Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 144

primarily on their solubility, but also on their capacity to adsorb to soil organic matter and mineral
surfaces. In mixtures, competitive sorption occurs, in which compounds with greater afi nities for
soil organic matter are preferentially adsorbed and those compounds with lower afi nities stay in
solution. Competitive sorption occurs between organic compounds in anthropogenic wastes and
natural dissolved organic substances such as humic and fulvic acids. Competitive sorption in soil
can cause compounds initially adsorbed to organic matter surfaces to become dislodged and replaced
by compounds with stronger afi nities for sorption (Snoeyink, 1999). Consequently, compounds that
migrate more rapidly because of higher solubility and weak capacity to adsorb to soil surfaces may
be displaced as compounds with higher K oc values arrive at the soil surface.
The potential adsorption of a compound determined from a pure-component test does not
necessarily predict its degree of removal from a dynamic, multicomponent mixture (Verschueren,
1996). The presence of other organic compounds in solution, whether natural or anthropogenic,
usually reduces the adsorption capacity of activated carbon or soil organic matter for a particular
organic compound.
In a study of competitive sorption of aqueous-phase solvent-stabilizer compounds onto granular
activated carbon, nitromethane was found to have an adsorption capacity similar to methyl ethyl
ketone (2-butanone), both of which have substantially larger capacities than n- butanol, which in turn
has a larger sorption capacity than 1,4-dioxane (McGuire and Suffett, 1978). The adsorption iso-
therms for the four-compound mixture were signii cantly lower than those for each individual com-
pound in solution. As the equilibrium concentration was reduced, the mixture isotherms for
n- butanol and 1,4-dioxane became identical to the single-compound isotherms. 1,4-Dioxane was
progressively desorbed by the three other compounds in the mixture. Nitromethane, n- butanol, and
methyl ethyl ketone continued to adsorb to the granular activated carbon after 1,4-dioxane had
completely broken through, indicating that they are more successful competitors for adsorption sites
than 1,4-dioxane. Initial adsorption followed by displacement by compounds with stronger afi nity
to adsorb to carbon is referred to as the “chromatographic effect” (McGuire and Suffett, 1978).
Figure 3.3 shows the effect of competitive sorption on the retention on granular activated carbon of
1,4-dioxane and its displacement by other solvents.

12

10

8

6

4

Nitromethane
n

2

-Butanol
Methyl ethyl ketone
1,4-dioxane

0

0 2 4 6 8 10 12 14 16 18 20

Time (h)

FIGURE 3.3 Breakthrough curves for a mixture of nitromethane, methyl ethyl ketone, n -butanol, and 1,4-
dioxane on granular activated carbon (GAC). Inl uent concentrations were 10 −4 mol/L each; l ow rate = 23 mL/
min, pH = 8.0, detention time = 2.1 min. (From McGuire, M.J., and Suffett, I.H., 1978, Journal of the American
Water Works Association 70(11): 621-635. With permission.)