Technology Background

Organoclays are typically smectite slays treated with an alkylamine or quaternarized cationic surfactant [22], Adsorptive properties of the clay are modified due to insertion of the surfactant molecules into interlamella, providing an extensive volume that apparently serves as a hydrophobic "phase" into which hydrophobic contaminants can then be partitioned [3]. Characteristics of this phase can be altered by appropriate matching of clay and surfactant, enabling in theory the customized design of such sorbents. Variables that may be considered include charge density of the clay [11,23] and properties of the surfactant such as alkyl chain length [24], number and basicity of positive charges [10,24], structural aspects (number of alkyl branches, presence of aromatic moieties), and polarity index of the alkyl groups [12]. Subsequent treatment of the organoclay with other inorganic materials can improve uptake as well (Figure 1).

Wyoming montmorillonite and hectorite are the base clays that have been used in research studies [e.g., 3,25-28] and also used as base materials in some commercially available organoclays (e.g., Bentones B27, B34, B38, produced by NL Industries for use as viscosity modifiers). These are smectites that possess deficits of positive charge due to isomorphic substitution of cations in the octahedral sheets. Substitution in this sheet distances the origin of charge from the interlayer and thus delocalizes the charge imbalance. Because there are no negatively charged sites per se in the sheets adjoining the interlayer, compensatory cations located in the interlayer do not serve as firm anchors between adjacent layers. The consequent disorder may enhance access to adsorption or exchange sites. On the other hand, it is possible that desorbability of inserted surfactant may also be facilitated. Montmorillonite, with a dioc-tahedral structure, differs from hectorite (which is trioctahedral) in allowing greater distortions in the sheet structures due to the unfilled cation sites. This is demonstrated by the substantial

Figure 1 Results of adsorption of 2,4,5-trichlorophenol onto calcium montmorillonite (Ca-M), montmorillonite modified with dioctadecyldimethylammonium (dd-M), and montmorillonite modified with di-octadecyldimethylammonium and then treated with CaCl2 (Ca,dd-M). Solid/liquid mass ratio 0.0005; equilibration time 15 hr, temperature 35°C, analyses by UV absorption. (From Demougeot [25] and Dentel et al. [26].)

Figure 1 Results of adsorption of 2,4,5-trichlorophenol onto calcium montmorillonite (Ca-M), montmorillonite modified with dioctadecyldimethylammonium (dd-M), and montmorillonite modified with di-octadecyldimethylammonium and then treated with CaCl2 (Ca,dd-M). Solid/liquid mass ratio 0.0005; equilibration time 15 hr, temperature 35°C, analyses by UV absorption. (From Demougeot [25] and Dentel et al. [26].)

dimensional differences between Al3 + and the substituting Mg2 +. Isomorphous substitution in hectorite instead replaces Li+ for Mg2+ because these have similar dimensions, and little distortion results. The more planar interlayer geometry may provide more uniform adsorption selectivity.

Charge density or cation-exchange capacity (CEC) of the smectite can have a significant effect on properties of the resulting organoclays. If charge density or the CEC is high, the spacing of intercalated surfactants is increased and the orientation is more likely to be parallel to the silica sheets [23], However, more recent findings [11] show superior removals of benzene and toluene using a smectite with a lower CEC (90 meq/100 g) than with a higher CEC (120 meq/ 100 g). Evidently, the greater amount of free volume provided between pillars is the critical factor, allowing placement of benzene and toluene parallel to the silica sheets in a more favorable configuration.

A range of cationic surfactants have been used in previous studies with organoclays. Early studies employed primary amines such as propylamine and octadecylamine [e.g., 29]. These were readily intercalated into interlayers, but the apparent conformation is with the alkyl chain parallel to the basal surfaces [30], The extent and control over ¿(001) spacing is thus limited. McBride et al. [3] suggested that it is the low basal spacing and thus close packing of n-alkylammonium ions that restricts the interlamellar volume available for adsorption. A further limitation is the pH dependency of the cationicity of these surfactants. Wolfe et al. [4,31] reported comparatively poor sorptive removal using 11 different organics with such materials (dodecylammonium and propylammonium).1 The study also used dodecyldiammonium [H3N + (CH2)i2N + H3] with the assumption that fewer of the divalent ions would be required to neutralize the anionic charge on the anionic silicate sheets, leaving more free interlamellar volume [22]. While this was apparently the case in adsorption from vapor, removals from aqueous suspensions were not improved. Plausibly, water molecules in the interlayer succeeded in shrinking the cavities regardless of the presence of any of these primary amines. It should be noted that some data presented by Wolfe et al. [4,31] are not consistent with more recent findings, possibly due to volatilization difficulties or the use of procedures in which organic solubility limits were exceeded.

Quaternary ammonium ions have also been employed as clay modifiers. A somewhat greater basal spacing may provide a looser packing in this case. Furthermore, a significant change in the properties of the modified clay may create a more favorable sorbent. The change includes the surface properties, which become organophilic as the mineral's surface area covered by the organic exchange cations increases [11]; this leads to an increase in the organic carbon content of the clay as well.

The porosity of the clay will also be altered. Since the porosity is related to the distance between the sheets of the clay and is dependent on the size and nature of the organic exchange cations [26], modified clays can be designed to have surface accessibility to adsorbates of various sizes. Tetraalkylammonium anions such as tetramethylammonium (TMA + ), tetra-propylammonium, and others with a variety of alkyl and aromatic groups have been used, as exemplified by Table 1, from Boyd et al. [32]. Boyd et al. found that the larger, more hydrophobic pillars led to the best removals of pentachlorophenol (PCP) from solution (Figure 2). The larger2 spacing appeared to enable partitioning of the sorbate into an organic "phase." (Thus the more general term sorption, rather than adsorption, appears to be more appropriate

'Low adsorption levels reported by Wolfe have been questioned by Cadena [9] due to the possibility of volatilization losses, emphasizing the need for zero-headspace procedures.

2The enlarged d( 100) spacings after insertion of the larger surfactants are considerable (e.g., 10-14 A increases with DODMA+ [28],

Table 1 Structure of Quaternarized Cationic Surfactants Used in the Research of Boyd et al. [32]

Ion name

Abbreviation

Structure

Dioctadecyldimethylammonium

DODMA+

Hexadecyltrimethylammonium

HDTMA s

Hexadecylpyridinium

Trimethylphenylammonium

Tetramethylammonium

TMAH

4-Mercaptopyridinium

Ammonium

ch3 I

ch3 I

ch3 I

0 0

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