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by Alfred R.
Conklin, Jr.
Any oxygen
containing organic molecule added to gasoline could be
considered an oxygenate. There are many oxygen containing
organic compounds, alcohols, ethers, aldehydes, ketones,
acids, esters, anhydrides, amides and more but few are
suitable as fuel additives. Two common oxygenates are ethanol
and METB (methyl- tert-butyl ether - see figure below). The
interaction of these compounds with the environment in general
and with water and soil in particular, depends on their
physical and chemical characteristics. The oxygen portion of
the molecule is particularly important in determining the
compounds’ characteristics and its interaction with water and
with the inorganic, organic and biological components of soil.
In discussing
with my students the concern over finding METB in water a
question arose. Ethers are insoluble in water so how can METB
be contaminating water? Students use ether (diethylether) to
extract organic compounds from water and they know that it is
not noticeably soluble during this process. Surely METB which
is larger than diethylether should be even less soluble in
water or at least not soluble enough to be of concern.
The answer is
in chemical instrumentation and the ever increasing demand for
greater sensitivity. In a macroscopic sense, ethers are
insoluble in water. However, in the world of instrumentation
which can measure ppm, ppb and ppt (parts per million, parts
per billion, parts per trillion respectively) and in some
cases even individual atoms and molecules, finding traces of
mutually insoluble compounds dissolved in each other is
common.
Once the
compound is discovered in the environment, its effect on human
health becomes a concern. This is especially true for
compounds found in water because it is essential for life.
Now that METB
has been found in water several questions arise. How does it
get to and into water? Over the long term, how does it
interact with and how long will it persist in the environment?
There are
different directions to take in looking at these questions.
These involve investigating the physical, chemical and
biological interaction of ethers (METB) with air, water and
soil. There is also the question of the interaction of these
compounds with living organisms. For our purposes here we will
let others concern themselves about interactions with air and
concentrate on interactions with water and soil. We know a
great deal about the physical and chemical characteristics of
ethers so it is straight forward to propose mechanisms of its
interaction with components of the environment. The question
we will concentrate on is how METB moves through soil and into
groundwater.
Ethers can
interact with various soil components by a variety of
mechanisms. First, we might expect it to be adsorbed onto
various inorganic surfaces particularly those of clays.
Secondly, it may be adsorbed onto organic matter surfaces.
However, in terms of organic matter the interaction can often
be more accurately described as an absorption into organic
matter by a slovation process where the ether dissolves in the
organic matter. The data that leads us to this conclusion is
that organic matter sorption does not follow typical surface
adsorption mechanisms. That is, it cannot be described by
equations that generally describe surface adsorption.
Alcohols can
interact with each other and their surroundings through
relatively strong hydrogen bonding. This is the attraction
between slightly positive protons attached to oxygen with
slightly negative oxygens and their lone pairs of electrons in
water and alcohols (see diagram above). While ethers also
have lone pairs of electrons that can interact with positive
species they do not form hydrogen bonds. Interactions between
electron pairs on the oxygen of ethers and positive species is
not particularly strong. There is little attraction between
ether molecules resulting in their having low boiling points.
Thus, most interactions of ethers are weak.
Soils have
cation exchange capacity. This means that soil particles have
a net negative charge which attracts cations. These same
negative charges would be expected to repel the lone pairs of
electrons on the oxygen of ethers. Thus, we would expect that
ethers would not be attracted to the inorganic components of
soil. Or if they are attracted the attraction would be very
weak. The chief interaction then would be expected to be
between soil organic matter and ether. The more organic matter
a soil has the more effective it should be in adsorbing
ethers. Soils in low elevations and high moisture conditions
tend to be higher in organic matter and thus would be an
important sink for any ether moving into them.
The
carbon-oxygen-carbon bond in ethers is strong and is broken
only under harsh chemical conditions. These involve strong
acid, usually sulfuric, and high temperatures. Because these
conditions are not common in nature ethers are not subject to
chemical breakdown in the environment. If unavailable for
biological breakdown they persist for a long time.
Because
ethers are chemically stable, microorganisms are important in
removing or decomposing them. In soil, the intimate mixture of
solid, liquid and gaseous phases provides an excellent medium
for the decomposition of organic matter. Generally speaking,
small simple organic molecules, such as ethanol and METB, are
readily available carbon sources for soil microorganism. In an
aerobic soil, ethers would be readily oxidized to carbon
dioxide and water. In anaerobic soils, the decomposition might
be a little slower but should readily take place and ethers
eventually converted to methane. We could expect
bioremediation of ethers to be slower than physical adsorption
or absorption but it would be an effective decomposition
process as long as the ether containing water move through
soil.
The main way
in which water becomes contaminated with METB is through
direct exposure to fuel containing METB or with METB itself.
It is possible that large pores and cracks in soil or in
decomposed rock or clay under soil would allow rapid movement
of METB containing fuel or water into subsoil water and
subsequently into streams, lakes and general water supplies.
Controlling water flow so that it moves slowly through soil,
typical sewage disposal plants or wet lands should be
sufficient to remove METB from water.
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