Organic sorbates

Organic Sorbate-Natural Organic Matter Reactions... 

Organic Sorbate-Inorganic Solid Surface Reactions Illustrative Example 11.7 Estimating the Adsorption of Benzoic Acid to Goethite... 

In all of these cases, the structure of the organic sorbate, the composition of the surface, and the conditions of the vapor or solution exchanging with the solid must be considered. However, it is important to note that with some experience in thinking about the organic chemicals and environmental situation involved, we can usually anticipate which one or two sorption mechanisms will predominate. For example, in Chapter 9 we wrote an expression reflecting several simultaneously active sorption mechanisms, each with their own equilibrium descriptor, to estimate an overall solid-water distribution coefficient for cases of interest (Eq. 9-16) ... 

Two alternative explanations have been suggested which are both quite speculative. First, portions of mineral surfaces of intermediate polarity (e.g., siloxane regions, -Si-O-Si-) may permit some exchange of polar water and nonpolar organic sorbates (Hundal et al., 2001). Such surfaces occur in minerals like the faces of aluminosilicates. However, amorphous solids like silica (-Si-OH) and alumina (-A1-OH) have very hydrophilic exteriors when these inorganic materials are suspended in water. Yet these amorphous materials still clearly show sorption of apolar substances (e.g., Mills and Biggar, 1969a Schwarzenbach and Westall, 1981 Estes et al., 1988 Szecsody and Bales, 1989 Farrell et al., 1999). 

Some nonionic organic compounds exhibit much stronger mineral surface affinities than we see for apolar and weakly monopolar compounds like chlorobenzenes and PAHs. In these cases, the organic sorbates are able to displace water from the mineral surface and participate in fairly strong sorbate sorbent intermolecular interactions. Example compounds include mtroaromatic compounds (NACs) such as the explosive, trinitrotoluene (TNT), or the herbicide, 2,4-dinitro-6-methyl-phenol, also called dinitro-o-cresol (DNOC). 

Figure 11.14 shows the variation of-7 Tln Kid for the alkyl ammonium ions (when these organic sorbates are present at levels much less than Na+) as a function of the number of methylenes in the alkyl chains. The least-squares correlation line through the data yields ... 

First, some organic sorbates can react with organic moieties contained within the natural organic matter of a particulate phase. Especially prominent in this regard are organic bases like substituted anilines (Hsu and Bartha, 1974,1976 Fabrega-Duque et al., 2000 Li et al., 2000 Weber et al., 2001). Due to their low pKias ( 5), the aromatic amine functionality is mostly not protonated at natural water pHs, and the nonbonded electrons can therefore attack carbonyl moieties in the NOM ... 

Organic Sorbate-lnorganic Solid Surface Reactions... 

A second type of surface reaction involves bonding of the organic compounds with atoms (e.g., metals) exposed on the surface of the solid (Table 11.4). In these cases a water or a hydroxyl bound to a metal on the solid is displaced by the organic sorbate ... 

Table 11.4 Examples of Organic Sorbates Reacting with Mineral Surfaces ...

With the empirical measures of AT 1 reported in the growing literature and understandings of the stoichiometries of both the ion exchange and ligand exchange processes, we can now estimate the solid-water distribution ratios of such ionic organic sorbates (see Illustrative Example 11.7). 

We should point out that many organic sorbates, and especially bidentate ones like phthalate and salicylate, can apparently form more than one bound surface species. 

The organic sorbates used were the purest (>99%) available commercially, as were the non-sorbable solvents and internal standards 1,3,5-tri-iso-propylbenzene, 1,3-di(trifluoromethyl)benzene, mesitylene, tetramethylsilane, and cis- and trans-decalin. 

For organic sorbates, the distribution coefficient is related to Koc through the expression... 

On the basis of the correlations shown in Figures 10.16 and 10.17, the partitioning coefficient of any nonpolar organic sorbate is predicted by the empirical equation ... 

Large soil particles reach equilibrium with a given organic sorbate more slowly than do small particles (due to slow diffusion through micropores). 

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