Objective  To explore the differences in biological activity of humic acids derived from weathered coals of different sources, this study clarified the relationship between their biological activities and the properties of weathered coal, so as to provide a reference for finding out active humic acids from weathered coal.

  Method  The weathered coal samples from Zuoquan, Wutai and Jingle in Shanxi Province were collected , treated with different methods and subjected to plant culture tests. The processing chemical characteristics and biological activity of humic acids derived from weathered coal samples of different origins were compared and analyzed with ultraviolet spectrometry, infrared spectrometry and root scanning.

  Result  The results showed that the total humic acid contents of weathered coal from Zuoquan, Wutai and Jingle were between 37.1% and 40.9%, but the free humic acid contents varied greatly (1.92%, 10.09% and 30.62%, respectively). The content of total acidic functional group phenolic hydroxyl was significantly different, and the content ratio of the three samples was 1.00∶1.22∶1.83. The hydroxyl group content and carboxyl group content of weathered coal samples from Jingle was the highest. The content ratio of calcium to magnesium among the three samples was 2.65∶1.25∶1.00. There was small difference in the E4/E6 value of different weathered coal samples, but the E4/E6 value of humic acids extracted was 7.5, 6.2 and 4.39, respectively. Bioactivity assays showed that humic acids derived from the weathered coal samples of the three places promoted the regeneration of mung bean roots, showing a typical hormone response similar to that induced by IAA. The weather coals were ranked as Jingle sample > Wutai sample > Zuoquan sample based on the growth-promoting activities of their humic acids. The highest growth-promoting activity of humic acid was found at 0.01% in Zuoquan sample and Wutai sample, 0.05% in Jingle sample.

  Conclusion  Among the humic acids extracted from the weathered coals of three origins, the Jingle sample has the highest biological activity, followed by the Wutai and Zuoquan samples. The lower the contents of calcium and magnesium are in weathered coal, the higher the contents of free humic acids are. The wider the absorption peak of hydroxyl is in infrared spectrum, the higher the content of total acidic groups is, the higher the biological activity of humic acid is. The results provide references for the selection of high-quality weathered coal used as raw materials for humic acid production, and the appropriate concentrations of humic acids in the application.