Introduction - Principles of Chemical Separations with Environmental Applications - page 4

which the vapor is soluble. Desorption, or stripping, on the other hand, is the removal
of a volatile gas from a liquid by means of a gas in which the volatile gas is soluble.
Adsorption consists of the removal of a species from a fluid stream by means of a solid
adsorbent with which the species has a higher affinity. Ion exchange is similar to adsorp-
tion, except that the species removed from solution is replaced with a species from the
solid resin matrix so that electroneutrality is maintained. Lastly, membrane separations
are based upon differences in permeability (transport through the membrane) between
components of a feed stream due to size and chemical selectivity for the membrane
material.

1.4 Pollution sources

Sources of pollution vary from small-scale businesses, such as dry cleaners and gas sta-
tions, to very large-scale operations, such as power plants and petrochemical facilities. The
effluent streams of industry are particularly noticeable because of their large volumes [1].
Sources include both point-source and non-point-source pollution. Point-source pollution
can be traced directly to single outlet points, such as a pipe releasing into a waterway.
Non-point-source pollutants, on the other hand, such as agricultural run-off, cannot be
traced to a single definite source. The emissions from both span a wide range of gas,
liquid, and solid compounds.
A large majority of air-polluting emissions come from mobile sources. The automobile
is an obvious example, but other vehicles, such as trucks, trains, and aircraft also con-
tribute. Emissions from mobile sources include CO 2 , volatile organic compounds (VOCs),
NO x , and particulates. The last may also have heavy metals, such as lead or mercury, or
hazardous organics attached. Stationary sources typically burn or produce fossil fuels -
coal, gasolines, and natural gas. This produces gaseous sulfur compounds (H 2 S, SO 2 ,
etc.), nitrogen oxides (NO x ), CO 2 and particulates. Fuel producers and distributors also
typically produce VOCs. Most of these pose human health concerns and many contribute
to the acid-rain problem and global warming effect.
Water pollution also comes from a variety of sources. Agricultural chemicals (fertil-
izers, pesticides, herbicides) find their way into groundwater and surface water due to
water run-off from farming areas. For example, agricultural drainage water with high con-
centrations of selenium threatens the Kesterson National Wildlife Refuge in California.
Chemical discharge from sources ranging from household releases (lawn fertilizers, deter-
gents, motor oil) to industrial releases into surface or groundwater supplies is an obvious
problem. Industrial discharges can occur due to leaking storage facilities as well as process
effluent. Municipal water treatment effluent is another prevalent source. MTBE, a gaso-
line additive used until recently to reduce air pollution, has been identified as a source of
water pollution, demonstrating that the solution to one environmental concern can create
a problem elsewhere. Isolation and recovery of these and other water pollutants pose a
challenge to develop innovative separation techniques.