Biomedical Engineering Reference
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ethanol in the gasoline and on mass transfer limitations in subsurface
systems. Heermann showed that when the concentration of ethanol was
10% (by volume) in a gasoline, > 99% of the ethanol partitioned into the
water phase. Heermann did not quantify the time required for dissolu-
tion but determined that it is limited by the rate of ethanol liquid
molecular diffusion to the gasoline/groundwater boundary, which may
result in longer dissolution times ( > 1 year).
In conclusion, pure methanol would be expected to quickly dissolve
into groundwater and disperse at a rate directly proportional to the local
velocity of the groundwater. All of the methanol present in an M85
gasoline would likely be released into the groundwater, but the time for
complete dissolution would be site-specific. It is unlikely that either M85
or M100 would exhibit a slow dissolution time ( > 1year)becauseofthe
large quantities of methanol in contact with the water phase, methanol's
infinite solubility inwater, andmethanol's affinity for thewater relative to
soil, thus reducing the effects of diffusion-controlled dissolution. How-
ever, the time required for dissolution is highly dependent on the source
area shape and the types of soils in the subsurface. Therefore, if methanol
enters a low-permeability soil horizonwithminimal groundwater contact,
particularly one with relatively high organic carbon content, dissolution
could proceed slowly.
2.2.5 Commingling/Cosolvency Effects
Cosolvency is a chemical phenomenon whereby one chemical dis-
solved in water increases the aqueous solubility of a second chemical.
For example, BTEX compounds tend to be more soluble in concen-
trated aqueous methanol solutions than in water alone. This preferential
solubility is referred to as the cosolubility effect. The possibility of
enhanced aqueous solubility of BTEX due to a neat or pure methanol
plume that encounters existing subsurface BTEX contamination is an
issue for methanol releases from underground storage tanks at existing
gasoline stations or otherwise comingled with gasoline/BTEX plumes
(Scenario 3) (Donbaldson et al., 1993).
The majority of studies on the effects of methanol cosolvency have
focused on increased BTEX solubility within methanol/gasoline
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