Research progress on extraction technology and application of humic acid from weathered coal in desertification area of Northwest China

The weathered coal in China is rich in reserves and has high humic acid content. Due to its high oxygen content and low calorific value, it is not cost-effective for industrial production such as power generation and coking, direct combustion or as raw powder used to improve soil and other traditional utilization methods will produce environmental pollution and low efficiency, but the humic acid extracted from high biochemical activity can be widely used in agriculture, industry, medicine and materials and other fields. Firstly, the extraction techniques of humic acid from weathered coal such as alkali extraction, acid extraction and microbial dissolution are discussed, including the reaction mechanism, advantages and disadvantages and research progress of each method. In alkali extraction method, the pretreatment of nitric acid oxidation, hydrogen peroxide oxidation, ultrasonic activation oxidation and other oxidation pretreatment methods are emphasized. The mechanism and research status of extracting fulvic acid from weathered coal in acid extraction method are analyzed. In the method of microbial dissolution, the selection method of microbial strains, the available types, the degradation mechanism and the influencing factors of degradation are reviewed, the degradation mechanism including alkali mechanism, chelating agent mechanism, surfactant mechanism, enzyme mechanism and ABCDE comprehensive mechanism. Then, the applications of humic acid in agriculture, industry, environmental protection and medicine are summarized. Finally, based on the huge development advantages of humic acid in the desertification area of Northwest China, the extraction and application of humic acid are prospected, in order to find a cleaner and more efficient extraction technology of humic acid from weathered coal and promote the efficient utilization of low-rank coal.