Geoscience Reference
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finding suggests that the heat involved in the adsorption process mainly affects
solute solubility. Similar results emphasizing the significant influence of temper-
ature on adsorption through its solubility effect were reported by Yamane and
Green (
1972
) for atrazine and by Yaron and Saltzman (
1978
) for parathion.
Strong, sometimes irreversible retention of organic contaminants on hydrated
humic substances can be explained by penetration and trapping in the internal
structure. Burchill et al. (
1981
) showed that hydrated exchangeable cations and
some dissociated functional groups, as well as water held by various polar groups
of the humic substances, provide adsorption sites for organic contaminants. At low
moisture content, the hydrophobic portions of the organic matter structure may
bind hydrophobic nonionic organic contaminants. Pignatello (
2012
) showed that in
some soils, it is possible that there are (qualitatively) many different kinds of
natural organic matter (NOM) microdomains, which form microstructures that
play an important role in selectivity, kinetics and reversibility of organic con-
taminant sorption. The physical microstructure of the NOM responds to changes in
environmental conditions, such as concentration of sorbing chemicals, tempera-
ture, and moisture content, thus developing a dynamic system that evolves with
time. Pignatello (
2012
) considers that NOM apparently has a ''memory'' of its
exposure history to environmental stresses, which ''challenges the historical par-
adigm of NOM as a passive sorbent with immutable structure.''
5.7 Nonadsorptive Retention of Contaminants
Nonadsorptive (physical) retention of chemicals in the subsurface has received less
attention, despite the fact that significant quantities of contaminants can be
retained by processes other than purely adsorptive ones.
5.7.1 Contaminant Precipitation
Contaminant precipitation involves accumulation of a substance to form a new
bulk solid phase. Sposito (
1984
) noted that both adsorption and precipitation imply
a loss of material from the aqueous phase, but adsorption is inherently two-
dimensional (occurring on the solid-phase surface), while precipitation is inher-
ently three-dimensional (occurring within pores and along solid-phase bound-
aries). The chemical bonds that develop due to formation of the solid phase in both
cases can be very similar. Moreover, mixtures of precipitates can result in heter-
ogeneous solids with one component restricted to a thin outer layer, because of
poor diffusion. Precipitate formation takes place when solubility limits are reached
and occurs on a microscale between and within aggregates that constitute the
subsurface
solid
phase.
In
the
presence
of
lamellar
charged
particles
with
