. The slighter higher efficiency of O2− plasma bubbles compared to air–plasma bubbles is linked to the increased concentration of ROS (O3, H2O2, ·OH), as shown in Figure 2. Given their performance and the lower cost of air compared to O2, air was selected for subsequent experiments in this study.
In addition to the type of feed gas, the gas flow rate also significantly influences pollutant degradation [48,49]. To investigate the impact of gas flow rate in the pilot-scale plasma bubble system, experiments were conducted in an air atmosphere at three different gas flow rates: 5, 10, and 15 L/min (Figure 5b). The results showed that increasing the gas flow rate from 5 to 10 L/min enhanced MB degradation at both short and intermediate treatment times.
However, at extended treatment times, complete degradation was achieved regardless of the gas flow rate. Specifically, after 4 min of treatment, the degradation efficiencies were 67.9% at 5 L/min and 80.4% at 10 L/min, with complete degradation observed for both flow rates after 15 min. This improvement can be attributed to the higher concentration of plasma-generated species and enhanced mass transfer due to increased fluid circulation at higher gas flow rates [33]. Indeed, the concentration of plasma species increased as the flow rate rose from 5 to 10 L/min (Figure S5). At 5 L/min, the maximum concentrations of H2O2 and ·OH were 0.31 and 3.7 mg/L, respectively, with corresponding values at 10 L/min being 0.49 and 20.5 mg/L. However, further increasing the gas flow rate to 15 L/min resulted in poorer performance compared to 10 L/min, despite plasma species concentrations being nearly identical between 10 and 15 L/min (Figure S5). This suggests that at even higher flow rates, the mass transfer of reactive species occurs too rapidly, causing them to be quickly expelled and reducing their interaction with organic molecules.

3.5. Effect of the Water Matrix on MB Degradation

Given that the plasma bubbles are intended for treating real water samples, all pollutant degradation experiments in the pilot-scale reactor were conducted using tap water. However, it is also valuable to evaluate pollutant degradation in ultrapure water. Interestingly, as observed in the lab-scale reactor [21], the pilot-scale system proved effective in degrading MB in tap water, with results comparable to or even slightly better than those in ultrapure water, suggesting the method’s potential applicability under realistic conditions (Figure 6a). For instance, after 4 min of plasma bubbles, the degradation efficiency was 80.6% and 77.9% for tap and ultrapure water, respectively, while after 10 min, degradation efficiency greater than 99% was observed for both tap and ultrapure water. The apparent rate constant for MB degradation in tap water was slightly higher, at 0.49 min−1, compared to 0.45 min−1 in ultrapure water (Figure 6b). In other words, the co-existing salts in the tap water most likely supported the degradation pathways by generating additional RONS, also previously noticed [50].

3.6. Application of Pilot-Scale Plasma Bubbles to Different Classes of Organic Contaminants, and Comparison to Lab-Scale Results

3.6.1. Degradation Efficiency

The degradation efficiency of lab-scale and pilot-scale nanopulsed plasma bubbles was compared across different classes of pollutants, including the dye methylene blue (MB), the antibiotic sulfamethoxazole (SMX), and the angiotensin receptor blocker valsartan (VAL). In the lab-scale nanopulsed plasma bubble system, MB and SMX exhibited rapid degradation (Figure 7a). After just 4 min of treatment, MB and SMX were degraded by 97.4% and 92.8%, respectively, achieving nearly complete degradation (100% for MB and >99% for SMX) after 8 and 10 min, respectively. This rapid degradation is reflected in the high apparent rate constants of 0.88 min−1 for MB and 0.76 min−1 for SMX (inset of Figure 7a). Although the degradation of VAL was slower, it was still effective, consistent with previous observations during photocatalytic degradation, where the complex nature of transformation products and different degradation mechanisms were noted [51]. In this study, using lab-scale nanopulsed plasma bubbles, 47.2% of VAL was degraded after 4 min, with >99% degradation achieved after 20 min of treatment. The corresponding apparent degradation rate constant was 0.23 min−1 (inset of Figure 7a).
A similar trend was observed for the three pollutants when using the pilot-scale nanopulsed plasma bubble reactor (Figure 7b). After 4 min of treatment, MB showed a degradation of 80.6%, reaching degradation >99% after 10 min. Similarly, SMX was 52.6% degraded after 4 min, achieving >99% degradation after 20 min. VAL, on the other hand, was degraded by 20.7% after 4 min and >98% after 40 min of treatment. The apparent rate constants for MB, SMX, and VAL were 0.49, 0.19, and 0.10 min−1, respectively (inset of Figure 7b).
When comparing the pseudo-first-order kinetic constants between the lab-scale and pilot-scale reactors, the pilot-scale reactor exhibited lower values (0.88 vs. 0.49 min−1 for MB, 0.76 vs. 0.19 min−1 for SMX, and 0.23 vs. 0.10 min−1 for VAL). However, this comparison is not entirely conclusive due to the different volumes treated by the two reactors (70 mL for the lab-scale and 2.5 L for the pilot-scale reactor). To provide a more accurate comparison, the apparent rate constants could be normalized based on the treated volume. When adjusted for treated volume, the volume-normalized constants ( k ′ ) were 61.6 vs. 1225 mL min−1 for MB, 53.2 vs. 475 mL min−1 for SMX, and 16.1 vs. 250 mL min−1 for VAL in the lab- and pilot-scale reactors, respectively.
To fully understand the relevance of the two reactor scales, it is important to directly compare the energy requirements of the two systems, as discussed in the following section.

