Optimizing Nitrate Fertilizer Production Using Plasma-Activated Water (PAW) Technology: An Analysis of Dielectric Properties

Abstract

This study investigates the potential of Plasma-Activated Water (PAW) technology for the production of nitrate fertilizer, focusing on the dielectric properties of plasma-treated water, such as the dielectric constant and dielectric loss factor. The research aims to elucidate the impact of plasma treatment and varying water flow rates on these properties. An experimental approach was employed wherein ion-free water was subjected to plasma treatment at different flow rates (0.5, 1.0, and 2.0 L/min) and durations (1, 2, and 3 h). The results reveal a marked enhancement in the dielectric properties of the water following plasma treatment, with the most significant improvements observed at a flow rate of 0.5 L per minute and a treatment duration of 3 h, and dielectric efficiencies of 97.82%, 97.21%, and 96.61% achieved at flow rates of 0.5, 1.0, and 2.0 L/min, respectively. These findings demonstrate that PAW technology enhances the efficiency of nitrate fertilizer production by optimizing energy storage and reducing energy losses. The study underscores the potential of PAW as a sustainable, environmentally benign alternative to conventional chemical fertilizers, contributing to more efficient and sustainable agricultural practices.

1. Introduction

Agriculture is a cornerstone of economic development and a critical component in ensuring global food security [1]. The use of agricultural fertilizers is fundamental in preserving soil fertility, promoting plant growth, and enhancing overall agricultural productivity [2,3,4]. However, the widespread use of inorganic chemical fertilizers has led to significant environmental issues, such as the depletion of soil organic matter and the degradation of soil quality [5,6,7]. Continuous and extensive application of these fertilizers disrupts the natural nutrient balance in the soil, leading to the accumulation of harmful chemicals in the environment. A significant consequence is water pollution, where runoff and discharge into aquatic ecosystems lead to excess nutrient loads, particularly nitrates [8,9,10]. High nitrate levels in drinking water pose serious health risks, including methemoglobinemia and increased cancer risks in humans and animals [11,12,13,14,15,16].

To address these concerns, modern agriculture must adopt more efficient and environmentally friendly fertilizers. Advanced technologies, like Plasma-Activated Water (PAW), and organic alternatives, can reduce greenhouse gas emissions while maintaining long-term agricultural productivity and improving soil quality [17,18,19]. These technologies also promote sustainable farming practices by enhancing nutrient use efficiency and minimizing adverse environmental effects such as nutrient runoff and soil degradation.

Nutrient management techniques such as cover cropping, crop rotation, and the use of organic fertilizers play a pivotal role in sustainable agriculture. These methods maintain soil health by enhancing organic matter, promoting beneficial microbial activity, and reducing dependency on synthetic fertilizers. For example, nitrogen-fixing microbes naturally convert atmospheric nitrogen into forms usable by plants, decreasing reliance on energy-intensive synthetic fertilizers and reducing greenhouse gas emissions [20,21,22]. However, these strategies require careful planning and adaptation to local conditions. Moreover, despite their benefits, certain nutrient management techniques have limitations. Cover cropping may require increased water and management resources, while organic fertilizers may not meet the nutrient demands of crops during critical growth periods, potentially leading to yield reductions [23,24].

In addition, Controlled Release Fertilizers (CRFs), designed to release nutrients gradually, offer a solution to ensure a steady supply of nutrients during vital growth stages. However, they may not always perfectly align with a plant’s nutritional needs, potentially resulting in nutrient deficiencies or limited yield improvements. Furthermore, some organic fertilizers can emit methane and nitrous oxide, potent greenhouse gases that contribute to global warming. In high-yield agricultural systems, the time investment and meticulous planning required further hinder the adoption of crop rotation and cover cropping [25,26,27].

On the other hand, Plasma-Activated Water (PAW) technology has emerged as a promising alternative due to its ability to generate reactive species that enhance plant growth and nutrient uptake while reducing reliance on conventional chemical fertilizers. Plasma treatment of water generates reactive oxygen species (ROS) and reactive nitrogen species (RNS), which, when properly managed, serve as beneficial signaling molecules in plant biology. These species stimulate seed germination, promote root development, and enhance crop resilience to stress [28,29,30,31]. PAW can be easily integrated into agricultural systems via foliar spraying or drip irrigation, making it a versatile tool for enhancing crop yields and nutrient efficiency in both soil-based and hydroponic systems.

