Effects of Plasma-Activated Water via Different Gas Treatments on Coffee Ground Decomposition

Plasma-activated water (PAW) has been widely used in numerous fields recently. In this study, DI water was activated by atmosphere pressure plasma jet (APPJ) through pure argon, argon/nitrogen, and argon/air treating gas and then immersed coffee grounds into PAWs for an hour. The results indicated that plasma-activated coffee ground solutions characterize an obvious increase in nitrate concentration—the major nutrition for plant growth. Especially for argon/air plasma-activated coffee ground solution, the nitrate concentration was 23 times higher than that of none plasma-treated coffee ground solution. From high-performance liquid chromatography (HPLC/MS) analysis, amino acids dissolved in the plasma-activated coffee ground solutions were detected to be the nitrogen-containing substances that participated in the reaction with reactive species in PAWs. Afterward, since amino acids contain nitrogen, the amino acids were treated by the same methods as coffee ground so as to prove that amino acid was the nitrogen source to produce nitrate. Interestingly, the nitrate concentration of the plasma-activated amino acid solution characterized the same trend as that of plasma-activated coffee ground solutions. In conclusion, the amino acid was one of the nitrogen providers participating in the reaction with reactive species to generate nitrate.