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

Abstract

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.

Keywords:

• Characteristic
• humic acid
• lignite
• nitric acid
• soil conditioner

Acknowledgments

The authors acknowledge the financial support of the Shandong Provincial Doctoral Foundation (No. 2007BS08013) and the National Key Technology Research and Development Program of China (No. 2006BAD03A1505-3) as well as anonymous reviewers for useful comments on the manuscript.