In addition to promoting plant growth, PAW offers significant environmental benefits. The reactive species in PAW have antimicrobial properties, which help activate plant defense mechanisms and increase resistance to environmental stressors such as drought, salinity, and extreme temperatures [32,33,34,35]. Beyond its agricultural benefits, PAW has been demonstrated to extend the shelf life of food products, inhibit microbial growth, and improve food safety and quality during food processing [32,33,34,35].

The production of conventional nitrogen fertilizers, like urea, is highly energy-intensive and contributes significantly to carbon dioxide emissions [36,37,38,39]. PAW technology offers a potential reduction in energy consumption of up to 68% compared to traditional urea synthesis. Unlike conventional methods, PAW production does not emit greenhouse gases, making it a highly sustainable alternative for nitrate fertilizer production [39]. By infusing water with RNS through plasma treatment, PAW generates a nitrate-rich solution that can be applied directly to crops, promoting efficient nutrient uptake and reducing dependency on chemical fertilizers. Moreover, PAW exhibits high stability, with nitrogen compounds remaining active for up to four weeks without degradation, making it a reliable solution for sustained nutrient supply [40,41,42,43].

The reactive species generated during plasma treatment include singlet oxygen, superoxides, electrons, ozone, and ultraviolet radiation [39,44]. These species initiate a series of complex chemical reactions within the liquid, producing key reactive intermediates that contribute to nutrient enrichment. Analytical techniques such as electron paramagnetic resonance (EPR), UV–visible spectrophotometry, and liquid chromatography are commonly used to identify and quantify these reactive species in plasma-treated liquids. Additionally, techniques like gas sensors, Fourier-transform infrared (FTIR) spectroscopy, laser-induced fluorescence (LIF), and mass spectrometry are utilized to detect gaseous products generated during the plasma-water interaction [45,46].

The production of Plasma-Activated Water (PAW) involves several critical processes at the liquid–gas interface. Reactive species generated in the plasma discharge transfer into the liquid phase, where they induce dynamic chemical reactions with water molecules [47]. These reactions result in the formation of highly reactive short-lived species, such as hydroxide ions (OH−) and hydrated electrons (e− solv). These unstable species rapidly transform into more stable compounds like superoxides (O₂−), ozone (O₃), and hydrogen peroxide (H₂O₂), all of which play crucial roles in nitrate production [48]. The non-equilibrium conditions created by the plasma lead to efficient energy transfer and enhanced fertilizer production.

Compared to traditional methods, PAW technology offers substantial energy savings and environmental advantages. The nitrate solution produced through PAW can be formulated into liquid or granular fertilizers, depending on the application. In addition, its high stability ensures that nitrogen compounds remain effective over extended periods, supporting nutrient availability throughout the crop cycle [40,41,42,43].

Furthermore, PAW technology enhances seed germination rates, plant growth, and nutrient uptake without compromising yield quality. This makes PAW a highly efficient and environmentally sustainable solution applicable across a wide range of agricultural practices, including both hydroponic and soil-based farming systems [49].

This study investigates the application of Plasma-Activated Water (PAW) technology in producing nitrate fertilizer. By analyzing the dielectric properties of plasma-treated water, the research aims to optimize the efficiency of nitrate production through improved plasma exposure times and water flow rates. These insights will help develop more sustainable, efficient nitrate fertilizers that support environmentally friendly agricultural practices.

2. Material and Methods

2.1. Water Sample Preparation

To minimize contamination, water samples were prepared using ion-free or reverse osmosis (RO) water [50] sourced from The Center for Scientific and Technological Equipment, Suranaree University of Technology, Nakhon Ratchasima, Thailand. These samples were then divided into three groups for analysis: (1) untreated water (non-plasma), (2) water mixed with 10 g of nitrate fertilizer per liter, sourced from Chia Tai Co., Ltd., Bangkok, Thailand, and (3) water treated with plasma [51,52]. The plasma-treated samples were exposed to varying flow rates of 0.5, 1, and 2 L per minute for durations of 1, 2, and 3 h.

