Innovative urea formation using microwave plasma-water interaction: a study on reactive species and plant growth

Abstract This study explores the production of urea using microwave (MW) plasma-water interaction with air, N 2 , CO 2 , and a N 2 + CO 2 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 2 + CO 2 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 2 − (51 mg l −1 ) and NO 3 − (295 mg l −1 ) concentrations, while the N 2 + CO 2 mixture generated the most NH 4 + (2250 mg l −1 ), and CO 2 plasma produced the most CO 3 2− ions. Notably, urea formation (plasma urea) was observed only with CO 2 and N 2 + CO 2 plasmas, attributed to the formation of stable compounds like NH 4 + and NH 2 COO − . In this process, NH 4 + ions formed via the reaction between atomic nitrogen and water, and their subsequent reaction with NH 2 COO − ions in the aqueous phase led to urea synthesis. The N 2 + CO 2 plasma produced 2991% more urea than CO 2 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.