Methods of determining cation exchange capacities for clinoptilolite-rich rocks of the Logudoro region in northern Sardinia, Italy

Abstract

Clinoptilolite-rich materials from widespread pyroclastic and epiclastic deposits of northern Sardinia were investigated to assess their cation exchange properties and to find the most reliable experimental method to determine their cation exchange capacity (CEC). The CECs were evaluated using a cross exchange method (CEM) and a batch exchange method (BEM). The CEM resulted in values 30–35% lower than the theoretical or expected CEC calculated from the chemical analyses of the clinoptilolite-rich materials. The BEM resulted in CECs 6–12% lower than the expected CECs. Various parameters, such as the grain-size of powders (<64 µm and 125–250 µm) and replacement cation (Na⁺, K⁺, Li⁺, Cs⁺, NH ⁺₄ , Ca²⁺, Mg²⁺, Sr²⁺) were evaluated in order to optimize the cation exchange process, i.e. enhance complete exchange. The particle size did not affect the exchange process appreciably. The type of replacement cation had a substantial effect on the experimental CEC determined by the BEM. The NH ⁺₄ and Cs⁺ replacement solutions resulted in the best experimental CECs ranging between 75% and 94% of the theoretical CEC with NH ⁺₄ as the replacement cation and 79% and 88% of the theoretical CEC with Cs⁺ as the replacement cation. The exchange efficiency was also measured as a function of ammonium concentration in the replacement solution (0.50, 1.00, 2.00 and 3.00 M). Experimental CECs ranged between 94% (0.5 M NH₄Cl solution) and 99% (1 M NH₄Cl solution) of the theoretical CEC for one epiclastic rock sample and between 79% (3 M NH₄Cl solution) and 87% (2 M NH₄Cl solution) of the theoretical CEC for one pyroclastic rock sample.