Partially Cd+ Ion-Exchanged Type-A Zeolites and Their Thermal Analysis

Abstract

A series of partially exchanged Cd2+ A-zeolites (Cd0.86 A, Cd1.7 A, Cd2.46 A, Cd2.86 A, Cd3.25 A) were prepared by aqueous cation exchange technique from synthetic NaA-zeolite. The percentage of Cd2+ ions exchanged increased with decreasing the concentration of Cd2+ions and increasing temperature of the exchanging solution. This behaviour of Cd2+ ion exchange with concentration is attributed due to ‘volume effect,’ ‘salt imbibent’ and electroselectivity effect.’ The uptake of Cd2+ ions increased with rise in temperature indicated the process to be endothermic. Ion-exchange isotherms plotted at different temperatures showed the type ‘a’ isotherm i.e., the entering ion (Cd2+) indicated selectivity to the outgoing ions (Na+). The ion-exchange equilibrium data also followed the Freundlich and Langmuir adsorption isotherms. Thermogravimetric analysis (TGA) showed that the zeolitic water depends on the number of Na+ and Cd2+ ions per unit cell (p.u.c.) Thermal analysis curves showed that the minimum endothermic peak of dehydration slightly shifted towards higher temperature for CdA-zeolites indicated that water molecules are more strongly bonded to Cd2+ than Na+. Three exotherms were clearly observed within the temperature range 780-1000 °C, which are indicative of more than one reaction or crystallization of more phases. The differential thermal analysis (DTA) curves showed that dehydrated NaA zeolite retained its stability from 340 to 780 °C and CdNaA-zeolites from 330-380 °C to 800-820 °C, NaA amorphisized at 920°C and CdNaA-zeolite, from 920 to above 1000 °C.