I

Source: Boyd et al. [32]. Reprinted with permission.

in describing uptake into organoclays.) Consistent with this hypothesis was the lack of competitive effect in PCP uptake when up to 0.5 mmol of 3,4,5-trichlorophenol was also present. Boyd et al. [32] suggested that the uptake of hydrophobic organic compounds by smectite modified with HDTMA was due to solute partitioning into a highly nonpolar organic phase formed by the hydrophobic HDTMA ions. The partitioning behavior of HDTMA-smectite was supported by (1) the highly linear isotherms for the adsorption of the organic compounds, (2) the dependence of the sorption coefficient on the solubility of the organic compounds, (3) the general agreement between the organic matter normalized partition coefficient of the organoclay and the corresponding octanol/water partition coefficient of the organic compounds, and (4) the dependence of the adsorption capacity on the organic carbon content of the modified clay. Smith and Jaffé [33] used thermodynamic arguments—low or negative heats of sorption as

0.14

0.02

TMA*

0 0.001 0.002 0.003 0.004 0.005 0.006 0 007 0.008 PCP EQUILIBRIUM CONCENTRATION (mmole/100 ml)

Figure 2 Results of Boyd et al. [32] for adsorption of pentachlorophenol onto smectite modified with organics listed in Table 1 and onto activated carbon. Temperature 20°C. [From Clays and Clay Minerals, 36 (2), with permission.]

0.12

0.12

r cos

Ui a oc

0.02

TMA*

0 0.001 0.002 0.003 0.004 0.005 0.006 0 007 0.008 PCP EQUILIBRIUM CONCENTRATION (mmole/100 ml)

Figure 2 Results of Boyd et al. [32] for adsorption of pentachlorophenol onto smectite modified with organics listed in Table 1 and onto activated carbon. Temperature 20°C. [From Clays and Clay Minerals, 36 (2), with permission.]

evidenced by isotherms at different temperatures—to support partitioning as the uptake mechanism of tetrachloromethane using decyltrimethylammonium bentonite. If a decyltrimethyldi-ammonium bentonite organoclay was used, however, the predominant mechanism appeared to be adsorption rather than partitioning. Consistent with the work of Wolfe et al. [4] is a lesser d spacing due to attachment of the diammonium structure to the same mineral surface. The diammonium clay exhibited competitive adsorption in the presence of benzene with the tetrachloromethane, while the decyltrimethyldiammonium bentonite sorbed equal amounts of tetrachloromethane whether the benzene was present or not.

Such sorbent materials thus show promise in separation of more hydrophobic classes of contaminants from water as a class. One the other hand, Lee et al. [11] reported that the use of smaller TMA+ ions allowed quite selective sorption, removing benzene while excluding toluene because of its somewhat larger dimensions. Cadena [9] showed the extent of organic uptake by a TMA bentonite to be in the order benzene >> toluene > o-xylene, although each was removed to a much greater extent than by an unmodified bentonite. This is consistent with the findings of McBride et al. [3], who observed high selectivity of TMA +-exchanged montmoril-lonites exposed to single-ring aromatic hydrocarbons and was greatly favored by small planar molecules such as phenol and benzene. Although Cadena surmised a partitioning mechanism, the consensus of other investigators is that adsorption predominates with the smaller cationic modifiers. Lee et al. [6] suggested "adsorption" as the mechanism of uptake for nonionic organic compounds by TMA smectite. Their conclusion was based on the following: (1) A curvilinear isotherm was observed for sorption of benzene by TMA smectite, while a linear isotherm was observed by HDTMA smectite; (2) more soluble compounds such as benzene showed strong affinity for TMA smectite, despite the fact that with a partitioning mechanism the higher the water solubility, the lower the partitioning coefficient; and (3) the uptake of benzene from aqueous solution was higher for TMA smectite than for HDTMA smectite, although TMA smectite has a much lower organic carbon content than that of HDTMA smectite. Smith et al. [34] in comparing the behavior of 10 different organoclays, indicated that those with alkyl carbon content less than 3 functioned by adsorptive uptake; those with 12 carbons or more exhibited removal by partitioning (no carbon contents of 3-11 were characterized).

These studies have made clear the potential for optimizing organoclay configuration for a given use or in idealized experimental systems. Results such as those in Figure 2 indicate that properly synthesized organoclays approach within an order of magnitude the performance of activated carbon [36] as an adsorbent in a "clean" system. Evidence to date suggests that the modified clays may perform equally as well as carbon in a complex system where selective adsorptive properties are advantageous. Cost advantages may also be significant in comparing organoclays to carbons, due to energy costs in production and regeneration of the latter.

Such comparisons may well extend to use in the more complex chemical environment of water treatment. Recent work [7] suggests the practicability of using these materials in actual surface waters; however, analytical difficulties may be encountered in determining removals for targeted components if measures are not taken for proper extraction and separation from a complex water or wastewater matrix.

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