Co-production of water-soluble humic acid fertilizer and crude cellulose from rice straw via urea assisted artificial humification under room temperature

This study investigated urea assisted artificial humification of rice straw under room temperature, aiming at producing water-soluble humic acid fertilizer and crude cellulose from agricultural wastes with a single treat-ment. Artificial humification parameters including potassium hydroxide concentration (1-2 mol/L), retention time (4-8 h) and liquid-to-solid ratio (6-8) were investigated using response surface methodology to achieve maximum humic acid yield and cellulose retention. The optimum condition was 1.5 mol/L potassium hydroxide at a liquid-to-solid ratio of 6 for 4 h. At such conditions, the yield of humic acid was 18.36 % with a concentration of 30.60 g/L, which was sufficient for China's national standard of water-soluble humic acid fertilizers. Mean-while, cellulose retention was 93.15 %. Moreover, the addition of 1.0 mol/L urea significantly increased the yield of artificial humic acid by 20.71 %. Urea involved artificial humification as a precursor, forming C-N bond with humic acid. The product with fortified the nitrogen containing functional groups, enhanced the solubility at acidic pH, reduced the particle size, and improved the porosity, demonstrated advantages in agricultural applications.