To prevent contamination and maintain the physical and chemical integrity of the samples, all were stored in clean, sealed containers after testing. The dielectric constant and dielectric loss factor of the samples were measured, as these properties directly affect the efficiency of the plasma system. Understanding how water responds to an electric field is crucial for establishing optimal plasma conditions [53,54]. Thus, assessing dielectric properties is essential for optimizing plasma system design and operation to enhance the efficiency of nitrate fertilizer production. Additionally, this evaluation aids in refining the production process to meet the standards of environmentally sustainable agricultural practices.

2.2. Experimental Setup of a Nitrate Production System Using Plasma Technology

The plasma system designed for nitrate production from water consists of two main components. The first component is a high-voltage generator that supplies 100 W of power to the electrodes, creating an electric field that directs electrons toward the positive electrode, generating an electric current between the electrodes. The second component is a series of five sawtooth-shaped electrode emitters, each arranged in parallel with a positive charge characteristic. The cathode is a rectangular aluminum ground plate positioned parallel to the sawtooth electrode lines.

Selecting the appropriate electrode materials is crucial for efficient plasma generation, as they must exhibit excellent electrical conductivity. Among the materials evaluated, copper (Cu) proved to be the most suitable for the anode electrode due to its high electrical conductivity (5.96 × 10⁷ S/m). The ground plane is made of aluminum, which, while having a slightly lower conductivity than copper (3.5 × 10⁷ S/m), offers the advantages of being lightweight and cost-effective, making it ideal for use in ground planes and reducing construction costs for industrial applications.

The block diagram of the plasma system used for nitrate production from water is shown in Figure 1. The system consists of a high-voltage power source (100 W) with a voltage range of 2–30 kV and a control system to supply power to the discharge electrode. The discharge electrode is designed to generate a high-intensity electric field within a plasma state. The system also includes a fan to direct steam through the electrodes, which are exposed to high electric field stress, and an ultrasonic head to facilitate the conversion of water into steam.

The electrodes are designed with a pointed tip, using copper for the tip and aluminum for the ground plane component, to optimize electrode size and geometry. To evaluate the electric field intensity between the sharp-tipped electrode and the plate, a tungsten needle with a 1.5 mm diameter was used as the electrode. The distance between the electrodes was maintained at a constant 21 mm, and the voltage applied during the experiments ranged from 2 to 30 kV. The primary objective of this investigation was to determine the intensity of the electric field during the streamer formation phase, which occurs at voltages between 13 and 23 kV. The field intensity was calculated using Equations (1) and (2).

$$E_{\max }={\displaystyle \frac{V}{d{\eta }^{*}}}$$

(1)

$${\eta }^{*}={\displaystyle \frac{E_{av}}{E_{\max }}}\hspace{1em};\hspace{1em}0\le {\eta }^{*}\le 1$$

(2)

where ${\eta }^{*}$ is the field utilization factor, $E_{av}$ is the average of the electric field intensity, and $d$ is the distance between the electrodes.

The analysis and design of a nitrate production system from water using electric field technology involve the implementation of electromagnetic technology in the form of plasma, utilizing an electric field intensity of $6.31 \times {10}^5$ V/m, as applied in the experiments.

2.3. Measurement of Dielectric Properties

To investigate the dielectric characteristics of different materials, appropriate measurement techniques are required to evaluate their ability to store charge and conduct electricity. Dielectric measurements are essential for determining how materials respond to electric and electromagnetic fields, providing critical information on their capacity to store charges, insulate, and dissipate energy as heat. These measurements can be applied to materials in various states, including solid, liquid, or semi-solid.

Several techniques are available for evaluating dielectric properties, such as coaxial probes, transmission lines, and resonant cavity studies, each suited to specific material types and frequency ranges. For instance, the coaxial probe method can measure dielectric characteristics over a frequency range of 20 MHz to 20 GHz. To fully understand a material’s properties and potential applications, it is crucial to use precise and accurate measurement methods.

