Preparation of Fulvic Acid and Humic Acid by Nitric Acid Oxidized Rice Straw

Preparation of fulvic acid (FA) and humic acid (HA) from nitric acid-oxidized rice straw was studied through single factor analysis and orthogonal experiment. The effects of concentration of nitric acid, temperature, time and liquid-solid ratio on the yields of FA and HA were investigated, and optimal process conditions for FA were revealed. The results of single analysis showed that the increases of concentration of HNO₃ within a certain range, reaction temperature or reaction time could all increase the yield of fulvic acid, the effects of liquid-solid ratio on the yields of FA and HA were not significant. The same conclusion was obtained by the range analysis of the FA yield in the orthogonal experiment. The optimal conditions obtained by the orthogonal experiment were: HNO₃ mass fraction 5%, reaction temperature 90 ℃, liquid-solid ratio 13 mL/g. The average yield of FA of the reaction for 1 h was 32.63%, and the average yield of HA was 12.77% under the optimal condition. Elemental analysis showed that n_C/n_O of the FA and HA was 0.87 and 1.93, respectively, and n_C/n_H of the FA and HA was 0.78 and 0.73, respectively, indicating the high biological activity and obvious aroma characteristics of the product. Infrared spectroscopy showed that FA contained obvious aromatic characteristics, acidic groups and nitro groups, suggesting that the oxidation and nitrification of HNO₃, and the nitro group of HA was not obvious. The UV-visible spectrum analysis showed that the E₄₆₅/E₆₆₅ of the FA and HA was 13.87 and 8.47, respectively, higher than those of FA and HA that were prepared from low rank coal, which further indicated its higher activity.