3.6.2. Comparison between Lab- and Pilot-Scale Nanopulsed Plasma Bubbles in Terms of Energy Requirements

Energy efficiency is a crucial factor for the broader adoption of any treatment method, as it directly impacts energy cost requirements. A useful metric for evaluating the energy footprint of a method is the electrical energy per order (EEO), defined as the amount of electrical energy required to reduce the pollutant concentration by one order of magnitude in a unit volume of water (Equation (4)) [52,53]. This key parameter can be used to assess the energy performance of different advanced oxidation processes (AOPs), regardless of the specific experimental setup or the prevailing energy costs. In this study, EEO values were calculated to compare the energy efficiency of lab-scale and pilot-scale nanopulsed plasma bubbles for three different pollutants (Figure 8). For instance, the EEO for the degradation of MB was 0.37 kWh/m3 in the lab-scale treatment, while it was 0.18 kWh/m3 in the pilot-scale. A similar trend was observed for the other two pollutants: for SMX, the EEO was 0.50 kWh/m3 at the lab-scale and 0.42 kWh/m3 at the pilot-scale, while for VAL, the corresponding values were 1.54 and 0.88 kWh/m3, respectively.
Given the above findings, it is important to highlight that the EEO values obtained in both the lab-scale and pilot-scale setups are among the lowest reported in the literature (see also Section 3.7). Additionally, when comparing the energy requirements of the lab- and pilot-scale units of this study, it is evident that the pilot scale not only successfully scales up the technology without energy losses, but actually achieves a lower energy cost compared to the lab scale. This improved efficiency may be attributed to the more effective mass transfer of the plasma species and bubbles, as well as enhanced fluid recirculation in the larger volume of polluted water. These factors likely facilitate quicker and more accessible interactions between plasma reactive species and pollutants.

3.7. Comparison of Pilot-Scale Nanopulsed Plasma Bubbles with Other Methods in Terms of Electrical Energy per Order

To assess the dynamics of the pilot-scale plasma bubbles compared to other pilot-scale water remediation methods, it is useful to compare the energy requirements for pollutant degradation across various treatment methods applied to larger volumes (Table 1). The superiority of pilot-scale nanopulsed plasma bubbles in terms of energy efficiency is evident when considering the EEO values from this study (0.18–0.88 kWh/m3, depending on the pollutant). Specifically, the pilot-scale nanopulsed plasma bubble system used in this study outperforms other methods such as photocatalysis and ozonation combined with UV. Notably, this pilot-scale system demonstrated up to three-orders-of-magnitude-higher performance compared to other widely used methods, including photocatalysis [54,55], UV/Fenton treatment [56], and UV/ozonation [57].

