Extraction of humic acid derived from sludge and mechanism of low concentration rare earth precipitation

Extracted from surplus sludge by the sodium pyrophosphate and alkaline solution-acid extraction methods, respectively, sludge-derived humic acids were characterized by scanning electron microscopy, infrared spectroscopy and three-dimensional fluorescence. The precipitation effect of the two humic acids on the low concentration of rare earth ions was also compared. The results show that both humic acids contain a large amount of carboxylic acids, amines and hydroxyl groups, but the types and contents of functional groups are higher in S-HA-1 extracted by the sodium pyrophosphate method. When the rare earth ion concentration is 103.40 mg/L, the precipitation rates of rare earth elements are 97.30% and 91.18%, and the total weight percentages of rare earth elements in the sediment are 31.73% and 28.64%, respectively, with the best precipitation effec of S-HA-1. With S-HA-1 as a precipitatant, determined by a single-factor experiment and response surface optimization experiment, the optimal reaction conditions are the rare earth ion concentration of 51.70 mg/L, humic acid concentration of 0.25 g/L, the reaction time of 56.53 min, the stirring rate of 106.27 r/min, the reaction temperature at 40 ℃, pH of 5.90, and the rare earth precipitation rate ≥ 98%. The analysis of rare earth precipitates by energy spectrum and infrared spectrum revealed that a series of physical and chemical reactions occured between rare earth ions and functional groups such as phenolic hydroxyl groups and carboxyl groups on the surface of S-HA during precipitation, forming humic acid-rare earth flocculent precipitates. The results show that humic acids can be extracted from residual sludge to be the low-concentration rare earth ion precipitatants, which can not only achieve precipitant reuse, but also recover rare earth resources.