Page 165 - Partition & Adsorption of Organic Contaminants in Environmental Systems

156 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

exhibits a small depression of DUN sorption at the EDB concentration of
1070mg/L (C e/S w = 0.30). MON at 80mg/L (C e/S w = 0.29) exhibits a significant
but incomplete suppression of the DUN nonlinear capacity while MON at
230mg/L (C e/S w = 0.84) and DCP at 260mg/L and 700mg/L (C e/S w = 0.032 and
0.087, respectively) suppress most of the nonlinear capacity. In the latter case,
the resultant DUN isotherms are relatively linear with slopes about equal to
that of the upper linear DUN single-solute isotherm. The response of DCP to
co-solute PHL on Woodburn soil (Figure 7.21) is similar to that with peat soil
(Figure 7.19), in which PHL as the co-solute at 12,000mg/L (C e/S w = 0.14)
erases most of the DCP nonlinear capacity.
The results above illustrate that the nonlinear behavior of a nominal solute
in binary-solute systems is influenced by both the co-solute type and its con-
centration and that a polar co-solute (e.g., DCP) of one chemical class may
effectively suppress the nonlinear sorption of a nominal polar solute of a dif-
ferent class (e.g., DUN). In this respect, phenolic compounds are more pow-
erful sorbates and competitors than are substituted ureas, which is consistent
with their relative nonlinear sorption capacities. If the nominal solute is a polar
compound, suppression by various co-solutes occurs in a highly selective
manner. Here the large nonlinear capacities of polar solutes (e.g., DCP and
DUN) are not strongly affected by nonpolar co-solutes at relatively low C e/S w
values; a large suppression occurs if the co-solute is of high polarity even at
relatively low C e/S w values, as illustrated by DCP on DUN (Figure 7.18). For
the polar solutes studied, the relative suppressive power follows the order
PHL ≥ DCP > MON > DUN, which is essentially the order of their S w values.
By contrast, the small nonlinear capacities of nonpolar solutes (e.g., EDB and
TCE) are more effectively depressed by either polar or nonpolar co-solutes if
the C e/S w of the co-solute is appreciably higher than the upper C e/S w limit
(0.010 to 0.015) of the sorption nonlinearity for nonpolar solutes.
Some important isotherm features for solutes at low C e/S w values, as
revealed explicitly by Chiou and Kile (1998) and less explicitly by Xing et al.
(1996) and Xing and Pignatello (1997), include (1) the smaller nonlinearity
effects for nonpolar than for polar solutes; (2) the relatively small suppression
of the sorption of a polar solute (e.g., atrazine) by a nonpolar co-solute (e.g.,
TCE) over a range of co-solute concentrations; and (3) the significant sup-
pression of the sorption of a polar solute (atrazine) by other polar co-solutes
(e.g., prometon and other triazines). Here the linear plots, as employed by
Chiou and Kile (1998), are better adapted than the log-log plots used in related
studies for displaying the different extents of nonlinear sorption in the differ-
ent systems.
The unique isotherm shape for single-solute systems (i.e., nonlinear at low
C e /S w but virtually linear at other C e /S w values) suggests that more than one
mechanism is operative over the entire concentration range. Moreover, since
the (apparent) nonlinear capacity and the point of nonlinear saturation are
not the same for polar and nonpolar solutes, the data suggest that the primary
causes for their nonlinear behaviors at low concentrations are not the same.