Adsorption of ammonia nitrogen on lignite and its influence on coal water slurry preparation

Abstract

Using lignite as an adsorbent and subsequently as a material for coal water slurry (CWS) preparation represents an alternative method for coal chemical wastewater treatment. However, the components in these wastewaters could affect the CWS performance, especially ammonia nitrogen (NH4+-N). The influencing mechanism of NH4+-N was studied through adsorption method. Associated with adsorption reaction model, Fourier transform infrared spectrum and zeta potential detection, it was found that the adsorption reaction of NH4+-N, driven by ion-exchange interactions with H+ of –OH and –COOH on the lignite surface, displayed a typical monolayer mode and was much faster than that of NSF which displayed a double-layer mode and was adsorbed preferentially at hydrophobic sites. In binary adsorption, the adsorption amount of NSF was increased remarkably by adding NH4+-N. This was attributed to the modified lignite surface and hydrophilic group by NH4+-N that decreased the surface electronegativity as well as the electrostatic repulsion among –SO3− in NSF molecules. The enhanced adsorption of NSF resulted in capturing more water molecules in hydration film surrounding the particles in CWS. The hydration film was then thickened and stabilized, thus leading to a decrease in free water for CWS flowing. As a result, the apparent viscosity was increased, and CWS tended to be dilatant. In addition, the thickened and stabilized hydration film also increased the yield stress of CWS prepared with NH4+-N, resulting in improved stability.