Abstract

This study explores the production of urea using microwave (MW) plasma-water interaction with air, N<inf>2</inf>, CO<inf>2</inf>, and a N<inf>2</inf> + CO<inf>2</inf> gas mixture to generate plasma-activated water (PAW). After 180 s of plasma exposure, air plasma reduced the pH to 3.3 and increased the oxidizing potential by 127.1%, making the water acidic and oxidizing, while the N<inf>2</inf> + CO<inf>2</inf> plasma raised the pH to 10.1 and reduced the oxidizing potential by 33.8%, creating basic and reducing conditions. PAW from air plasma produced the highest NO<inf>2</inf>⁻ (51 mg l⁻¹) and NO<inf>3</inf>⁻ (295 mg l⁻¹) concentrations, while the N<inf>2</inf> + CO<inf>2</inf> mixture generated the most NH<inf>4</inf>⁺ (2250 mg l⁻¹), and CO<inf>2</inf> plasma produced the most CO<inf>3</inf>²⁻ ions. Notably, urea formation (plasma urea) was observed only with CO<inf>2</inf> and N<inf>2</inf> + CO<inf>2</inf> plasmas, attributed to the formation of stable compounds like NH<inf>4</inf>⁺ and NH<inf>2</inf>COO⁻. In this process, NH<inf>4</inf>⁺ ions formed via the reaction between atomic nitrogen and water, and their subsequent reaction with NH<inf>2</inf>COO⁻ ions in the aqueous phase led to urea synthesis. The N<inf>2</inf> + CO<inf>2</inf> plasma produced 2991% more urea than CO<inf>2</inf> plasma. Plasma urea enhanced seed germination and plant growth, increasing germination rates for carrots by 10.67% and coriander by 15.6%. Shoot lengths grew by 38.6% for carrots and 30.8% for coriander, while root lengths improved by 24.24% and 37.5%, respectively, compared to controls. This study highlights MW plasma-water interaction as a sustainable, energy-efficient alternative to conventional urea production, offering significant environmental benefits and improved agricultural performance.