Environ. Sci. Technol. 1995, 29, 3022-3028
Alternative Model for Cationic Surfactant Adsoqition by Layer Silicates AND STEPHEN A. BOYD* and Soil Sciences, Department of Crop Michigan State University, East Lansing, Michigan 48824-1325
Environ. Sci. Technol. 1995.29:3022-3028. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/23/19. For personal use only.
SHIHE
XU
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 predict the 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.
Introduction Cationic surfactants are used in a wide range of industrial and household activities (1, 2). The adsorption and desorption of these compounds in soils and sediments strongly influences their fate and transport in environment (1, 3). Several studies have examined the adsorption of a variety of organic cations on mineral surfaces [4-11). More recently, the potential utility of a certain type of cationic surfactant, namely, quaternary ammonium compounds (QACs), for in-situ remediation of organic contaminated subsoils and aquifer materials has stimulated interest in understanding their chemical interactions with soil components {8, 12—15). The in-situ remediation technique under study involves direct injection of QAC solutions into the subsurface to create sorptive zones for immobilizing organic groundwater contaminants. Previous studies have demonstrated that *
Corresponding author telephone: (517) 353-3993; fax: (517)355-
0270.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 12, 1995
replacing native exchangeable cations of soil clays with long-chain QACs (e.g., hexadecyltrimethylammonium, HDTMA) results in dramatic increases in the sorption of hydrophobic organic compounds (HOCs) by the resultant modified soil or subsoil {16—19). If properly placed, such sorbent zones could intercept and immobilize contaminant plumes containing HOCs. This, coupled with subsequent biodegradation of the immobilized contaminants may provide a comprehensive remediation approach for contaminated aquifers (20). The feasibility of in-situ soil modification and contaminant plume immobilization has been demonstrated recently using an aquifer box model {21, 22).
One major need in applying the soil modification technique to environmental restoration is the ability to predict QAC adsorption by soil components because this determines the efficiency of soil modification. Once surfactant solutions are delivered into the subsurface, it is important to know the surfactant distribution among the liquid and solid phases. Obviously from an environmental safety prespective, the goal is to maximize surfactant binding; this also has the advantage of increasing the effectiveness of the sorbent zone. Furthermore, dissolved QAC (e.g., HDTMA) may be toxic to pollutant degrading bacteria (20), thwarting efforts to bioremediate immobilized contaminants. It is also important how much surfactant is bound via cation exchange versus the nonexchange mechanism at a given surfactant loading. Surfactant adsorbed via the latter mechanism (e.g., hydrophobic bonding) is more susceptible to desorption than that adsorbed directly on exchange sites {14). QAC adsorption by clays depends on the clay type, the nature of exchangeable cations initially saturating the clay, and the ionic strength of the aqueous solution (15). For nonswelling clays (e.g., kaolinite), the adsorption isotherm is generally monotonic (15). For swelling clays (e.g., montmorillonite), in dilute salt solutions (e.g., 2.5 mM CaCl2 or 5 mM NaCl), monotonic HDTMA adsorption isotherms have been observed for Ca-montmorillonites, but s-shaped non-monotonic isotherms were observed for Na-montmorillonites. The s-shaped HDTMA adsorption isotherms changed to monotonic as the ionic strength of the NaCl solution increased to greater than 0.01 M (15). The s-shaped HDTMA adsorption isotherm and the dependence of isotherm shape on solution conditions are unique characteristics of HDTMA adsorption by swelling clays. Previous surfactant adsorption models (9-11, 23) were developed using data obtained from nonswelling clays and cannot account for these phenomena (15). Based on X-ray diffraction patterns, electrophoretic mobility of clays with various HDTMA loadings, Xu and Boyd (15) attributed the s-shaped adsorption isotherms of swelling clays to a random distribution of inorganic and HDTMA cations in the interlayer region and attributed the monotonic isotherms to layer segregation of HDTMA and inorganic cations. Swelling clays also behave differently from nonswelling clays with regard to the hydrophobic adsorption of surfactants. For example, Xu and Boyd {14,15) tested several ionic surfactant adsorption models using HDTMA adsorption and cation release data, X-ray diffraction, electro-
0013-936X/95/0929-3022$09.00/0
S 1995 American Chemical Society
phoretic mobility, and clay suspension turbidity measure-
HDTMA adsorption via hydrophobic bonding for swelling
(Kyüql aA) / (1 + KyQql Up)
—
1VqX
ments and found that the existing models could not describe
and then
clays.
In this study, we have developed and tested a new model for cationic surfactant adsorption using HDTMA as a model compound. The model consists of two submodels describing cation exchange and hydrophobic bonding. The cation exchange submodel is based on the hypothesis that variations in the strength of the HDTMA- clay binding result from the interlayer distribution of inorganic/organic cations and the structure of the HDTMA adsorption layer. The hydrophobic bonding submodel is based on an empirical observation that HDTMA adsorption at high loadings is linearly related to square root of aqueous concentration of HDTMA. The model is conceptually simple and accounts for most of the observed variations in HDTMA adsorption under different experimental conditions. Model Description Cationic surfactants maybe adsorbed by two mechanisms, viz., cation exchange and hydrophobic bonding. Therefore =
Íce
4"
Íhb
(1)
where eft, qcE, and qm denote the total surfactant adsorbed (e.g., in mol kg™1), surfactant adsorbed by cation exchange, and surfactant adsorbed by hydrophobic bonding, respectively. In the following paragraphs, we will describe how to obtain these two components for monovalent hydrophobic cationic surfactant (e.g., HDTMA) adsorption by layer silicates. Cation Exchange ()
+ v]
For monovalent inorganic cations, eqs 6 and reduced to
Ky¡
=
Xo
exp(-coi/RT)
(16)
where K° is the combined contribution of the electrostatic interaction and the hydrophobic bonding of the organic cation to the selectivity coefficient; tu, is the free energy of hydrophobic lateral interactions and depends mainly on length of alkyl chain, surfactant organization, and loading on the clay surfaces. The term cu,· can be defined as
(7)
7 can be
,.
=
w°EQXl2e
(17)
where ° is the free energy of lateral interactions at 100% VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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surfactant- saturation of the exchange sites; is a parameter accounting for how randomly the surfactant is distributed on surfaces comprising the interlayers. For a random > 0, and Kvi increases with surfactant organization, loading. For complete segregation, = 0, , = °, and Kvi remains constant as loading increases. Hydrophobic Bonding (¿¡ ß). The submodel for predicting
