Page 253 - Soil and water contamination, 2nd edition

240 Soil and Water Contamination

t=0 Centre of mass at times t , t , and t
x=0 1 2 3
t t t
1 2 3

L
1

L 2 x - direction

L
3

max
C at t 1

Concentration C at t 2 C at t 3
max

max

6642 6642 6642
0 L L L
1 2 3
Distance (x)
Figure 13.1 Downstream propagation of a pulse injection into a river due to advection and dispersion. In addition,
the injected chemical is subject to first-order removal . Note that the chemographs become broader while travelling
downstream (dispersion) and their size decreases.

13.2 SORPTION EQUILIBRIUM AND KINETICS

Adsorption and desorption reactions between solute and the surfaces of solids play a very
important role in the retention or even immobilisation of chemicals by solids. The sorption
mechanisms of dissolved constituents onto solid particles include cation exchange at
negatively charged surfaces of clay minerals , and hydrophobic sorption of organic compounds
to organic coatings or organic matter . These mechanisms are controlled by the physico-
chemical properties of both the solute and the sorbent; many solids can preferentially adsorb
some types of dissolved constituents.
In Section 2.5.4 we saw that when water containing a dissolved chemical is mixed with a
solid medium, the total mass of the chemical partitions between the solution and the solid.
The following equation represents a mass balance of this process in a given volume of water:
M V
C C C s C ( C C ) (13.3)
tot w s s tot w
V M
s
-3
where C = the total concentration of the chemical in the water [M L ], C = the
tot w
-3
concentration of the chemical in solution [M L ], C = the concentration of the chemical
s

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