Adsorption of Ethylenediamine (EDA) on Montmorillonite Saturated with Different Cations—II. Hydrogen- and Ethylenediammonium-Montmorillonite: Protonation and Hydrogen Bonding

Abstract

AbstractThe form under which ethylenediamine (EDA) is adsorbed from aqueous solution by hydrogen- and ethylenediammonium-montmorillonite was studied as a function of the amount of amine present in the system.EDA added to the acid clay in quantities lower than or equal to the cation exchange capacity (C.E.C.) was exclusively adsorbed as ethylenediammonium (EDAH22+) ion. On further addition of diamine the pH of the suspension rose to alkaline values and the monoprotonated species (EDAH+) was the main charge balancing cation.Evaporating at room temperature “EDA-H-montmorillonite-H2O” or “EDA-EDAH2-mont- morillonite-H2O” systems containing 300 me EDA/100 g clay did not cause loss of nitrogen, but degassing under high vacuum (10−5 mm Hg) did. Nevertheless, excess EDA molecules with respect to the C.E.C. were retained on the clay surface, at the expense of water molecules, through strong asymmetrical hydrogen bonds between their NH2 groups and the NH3+ groups of EDAH+ ions. On heating up to 160°C under vacuum the nitrogen content decreased further, but still remained at a level significantly higher than the C.E.C. value, all NH3+ groups remaining involved in strong hydrogen bonding. It is suggested that a “condensation” process takes place, implying evolution of EDA molecules and giving rise to “polymeric” associations between protonated and unprotonated diamine.Washing the clay suspensions with distilled water did not completely remove excess EDA either, as a consequence of the equilibrium existing between the ionic species in solution (EDAH22+ and EDAH+) and on the clay surface. It seems that these species were preferentially adsorbed as “trimeric” associations in which two out of four NH3+ groups are hydrogen bonded to NH2 groups.After heating at 200°C, nitrogen retained on the clay surface was mainly in the form of NH4+ ions. Ammonium formation was enhanced by the presence of excess EDA and was considerably faster than in montmorillonite systems containing EDA coordinated to Cu2+ ions.