Page 186 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 177

content could thus significantly influence the apparent solubility (i.e., the K dom
or K doc value) of a PAH in addition to the DOM’s polarity and molecular size.
Alternatively stated, one expects a PAH solute to exhibit a higher K doc value
than a non-PAH solute with a given DOM, if the two solutes have similar K ow
or supercooled S w value. Chin et al. (1997) found that the K doc value of pyrene
with dissolved Aldrich humic acid, a commercial humic acid surrogate with a
very high aromatic content, is only slightly smaller than that of DDT (by about
a factor of 2), whereas the K ow value of DDT is about 15 times as large as that
of pyrene. The measured K doc values for pyrene with a series of dissolved river
and lake humic materials, which exhibit minor differences in oxygen content,
appear to correlate well with the DOM’s aromatic content as well as its molec-
ular weight (Chin et al., 1997). This suggests that DOM in natural water might
have a far greater enhancing effect on the concentration and mobility of some
PAHs.
In natural systems, the K dom (or K doc) values for all low-solubility solutes
would vary with the DOM source. Based on the limited data available, the
K dom values for natural (not human-made) DOM values in rivers and streams
appear to fall between values for soil humic acid (SSHA) and for highly acidic
aquatic fulvic acid (SRFA) (Chiou et al., 1987), which vary by a factor of about
5. A more comprehensive account of the K dom variance requires further studies
with purified DOM samples. The earlier difficulties in extracting relatively
pure aquatic humic materials for solute–DOM interaction studies have led
some researchers to the use of commercial humic acids as surrogates for
aquatic humic materials. Since the elemental compositions of certain com-
mercial humic acids are vastly different from those of aquatic and soil-derived
humic substances (Malcolm and MacCarthy, 1986), the data derived from
these samples would not serve as a realistic reference for aquatic humic mate-
rials. For example, the K dom (or K doc) values of DDT and PCBs with Aldrich
humic acid are about three times higher than with soil humic acid and some
4 to 20 times higher than with aquatic humic acid and fulvic acid extracts
(Chiou et al., 1987). The exceptionally high K dom (or K doc) values with Aldrich
humic acid are much a result of its exceptionally high carbon (65.3%) and low
oxygen (25.1%) contents on an ash-free basis of the sample (Malcolm and
MacCarthy, 1986). Thus the K dom (or K doc) data with commercial humic acids
could grossly overestimate the actual impact of aquatic humic materials on
solute behavior in natural water.
As noted, both suspended and dissolved organic matter can affect the
apparent solute solubility or solute sorption coefficient. Whereas the effect of
suspended organic matter in soil–water mixtures on K d may be viewed effec-
tively as the third-phase effect (Gschwend and Wu, 1985), the weaker effect as
exhibited by truly dissolved organic matter of both natural and human-made
origins cannot be well reconciled with a phase concept according to our con-
vention. Rather, it would be more appropriate to treat a water solution con-
taining dissolved organic matter as a mixed solvent, in which the solvency of
water for solutes is altered by the dissolved organic substance. In natural