1-14　Preparation of humic substances from biomass by acid hydrolysis and evaluation of plant-growth activity

Abstract

Humic acid, a plant growth promoter, was prepared from hydrochar obtained by acidic hydrothermal treatment of sugarcane leaves. Effect of hydrochar oxidation with HNO₃ on humic acid yield was investigated by varying the HNO₃ concentration, reaction temperature, and reaction time. The results showed that under conditions of high HNO₃ concentration and high reaction temperature, humic acid was decomposed into smaller molecules, resulting in a decrease in yield. Humic acid obtained from hydrochar oxidized with HNO₃ had lower C content, and higher N and O contents than humic acid prepared from hydrochar. In the presence of humic acid obtained from oxidized hydrochar, obtained by oxidation with 10 % HNO₃ at 50°C for 1 h, the root length of a model plant (Arabidopsis thaliana) was 1.8 times longer than in the case of humic acid from hydrochar. It was confirmed that oxidation of hydrochar with nitric acid increased the yield of humic acid and improved the plant-growth activity of humic acid.