Effect of Na+ and K+ on the Modification of Low-Rank Coal by an Ionic Surfactant and Its Adsorption Mechanism

This study is dedicated to investigating the impacts of the inorganic salt concentration on low-rank coal modified by ionic surfactants [dodecyltrimethylammonium chloride (DTAC) and sodium dodecyl sulfate (SDS)] and its adsorption mechanism using ζ potential determination, total organic carbon test, contact angle measurement, and Fourier transform infrared (FTIR) analysis, in addition to the interaction theory calculations and adsorption experiments. The results of interaction model calculation, adsorption capacity and contact angle showed that the salt solution (KCl and NaCl) could effectively increase the adsorption capacity of the surfactant on coal and improve the hydrophobicity of coal. Moreover, the relatively optimal salt concentrations for DTAC- and SDS-modified coal were different, which were 500 g/tcoal of KCl and 5000 g/tcoal of NaCl, respectively. The FTIR analysis results indicated that DTAC was adsorbed on the coal by strong electrostatic force and weak hydrogen bonding force, but the way of SDS molecule adsorption was mainly hydrogen bonds. The role of salt ions is to break the hydrogen bonds of coal–water and surfactant–water, in addition to enhance the electrostatic interaction, which makes the electrostatic adsorption of DTAC and hydrogen bond adsorption of SDS on the coal surface easier to occur.