Surfactant sorption

Physical and ionic adsorption may be either monolayer or multilayer (12). Capillary stmctures in which the diameters of the capillaries are small, ie, one to two molecular diameters, exhibit a marked hysteresis effect on desorption. Sorbed surfactant solutes do not necessarily cover ah. of a sohd iaterface and their presence does not preclude adsorption of solvent molecules. The strength of surfactant sorption generally foUows the order cationic > anionic > nonionic. Surfaces to which this rule apphes include metals, glass, plastics, textiles (13), paper, and many minerals. The pH is an important modifying factor in the adsorption of all ionic surfactants but especially for amphoteric surfactants which are least soluble at their isoelectric point. The speed and degree of adsorption are increased by the presence of dissolved inorganic salts in surfactant solutions (14). 

SURFACTANT SORPTION ON NATURAL SEDIMENTS Eduardo Gonzalez-Mazo and Victor M. Leon... 

Fig. 5.4.5. Schematic diagram of the typical surfactant sorption behaviour [5,11] related to surfactant concentration at constant temperature.

Nevertheless, surfactant sorption isotherms on natural surfaces (sediments and biota) are generally non-linear, even at very low concentrations. Their behaviour may be explained by a Freundlich isotherm, which is adequate for anionic [3,8,14,20,30], cationic [7] and non-ionic surfactants [2,4,15,17] sorbed onto solids with heterogeneous surfaces. Recently, the virial-electrostatic isotherm has been proposed to explain anionic surfactant sorption this is of special interest since it can be interpreted on a mechanistic basis [20]. The virial equation is similar to a linear isotherm with an exponential factor, i.e. with a correction for the deviation caused by the heterogeneity of the surface or the energy of sorption. 

Apart from the structural properties of the molecule, surfactant sorption is affected by physico-chemical parameters, such as temperature, pH, organic carbon content of sediment, suspended solids concentration, ionic strength, etc. The most relevant of these are discussed next. 

Surfactants are selected based primarily on the degree of solubilization. Other factors to be considered include toxicity, biodegradability, surfactant sorption, and surfactant solubility and compatibility with the separation process. Surfactants have the ability to lower the interfacial tension between water and the contaminant by as little as a factor of three to four orders of magnitude. Combined with a sufficient reduction in capillary forces, this allows pumped groundwater theoretically to move the DNAPL toward the recovery or extraction well. This is accomplished by injecting surfactant solution into the contaminated zone. Impacted groundwater characterized by an increase in the concentration of the contaminant is then recovered and treated. 

The fate of surfactants in soils and surface waters is determined primarily by biodegradation and sorption. Biodegradability depends on the presence of functional groups in surfactants which can be metabolized by microorganisms. Surfactant sorption depends on a) the ability of a surfactant headgroup to undergo interactions with the sorbent and b) the interactions of its hydrophobic tail which is repelled from the water. 

Since surfactants are designed to be enriched at interfaces, surfactant sorption onto environmental solids should be of major importance particularly when the ratio of water volume to water-solid interface is small. Those conditions exist in wastewater treatment and in soil. 

Sorption is measured by recording sorption isotherms, which themselves are a way to express the amount of surfactant sorbed as function of the concentration of the compound in the solution. The Freundlich isotherm (Equation II) is a general sorption isotherm which describes sorption behavior and often is used in studies of surfactant sorption. KF is the Freundlich sorption coefficient which expresses the affinity of a surfactant for a given solid... 

Figure 17.4 shows a surfactant sorption isotherm from low to high (>CMC) concentrations of the surfactant. It can be divided into three parts (Figure 17.5). In Region 1, individual surfactant molecules are in equilibrium with the surfactant molecules adsorbed to the solid sorbent. In Region 2, the surfactant concentration in the water has exceeded the CMC. That is equivalent to saturation of the air/water interface with surfactant molecules. Subsequent addition of surfactant molecules leads to increased sorption due to formation of sorbed surfactant aggregates (Region 2). In Region 3, the aggregates in solution (micelles) are in equilibrium with the sorbed aggregates, the so-called admicelles..

