Page 41 - Soil and water contamination, 2nd edition

28 Soil and Water Contamination

For the water phase (dissolved phase) ) use Equation (2.27):

1 1 -1 -3
Z 180 mol atm m
water 3
H . 5 55 10

For the sediment phase (adsorbed phase) , use Equation (2.28):
K d 1100 6 . 0 10 3 -1 -3
s
Z 119 mol atm m
sed 3
H . 5 55 10
Third, calculate the fugacity for the whole system using equation (2.25):
112 7
f . 2 68 10 atm
4
7
40 9 . 10 180 5 10 119 400
Finally, from Equation (2.29) the amount of benzene in the three different phases can be
calculated:

M . 2 68 10 7 10 7 40 9 . 110 mol
air
M . 2 68 10 7 5 10 4 180 4 . 2 mol
water
M sed . 2 68 10 7 400 119 . 0 013 mol

Thus, most of the benzene is present in the air. Nevertheless, the benzene concentrations
in the different phases are proportional to the fugacity capacity and, therefore, the highest
-1
benzene concentration is found in the water (0.048 μmol l ).

2.6 CHEMICAL EQUILIBRIUM AND KINETICS

2.6.1 Equilibrium

Chemical equilibrium in a closed system describes the state of maximum thermodynamic
stability, which means that there is no chemical energy available to alter the relative
distribution of mass between the reactants and products in a reaction, and the final expected
chemical composition has been reached. Consider a reversible reaction where components A
and B (reactants) react to produce components C and D (products):

aA bB cC dD (2.30)

where the uppercase letters represent the chemical compounds and the lowercase letters
the stoichiometric coefficients. Similarly to Equation (2.17), the change in free energy per
additional mole reacting as a system proceeds towards equilibrium can be expressed as:
G G 0 RT ln Q (2.31)

2 -2
0
where ΔG = the standard free energy change at 25 °C and 1 atm pressure [M L T ], which
is constant for a given reaction, and Q = the reaction quotient [-]The change of the standard
free energy for a given reaction can be calculated from the free energies of formation (i.e. the
energy needed to produce one mole of a substance from the elements in their most stable
form) of the substances participating in the reaction:

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