Thermal Stability of Organoclays: Effects of Duration and Atmosphere of Isothermal Heating on Iodide Sorption

Abstract

Heating periods necessary to destroy iodide sorption capacity of the quaternary (alkyl) ammonium and phosphonium modified bentonites were determined using iodide sorption batches. For this purpose, prior to the batches the studied organoclays were isothermally heated in air in the temperature ranges of 110-180 °C and 160-300 °C, respectively. The temperature dependence of the heating periods was found to follow the Arrhenius relationship, which allowed a determination of Arrhenius parameters for the reaction leading to the loss of the iodide sorption capacity of a bentonite modified by CP(+) (cetylpyridinium), BE(+) (benzethonium), CTMA(+) (cetyltrimethylammonium), or TPP(+) (tetraphenylphosphonium) surfactant. At 160 °C, the thermal stability of the iodide sorption capacity of TPP(+)-bentonite is much higher than that of the second most stable CTMA(+)-bentonite (80 days vs 5 days). However, the obtained Arrhenius parameters predict that CTMA(+)-bentonite becomes the most stable one as the heating temperature decreases to 40 °C with iodide sorption still available for ∼12000 years as compared to ∼8000 years for TPP(+)-bentonite. Heating of the organoclays in a N(2)-atmosphere (<70 ppm O(2)) at 160 °C revealed that the strong deficit of molecular oxygen in the contacting atmosphere resulted in a strong increase of their thermal stability. For CTMA(+)-bentonite, this increase is equivalent to the stability increase due to a decrease of the heating temperature by ∼20 °C (from 160 °C to ∼140 °C). Accordingly, the iodide sorption capacity of CTMA(+)-bentonite at a heating temperature of 40 °C is predicted to be retained for ∼350,000 years in the absence of molecular oxygen.