Dye adsorption by resins: Effect of ionic strength on hydrophobic and electrostatic interactions

Abstract

Dyes are adsorbed mainly through hydrophobic and electrostatic attractions and hydrogen bonding between the dyes and the adsorbents. pH and ionic strength are two important factors affecting dye adsorption. Solution pH affects the adsorption mainly through adjusting the electrostatic interactions. While the effect of ionic strength on the dye adsorption is more complicated and is not so clear. Ionic strength may affect not only the electrostatic interactions, but also the hydrophobic interactions. In this study, the effect of ionic strength on the adsorption of three dyes onto three resins were designed and investigated comparatively and comprehensively. The selected resins with a same matrix were negatively charged D001, neutral AB-8 and positively charged 717 resins. The selected dyes were methyl orange (MO), methylene blue (MB) and neutral red (NR) with negative, positive and weak positive charges, respectively. Nine groups of combinational batch adsorption experiments were carried out using the three resins and the three dyes. There are two experiments in each group. In one experiment a solution with 1mol/L NaCl was used, while in the other experiment a solution without NaCl was used for comparison. Our experimental results confirmed the importance of hydrophobic and electrostatic interactions on the dye adsorption. The hydrophobic attraction was found to increase with increasing ionic strength. While the electrostatic interactions were found to decrease with increasing ionic strength.