Characterization and iodide adsorption behaviour of HDPY⁺ modified bentonite

Owing to its favourable physical, chemical and rheological properties, densely compacted bentonite or bentonite-sand mix is considered as a suitable buffer material in deep geological repositories to store high level nuclear waste. Iodine-129 is one of the significant nuclides in the high level waste owing to its long half life and poor sorption onto most geologic media. Bentonite by virtue of negatively charged surface has negligible affinity to retain iodide ions. As organo-bentonites are known to retain iodide ions, the present study characterizes hexadecylpyridinium chloride (HDPyCl.H₂O) treated bentonite from Barmer India (referred as HDPy⁺B) for physico-chemical properties, engineering properties and the iodide adsorption behavior of the organo clay. Batch experiments revealed that HDPy⁺ ions are largely retained (94 % retention) via cation exchange; the ion-exchange process neutralizes the negative surface charge and bridges clay particles leading to reduction in Atterberg limits, clay content and sediment volume. The organo clay retains iodide by Coulombic attraction (at primary sites) and anion exchange (at secondary sites). The free-energy change (ΔG ^o = −25.5 kJ/mol) value indicated that iodide retention by organo clay is favored physical adsorption process. Iodide adsorption capacity of organo clay decreased significantly (85–100 %) on dilution with 50–80 % bentonite. On the other hand, dilution of bentonite with 50 % organo clay caused 58 % reduction in swell potential and 21 % reduction in swell pressure.