Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 153

Great Lakes and New Bedford Harbor contaminated with PCBs, Indiana Harbor
contaminated with PAHs, and Hudson River sediment contaminated with metals and
organic compounds. Developing countries also suffer extensive contamination of
major rivers and tributaries. There are many areas in the United States and elsewhere,
where extensive sediment contamination has led to an outright ban on fishing and
recreation. Some well-known examples of sediment contamination are the oil spill in
Prince William Sound near Alaska from the Exxon-Valdez oil tanker disaster and the
Kuwait oil fires after the First Persian Gulf War.
Sediments generally accumulate contaminants over a period of time. Over time
as inputs cease, the previously contaminated sediments become sources of pollu-
tants to the water column. Contaminants enter the marine food chain and further
bioaccumulate in birds and mammals. The soil-water environment is another area
of significance. In the United States and other parts of the world, groundwater is
a major source of clean water. Improperly buried waste (e.g., old landfills, leaking
undergroundpetroleumstoragetanks)releasescontaminantsthatleakintotheground-
water. Inadvertent solvent spills and intentional dumping also create major threats to
the limited clean water supplies in the world. In the groundwater environment, the
equilibrium between the soil and porewater is the issue of concern.
4.3.1 P ARTITIONING INTO S OILS AND S EDIMENTS FROM W ATER
The soil and sediment environments can be characterized by a solid phase composed
mainly of mineral matter (kaolinite, illite, and montmorillonite), organic macro-
molecules (humic and fulvic acids of both plant and animal origin), and porewater.
The mineral matter is composed of oxides of Al, Si, and Fe. Typical Louisiana soil
and sediment have compositions given in Table 4.10. The sand, silt, and clay frac-
tions predominate whereas the total organic carbon fraction is only a small fraction
of the total.
A soil or sediment has micro-, meso-, and macro-pores that contain porewater.
Hydrogen and hydroxyl ions in porewater control the pH. Different metal ions con-
trol the ionic strength of the porewater. Both pH and ionic strength are important in
controlling the dissolution and precipitation reactions at the mineral-water inter-
faces. The most important process as far as transport of organic and metal ions
in the sedimentary and subsurface soil environment is concerned is sorption .By
sorption we mean both adsorption on the surface and absorption (physical encapsu-
lation) within the solids. Metal ions and organic molecules in porewater will establish
sorption equilibria with the solid phase. The mineral matter in the soil develops
coatings of organic matter, thereby providing both an inorganic mineral surface and
an organic coated surface for sorption. Figure 4.8 is a schematic of the different
sorption sites.
The mineral surfaces (oxides of Al, Si, and Fe) are electrically charged and
have electrical double layers arising from the ions present in porewater. Water
strongly interacts with the charged surfaces and adopts a three-dimensional hydrogen-
bonded network near the surface in which some water molecules are in direct
contact with the metal oxide and others attached via H-bonds to other molecules