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    ity of 1.8g/L.  Most gasoline constituents have solubilities of less than 1 g/L, and some dissolve to
    only a few mg/L.  This is the key characteristic that causes so much of the mass of an MTBE release
    to dissolve in groundwater.  The higher solubility is associated with the greater polarity of
    the compound.
    The Henry’s Law constant is an indicator of the equilibrium distribution of a compound be-
    tween water and air.  When compared to benzene, MTBE tends to partition strongly into the water
    phase.  If the same units are selected for the air and water concentrations, then Henry’s Law
    constant is dimensionless. The value of the dimensionless Henry’s Law constant for MTBE is
    0.0216 at 25o
    C (Robbins et al., 1993).  In contrast, the dimensionless Henry’s Law constant for
    benzene is 0.22 at 25o
    C, which indicates that it transfers easily from water and can be removed by
    aeration.  Compared to benzene, MTBE tends to stay in the water phase, which explains why
    MTBE is somewhat difficult to remove from water by aeration.
    The organic carbon water partition coefficient (Koc
    ) is a reflection of the compound’s
    tendency to sorb to the organic carbon matrix within soil systems. The organic carbon sorption will
    retard the migration of the compound.  With a value of Koc
     equal to 11 and an octanol water parti-
    tion coefficient (Kow) of 17.4 (Zhang et al., 1998a; Squillace et al., 1997), MTBE is less retarded
    than other gasoline constituents.
    Compared to other gasoline constituents, the physicochemical properties of MTBE and
    many other oxygenates present significant issues when considering treatment options and the fate
    and transport of these pollutants in the environment.  Given the high water solubility, MTBE is quite
    mobile in the environment.  It partitions weakly to the organic fraction in soils, sediments, and
    suspended particles, preferentially remaining in the aqueous phase.  It is expected to move essen-
    tially at the same rate as groundwater flow, with practically no retardation due to sorption.
    Ethers are a class of compounds that are characteristically unreactive over a wide range of
    industrial and laboratory conditions, so it is unlikely that MTBE will be degraded rapidly in theaquatic environment (Church et al., 1997).  MTBE is a persistent molecule in the environment for
    several reasons: (1) the ether bond is stable and requires acidic conditions to cleave it; (2) the bulky
    tert-butyl group does not allow easy access to the ether linkage; (3) MTBE is not a naturally
    occurring hydrocarbon unlike most oil and gasoline constituents; and (4) it has only been in the
    environment for a relatively short time, so there has been little selection for indigenous microbes to
    transform MTBE.  The biodegradability of MTBE is generally presumed to be significantly less than
    the degradability of other gasoline constituents.  Initial studies indicate that biodegradation of MTBE
    in the environment is slow (Borden et al., 1997; Mormile et al., 1994; Suflita and Mormile, 1993).
    A field study by LeBrun (1993) utilized a contaminant mass flux approach to estimate rates of
    intrinsic bioremediation for MTBE and BTEX at a gasoline-contaminated shallow aquifer in
    Sampson County, N.C.  His results suggested that MTBE degradation was occurring near the
    source area at a rate of 0.13%/day.  Since the contaminant plumes appear to have been degrading
    under a mixture of aerobic and denitrifying conditions, it is unclear which mechanism would have
    contributed to MTBE decay.
    However, a substantial record of MTBE biodegradation in both laboratory and full-scale
    treatment operations has now been accumulated.  At least a few bacterial species are able to use
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