Geoscience Reference
In-Depth Information
7.4 Volatilization of Multicomponent Contaminants
In general, contaminants reach the subsurface not as a single component but as a
mixture of various components. Each of the chemicals in the mixture has its own
physicochemical characteristics and therefore each behaves differently in its par-
titioning into the subsurface gas phase. Petroleum products, for example, contain
hydrocarbons whose vapor pressures range over several orders of magnitude. The
composition of a volatile organic mixture in the subsurface changes with time, due
to differential volatilization of the components into the gas phase. Also, different
rates of volatilization create concentration gradients among compounds in the
liquid, which leads to mass diffusion. This mechanism may affect the physical
properties of the liquid and their redistribution in the subsurface.
Woodrow et al. (
1986
) assumed that the mixture of components (W
L
) is ideal,
so that the volatilization of each component (W
i
) proceeds by a first-order process
that is described by
W
i
¼ W
L
exp
k
i
t
ð
7
:
8
Þ
where k
i
is the volatilization rate for component i, and W
0
denotes initial state. In
such mixtures, the more volatile components volatilize faster, causing a decrease
in the volatilization rate for the mixture as its ''total'' vapor pressure decreases.
It is known that, in a water phase, immiscible liquids such as gasoline or other
petroleum products may form multicomponent droplets of various forms and sizes,
under dispersive conditions. These droplets are transported by convection and
diffusion, which contributes to the contamination of fresh water systems. However,
during droplet transport, more volatile substances partition to the gas phase at the
droplet surface, leaving less volatile material that volatilizes more slowly. More
volatile material still exists in the droplet interiors, and it tends to diffuse toward
the surface because of concentration gradients created by prior volatilization.
Different components in a droplet have different volatilization rates, which may
vary significantly during droplet transport, and as a result, the contamination of
fresh water is affected accordingly.
Nye et al. (
1994
) studied the volatilization of single- and multicomponent
liquids through soil columns, emphasizing differences between the two cases. For
a single liquid, three stages were identified: (1) soil sorbs the gas as it diffuses
upward through the column; (2) steady-state conditions are reached in which the
vapor escapes to the atmosphere at a constant rate, after diffusing upward through
the column under a constant concentration gradient; and (3) the soil reaches a stage
of depletion when the gas desorbs from the soil and diffuses to the atmosphere.
When a mixture of components characterized by various vapor pressures volatil-
izes, similar stages are observed, but important variations may exist. In the
accumulation stage, each component from the mixture is sorbed according to its
volatility, diffusivity, and the competitive adsorption isotherms. As a consequence,
no true steady state is reached because the composition of the source liquid
