Adsorption kinetics of an organic dye by wet hybrid gel monoliths

Wet hybrid gel monoliths are prepared with bis(trimethoxysilylpropyl)amine (TSPA) or the mixture of TSPA with n-propyltriethoxysilane (PTES) or bis(trimethoxysilyl)hexane (TSH) or tetraethoxysilane (TEOS) as precursors. The adsorption kinetics of an organic dye (erioglaucine disodium salt, EDS) by the gel monoliths in aqueous solutions is studied comprehensively. The effects of temperature, pH, and ionic strength on the adsorption kinetics are investigated. Kinetic studies show that in general the kinetic data are well described by the pseudo second-order kinetic model. Initial adsorption rate increases with the increase in temperature, but decreases with the increase in solution pH and ionic strength. The adsorption activation energy is found to be 17–51 kJ mol⁻¹ under our experimental conditions. The internal diffusion of the dye into the hybrid gels appears to be the rate-limiting step of the overall adsorption process. The adsorption is promoted by hydrogen bonding, hydrophobic and electrostatic attractions in acidic or neutral solutions, suppressed by the electrostatic repulsion in basic solutions and by the ionic exchange competition of Cl⁻ with the dye anions in solutions with a high NaCl concentration. After adsorption for 165 h, all the gel monoliths present a linear shrinkage less than 10%.