Reaction Pathway Engineering for Nitrate Production in Atmospheric-Pressure Plasma via an Alternative Organic Nitrogen Source

Technical Summary: Reaction Pathway Engineering for Nitrate Production in Atmospheric-Pressure Plasma via an Alternative Organic Nitrogen Source

Problem Statement
Plasma-activated water (PAW) is a promising technology for agricultural applications, capable of generating reactive oxygen and nitrogen species (RONS) to produce nitrate fertilizers. However, a significant limitation exists: increasing plasma input power to enhance nitrate production yields diminishing returns while drastically increasing energy consumption. The efficiency of nitrate formation in conventional PAW is strictly dependent on the plasma-induced conversion of atmospheric nitrogen ($N_2$), a process hindered by the high energy threshold required to dissociate the strong $N \equiv N$ triple bond. Consequently, relying solely on power escalation is not an effective strategy for scalable nitrate production. This study addresses the need to explore alternative reaction pathways and nitrogen sources that can improve nitrate yields without proportionally increasing energy input.

Methodology
The researchers employed an atmospheric-pressure plasma jet (APPJ) using compressed air as the working gas. The study compared two distinct treatment systems under identical plasma conditions (80 W, 130 W, and 180 W):

1. Plasma-Activated Water (PAW): 100 mL of deionized water treated directly with the APPJ for 10 minutes.
2. Plasma-Activated Coffee Ground Solution (PAC): A suspension of 3 g of spent coffee grounds in 100 mL of deionized water, treated with the APPJ for 10 minutes without prior filtration.

Key characterization techniques included:

• Optical Emission Spectroscopy (OES): To identify and compare the relative intensities of reactive species (ROS and RNS) generated in the plasma at different power levels.
• Nitrate Concentration Measurement: Using an ion-selective electrode to quantify $NO_3^-$ production.
• Oxidation-Reduction Potential (ORP): To assess the solution's oxidizing capability and the consumption of reactive species.
• Conceptual Electron Energy Distribution Function (EEDF) Analysis: A theoretical framework used to correlate electron energy populations with the dissociation cross-sections of $O_2$, $H_2O$, and $N_2$.

Key Results

• Nitrate Production: While nitrate concentration in PAW increased with plasma power, the magnitude of the increase was limited. In contrast, the PAC system demonstrated a substantial enhancement in nitrate production. Notably, PAC treated at the lowest power (80 W) yielded 56.1 ppm of nitrate, exceeding the yield of PAW treated at the highest power (180 W), which was 51.1 ppm.
• Plasma Characteristics (OES): Spectral analysis revealed that oxygen-related emissions (atomic oxygen at 777 nm and 844 nm) and hydroxyl radicals (OH at 310 nm) were prominent across all power levels. Conversely, nitrogen-related emissions were comparatively weak. The relative intensity ratios of species remained consistent across power levels, suggesting that increasing power primarily increased the electron population rather than significantly altering the electron energy distribution or the dominant excitation characteristics.
• Redox Properties (ORP): PAW exhibited high and relatively stable ORP values across power levels, indicating a saturation limit in the dissolution of reactive species. In the PAC system, ORP values were significantly lower at 80 W, indicating the rapid consumption of plasma-generated oxidizing species by dissolved organic constituents from the coffee grounds. As power increased, ORP recovered alongside a proportional rise in nitrate, suggesting a dynamic balance where the continuous supply of ROS surpassed the consumption rate by organics.
• Reaction Pathway Analysis: The conceptual EEDF analysis indicated that the energy distribution of electrons in the APPJ strongly overlaps with the dissociation thresholds for $O_2$ and $H_2O$ (5–7 eV) but has minimal overlap with the high threshold for $N_2$ dissociation (~9.8 eV). This confirms that the plasma environment inherently favors ROS generation over RNS generation.

Key Contributions
The study proposes and validates a "reaction pathway engineering" approach. Instead of attempting to force the inefficient dissociation of atmospheric $N_2$ by increasing power, the authors demonstrate that introducing an organic nitrogen source (spent coffee grounds) shifts the reaction mechanism. In the PAC system, the abundant plasma-generated ROS oxidize dissolved organic nitrogen compounds (such as amino acids) directly in the liquid phase. This pathway bypasses the energy-intensive gas-phase $N_2$ activation and the solubility limitations of nitrogen oxide intermediates ($NO$, $NO_2$).

Significance and Claims
The paper claims that nitrate production can be significantly enhanced not by increasing plasma energy input, but by modifying the nitrogen source and reaction pathway. By utilizing an organic nitrogen source, the system effectively harnesses the preferentially generated ROS to drive nitrogen conversion. This approach offers a dual benefit:

1. Energy Efficiency: It reduces the reliance on additional plasma power to achieve high nitrate yields.
2. Waste Valorization: It provides a potential route for the recycling and valorization of nitrogen-containing organic waste (specifically spent coffee grounds) into a valuable agricultural resource.

The authors conclude that this strategy represents a viable alternative for plasma-assisted nitrate production, moving beyond the limitations of conventional PAW systems that depend solely on atmospheric nitrogen fixation.