Nitric Acid Oxidation for Improvement of a Chinese Lignite as Soil Conditioner

Oxidation is known commonly to enhance humic acid (HAs) contents of coal, but in China, most humic substances are used directly as soil conditioners or applied in combination with fertilizers without oxidation. Therefore, we investigated the impact of nitric acid (HNO3) oxidation on the characteristics of HAs derived from a Chinese lignite. The results showed that total HA content was increased by HNO3 oxidation, thus consequently increasing its cation exchange capacity and moderate humification. Besides, HAs extracted from oxidized lignite were richer in oxygen-containing functional groups with smaller molecular size than natural lignite. Compared with natural lignite, retention capacities of nitrogen and potassium by oxidized lignite were significantly greater. However, phosphorus-retention capacity was decreased to near zero, which might enhance the availability of phosphorus in soil. In general, optimum HNO3 oxidation had favorable effects on lignite and improved its characteristics for use as a soil conditioner.