Biomedical Engineering Reference
In-Depth Information
aerobic microbial processes. Even in oxygen-limited environments such
as the bottom layers of stratified lakes or reservoirs, anaerobic bio-
degradation is expected to proceed at rapid rates; the reported half-life
for methanol biodegradation under anaerobic conditions ranges from 1
to 5 days (Howard et al., 1991). In addition, the nutrient supply in rivers
and lakes is generally not expected to restrict the rate of methanol
metabolism because the required nutrient supplies are often constantly
recharged by runoff (Alexander, 1994). However, high concentrations
of methanol resulting from a large spill in an enclosed area could
deplete the surface water of oxygen required to sustain aquatic life.
Abiotic Degradation Abiotic degradation (i.e., nonbiological or chem-
ical) reactions are not likely to contribute significantly to methanol
removal from surface water bodies. Hydrolysis reactions usually trans-
form compounds into more polar products; methanol is a very polar
molecule and is stable in water. Photolysis involves the transformation
of a compound as a consequence of its direct absorption of light.
Methanol does not absorb light in the visible spectrum and long-
wavelength ultraviolet light (UV). It does absorb very short-wavelength
UV, which is present in negligible amounts in solar radiation. In
addition, methanol can be naturally oxidized by hydroxyl radicals
formed in water by the photolysis of nitrate, nitrite, and hydrogen
peroxide resulting from reactions with excited humic materials or from
the reaction of H
2
O
2
with Fe(II) (Schwarzenbach et al., 1993). Thus,
naturally occurring photooxidation of methanol via direct or indirect
oxidation is possible; however, these reactions are slow and, thus, not
expected to be significant in surface waters.
Bioaccumulation The bioaccumulation of fuel hydrocarbons and cer-
tain additives has been studied extensively (e.g., Brault et al., 1994). As
shown by various studies, uptake and elimination of organic pollutants
in freshwater fish and invertebrates may occur through gill membranes,
the efficiency of which depends on blood proteins and the hydropho-
bicity of the compounds. Bioaccumulation in aquatic organisms is
expressed by bioconcentration factors (BCF) (Streit, 1994). To the
authors' knowledge, no studies have been reported for methanol