The change in the dielectric constant with frequency in an electric field is caused by the alignment of charges through polarization and the oscillations that occur when the electric field reverses. The speed of these oscillations depends on both the frequency and the temperature. This behavior can be described using the Debye equation, which is used to calculate the dielectric constant of polar materials, as shown in Equation (3) [55,56,57,58,59].

$$\epsilon ={\epsilon }_{\infty }+{\displaystyle \frac{{\epsilon }_s-{\epsilon }_{\infty }}{1+j\omega \tau }}$$

(3)

where ${\epsilon }_s$ represents the dielectric constant, ${\epsilon }_{\infty }$ is the dielectric constant at high frequency, $\omega$ is the angular frequency, and $\tau$ is the relaxation time. Equations (4) and (5) can be divided into two main parts: ${\epsilon }^{\prime }$, the dielectric constant, and ${\epsilon }^{″}$, the dielectric loss factor, as follows [60].

$${\epsilon }^{\prime }={\epsilon }_{\infty }+{\displaystyle \frac{{\epsilon }_s-{\epsilon }_{\infty }}{1+{(j\omega \tau )}^2}}$$

(4)

$${\epsilon }^{″}={\displaystyle \frac{({\epsilon }_s-{\epsilon }_{\infty })\omega \tau }{1+{(j\omega \tau )}^2}}$$

(5)

2.4. Statistical Analysis

The statistical analysis in this study was conducted using an Agilent Technologies network analyzer (E5071C, Santa Rosa, CA, USA) along with a computer equipped with Keysight N1500A Materials Measurement Software Suite, version 16.1, to ensure high accuracy. The Agilent E5071C network analyzer, known for its high dynamic range and user-friendly interface, is ideal for measuring the electrical characteristics of RF and microwave components. It was used to evaluate the electrical properties of the water samples tested with plasma, as illustrated in Figure 2.

The experiments were replicated three times to ensure the validity and reliability of the results. Significant differences in the electrical properties of the plasma-treated water samples were observed. Statistical methods were applied to the data from these repeated experiments to confirm that the observed changes were meaningful. The analyses revealed that the variations in electrical properties were directly related to the amount of nitrate fertilizer present in the water.

3. Results and Discussion

3.1. Dielectric Properties Measurement Results of Test Time

Measuring dielectric properties is essential for verifying and evaluating experimental results, especially in the synthesis of nitrate fertilizers using plasma technology [61]. Determining the dielectric constant and dielectric loss factor provides valuable insights into how water or solutions react to the electric field applied during the plasma process [62]. These properties directly influence plasma generation and the production of reactive compounds, such as nitrates, which are crucial components of fertilizers.

This study examined the dielectric properties of water samples treated with Plasma-Activated Water (PAW) and compared them with untreated water and nitrate-mixed water (2.2 mg/L). The aim was to understand the impact of the plasma process on dielectric properties and its effect on the efficiency of nitrate fertilizer production. The results are presented graphically to demonstrate the relationship between dielectric properties and different electric field frequencies.

The results presented in Figure 3a, Figure 4a, and Figure 5a provide a detailed analysis of the dielectric constant (ε′), while Figure 3b, Figure 4b, and Figure 5b illustrate the dielectric loss coefficient (ε″) of water samples treated with plasma for 1, 2, and 3 h. The samples were categorized into three groups: (1) plasma-activated water (PAW), (2) nitrate-treated water (NO₃ + RO), and (3) untreated reverse osmosis (RO) water. The results show significant changes in dielectric properties depending on the treatment duration and flow rate used in the experiments.

At a frequency of 0.2 GHz and a flow rate of 0.5 L/min, the dielectric constant of nitrate-treated water increased from 78.75 ± 0.37% after 1 h of treatment to 79.75 ± 0.26% after 2 h and further to 79.79 ± 0.41 after 3 h, indicating the impact of treatment time on the dielectric properties. Across all frequencies, the dielectric constant (ε′) of nitrate-treated water consistently exhibited higher values than those of PAW-treated water and untreated RO water. Similarly, the dielectric loss coefficient (ε″) for nitrate-treated water rose from 7.03 ± 0.11% after 1 h to 7.53 ± 0.33% after 2 h, and 5.64 ± 0.10% after 3 h, demonstrating a trend of increasing dielectric loss with treatment duration.

