Soils and Environmental Pollution

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Soils and Environmental Pollution Chapter 18
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Of these 5 fates, inorganics are subject to 4
all but degradation. The notion is that the
inorganic contaminant is actually an element. As
such, it may appear in one of many molecules or
ions but it does not degrade unless it is
radioactive. Another point, too, is that
phytoremediation is a type of bioremediation.
Also, there are cases in which an organic
contaminant is removed by use of plants. So
bioremediation applies also to inorganic
and phytoremediation to organics.
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By 1 kg, what is meant is active ingredient, and
that is a lot. The benefits are obvious,
however, the detriments are probably mostly
unknown.
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Lethal dose 50 kills ½ the population. Lethal
concentration 50 is analogous.
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Comparing the mammalian toxicities of 2,4-D and
atrizine, the latter is much less toxic. However,
it is much more toxic to fish, no?
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?
?
The basic idea is that the more of the substance
(in a volume of soil) that is associated with
the solids, the less there is in solution, so
there is less mobility.
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Its an organic cation so it is adsorbed onto
charged sites on soil particles and strongly so.
Look back at 2,4-D. Its an anion (on
dissociation of the acid), so just the opposite,
right?
Hydrophobic means low water solubility.
Couple this with the concept of like dissolves
like and you get.
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Two differences between the Dundee and
Sharkey soils is that the Sharkey has a higher
content of clay ( more surface area for
adsorption) and more organic matter. Thus,
the compound (a derivative of the herbicide
bentazon) is more highly adsorbed in the
Sharkey. The plot is called an isotherm and
relates the concentration of a chemical associated
with solids to the concentration of the chemical
in solution.
The terms adsorbed and sorbed are not necessarily
synonymous. The former implies chemical
adherence to a surface, while the latter is
broader, also including precipitation. Use of
adsorbed on the y-axis of the graph would be more
apt.
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After 1 half-life, ½ remains, after 2 half-lives,
¼, etc. Strictly speaking, the concept of a
half-life is applicable only to a degradation
process that follows 1st-order kinetics, i.e.,
the rate of degradation is directly proportional
to the mass of the chemical at all times. Often,
degradation does not follow 1st-order kinetics so
use of half-life in these cases is meaningless.
Even so, the word half-life is often used.
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Degradation of compound X can be abiotic, biotic
or both. Usually the latter is more important,
i.e., most of the degradation that occurs is
biologically mediated.
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For most organic chemicals, degrada- tion is
relatively fast. Personally, I dont trust one
of these data sets.
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First, water movement is faster. Second,
where this occurs, the soil is macroporous,
perhaps sandy, and if sandy, there is little
surface area or organic matter for adsorption.
Compare the mobility (mass and velocity)
of compounds A, B, C and D that differ in
the extent to which they are adsorbed and rate at
which they are degraded.
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A contaminant may move in runoff as the dissolved
chemical species or bound to suspended solids
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Adding N, P, K, etc. adds essential elements
needed by the population of microorganisms that
degrade the hydrocarbons. They cant live by C
and H alone (see earlier discussion on C / N
ratio effect).
Better aeration faster metabolism. Ditto for
nutrients.
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Early work of this type led to isolation of
organisms that had a great capacity to degrade a
specific contaminant but that when introduced
into soil at the contaminated site, died
off. Isolation of superior degraders from
the site worked. site worked.
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Some sources of inorganic contaminants.
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A mitigating fact is that inorganics tend to
exist in forms that are the least soluble,
hence, least mobile and plant-available. These
are the general forms, and their relative
occurrence and mobility.
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This is what you want to do and often simple
prescriptions are effective, like
See next slide for common effect of soil pH on
the solubility of many metals. Recall discussion
on metal micronutrient solubility.
Perhaps surprising but true, the chemically
reduced form is more soluble at any pH than the
oxidized form. Thus, wet, reducing conditions in
the soil favor solubility, mobility and uptake.
So, improve aeration at the site. As
above, recall related discussion on redox metal
micronutrients.
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Curves like these, relating extent of adsorption
to pH, are called adsorption edges (the figure
doesnt look very edgy but imagine the pH range
was expanded and the spacing shortened more
edgy). You get the point, though, decreasing
mass in solution with increasing pH.
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OK, first, really, the phytoremediating plant
must survive, i.e., not experience toxicity.
Beyond this minimum, the plant must take up the
contaminant in high amounts (high tissue
concentration) and produce a lot of biomass.
What is removed is tissue concentration x tissue
mass per area, per time. Make sense?