Adsorption of tungstate using cationic gemini surfactant-modified montmorillonite: Influence of alkyl chain length

Influence of alkyl chain length on the adsorption characteristics of organic pollutants by conventional cationic surfactant-modified montmorillonite (Mt) has been well explored. However, applying different cationic gemini surfactants to modify Mt for the adsorption of pollutant that possesses different species in aqueous solution to insightfully understand the structure-property relationship is rarely reported. In this study, three gemini surfactants with different alkyl chain lengths (number of carbon atoms, m = 12, 14, and 18) are selected to regulate the distribution and configuration in the composites. Tungstate, an emerging pollutant, is chosen as a probe, and its different species are modulated by changing the adsorption temperature and pH. The molar contents of the three surfactants are negligibly affected by the alkyl chain length, while the distributions of surfactants in the interlayer space and on the external surface are different. Lengthening the alkyl chain increases the packing density, lowering the accessibility of adsorption sites and decreasing the uptake of tungstates. At lower temperatures and pH values, polytungstates are the predominant species in the solution and are more favorably immobilized by the modified Mt. Ion exchange with Br- ions and desorption-adsorption are jointly responsible for the adsorption of tungstates, and both of these processes are configuration-dependent. The modified Mt by the gemini surfactants with longer alkyl chains demonstrates a lower ion exchange percentage and a more remarkable uptake of tungstates on the external surface. The adsorption of tungstates is accompanied by the rearrangement of gemini surfactants. Thus, elucidating the self-accommodation mechanism of cationic gemini surfactants during tungstates adsorption on modified Mt will provide a significant reference for the design of cationic surfactant-modified clay minerals used for pollutant immobilization.