Under environmental conditions, Cw is in the low pM-range or below. Table 17.10 is a compilation of data on surfactant sorption for a variety of surfactants to soils, sediments, or minerals, employing information generated at low surfactant concentrations. 

Schematic of the concentration-dependent processes during surfactant sorption (Reprinted from Edwards et al., 1994).

The correlations of KF-values with sorbent properties by Hand and Williams (1987) as well as Matthijs and De Henau (1987) give no clear indication which sorbent properties govern surfactant sorption. LAS sorption appears brought about by specific as well as unspecific hydrophobic interactions, and the prevalent mechanism depends on the nature of the sorbent. 

Effects of Surfactant Sorption on the Equilibrium Distribution of Organic Pollutants in Contaminated Subsurface Environments... 

Key words surfactant-enhanced aquifer remediation surfactant sorption distribution... 

Figure I. Surfactant sorption isotherms on kaolinite (pH 4.6, I S. = 0.1 M). Error bars for some data points are smaller than the symbols. Kaolinite concentrations were 100 g/L (SDS, Tween 80) or 250 g/L (SDS). Adapted from Ko et al. (1998b).

The influence of ionic strength on surfactant sorption is shown in Fig. 2. In general, SDS sorption at 0.1 M NaCl was greater than for no added NaCl, consistent with previous observations (Xu and Boyd, 1995). Increased SDS sorption at the higher ionic strength can be explained by a decrease in the electrostatic repulsion between sorbed SDS molecules as well as between... 

Figure 4 shows phenanthrene and naphthalene sorption isotherms to kaolinite covered with varying levels of sorbed surfactant these levels of surfactant coverage correspond to the different regions existing in the surfactant sorption isotherms discussed earlier (Fig. 1). The linearity of each isotherm was evaluated using Freundlich and linear sorption models. It is apparent from Fig. 4 and Table 4 that HOC partitioning to kaolinite with and without adsorbed surfactants results in linear or near-linear isotherms. As the amount of surfactant adsorbed on the kaolinite surface increased, the sorption of phenanthrene and naphthalene to the solid phase also increased. However, upon normalizing by the amount of sorbed surfactant present, the sorbed surfactant partition coefficient (Kss) decreased with increasing sorbed surfactant amounts (Table 4).

Depending on the desired treatment methodology and goals, addition of surfactants to a subsurface system should either increase HOC distribution coefficients (i.e., immobilization approach) or decrease them (i.e., mobilization objective as in many SEAR applications). For example, distribution coefficients for phenanthrene and naphthalene to kaoiinite are 0.002 and 0.0003 L/g, respectively (Table 4). Therefore, if enhanced mobilization of these HOCs in a similar type of aquifer system was desired, addition of a surfactant would have to bring the distribution coefficients below these values. However, as can be seen in Fig. 5, all distribution coefficients for the surfactant doses investigated here are larger than these values, even when the doses and subsequent aqueous surfactant concentrations are well above the CMC. This observation results from a combination of surfactant sorption followed by HOC partitioning to the sorbed surfactant. 

In any evaluation of a remediation scheme utilizing surfactants, the effect of dose on HOC distribution coefficients must be quantified. Very often, only one coefficient value for HOC partitioning to sorbed surfactants has been reported in the literature, presumably because the experimental data covers only the sorption regions where the surfactant molecule interactions dominate at the surface (Nayyar et al., 1994 Park and Jaffe, 1993). However, all of the characteristic sorption regions will develop during an in-situ SEAR application as the surfactant front (i.e., mass transfer zone) advances through the porous medium. Therefore, the relative role ofregional HOC partition coefficients to sorbed surfactant should be considered in any remediation process. Finally, the porosity or solid volume fraction for the particular subsurface system must be taken into account when surfactant sorption is quantified. 

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