Temperature effects on iodine adsorption on organo-clay minerals

Smectites and vermiculites modified with hexadecylpyridinium (HDPy ⁺), hexadecyltrimethylammonium (HDTMA ⁺), benzethonium (BE ⁺) and dipyridinododecane (DPyDD ²⁺ ) cations exhibit high adsorption capabilities for the anionic radionuclide ¹²⁵I ⁻ (Part I of this article). In some cases, exposure to higher temperature decreased the anion adsorption. To clarify the mechanisms of temperature dependence of the investigated organo-clay minerals, structural properties and thermal stabilities were examined by in situ powder X-ray diffraction (XRD), thermogravimetry (TG), thermocalorimetry (DTA) and FTIR spectral analyses. The organic cations HDPy, HDTMA and BE form bilayers, pseudotrimolecular layers and paraffin-type structures. The basal spacings of DPyDD-smectite and DPyDD-vermiculite were more or less unchanged in comparison to the original clay minerals (∼1.4 nm). It is assumed that the anion adsorption is due to the uptake of organic cations in an excess of the CEC, predominantly in the interlayer spaces as ion pairs. The decreasing iodide adsorption with increasing temperature is probably due to conformational changes of the alkyl chains, which are most pronounced for DPyDD-vermiculite at 60 °C (1.48→1.20 nm). Additionally, dehydration reactions, which occur at distinct temperatures for the organo-vermiculites (HDPy, 71; HDTMA, 57, 72; BE, 61; DPyDD, 41 °C), may be another reason for a decrease in iodide adsorption. The thermal decomposition of the organic cation starts at 200 °C. The HDPy, HDTMA and DPyDD-smectites and the HDTMA and BE-vermiculites show a high thermal stability with the basal spacing remaining constant up to 150 °C, which recommends the use for anion retention in engineered barriers for heat-producing waste.