Effects of HCl–HF purification treatment on chemical composition and structure of humic acids

Summary Sample preparation for the study of humic acids involves a purification treatment to remove both mineral and organic components extracted along with the humic substances. The effects of HCl–HF purification on the chemical composition and structure of humic acids were studied in samples from eight different composts. Only small increases in the concentrations of C, N and H and a decrease in O were found in purified humic acids when compared with the unpurified materials, no more than 10% of average concentration. Total and phenolic acidity increased in the purified materials, whereas carboxylic and carbonyl acidity showed no statistically significant differences as a result of the purification. Gel permeation chromatography showed that only minor changes occurred in molecular size distribution of the humic acids, and the infrared spectra showed no appreciable differences between the purified humic acids and the untreated materials. We conclude that the HCl–HF treatment successfully reduced the ash contents of the humic acids without modifying their chemical composition and structure significantly.