Effect of Mg2 + content in ion-exchanged Shengli lignite on its equilibrium adsorption water content and its mechanism

Mg ion-exchanged samples were prepared with acid-washed Shengli lignite. The chemical composition of the ash of the raw sample was determined by X-ray fluorescence. The equilibrium adsorption water contents of samples were determined in a range of relative humidity. The ion-exchange process was characterized by FT-IR, ash content, and pH value. A possible mechanism is proposed for equilibrium adsorption water of ion-exchanged samples at different humidities. The extent of ion-exchange reaction between Mg2 + and lignite is controlled by the concentration of Mg2 + in MgSO4 solution. The effect of Mg2 + on equilibrium adsorption water content varies with relative humidity and content of Mg2 +. The factor that controls equilibrium adsorption water content at low relative humidity is water interactions with sorption sites, which are Mg2+–carboxyl group complex. At middle relative humidity capillary force between Mg2 +–water clusters Mg+(H2O)n and capillary is more important. At high relative humidity, free water –free water interactions are more significant.

At low humidity, water adsorbs on the primary sites in the form of monolayer water, the content of which is strongly influenced by the Mg2+–carboxyl group complex. Middle humidity represents multilayer adsorption, Mg2 +–water clusters are bound in capillaries, and the factor that controls water adsorption is capillary force attributed to Mg2 +–water clusters condensed in the capillary with diameters of less than several microns. At high humidity, water is considered to be in the form of free water without specific interactions with lignite, the content of which is strongly influenced by free water –free water interactions.Figure optionsDownload as PowerPoint slide