For a more precise comparison, the dielectric properties were also evaluated at a higher frequency of 3.0 GHz and the same flow rate of 0.5 L/min. The dielectric constant of nitrate-treated water was 76.55 ± 0.18% after 1 h of plasma treatment, increasing to 77.55 ± 0.16% after 2 h, and reaching 77.89 ± 0.13% after 3 h, highlighting the influence of plasma exposure time on these properties across different frequency ranges.

These findings suggest that plasma-activated water (PAW) has a higher energy storage capacity as the plasma treatment time increases, indicated by an increased dielectric constant. This increase may result from the enhanced polarization of nitrate ions when subjected to an electric field. As the frequency rises, the dielectric constant of all samples decreases, which aligns with the typical behavior of dielectric materials. This decrease indicates a reduced ability of water molecules to align themselves in response to rapidly fluctuating electric fields at higher frequencies. The dielectric loss factor (ε″) is high in the plasma-activated water (PAW vs. RO) across all frequencies and flow rates. This can be attributed to the presence of reactive species, such as reactive oxygen species (ROS) and reactive nitrogen species (RNS), generated during plasma treatment, which increase electrical conductivity and dielectric loss, leading to greater energy dissipation as heat. At higher frequencies, the increase in the dielectric loss factor suggests that loss mechanisms, such as dipole relaxation and ion conduction, become more significant.

The analysis of flow rate effects on the dielectric properties of PAW revealed notable results. At a flow rate of 1 L/min, the PAW sample exhibited dielectric properties that were intermediate between those of nitrate-mixed water (NO₃ + RO) and untreated RO water, with a balanced increase in both the dielectric constant and dielectric loss factor. This balance suggests optimal energy storage with controlled energy dissipation, enhancing the efficiency of the plasma treatment process. Conversely, higher flow rates tend to increase the dielectric loss factor, indicating greater energy loss, likely due to reduced interaction time between water molecules and the plasma field, as shown in

Table 1. Dielectric properties of nitrate-mixed water (NO₃ + RO), untreated RO water, and plasma-treated RO water (PAW) samples for nitrate production at different flow rates.

