78    SORPTION OF 2,4-DICHLOROPHENOL AND
1,1,1-TRICHLOROETH ANE
ONTO THREE SOILS
William M. Ollinger, Assistant Research Professor
Robert C. Ahlert, Professor II
Chemical & Biochemical Engineering
Rutgers University
Piscataway, New Jersey 08855
Introduction
Sorptive interactions play a primary role in determining the transport and ultimate fate of many
substances originating as components of an aqueous solution in unsaturated soil systems. These
interactions may retard the flow of those solutes that have an affinity for the sorbent, relative to the
flow of unretained solvent. The interactions between organic solutes and soil have been described by
both non-specific hydrophobic sorption mechanisms and specific, chemical interactions, such as
hydrogen bonding and ion exchange.
Chiou et al.,1 proposed a partition mechanism for sorptive interactions between hydrophobic
components of an aqueous matrix and a silt loam. Batch experiments were performed to describe the
fluid-solid distribution of a series of haloalkanes. The distribution was observed to be linear at
aqueous phase solute concentrations ranging from a few parts per million to approximately ninety-
five percent of solubility for some solutes. The ratio of mass of solute sorbed within the soil phase to
the equilibrium aqueous phase solute concentration is called the soil-water partition coefficient, K.
The dimensions depend on the reported concentration units, i.e.,
q(C) = K C (1)
where q(C) is the mass of sorbed solute per unit mass of sorbent and C is the aqueous phase solute
concentration.
The partition analogy requires the existence of an interphase, a three dimensional region in which
the molecular interactions differ from those in the bulk aqueous phase. The primary constituent of
this interphase region is proposed to be the carbonaceous material present in many soil types, operationally described as humic and fulvic fractions. The strong positive correlation between the organic
carbon content of soils and sediments and the uptake of hydrophobic non-ionic organic solutes has
been observed by many researchers.
Some researchers have found that sorptive interactions are not adequately described by linear
isotherms. Boyd 2 used the Freundlich isotherm to model the sorptive interactions of structural
isomers of cresol and several substituted phenolics with a clay loam. The Freundlich isotherm is
written as
q(C) = KF C"" (2)
where KF and n are empirical parameters. This model was also chosen by Dao et al.} for the sorption
of aniline and four mono-, di-, tri-and tetrachloro-derivatives. Boyd2 observed a sorption capacity for
the phenolic solutes which was greater than that predicted by an empirical relation based on the
hydrophobic mechanism presented by Means.4 This was attributed to specific chemical interactions
such as hydrogen bonding. Another common non-linear sorption isotherm is the Langmuir form:
q(C) =      Q°bC (3)
(I  +b C)
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