4. Conclusions

This study demonstrates the scalability of lab-scale nanopulsed plasma bubbles for water remediation by evaluating their performance at an upscaled level. Various classes of toxic organic pollutants were examined, including dyes (methylene blue, MB) antibiotics (sulfamethoxazole, SMX), and the angiotensin receptor blocker valsartan (VAL).
The enhanced production of ⋅OH was confirmed (up to ~25 mg/L under O2 and up to ~20 mg/L under air), with negligible production of long-lived species such as O3, H2O2, and NO3−/NO2. Increasing the gas flow rate from 5 to 10 L/min improved MB degradation; however, a further increase to 15 L/min led to diminished performance, likely due to the rapid mass transfer of reactive species, which reduced the interaction time with organic molecules. Additionally, degradation efficiency increased with higher pulse voltage and pulse repetition rates.
Pilot-scale experiments showed high degradation efficiency for both O2– and air–plasma bubbles, even with short treatment times (93.0% and 80.7%, respectively, after 4 min of treatment). MB degradation was equally effective in both tap and ultrapure water, with slightly better results in tap water, likely due to co-existing salts that enhanced degradation pathways by generating additional reactive plasma species. Almost complete degradation was observed for MB, SMX, and VAL after 10, 20, and 40 min of treatment, respectively, using the pilot-scale plasma bubbles. The electrical energy per order (EEO) for MB degradation was 0.37 kWh/m3 at the lab scale and 0.18 kWh/m3 at the pilot scale, while the corresponding EEO for SMX degradation was 0.50 and 0.42 kWh/m3, and for VAL, 1.54 and 0.88 kWh/m3, respectively.
These results indicate that the pilot scale not only successfully scales up the technology without energy losses, but actually achieves lower energy costs compared to lab-scale operation. Notably, this pilot-scale system demonstrated performance up to three orders of magnitude higher than other widely used methods. This study highlights the potential of nanopulsed plasma bubbles for large-scale water remediation, showcasing their ability to rapidly, efficiently, and cost-effectively degrade various organic pollutants.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/technologies12100189/s1, Figure S1: Schematic illustration of the general experimental setup.; Figure S2: Temperature evolution of plasma-treated water samples in the pilot-scale plasma bubble reactor. Operating conditions: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; plasma gas, air; gas flow rate, 10 L/min.; Figure S3. Instantaneous voltage and current waveforms of the (a) pilot-scale and (b) lab-scale plasma bubble reactors. Operating conditions for the pilot scale: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; plasma gas, argon; gas flow rate, 10 L/min. Operating conditions for lab-scale: pulse voltage, 26.0 kV; pulse frequency, 200 Hz; plasma gas, air; gas flow rate, 3 L/min.; Figure S4: Pseudo-first-order kinetics of MB degradation under O2, air, and Ar atmospheres in the pilot-scale plasma bubble reactor. Operating conditions: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; gas flow rate, 10 L/min; initial pollutant concentration, 10 mg/L; Figure S5. The effect of gas flow rate on the composition of plasma-activated water in the pilot-scale nanopulsed plasma bubble reactor: (a) H2O2 and (b) ⋅OH. Operating conditions: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; plasma gas, air; gas flow rate, 10 L/min.

Author Contributions

S.M.: investigation, methodology and visualization; C.A.A.: conceptualization, methodology, investigation, visualization, supervision, resources, writing, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This project has received funding from the Hellenic Foundation for Research and Innovation (HFRI) under grant agreement no. [3560].

Informed Consent Statement

Not applicable.