Flow Rate (L/min)	Frequency (GHz)	NO3 + RO (10 g)		RO Water		Plasma-Activated Water (PAW)
						1 h		2 h		3 h
		ε′ (%Err.)	ε″ (%Err.)	ε′ (%Err.)	ε″ (%Err.)	ε′ (%Err.)	ε″ (%Err.)	ε′ (%Err.)	ε″ (%Err.)	ε′ (%Err.)	ε″ (%Err.)
	0.2	82.55 ± 0.58	9.19 ± 0.28	75.39 ± 0.24	6.49 ± 0.23	78.75 ± 0.37	7.03 ± 0.11	79.75 ± 0.26	7.53 ± 0.33	79.79 ± 0.41	5.64 ± 0.10
	0.5	82.41 ± 0.42	9.01 ± 0.32	75.30 ± 0.33	6.31 ± 0.32	78.54 ± 0.40	7.51 ± 0.13	79.54 ± 0.29	6.87 ± 0.34	79.70 ± 0.34	5.47 ± 0.18
	1.0	82.13 ± 0.53	8.96 ± 0.19	75.07 ± 0.29	6.26 ± 0.30	78.33 ± 0.27	6.05 ± 0.18	79.33 ± 0.41	6.55 ± 0.28	79.47 ± 0.28	5.42 ± 0.13
0.5	1.5	81.78 ± 0.41	9.85 ± 0.19	74.78 ± 0.23	7.15 ± 0.22	77.73 ± 0.38	7.61 ± 0.14	78.88 ± 0.38	6.13 ± 0.38	79.18 ± 0.38	6.35 ± 0.15
	2.0	81.37 ± 0.23	11.39 ± 0.25	74.42 ± 0.19	8.69 ± 0.18	77.14 ± 0.23	9.85 ± 0.16	78.43 ± 0.25	7.68 ± 0.23	78.8 ± 0.23	7.89 ± 0.22
	2.5	80.89 ± 0.27	12.99 ± 0.17	73.99 ± 0.22	10.29 ± 0.21	76.84 ± 0.12	8.74 ± 0.23	77.98 ± 0.11	9.54 ± 0.31	78.39 ± 0.31	9.49 ± 0.16
	3.0	80.35 ± 0.14	14.59 ± 0.23	73.49 ± 0.11	11.89 ± 0.15	76.55 ± 0.18	12.30 ± 0.19	77.55 ± 0.16	12.80 ± 0.13	77.89 ± 0.13	11.09 ± 0.19
	0.2	82.55 ± 0.58	9.19 ± 0.28	75.39 ± 0.24	6.49 ± 0.23	77.99 ± 0.23	5.69 ± 0.22	78.49 ± 0.58	5.77 ± 0.28	78.99 ± 0.25	7.39 ± 0.23
	0.5	82.41 ± 0.42	9.01 ± 0.32	75.30 ± 0.33	6.31 ± 0.32	77.90 ± 0.32	5.51 ± 0.34	78.40 ± 0.42	5.59 ± 0.32	78.90 ± 0.34	7.33 ± 0.32
	1.0	82.13 ± 0.53	8.96 ± 0.19	75.07 ± 0.29	6.26 ± 0.30	77.67 ± 0.30	5.46 ± 0.28	78.17 ± 0.53	5.53 ± 0.19	78.67 ± 0.32	8.15 ± 0.30
1.0	1.5	81.78 ± 0.41	9.85 ± 0.16	74.78 ± 0.23	7.15 ± 0.22	77.38 ± 0.22	6.35 ± 0.21	77.88 ± 0.41	6.35 ± 0.16	78.38 ± 0.24	9.69 ± 0.22
	2.0	81.37 ± 0.23	11.39 ± 0.25	74.42 ± 0.19	8.69 ± 0.18	77.02 ± 0.18	7.89 ± 0.17	77.52 ± 0.23	7.89 ± 0.25	78.02 ± 0.20	11.29 ± 0.18
	2.5	80.89 ± 0.27	12.99 ± 0.17	73.99 ± 0.22	10.29 ± 0.21	76.59 ± 0.21	9.49 ± 0.20	77.09 ± 0.27	9.49 ± 0.17	77.59 ± 0.23	12.89 ± 0.21
	3.0	80.35 ± 0.14	14.59 ± 0.23	73.49 ± 0.11	11.89 ± 0.15	76.09 ± 0.15	11.09 ± 0.14	76.59 ± 0.14	11.09 ± 0.23	77.09 ± 0.17	7.39 ± 0.15
	0.2	82.55 ± 0.58	9.19 ± 0.28	75.39 ± 0.24	6.49 ± 0.23	77.75 ± 0.31	6.53 ± 0.28	78.75 ± 0.29	7.03 ± 0.14	78.19 ± 0.34	7.44 ± 0.12
	0.5	82.41 ± 0.42	9.01 ± 0.32	75.30 ± 0.33	6.31 ± 0.32	77.54 ± 0.18	6.04 ± 0.19	78.43 ± 0.18	6.85 ± 0.25	78.10 ± 0.18	7.27 ± 0.20
	1.0	82.13 ± 0.53	8.96 ± 0.19	75.07 ± 0.29	6.26 ± 0.30	77.33 ± 0.15	5.55 ± 0.14	78.33 ± 0.27	6.05 ± 0.12	77.87 ± 0.31	7.22 ± 0.13
2.0	1.5	81.78 ± 0.41	9.85 ± 0.16	74.78 ± 0.23	7.15 ± 0.22	76.89 ± 0.18	6.13 ± 0.17	77.87 ± 0.15	8.23 ± 0.16	77.58 ± 0.15	8.15 ± 0.16
	2.0	81.37 ± 0.23	11.39 ± 0.25	74.42 ± 0.19	8.69 ± 0.18	76.44 ± 0.34	7.12 ± 0.25	77.43 ± 0.19	8.89 ± 0.11	77.22 ± 0.18	9.69 ± 0.10
	2.5	80.89 ± 0.27	12.99 ± 0.17	73.99 ± 0.22	10.29 ± 0.21	75.99 ± 0.17	6.58 ± 0.18	77.15 ± 0.14	11.25 ± 0.15	76.79 ± 0.17	11.29 ± 0.14
	3.0	80.35 ± 0.14	14.59 ± 0.23	73.49 ± 0.11	11.89 ± 0.15	75.55 ± 0.11	11.80 ± 0.15	76.55 ± 0.09	12.30 ± 0.13	76.29 ± 0.11	12.89 ± 0.11

