Effect of dilute acid on fulvic acid extracted from lignite

To explore the effect of dilute acid on the extraction of fulvic acid by lignite using an alkali-soluble acid precipitation method, lignite was acid-precipitated by H2SO4, HNO3, HCl, and H3PO4, and the content, extraction rate, and structural characteristics of fulvic acid were analyzed. The results showed that the highest content of humic acid extracted by HNO3 is 0.3383 mg/mL, and the lowest content of humic acid extracted by sulfuric acid is 0.0817 mg/mL. The highest extraction rate of humic acid derived from lignite extracted by hydrochloric acid is 3.84%, and the lowest extraction rate of humic acid extracted by sulfuric acid is 0.89%. Fulvic acid extracted from lignite contained functional groups such as benzyl, carbonyl, methyl, and hydroxyl, but their contents varied; the benzene ring structure in fulvic acid precipitated by HNO3 was the most pronounced, but the aromatization degree was the lowest. By contrast, the aromatization degree of fulvic acid precipitated by H2SO4 was the highest. This indicates that the functional group contents in humic acid extracted from different dilute acids vary, although the types of functional groups are similar.