Humic acid: research progress in its structural properties, sources, preparation and application

Humic acid is a representative class of functional natural polymer compound with high commercial value and good development prospects.However, the formation of natural humic acid takes a long time, so the synthesis of humic acid by the artificial methods is considered to be an excellent value-added method.This study reviews the research progress in structural properties, sources, preparation and applications of humic acids.Firstly, we introduce the structural properties and function of humic acids; and then by classifing it into two categories according to their sources: mineral humic acids and biological humic acids, we describe the current research status of four enhanced humic acid preparation methods: microbial decomposition, hydrothermal method, catalytic/oxidation method and pyrolysis method, as well as the applications of humic acids in agriculture, industry, medicine, environmental protection and new materials.Finally, we give a outlook according to the challenges in the enhanced preparation and applications of humic acids.