Data Availability Statement

Data supporting the work are contained within the article or Supplementary Materials. Any other raw data are available on request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Photos of the pilot-scale nanopulsed plasma bubble reactor during pollutant degradation experiments shown (a) with and (b) without the stainless-steel grid used as the grounded electrode. Figure 1b is provided solely to help the reader visualize the bubbles formed.
Figure 1. Photos of the pilot-scale nanopulsed plasma bubble reactor during pollutant degradation experiments shown (a) with and (b) without the stainless-steel grid used as the grounded electrode. Figure 1b is provided solely to help the reader visualize the bubbles formed.
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Figure 2. Instantaneous voltage and current waveforms of the (a) pilot-scale and (b) lab-scale plasma bubble reactors. Operating conditions for pilot-scale: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; plasma gas, air; gas flow rate, 10 L/min. Operating conditions for lab-scale: pulse voltage, 26.0 kV; pulse frequency, 200 Hz; plasma gas, air; gas flow rate, 3 L/min.
Figure 2. Instantaneous voltage and current waveforms of the (a) pilot-scale and (b) lab-scale plasma bubble reactors. Operating conditions for pilot-scale: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; plasma gas, air; gas flow rate, 10 L/min. Operating conditions for lab-scale: pulse voltage, 26.0 kV; pulse frequency, 200 Hz; plasma gas, air; gas flow rate, 3 L/min.
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Figure 3. Composition of plasma-activated water in the pilot-scale nanopulsed plasma bubble reactor under an air or O2 atmosphere: (a) O3; (b) H2O2; (c) ⋅OH; and (d) pH evolution under air–plasma bubbles. Operating conditions: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; gas flow rate, 10 L/min.
Figure 3. Composition of plasma-activated water in the pilot-scale nanopulsed plasma bubble reactor under an air or O2 atmosphere: (a) O3; (b) H2O2; (c) ⋅OH; and (d) pH evolution under air–plasma bubbles. Operating conditions: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; gas flow rate, 10 L/min.
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Figure 4. (a) MB degradation efficiency at different pulse voltages and (b) pseudo-first-order degradation kinetics (pulse frequency: 400 Hz). (c) MB degradation efficiency at different pulse frequencies and (d) pseudo-first-order degradation kinetics (pulse voltage: 24.0 kV). Operating conditions: air flow rate, 10 L/min; initial MB concentration, 10 mg/L.
Figure 4. (a) MB degradation efficiency at different pulse voltages and (b) pseudo-first-order degradation kinetics (pulse frequency: 400 Hz). (c) MB degradation efficiency at different pulse frequencies and (d) pseudo-first-order degradation kinetics (pulse voltage: 24.0 kV). Operating conditions: air flow rate, 10 L/min; initial MB concentration, 10 mg/L.
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Figure 5. The effect of (a) plasma gas (gas flow rate: 10 L/min) and (b) gas flow rate (plasma gas: air) on MB degradation. Operating conditions: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; initial MB concentration, 10 mg/L.
Figure 5. The effect of (a) plasma gas (gas flow rate: 10 L/min) and (b) gas flow rate (plasma gas: air) on MB degradation. Operating conditions: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; initial MB concentration, 10 mg/L.
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Figure 6. (a) Comparison of MB degradation in tap versus ultrapure water using the pilot-scale nanopulsed plasma bubble reactor and (b) the corresponding pseudo-first-order degradation kinetics. Operating conditions: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; plasma gas, air; gas flow rate, 10 L/min; initial MB concentration, 10 mg/L.
Figure 6. (a) Comparison of MB degradation in tap versus ultrapure water using the pilot-scale nanopulsed plasma bubble reactor and (b) the corresponding pseudo-first-order degradation kinetics. Operating conditions: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; plasma gas, air; gas flow rate, 10 L/min; initial MB concentration, 10 mg/L.
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Figure 7. Degradation efficiencies of MB, SMX, and VAL using (a) the lab-scale and (b) pilot-scale plasma bubbles, with corresponding pseudo-first-order kinetic constants shown in the inset. Operating conditions for lab-scale experiments: pulse voltage, 26.0 kV; pulse frequency, 200 Hz; plasma gas, air; gas flow rate, 3 L/min; initial pollutant concentration, 10 mg/L. Operating conditions for pilot-scale experiments: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; plasma gas, air; gas flow rate, 10 L/min; initial pollutant concentration, 10 mg/L.
Figure 7. Degradation efficiencies of MB, SMX, and VAL using (a) the lab-scale and (b) pilot-scale plasma bubbles, with corresponding pseudo-first-order kinetic constants shown in the inset. Operating conditions for lab-scale experiments: pulse voltage, 26.0 kV; pulse frequency, 200 Hz; plasma gas, air; gas flow rate, 3 L/min; initial pollutant concentration, 10 mg/L. Operating conditions for pilot-scale experiments: pulse voltage, 24.0 kV; pulse frequency, 400 Hz; plasma gas, air; gas flow rate, 10 L/min; initial pollutant concentration, 10 mg/L.
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Figure 8. The EEO values for MB, SMX, and VAL in the lab-scale and the pilot-scale plasma bubbles.
Figure 8. The EEO values for MB, SMX, and VAL in the lab-scale and the pilot-scale plasma bubbles.
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Table 1. Comparison of different AOPs at the pilot scale in terms of electrical energy per order (EEO).
Table 1. Comparison of different AOPs at the pilot scale in terms of electrical energy per order (EEO).
Upscaled Treatment MethodPollutantWater Volume
(L)		Treatment Time
(min)		DE
(%)		EEO
(kWh/m3)		Ref.
Pilot-scale nanopulsed plasma bubbles MB,
10 mg/L		2.51099.30.18This study
Pilot-scale nanopulsed plasma bubbles SMX,
10 mg/L		2.52099.20.42This study
Pilot-scale nanopulsed plasma bubbles VAL
10 mg/L		2.54098.20.88This study
Pilot-scale plasma PFOS
80 mg/L		25 60>991.6 to 8.4 [58]
Photocatalysis/ZnOAcid
Yellow 36
50 mg/L		11229036.6[54]
Photocatalysis/
WO3 nanopowder		Acid Orange 7
5 mg/L		118099213[55]
UV/FentonIndustrial wastewater160>9925.04[56]
Combination of UV with ozonationAtrazine
5 mg/L		22094.9181.6[57]
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Meropoulis, S.; Aggelopoulos, C.A. Advancing Nanopulsed Plasma Bubbles for the Degradation of Organic Pollutants in Water: From Lab to Pilot Scale. Technologies 2024, 12, 189. https://doi.org/10.3390/technologies12100189

AMA Style

Meropoulis S, Aggelopoulos CA. Advancing Nanopulsed Plasma Bubbles for the Degradation of Organic Pollutants in Water: From Lab to Pilot Scale. Technologies. 2024; 12(10):189. https://doi.org/10.3390/technologies12100189

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Meropoulis, Stauros, and Christos A. Aggelopoulos. 2024. "Advancing Nanopulsed Plasma Bubbles for the Degradation of Organic Pollutants in Water: From Lab to Pilot Scale" Technologies 12, no. 10: 189. https://doi.org/10.3390/technologies12100189

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