Page 169 - Partition & Adsorption of Organic Contaminants in Environmental Systems

160 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

gests that the surface area of the charcoal-like substance in the peat sample is
2
relatively small (estimated to be about 15m /g). Only a trace amount of char-
coal-like content was found in the Woodburn soil, as expected.
Additional support for the HSACM model was given by Xia and Ball
(1999), who applied the Polanyi adsorption potential theory to evaluate the
nonlinear sorption components of nine nonpolar single solutes (benzene, chlo-
rinated benzenes, and PAHs) on an aquitard solid. A plot of the adsorbed
solute volume versus the solute adsorption potential per unit molar volume
(see Chapter 4) for all liquid solutes yielded essentially a single curve and thus
about the same limiting adsorption volume (see the data with activated carbon
in Chapter 6). A similar plot for solid solutes displayed reduced adsorption
capacities and limiting volumes, due supposedly to a less efficient packing of
solid compounds within the porous adsorbent structure. These unique char-
acteristics are typical of the organic-solute adsorption onto activated carbon
(Manes, 1998). In a subsequent binary-solute study, Xia and Ball (2000)
showed that suppression of the nonlinear sorption of a solute by other co-
solutes is similar to that described earlier.
The complete isolation of small amounts of charcoal-like particles
(HSACM) from ordinary soils and sediments is extremely difficult because of
the small HSACM mass and the lack of effective isolation methods. However,
it is possible to remove these particles by a density-fractionated technique,
since the charcoal particle, which resembles activated carbon, should have a
higher density than SOM, the latter estimated to be about 1.3g/mL (Chiou
et al., 1983). On this basis, Chiou et al. (2000) prepared relatively pure (i.e.,
HSACM-free) humic acids (HAs) and a presumably HSACM-enriched humin
(HM) from the (Florida) peat sample used in their sorption studies by a
density-fractionated method. A similar HA was extracted from a Michigan
muck. The raw HA was first extracted by base, followed by centrifugation to
precipitate and remove fine charcoal-like particles. The solution was neutral-
ized with acid to precipitate HA and the suspensions were removed. The HA
fraction was then redissolved with base and centrifuged to remove the solids
precipitated. By repeating this procedure several times, a relatively pure HA
was obtained. The base-unextracted portion of the peat, the HM fraction,
should thus be enriched with charcoal-like particles. The idea here is that if
the HSACM is the source of nonlinear sorption for nonpolar solutes on the
whole peat, the sorption of the same solutes on the purified HA is expected
to be essentially linear. Conversely, the sorption isotherm with HM should
exhibit a greater nonlinear effect than that for the whole peat sample. The
organic carbon contents and the surface areas of the prepared HAs and HM
are shown in Table 7.12.
The sorption isotherms of EDB from water on the two HA sorbents are
depicted in Figure 7.23, and the isotherms for EDB on whole peat and peat-
derived HM are shown in Figure 7.24. The EDB isotherms on both HAs
exhibit virtually no visible nonlinearity from low to high C e /S w ; the high
linearity indicates that solute partition into SOM is the governing sorption