Page 199 - Partition & Adsorption of Organic Contaminants in Environmental Systems

190 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

the correlation with S w for solid pesticide solutes is improved with the use of
their supercooled-liquid S w values (not shown). For more water-soluble BTEX
liquids, in which the S w ranges from 190mg/L for p-xylene to 1780mg/L for
benzene, the K*/K d ratios exceed 1 on all solids and increase with increasing
d
X up to two to three nominal CMCs. With solid f om decreasing from peat to
CSMS, TCS, and bentonite, the K*/K d ratios go up rapidly. Relative to BTEX
d
data, the K*/K d values for less soluble lindane (S w = 7.8mg/L), a-BHC (S w =
d
2.0mg/L), and HPOX (S w = 0.2mg/L) are much more variable. On bentonite
(f om ª 0), the K*/K d ratios are >1 and increase rapidly with X from below to
d
above the CMC. On TCS ( f om = 0.024), the K* values at X < CMC are about
d
the same as K d for all three chlorinated compounds; when X is about 2 CMCs,
the K*/K d values are about 1.5 for lindane and a-BHC, while the K*/K d value
d
d
of HPOX drops to about 0.7. On CSMS (f om = 0.148) and peat (f om = 0.864),
the K*/K d values are <1 for all three solutes and decrease steadily with increas-
d
ing X from below to above the CMC.
In terms of Eq. (7.29), the observed K*/K d values for solutes with TX100
d
on natural solids from all reports reveal the following trends: (1) the (1 +
f sfK sf /K d) term is greater than the (1 + X mnK mn + X mcK mc) term for relatively
water-soluble BTEX on all solids at low-to-moderate X, in which the ratio of
(1 + f sfK sf /K d) to (1 + X mnK mn + X mcK mc) increases with decreasing solid f om and
with increasing solute S w; (2) if the solid has a very low f om value, then (1 +
f sfK sf /K d) > (1 + X mnK mn + X mcK mc) holds for practically all solutes at low X
(<<CMC); (3) the reverse effect, that is, (1 + f sfK sf /K d) < (1 + X mnK mn + X mcK mc)
occurs at low X only for low-S w solutes (e.g., lindane and HPOX) on high-f om
solids (e.g., CSMS); and (4) the aromatic BTEX tend to exhibit higher K*/K d
d
than other relatively soluble nonaromatic compounds (e.g., TCE) at compar-
able X.
The diversity of the K*/K d data manifests the sensitive balance between
d
(1 + f sfK sf /K d) and (1 + X mnK mn + X mcK mc) for different solutes on different
sorbents (solids) with TX100 at low to moderate X. In the water phase, we
recall that the magnitude of (1 + X mnK mn + X mcK mc) increases sensitively with
the K ow or 1/S w of the solute (see the discussion on page 180); the TX100
applied at X < 400mg/L is not expected to have a significant effect on S w for
relatively soluble solutes, such as BTEX [i.e., the term (1 + X mnK mn + X mcK mc)
should be close to 1]. By contrast, the applied surfactant at the same X would
significantly affect the solubility of the less-soluble chlorinated solutes; the
estimated (1 + X mn K mn + X mc K mc ) for lindane at X = 200 to 400mg/L is about
1.6 to 3.0, and a higher value is anticipated for HPOX (Lee et al., 2000). To
the solid sorbent, the effect of the sorbed surfactant on solute uptake (i.e.,
f sf K sf /K d ) is expected to be about the same for all solutes on a sorbent because
K sf should be largely linearly related to K om ; the exception will be for certain
solutes that exhibit a special affinity for a specific type of surfactant. However,
as the solid f om increases, the amount of adsorbed surfactant on the mineral
matter may decrease, which could then reduce the impact of the surfactant on
solute uptake.