Decrease of hydrophilicity of lignite using CTAB: Effects of adsorption differences of surfactant onto mineral composition and functional groups

The interactions in the "lignite-surfactant" system were studied with the aim of disclosing the mechanism of the decrease in hydrophilicity of lignite using cetyltrimethylammonium bromide (CTAB). The inhibiting efficiency in the hydrophilicity of lignite by CTAB was studied by measuring the wetting heat using a Calvet calorimeter. The adsorption of CTAB to organic content and mineral matter was analyzed, and the relationship between the distribution characteristics of different functional groups and the inhibiting efficiency in hydrophilicity was discussed. The hydrophilicity of lignite was decreased significantly after adsorption of CTAB. CTAB decreased the hydrophilicity more effectively for a more strongly hydrophilic sorbate, and an excess adsorption was not beneficial. Competitive adsorption existed between the oxygen-containing groups and the silicon-containing mineral matter of lignite, and the adsorption to oxygen-containing groups occurred prior to reaching the mineral matter surface. The decrease in hydrophilicity of lignite by CTAB was shown to be closely associated with the distribution characteristics of the adsorbed layer. Coverage of the oxygen-containing groups by a directed surfactant layer was mainly devoted to the decrease in the wetting heat of lignite. Among these oxygen-containing groups, the COO/COOH group of lignite played a key role in the decrease of hydrophilicity due to its high adsorption capacity for CTAB. (C) 2017 Elsevier Ltd. All rights reserved.