Geoscience Reference
In-Depth Information
Transient
state
Reactants
Products
Reaction path
Figure 5.10
The kinetic isotope effect: a bond involving a light isotope is higher up on the energy scale than
the same bond involving a heavy isotope. Breaking the bond and proceeding along the reaction
path therefore requires less energy for an OH bond than for an OD bond.
reflect that changes in coordination are usually slow processes that rarely reach equilib-
rium and that isotope fractionation away from equilibrium therefore is the rule whenever
organisms such as bacteria are involved in biogeochemical processes.
5.5 Adsorption
Capture of atoms, ions, and molecules by surfaces is known as adsorption. This process
is critical to the understanding of geochemical cycles because it accounts for why the
bulk of many elements carried by rivers do not contribute to the inorganic salts dissolved
in seawater as much as expected: upon mixing of freshwater and seawater in estuaries,
iron hydroxides precipitate and entrain, adsorbed on their surface, most of the transition
elements, the rare-earth elements, and high-field-strength elements dissolved in the run-
off. This is also a process at work in carbonated springs: decompression removes CO
2
and
increases the pH; iron hydroxides precipitate and “clean up” the spring water from many
of its constituents. Likewise, the scavenging of shallow waters by wind-blown particles
carried from the deserts and by organic material depletes the surface ocean in the same
elements.
Adsorption processes are conventionally separated by the energy binding the element to
the surface. Adsorption with strong binding energies between ions and charged substrate
is referred to as chemisorption. Weak binding energies resulting in particular from van der
Waals forces characterize physisorption. Although the physical forces underlying these
two processes are different, they may be simply described by similar phenomenological
laws relating pressure in the gas or concentration in the liquid and the fraction
of active
sites on the solid surface filled by the adsorbed element and known as the coverage of the
surface.
As an illustration, let us suppose that adsorption of species
i
takes place as a single layer
on the surface of a solid phase in a gas at pressure
P
i
and that temperature is constant during
θ
