TRANSPORT WITH PRECIPITATION
AND DISSOLUTION
CONTAMINANTS MAY BE REMOVED BY:
1) Direct precipitation
2) Co-precipitation – isomorphous substitution with
an ion of similar size in a crystal that is forming
or that has formed.
Of particular importance in the removal of
contaminants from solution in ground water is the
formation of:
1)
2)
3)
4)
5)
metal carbonates (e.g., SrCO3, CdCO3)
phosphates (FePO4 • 2H2O)
sulfides (ZnS, PbS)
hydroxides (Fe(OH)3)
oxides (MnO2)
For these reactions, it is necessary to calculate
saturation indices for the species of interest to
determine whether dissolution or precipitation is likely
to occur.
TRANSPORT WITH REDOX PROCESSES
Oxidation and reduction (redox) processes are
important in governing the geochemistry of elements
which may gain or lose electrons in ground water.
(e.g., Fe, Mn, Cr, N, O, S, As, and U)
Oxidation is the removal of electrons from an atom or
group of atoms.
Fe2+→ Fe3+ + eReduction is the addition of electrons to an atom or
group of atoms
Fe3+ + e- → Fe2+
Redox potential (Eh) is used to describe the redox state
of ground water. Eh is defined as the energy gained in
the transfer of 1mol of electrons from an oxidant to
H2.
The h in Eh indictates that the potential is on the
hydrogen scale and E symbolizes the electromotive
force.
Redox potential also is measured by (pE).
pE = -log [e]
Positive values of Eh and pE indicate an oxidizing
environment (i.e., and environment relatively rich in
oxidants – O2, NO3-, SO42-)
Negative values of Eh and pE indicate a reducing
environment (i.e., relatively low concentrations of
oxidants and relatively high concentrations of reduced
species – H2S, CH4)
It is very important to understand the redox
environment in order to predict 1) the mobility of
elements which have variable valences or charges
and 2) the solubility of transition metal oxides which
may function as adsorbents in aquifer systems.
EXAMPLES
Example 1
Rain water saturated with dissolved oxygen
infiltrates through the soil zone during which time
HCO3-, SO42-, and NO3- become dissolved in the
water which eventually recharges a confined aquifer
containing MnO2, Fe(OH)3 and excess organic
material.
Example 2
Ground water contaminated with organic-rich
leachate from a landfill rapidly uses up the available
oxygen in the ground water.
In these cases, the following sequence of redox
processes should occur:
1) O2 will be reduced and will disappear from the
ground water.
2) denitrification of the ground water
(NO3- → N2O → N2) (reduction of NO3-)
3) appearance in the ground water of Mn2+ and
Fe2+
4) reduction of sulfate to sulfide, carbon dioxide
to methane and nitrogen to ammonia.
Example 3
The purification of leachate by oxygenated ground
water, the oxidation of spring water, and the
oxidation of sulfide minerals in a confined aquifer
being recharged by oxygenated water.
In these cases, the following sequence of redox
processes should occur:
1) oxidation of the organics
2) oxidation of sulfide to sulfate
3) oxidation of ferrous iron and the precipitation
of ferric oxide
4) oxidation of the ammonium ion to nitrate
5) oxidation of dissolved manganese and the
precipitation of MnO2 or some similar hydrous
oxide.
TRANSPORT WITH COMPLEXATION
Complex ion formation is important because the
concentration of potential contaminants and the
mobility of the contaminants are controlled to varying
degrees by the concentration and nature of the
complexes they form.
For example, contaminants that are complexed by
inorganic or organic ligands may not be immediately
available for adsorption or precipitation.
The atom or group that donates a pair of electrons is
called a ligand.
As a consequence, the mobility and concentration of
contaminants may be increased in the aquifer.
Just the opposite may occur where the mobility of
some contaminants would be reduced if the
contaminants become associated with adsorbing
complexing ligands (e.g., humic compounds).
Adsorbing and nonadsorbing ligands may compete for
contaminant ions and thus control the distribution of
complexed contaminants between adsorbed and
solution states.
ACID-BASE REACTIONS
Knowledge of the pH of ground water and its
buffering agents is important because the solubility of
many minerals, and therefore potential contaminant
sources and sinks, are dependent on the pH.
In addition, the surface charge of many adsorbents is
determined by the adsorption of H+ and OH- from
solution (e.g., increasing the pH from 2 to 4.5 will
increase the Kd for Ra-226 from 1 to 12.
MICROBIAL REACTIONS
BENEFICIAL EFFECTS INCLUDE:
1) The purification of contaminated water by which
organic contaminants are broken down to
relatively innocuous products such as CO2, H2O,
NO3-, and SO42-, in aerobic environments.
2) The cycling of N, S, C and P, which are essential
cell synthesis, and which therefore may be
removed from the ground water.
DETRIMENTAL EFFECTS INCLUDE:
1) Bound oxygen in the form of NO3- and SO42- is
used as an electron sink (oxidizing agent) and
reduced species such as CH4, H2, NH3, and H2S
are formed.
2) In anaerobic environments, various heavy metals
may become more soluble (e.g., Fe2+, Mn2+).
Bacteria tend to catalyze reactions so that they
proceed more quickly than normal.