Page 178 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SORPTION FROM WATER SOLUTION 169

or solubility by a given level of dissolved and/or suspended (colloidal) organic
matter in water derived from soil/sediment or other sources (i.e.,
g w/g w * = C*/C e = S* /S w), as described later. Even when present in trace quanti-
e
w
ties, dissolved or suspended high-molecular-weight humic material is known
to be able to significantly enhance the water solubility of otherwise extremely
insoluble organic compounds. Wershaw et al. (1969) observed that the ap-
parent solubility of DDT in 0.5% soil sodium–humate solution is more than
200 times greater than in pure water (5.5mg/L). Carter and Suffet (1982) found
that the added sediment humic acid in water solution significantly enhances
the concentration of DDT over that in pure water. This solubility enhance-
ment effect is attributed to a partition interaction of solutes with colloidal
organic matter (Gschwend and Wu, 1985) or to a partitionlike interaction with
the microscopic organic environment of the dissolved organic matter (Chiou
et al., 1986). A useful relationship between apparent solute concentration (or
solubility) in water with a given level of dissolved and/or suspended organic
matter (C* ) and solute concentration (or solubility) in pure water (C e) has
e
been established by Chiou et al. (1986), which gives

C* = C e + XK domC e = C e(1 + XK dom) (7.23)
e
or

S* = S w(1 + XK dom) (7.24)
w
where X is the total mass of dissolved and suspended organic matter per unit
weight (or volume) of water (usually in a dimensionless unit), which, for sim-
plicity, is operationally termed the concentration of dissolved organic matter
(DOM); K dom is the enhancement (or partition) coefficient of the solute
between DOM and water (dimensionless), which is a function of the type of
solute and the composition of DOM; S* is the apparent solute solubility in
w
water with X amount of DOM; and S w is the solute water solubility in pure
water at the same temperature. If the term X in Eqs. (7.23) and (7.24) is
expressed alternatively in terms of the dissolved organic carbon (DOC) mass
per unit weight of water, the term K dom in these equations is replaced by K doc.
Relating C* with solute concentration in soil (Q) at equilibrium, one obtains
e
an apparent soil–water distribution coefficient for the solute (K*) as
d
Q K d
*
K d = = (7.25)
C e (1 + XK dom ) 1 + XK dom
where K d = f omK om, as defined before. Normalization of K* to f om leads to
d

*
K om = K om (7.26)
1 + XK dom