Page 166 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 157

We consider first the probable source of the sorption nonlinearity for nonpo-
lar solutes, where the nonlinear capacity is relatively small and approaches
apparent saturation at very low C e/S w (0.010 to 0.015). On peat soil, the non-
linear capacities of about 0.18mg/g for EDB and 0.15mg/g for TCE are well
within the allowed monolayer adsorption capacity of the soil based on its BET-
2
N 2 surface area of 1.4m /g. The same is true for EDB and LND on Woodburn
soil, where the nonlinear capacities of EDB and LND are <0.008mg/g and the
2
surface area of the soil is 11.2m /g. Since there is little tendency for nonpolar
solutes to engage in specific interaction with SOM and since the peat soil has
a very low mineral content, these features might be ascribed to strong solute
adsorption at low C e/S w values on a small amount of HSACM (e.g., charcoal-
like materials) in soil (Chiou, 1995), on which water exhibits a weak compet-
itive adsorption (see Chapter 6, section 6.5, on the adsorption of water on
activated carbon). At moderate to high C e/S w values, this adsorption is largely
saturated and the partition in SOM predominates to make the isotherm essen-
tially linear.
The HSACM hypothesis is consistent with the characteristics of solute
adsorption on activated carbon. For adsorbates with a density of about 1g/mL
2
on a typical activated carbon, 1m /g of carbon surface area corresponds to
about 0.25mg/g for the adsorbate monolayer capacity, the saturation capacity
is about twice as high. Adsorption on activated carbon rises sharply at low
C e/S w values (see the related discussion in Chapter 6). At C e/S w = 0.01 to 0.02,
the adsorbed capacity is about 40 to 50% of the saturation capacity (Manes
and Hofer, 1969; Chiou and Manes, 1974). From this point up to C e/S w = 1,
adsorption approaches full saturation more gradually. With a small quantity
of assumed HSACM and a significant amount of SOM in soil, the isotherms
for nonpolar solutes at C e/S w > 0.01 to 0.02 would therefore become relatively
linear as the (linear) partition into SOM outweighs the adsorption on
HSACM. On peat soil, the nonlinear EDB and TCE capacities (0.15 to
0.18mg/g) are consistent with the HSACM hypothesis, if most of the soil
2
surface area (1.4m /g) comes from a small amount of HSACM. For Woodburn
soil, the surface area is much higher and the nonlinear capacity for nonpolar
EDB is much smaller. The large surface area results presumably from mineral
surfaces, on which the strong interaction with water minimizes the solute
adsorption (Chiou and Shoup, 1985; Chiou et al., 1985). The lower nonlinear
capacity of EDB on Woodburn soil may be attributed to a trace amount of
HSACM in the soil. Since adsorption on activated carbon occurs primarily by
London forces as discussed in Chapter 4, and the same is expected for
HSACM, the solute polarity would not be relevant for competitive adsorp-
tion. This is corroborated by the nonspecific suppression of the EDB sorption
by both polar and nonpolar co-solutes on peat.
The nonlinear sorption characteristics of nonpolar solutes on peat are inter-
nally consistent with the N 2 adsorption data on this sample. The N 2 adsorption
exhibits a similar sharp rise and a downward concavity at low relative pres-
sures (P/P°) (£0.02) with a monolayer capacity of 0.36mg/g (Chiou et al., 1993;