ENVIRONMENTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 231

adsorbed, and all aliphatic organic matter and inorganic matter are weakly
adsorbed, with the exception of chlorine, brome and iodine. If during convective
transport a part of the matter is being adsorbed, it shows up by the changes in
concentration occurring slower; one speaks of a retardation effect, which can be
incorporated as a time factor.
Chemical equilibrium
Chemical processes represent another significant aspect of changes in
concentration in soils. Such processes are related to the chemical equilibrium of
solid matter, i.e. the dissolution in water, of different fluids and fluid-gas, i.e.
solution of gas in the fluid. Because of suspension and solution, these processes
determine the mobility of specific matter. Many such processes may run parallel
and interact, making the system complex.
Specific chemical reactions mostly depend on the pH of the soil fluid, the CO 2 -
tension in soil and the redox potential. The redox potential reflects that oxidation is
always coupled to a reduction and expresses the potential of electron exchange
during oxidation. Important elements in soil are calcium, iron and aluminium. In
calcium-rich soils (with low ventilation) there is little to no (micro)biological
activity and the pH may reach 10 (maximum). If such a soil is well ventilated
(CO 2 ) the pH drops to 8. If the CO 2 -tension increases the pH becomes even lower,
down to 6.7, due to which more CaCO 3 dissolves. Iron is present in soil as oxide or
hydroxides and as organic and inorganic complexes. Both the ions and the Fe 3+
may occur; their activity is determined by the pH value. Aluminium is present in
Feldspath, and oxides and hydroxides, both crystalline and amorphous. Under low
pH Al and Fe oxides can dissolve, infiltrate to lower regimes and precipitate again.
C BIO - CHEMICAL PROCESSES IN SOIL
Biologically induced chemical processes in rock, including soil, are due to the
activity of flora and fauna within it, much of which is microbial. Mould, yeast, and
most of the microbes, such as bacteria are heterotrophic, i.e. they obtain carbon for
their growth from organic materials. Microorganisms such as algae and other
bacteria obtain carbon from CO 2 or CH 4 , and are called autotrophic. The energy for
heterotrophic life is obtained from oxidation-reduction reactions. Beside aerobic
breakdown with oxygen as electron acceptor, anaerobic breakdown occurs with
many kinds of organic or inorganic matter as electron acceptor (notably sulphates,
nitrates, Fe 3+ ). The redox potential is a measure of the likelihood of such reactions.
Bacteria and notably archea can prosper under large ranges of redox potential,
acidity ( pH ), temperature and pressure.
For about 3 billion years microbial life existed on earth before complex multi-
cellular life forms arose, only about 600 million years ago. About 2.5 billion years
ago oxygen entered the oceans after the rise of cyano-bacteria, which give oxygen
as a waste product; oxigen eventually also entered the atmosphere. Bacteria and
archea have contributed significantly to the present-day atmosphere, hydrosphere
and biosphere. They occur in unsaturated soil, but also in groundwater, and in oil
and gas reservoirs at depths over 5 km, in oceans to over 10 km deep and under
conditions of 100 o C. They catalyse many chemical reactions in the