Adsorption of malachite green on silica gel: effects of NaCl, pH and 2-propanol.

Adsorption of malachite green (MG) on silica gel was studied as a function of temperature (308-328 K), pH, different concentrations of NaCl and various weight percentages of binary mixtures of 2-propanol with water. It was observed that the adsorption capacity decreases with increase in weight percentage of 2-propanol and NaCl and HCl concentration compared to that of in water. Results showed that in the presence of 2-propanol, surface aggregates of MG form on silica gel due to aggregation of MG molecules in these solutions, reverse desorption occurs. Binding constants of MG to silica gel were calculated using the Langmuir and bilayer isotherms. The process is exothermic in water and endothermic in NaCl solutions. The mean adsorption energy (E) value 9.0-14.2 kJ mol(-1) indicated chemical adsorption.