Long-term adsorption of phenanthrene to soils was
characterized in a silt-loam (LHS), a sandy soil (SBS), and
a podzolized soil (CNS) by use of the Polanyi−Manes
model, a Langmuir-type model, and a black carbon−water
distribution coefficient (K
BC) at a relative aqueous
concentration (C
e/S
w) of 0.002−0.32. Aqueous desorption
kinetic tests and temperature-programmed desorption (TPD)
were also used to evaluate phenanthrene diffusivities
and desorption activation energies. Adsorption contribution
in soils was 48−70% after 30 days and 64−95% after
270 days. Significant increases in adsorption capacity with
aging suggest that accessibility of phenanthrene to
fractions of SBS soil matrix was controlled by sorptive
diffusion at narrow meso- and micropore constrictions.
Similar trends were not significant for LHS silt-loam or CNS
podzol. Analysis of TPD profiles reveal desorption
activation energies of 35−53 kJ/mol and diffusivities of
1.6 × 10-7-9.7 × 10-8 cm2/s. TPD tests also indicate that
the fraction of phenanthrene mass not diffusing from
soils was located within micropores and narrow width
mesopores with a corresponding volume of 1.83 × 10-5-6.37
× 10-5 cm3/g. These values were consistent with the
modeled adsorption contributions, thus demonstrating the
need for such complimentary analytical approach in the
risk assessment of organic contaminants.
Sequestration of phenanthrene and pyrene was investigated in two soils--a sandy soil designated SBS and a silt-loam designated LHS--by combining long-term batch sorption studies with thermal desorption and pyrolysis of amended soil samples. The Polanyi-based adsorption volume and the adsorbed solute mass increased with aging for both soils, thus demonstrating the mechanism for observed sequestration. Despite rigorous thermal analysis, 30-62% (SBS sand) and 8-30% (LHS silt-loam) of phenanthrene could not be recovered after 30-270 days of sorption, with the increase in desorption resistance showing greater significance in SBS sand. For both soils, these values were 20-65% of adsorbed phenanthrene mass. Activation energies estimated from the temperature-programmed desorption (TPD) of sorbed phenanthrene at < or = 375 degrees C were 51-53 kJ/mol, consistent with values derived for desorption of organic compounds from humic materials. The activated first-order model fitting of observed TPD data supports the conclusion that the desorption-resistant fraction of phenanthrene has become sequestered onto condensed organic domains and requires temperatures exceeding 600 degrees C to be released. The work demonstrates the use of thermal analysis in complementing the Polanyi-based adsorption modeling approach for assessing the mechanistic basis for sequestration of organic contaminants in soils.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.