Shihe Xu

Cationic surfactant sorption to a vermiculitic subsoil via hydrophobic bonding

Organoclays formed in soil from the addition of the cationic surfactant hexadecyltrimethylammonium (HDTMA) can effectively immobilize organic contaminants dissolved in water. The adsorption and desorption of HDTMA in a subsoil was studied to determine the stability of surfactant-soil clay complexes as affected by surfactant retention mechanism. We found that HDTMA was initially adsorbed by cation exchange in the interlayer, causing extensive clay aggregation. As the loading increased, HDTMA adsorbed to the external surfaces of aggregates via both cation exchange and hydrophobic bonding, the latter causing positive charge development on surfaces and ultimately clay dispersion. When the equilibrium concentration of HDTMA reached the critical micelle concentration, no further HDTMA adsorption occurred. Desorption of HDTMA was more significant when the HDTMA retention mechanism was hydrophobic bonding. HDTMA adsorption can be affected by cation type as well as the type and concentration of electrolytes, and these factors can be used to minimize the undesirable effects of hydrophobic bonding such as clay dispersion and HDTMA desorption.

Cosorption of organic contaminants from water by hexadecyltrimethylammonium-exchanged clays

The characteristics of organoclays as sorbents for neutral organic contaminants (NOCs) in aqueous systems containing multiple solutes were evaluated by measuring the sorption of trichloroethylene on hexadecyltrimethylammonium (HDTMA) exchanged illite and smectite in the absence and presence of carbon tetrachloride, nitrobenzene and ethyl ether. Trichloroethylene partitioned into HDTMA-derived phases present on the external surfaces of the clay particles manifesting type III isotherms. Carbon tetrachloride functioned to increase the solvency of the external HDTMA phase, and hence enhanced sorption of trichloroethylene. Nitrobenzene solvated HDTMA resulting in a more vertical orientation of the C-16 alkyl chains and interlayer expansion. This rendered the interlamellar region of HDTMA-smectite accessible for trichloroethylene sorption, and nitrobenzene sorption simultaneously increased the solvency of HDTMA phase for trichloroethylene. Ethyl ether sorption suppressed the uptake of trichloroethylene by decreasing the solvency of HDTMA phase for trichloroethylene. Sorption of trichloroethylene also facilitated sorption of carbon tetrachloride and nitrobenzene. The results suggest that multiple organic compounds generally produce a synergistic effect on the uptake of NOCs from water by HDTMA-clays, and support multiple mechanisms controlling the sorption process.

Alternative model for cationic surfactant adsorption by layer silicates.

Cationic surfactants are used in a wide range of household and industrial activities and have potential utility in in-situ remediation of contaminated soils and aquifers. Although many models have been proposed to describe the adsorption of cationic surfactants by soils and sediments, none can quantitatively account for the adsorption of quaternary ammonium compounds (QAC) by swelling clays or soils. In this study, we developed an alternative model for adsorption of hexadecyltrimethylammonium, a model QAC, by major clay types common to subsoils. The model uses a randomness parameter to account for the variation of cation exchange selectivity coefficients arising from differences in the distribution of inorganic and surfactant cations on the surfaces or in the interlayers of clays. In addition, the model employs an empirical relationship to predictthe amount of QAC adsorption by hydrophobic bonding. Experimental data demonstrate that the model quantitatively describes the major characteristics of QAC-clay interactions for both swelling and non-swelling clays and provides a quantitative linkage between QAC adsorption and the nature of the QAC and clay minerals as well as solution conditions.

Surface heterogeneity of trimethylphenylammonium-smectite as revealed by adsorption of aromatic hydrocarbons from water

Adsorption studies of aromatic hydrocarbons of various molecular sizes on organo-clays in aqueous solution were carried out for characterizing the surface heterogeneity of organo-clays. Benzene, toluene, p-xylene, ethylbenzene and n-propylbenzene adsorption by a smectite with 5 different exchange degrees of trimethylphenylammonium (TMPA) cations for Ca2+ was measured. The Langmuir isotherm equation did not adequately describe the experimental data, especially for small molecules, whereas the Dubinin-Radushkevich (DR) equation combined with a gamma-type adsorption energy distribution function described all experimental data well, suggesting the surface and structural heterogeneity of TMPA-smectites. The calculated adsorption energy distributions indicated that the apparent heterogeneity depends on the molecular size of adsorbates. Small adsorbate molecules such as benzene explore a highly heterogeneous surface of TMPA-smectites while large molecules such as n-propylbenzene detect a relatively homogeneous surface. The surface fractal dimension was dependent on the extent of TMPA exchange for Ca2+. When TMPA content is less than 75% of the cation exchange capacity (CEC) of the smectite, the heterogeneity decreases as TMPA content increases; it increases with TMPA content thereafter. These results are related to the size distribution of micropores in TMPA-smectites, which are defined by the 2 semi-infinite aluminosilicate sheets and the interlayer cations. The micropore size distributions and, hence, heterogeneity are created in part by the inhomogeneity of the charge density of clay surfaces and the tendency for cation segregation in these systems.