.

The duration of plasma treatment is also crucial in determining dielectric properties. The data show that a treatment time of 3 h leads to a more significant improvement in dielectric properties compared to shorter durations of 1 and 2 h. This longer treatment time allows for better interaction between plasma and water, facilitating nitrate generation and the stabilization of reactive species, which improves electrical properties.

This investigation into the electrical characteristics of water samples using plasma technology highlights the importance of optimizing plasma treatment parameters, such as flow rate and duration. The results indicate that plasma-activated water, especially at a flow rate of 0.5 L/min and a treatment duration of 3 h, shows substantial improvements in electrical characteristics, which are beneficial for nitrate fertilizer synthesis. The observed higher dielectric constant in nitrate-mixed water and the controlled dielectric loss factor in PAW samples underscore the potential of plasma technology for efficient and environmentally friendly fertilizer production. This study suggests a need for further research to refine these parameters to optimize the efficiency and sustainability of plasma-based fertilizer production systems.

3.2. Dielectric Properties Measurement Results at Different Flow Rates

Figure 6 presents the dielectric properties of plasma-treated water at flow rates of 0.5, 1, and 2 L/min, compared with nitrate-mixed water (NO₃ + RO) and untreated RO water. Starting with the dielectric constant (ε′) shown in Figure 6a, it is observed that the dielectric constant decreases as the frequency increases across all flow rates. At the lowest flow rate (0.5 L/min), the water samples exhibit the highest dielectric constant over the frequency range, indicating a greater electrical energy storage capacity.

Furthermore, the dielectric loss factor (ε″) increases with frequency at all flow rates, with particularly high values observed at higher flow rates, indicating increased energy loss in the form of heat, as shown in Figure 6b.

This analysis highlights the importance of controlling the flow rate during the plasma treatment process to modify the dielectric properties of water. A lower flow rate significantly enhances the efficiency of nitrate fertilizer production by increasing electrical energy storage capacity and reducing energy loss. Therefore, optimizing water flow rate management and control in the plasma process is crucial for improving production efficiency and energy use in the agricultural industry.

Table 1 shows the dielectric properties of pure water samples treated with plasma for nitrate production at different flow rates. The data reveal significant differences in the dielectric constant (ε′) and dielectric loss factor (ε″) under various conditions, including nitrate-mixed water (NO₃ 10 g), reverse osmosis filtered water (RO water), and plasma-activated water (PAW) at flow rates of 1.0, 2.0, and 3.0 L/min.

When comparing NO₃ + RO, RO water, and PAW samples, the nitrate-mixed water (NO₃ + RO) consistently exhibited the highest dielectric constant (ε′) values across all frequencies due to the direct addition of nitrate ions. This was followed by plasma-activated water (PAW) and then RO water, indicating that the increase in ε′ and ε″ correlates with a higher electrical energy storage capacity. The trend suggests that the polarizability of the material in an electric field is enhanced by the presence of nitrate ions. Notably, the dielectric constant decreases with increasing frequency, which is typical behavior for dielectric materials as their dipole alignment ability diminishes at higher frequencies.

The dielectric loss factor (ε″), which represents energy loss as heat, was higher in the nitrate-mixed water (NO₃ + RO 10g) sample compared to plasma-activated water (PAW) and RO water, particularly at higher frequencies. This indicates that the presence of nitrate ions contributes to increased dielectric losses, while plasma treatment introduces additional conductive pathways or reactive substances that also affect these properties.

