Thermodynamics and log–contrast analysis in fluid geochemistry
Full Text
Share
There are two principal chemical concepts that are important for studying the natural
environment. The first one is thermodynamics, which describes whether a system is at
equilibrium or can spontaneously change by chemical reactions. The second main concept
is how fast chemical reactions (kinetics or rate of chemical change) take place whenever
they start. In this work we examine a natural system in which both thermodynamics and
kinetic factors are important in determining the abundance of NH+4 , NO−2 and NO−3 in
superficial waters. Samples were collected in the Arno Basin (Tuscany, Italy), a system in
which natural and antrophic effects both contribute to highly modify the chemical composition
of water. Thermodynamical modelling based on the reduction-oxidation reactions
involving the passage NH+4 -> NO−2 -> NO−3 in equilibrium conditions has allowed to
determine the Eh redox potential values able to characterise the state of each sample and,
consequently, of the fluid environment from which it was drawn. Just as pH expresses
the concentration of H+ in solution, redox potential is used to express the tendency of an
environment to receive or supply electrons. In this context, oxic environments, as those
of river systems, are said to have a high redox potential because O2 is available as an
electron acceptor.
Principles of thermodynamics and chemical kinetics allow to obtain a model that often
does not completely describe the reality of natural systems. Chemical reactions may indeed
fail to achieve equilibrium because the products escape from the site of the rection
or because reactions involving the trasformation are very slow, so that non-equilibrium
conditions exist for long periods. Moreover, reaction rates can be sensitive to poorly understood
catalytic effects or to surface effects, while variables as concentration (a large
number of chemical species can coexist and interact concurrently), temperature and pressure
can have large gradients in natural systems. By taking into account this, data of 91
water samples have been modelled by using statistical methodologies for compositional
data. The application of log–contrast analysis has allowed to obtain statistical parameters
to be correlated with the calculated Eh values. In this way, natural conditions in which
chemical equilibrium is hypothesised, as well as underlying fast reactions, are compared
with those described by a stochastic approach
Tots els drets reservats