Impacts of Inherent O‑Containing Functional Groups on the Surface Properties of Shengli Lignite

Shengli (SL) lignite was thermally treated by heating at 200–350 °C in an effort to reduce the number of O-containing functional groups and water present. The presence of carboxyl groups, phenolic hydroxyl groups, and methoxy groups was characterized using a chemical titration method. The moisture holding capacity (MHC), wettability, and ζ potential of the SL lignite were measured before and after the low-temperature heat treatment. The results revealed that the main reactions that occurred below 350 °C were decarboxylation and dehydration, corresponding to the decomposition of more than 60% of the carboxyl groups and phenolic hydroxyl groups. SL lignite treated at 350 °C displayed an approximately 50% reduction in its MHC. The O-containing functional groups (the carboxyl groups, in particular) played an important role in water adsorption, indicating that the formation of the hydrogen bond between the O-containing functional groups and water contributed most significantly to the water adsorption process. The contact angle decreased as the number of hydrophilic sites decreased as a result of the decomposition of O-containing functional groups. The ζ potential of the SL lignite decreased significantly as the concentration of O-containing functional groups decreased. MHC was successfully correlated with the presence of O-containing functional groups and the quantity of surface area without oxygen groups, as follows: MHC = 2.655[−COOH] + 2.912[−OH] + 0.209[−OCH3] – 3.321S non‑O + 1.341, where S non‑O for the lignite is defined as S non‑O = S i (1 – C i /C 0), where S i is the surface area of the lignite and C i and C 0 are the total contents of O-containing functional groups in the heat-treatment or as-received lignites.