Effect of Alkali on the Extraction of Fulvic Acid From Weathered Coal

ABSTRACT The alkaline dissolution–acid precipitation method was employed to systematically investigate the effects of different alkali types (NaOH, KOH, LiOH, Na 2 CO 3 , Na 2 SO 3 , Na 4 P 2 O 7 , CH 3 COONa, NaHCO 3 , and NH 3 ·H 2 O) on the extraction efficiency and structural characteristics of fulvic acid (FA). The results showed that under the optimal conditions of 0.35 mol L −1 of NaOH, 80°C, 90 min, and a liquid–solid ratio of 12 mL g −1 , the yield of fulvic acid was 32.46% and the purity was 28.52%, which was significantly better than that of NH 3 ·H 2 O (9.58%, 17.90%). This result demonstrated a significant improvement over the outcomes obtained from the spectral characterization of the NH 3 ·H 2 O system. The FA extracted with NaOH had the highest E4/E6 ratio (2.090) and the lowest Δ lgK (0.813), with the lowest molecular weight and rich in carboxyl and phenolic hydroxyl groups. XRD analysis indicated that the degree of graphitization was the lowest. The residual FA content was negatively correlated with the extraction rate ( r = −0.92). This study provides the process parameters and a structural basis for the efficient and green extraction of fulvic acid from weathered coal.