Fourteen anions were selected to investigate their adsorption capacities and affinities to unwashed and washed hexadecyl pyridinium (HDPy)-modified montmorillonite (HDPy/Mt). Correlations between adsorption characteristics and anionic properties including radius and hydration energy were determined. With the exception of IO3−, H2AsO4−, and CrO42−, monovalent anions showed higher adsorption capacities and affinities to unwashed or washed HDPy/Mt than divalent anions, because the latter possess higher hydration energies. Unwashed HDPy/Mt demonstrated higher selectivity toward poorly hydrated monovalent anions because it presented a more hydrophobic environment, whereas strongly hydrated divalent anions more readily intercalate into the less hydrophobic washed HDPy/Mt. Hydration of the counter ion (Cl−) is the driving force for anion adsorption on HDPy/Mt. Higher energy consumption required to dehydrate divalent anions accounts for their lower affinity. Selectivity of HDPy/Mt to anions depends not only on anion size in different phases (from r1 in aqueous solution to r2 in the organic solvent-like HDPy/Mt), but also the dielectric constant of HDPy/Mt (ε2) after HDPy release.
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