Abstract

Non-ionic surfactants Brij 56 and Igepal CO 720, containing hydrophilic poly(ethylene oxide) (PEO) segments, expanded smectite from 1.5 nm to 1.7 nm at room temperature. The surfactant-smectite composites had larger layer spacings than Ca-smectite after heat treatment. The surfactant-smectite composites were solvated and expanded to 1.8–1.9 nm by polar solvents, glycerol and water, but were not affected by the non-polar or weakly polar solvents, toluene, hexane or octanol. The hydrophilic PEO segments of non-ionic surfactants would logically access the interlayer spaces of smectite whereas the hydrophobic segments extend away from the mineral. The molecular structure and solvation properties suggest that the surfactant molecules are probably concentrated in the margin area of the interlayer galleries forming an annular ring structure between two neighboring silicate sheets. Only two layers or less of the surfactants could access the interlayer galleries of smectite and layer spacings did not exceed 1.8 nm even where excess surfactant was introduced into the composites. The layer spacings of the surfactant-smectite composites were well preserved during water or electrolyte solution washings, indicating stability of most non-ionic surfactant molecules in the interlayer galleries even though ~30% of the adsorbed Igepal CO 720 was desorbed by exhaustive washing. The non-ionic surfactant treatment preserved >80% of the CEC of the smectite. The interlayer cations of the resulting surfactant-smectite were exchangeable as in the untreated smectite. Therefore, the non-ionic surfactant-smectite was much more efficient at removing heavy metal ions than activated carbon or cationic surfactant-treated smectite. The surfactant-smectite composites effectively removed aromatic chlorophenols from a pH 4.9 acetate buffer solution while untreated smectite did not adsorb these molecules. The enhanced adsorption of the aromatic compounds is attributed to the aliphatic segments of the two surfactants.

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