Page 184 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 175

molecular sizes and, perhaps to a greater extent, to their greater polarities
(which enhance their interactions with water) as measured by (O + N)/C.
The suggested molecular weights are 1000 to 5000 for soil fulvic acids (Buffle
et al., 1978; Underdown et al., 1981), 1000 to 10,000 for aquatic humic acids
(Thurman et al., 1982), and 500 to 1000 for aquatic fulvic acids (Aiken and
Malcolm, 1987).
The comparable effects as found with SRHA, SRFA, and SSFA, despite
some differences in their molecular sizes, suggest that the DOM polarity has
a greater impact on solubility enhancement in this situation. Further, poly-
acrylic acid (with MW = 2000 and 90,000) as a linear polyelectrolyte shows
no enhancement effect at all, notwithstanding that its C (50%), H (5.6%), and
O (44.4%) contents are comparable with those of SSFA. Here the inability of
polyacrylic acid to enhance solute solubility is attributed to the frequent and
orderly attachment of hydrophilic carboxyl groups to the carbon chain and to
an extended chain structure, which prohibit the formation of a sizable
intramolecular nonpolar environment. Thus, although the molecular size of
DOM is essential, this property is not the sole deciding factor in solubility
enhancement. To be a strong solubility enhancer, the DOM must possess
both a favorable size of nonpolar moiety and a sufficiently large molecular
weight.
By the results in Table 7.15, the K dom values with dissolved soil humic acid
(SSHA) are about half as large as the K om values with bulk soil organic matter
(SOM) in concentrating organic compounds on a unit weight basis. Compar-
atively, the particulate or colloidal organic matter derived from suspended soil
or sediment particles should give K dom K om. In soil–water mixtures, the total
amount of dissolved and suspended organic matter in water (X) usually
increases with increasing soil-to-water ratio. With high X value, the resulting
K* value may be significantly smaller than the K d value for certain solutes
d
because of their enhanced water solubility. Hence, if the aqueous phase con-
tains, say, X = 30mg/L of soil humic acid, the apparent K* value of DDT with
m
logK dom = 4.8 would be about three times lower than the intrinsic K om value.
Gschwend and Wu (1985) measured the K* values of highly water-insoluble
d
2,4,5,2¢,5¢-PCB and 2,3,4,5,6,2¢,5¢-PCB with sediments in relation to total
organic carbons of suspended sediment microparticles. By use of Eq. (7.25),
they showed that when precautions were taken either to eliminate or to
account for the suspended microparticles in water the calculated logK oc (or
logK om ) values remained essentially constant for both solutes over a wide
range of sediment-to-water ratios, whereas the logK* value decreased with
oc
the amount of suspended microparticles.
With the preceding account, the observed large variation in logK om
for highly water-insoluble solutes with the solid/water ratio in the sorption
experiment (Means et al., 1982; Karickhoff, 1984) could be attributed in part
to the different extents of solute solubility enhancement by dissolved and/or
suspended organic matter. It would also appear that the significant increase
in the apparent K* value of DDT with soils following the soil extraction with
om