Precipitation of humic substances from aqueous and alkaline solutions under the action of electrolytes: A review

The precipitation of humic substances by electrolytes from aqueous and alkaline solutions occurs in two stages: the first is the transformation of true solutions into colloidal solutions, and the second is the decrease in the aggregative stability, the adhesion of particles, and the formation of sediment. The term coagulation, which is commonly accepted in colloid chemistry, is most suitable for the characterization of the above processes because the formation of the sediments of humic substances is of colloid-chemical nature. Methods for the determination of a coagulation threshold are not standardized; therefore, its values for the same materials determined by different researchers were different. The coagulation thresholds of the humic acids of solid fuels are very typical for different stages of metamorphism, and they adequately reflect the nature of fossil fuels.