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<< Click to Display Table of Contents >> Sorption |
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Sorption is the process whereby contaminants are removed from solution by interaction with solid matter in the soil. Typical interactions are cation exchange in clays and the attraction of organic contaminants to organic matter in the soil. Sorption can be modelled as a function of the concentration of the contaminants in the soil. POLLUTEv8 can model three types of sorption; linear sorption, Freundlich non-linear sorption,and Langmuir non-linear sorption.
Freundlich Non-Linear Sorption
Freundlich Non-Linear Sorption is represented by the relationship:
S = Kf cE
where,
S = mass of solute sorbed per unit mass of soil,
Kf = empirically determined parameter,
E = empirically determined exponent.
The parameters Kf and E are best determined by performing batch tests on samples. When non-linear sorption is used, the program splits the deposit into sublayers and uses an iterative technique to determine an equivalent linear distribution coefficient (K) value of each layer. Since this is an empirical equation, particular care is required ensuring correct units, especially when E is not unity. Large errors can result from the use of mixed units or errors in converting from one set of units to another. This is done by:
(a) calculating the concentration at the top and bottom of each sublayer based on an estimated linear K value for each sublayer.
(b) determining a new secant K for each sublayer.
K = Kf c' E-1
where,
c’ = the average of the previous estimates of the concentration at the top and bottom of the sublayer.
(c) repeating steps (a) and (b) using the new estimate of K for each sublayer until the process converges.
The number of sublayers should be experimented with to ensure that the results obtained are sufficiently accurate.
Langmuir Non-Linear Sorption
Langmuir Non-Linear Sorption is represented by the relationship:
S = (Sm b c) / (1 + b c)
where,
Sm = solid phase concentration corresponding to all available sorption sites being occupied,
b = parameter representing the rate of sorption,
S = mass of solute sorbed per unit mass of soil,
c = concentration of solute.
The parameters Sm and b are best determined by performing batch tests on samples of the deposit.
When non-linear sorption is used, the program splits the deposit into sublayers and uses an iterative technique to determine an equivalent linear distribution coefficient (K) value of each layer. This is done by:
(a) calculating the concentration at the top and bottom of each sublayer based on an estimated linear K value for each sublayer.
(b) determining a new secant K for each sublayer:
K = (Sm b) / (1 + b c’)
where,
c’ = the average of the previous estimates of the concentration at the top and bottom of the sublayer.
(c) repeating steps (a) and (b) using the new estimate of K for each sublayer until the process converges.
The number of sublayers should be experimented with to ensure that the results obtained are sufficiently accurate.