The flow rate significantly impacts the dielectric properties of the PAW sample. At a flow rate of 0.5 L/min, the PAW sample showed the highest dielectric properties compared to flow rates of 1.0 and 2.0 L/min, with elevated values of both ε′ and ε″. This suggests efficient energy storage with increased losses, similar to those observed in nitrate-mixed water (NO₃ + RO). Additionally, the data indicates that ε″ values generally increase as the flow rate decreases across all frequency ranges, implying greater energy losses due to longer interaction times between plasma and water molecules at lower flow rates. The highest ε″ values at 0.5 L/min suggest that this flow rate is optimal for generating high concentrations of nitrate.

Furthermore, the percentage error for most of the measured dielectric properties was below 1%, which highlights the reliability and precision of the data. The low percentage error indicates that the variations in ε′ and ε″ are statistically significant and accurately reflect the effects of plasma treatment and flow rates on the dielectric properties of water.

These findings highlight the importance of optimizing parameters such as flow rate and treatment duration to improve nitrate production efficiency using plasma technology. Understanding the interaction mechanisms between plasma and water is crucial for designing an effective plasma-based nitrate production system, and the observed trends in ε′ and ε″ provide valuable insights for this optimization.

After determining that the proposed PAW fertilizer at a flow rate of 0.5 L/min for 3 h yielded dielectric constant (ε′) and dielectric loss factor (ε″) values close to those of aqueous nitrate fertilizer (NO₃ + RO), we measured the nitrate concentration using the cadmium reduction method and compared our results with those of other studies, as shown in

Table 2. Comparison of nitrate concentration and energy consumption in this study with previous research.

Ref.	Type	Power	Nitrate Concentration (ppm)
[1]	Plasma Electrolysis	700 W	2213.5
[2]	Continuous Flow Liquid-Phase Plasma Discharge (CFLPPD)	300 W	73–136.2
[3]	Plasma-Activated Water (PAW)	125 W	2.9–3.8
[49]	Plasma-Activated Water (PAW)	24 kV	30–40
This work	Plasma-Activated Water (PAW)	100 W	235.2

. The results indicate that the PAW method used in this study provides a significantly higher nitrate concentration (235.2 ppm) while consuming less energy (100 W) compared to other PAW methods and the Continuous Flow Liquid-Phase Plasma Discharge (CFLPPD) technique. Although plasma electrolysis produced the highest nitrate concentration (2213.5 ppm), it required considerably more energy (700 W), demonstrating that the PAW process proposed in this study is a more energy-efficient alternative for nitrate production.

4. Conclusions

This study investigates the use of Plasma-Activated Water (PAW) for nitrate fertilizer production without the addition of external nitrate ions. The dielectric properties of plasma-treated water were analyzed and compared to untreated water (RO Water). The results show that plasma treatment for 3 h at a flow rate of 0.5 L/min achieved a dielectric constant (ε′) of 79.79 ± 0.41%, which is close to nitrate-mixed water (NO₃ + RO 10g) with a ε′ of 82.55 ± 0.58% at a frequency of 0.2 GHz. Moreover, plasma treatment reduced energy loss, as the dielectric loss factor (ε″) of PAW after 1 h of treatment was 7.03 ± 0.11%, decreasing to 5.64 ± 0.10% after 3 h, which is lower than that of nitrate-mixed water at 9.19 ± 0.28%. The study further reveals that a longer treatment duration enhances nitrate production efficiency, while a lower flow rate (0.5 L/min) improves energy storage and reduces energy loss more effectively compared to higher flow rates, such as 2.0 L/min, where ε′ decreases and ε″ increases. The PAW process at a flow rate of 0.5 L/min and a 3 h treatment period produced a nitrate concentration of 235.2 ppm using only 100 W of power, which is more energy-efficient than traditional methods like electrolysis that require significantly more energy. Despite not adding any chemicals, plasma treatment generated nitrate concentrations comparable to direct nitrate mixing, making PAW a promising and sustainable technology for nitrate fertilizer production. It reduces environmental impacts while enhancing energy efficiency.