Transport Process




One of the fundamental processes related with contaminant transport in soil is sorption. Actually, an environment system is highly dependent on sorptive behavior. It obviously affects the pollutant transport, and it can directly or indirectly affect pollutant degradation. The chemical reactivity of a pollutant in sorbed state may be significantly different than in aqueous solution, both in the pathway and the rate.
Natural sorbent may be biotic or abiotic, organic, inorganic or chemical composite. It may range in size from macromolecular to gravel. The pollutants may vary from ions which are completely soluble in water to water insoluble polymers.
Estimating the behavior of a chemical is difficult. We can, however, arrive at relative figures by comparing physicochemical properties.
Let us discuss some of the pertinent sorption parameters. Transport of a contaminant in soil can occur through a number of processes: advection, hydrodynamic dispersion, liquid diffusion, gaseous diffusion, and filtration. (Advection is the transport of dissolved contaminants in water. Sorption of contaminant in soil solids plays an important role in controlling advective transport.)
Equilibrium Sorption - Partition Coefficient
Partition coefficient is the ratio of soluble concentration in the solid phase (soil) to its concentration in the aqueous phase at equilibrium. Graphically:


Thermodynamically, sorption equilibrium can be defined as the state at which pollutant fugacity in the sorbed and solution phase are equal:
¶s = ¶w
It goes without saying that if sorbent is composed of a mixture of subcomponents i :
¶si = ¶w
The fugacity can be related to pollutant concentration in each phase; e.g., for aqueous:
¶w = fwCw
where fw is fugacity coefficient. It is a product of an activity coefficient (g) and corresponding static fugacity (¶€).
fw = g¶šw
For sorbent mixture, the sorbed pollutant concentration can be taken as weight averaged sum of components:
Cs=\|I\|SU(i,,)YiCsi=¶s\|I\|SU(i,,)\|F(Yi,fsi)
where Yi denotes component weight fractions of i components.
The sorbed and solution concentrations are related through the expression:
Cs = \|F(fw,fs)Cw
= \|F(fw,fs) = Ksw = partition coefficient
Soil and Sediment Partition of Uncharged Organics
The distribution of pollutant in soil water systems is dependent on a number of soil parameters.
Soil parameters which affect partition coefficient are organic carbon content, particle size distribution, clay mineral composition (amount and type), pH and cation-exchange capacity.
However, the predominance of organic carbon in "controlling" the sorption of uncharged organic compounds has been extensively documented.
This means that partition coefficient for a soil or sediment consisting of multiple sorbent components:
Ksw = \|F(fw,fs) or Ksw = f\|I\|SU(i,,)\|F(Yi,fsi)
for composite sorbents can be reduced (first approximation) to form containing only sorption to organic matter:
Ksw @ fw\|F(OC,foc)
where organic matter is expressed as fraction organic carbon OC:
Koc Vm (activity coefficient)
Estimation of Koc for hydrophobic liquids Vm is reciprocal of mole fraction solubility:
log Vm = ­ log sol ­ \|F(Dsf(Tm ­ T),2.303 RT)
where Dsf is the solute entropy of fusion
Tm is the melting point
T is the absolute temperature
log Koc = ­a log sol ­ \|F(Dsf(Tm ­ T),2.303 RT) + b
where a and b are regression-fitting parameters. For polyaromatic hydrocarbons the expression has been reduced to:
log Koc = ­ 0.921 log sol ­ 0.00953 (mp ­ 25) ­ 1.405
mp in this relation is given in €C
The equation works well for low molecular weight compounds.
A more generalized form of the equation is:
log Koc = ­ 0.83 log sol ­ 0.01 (mp ­ 25) ­ 0.93
Koc from Kow

Kow is a common measure of hydrophobility of a chemical. It is a value that can be determined experimentally with a greater degree of accuracy and ease than Koc.
The octanol water partition coefficient (Kow) like Koc is coefficient of solute monomer between an aqueous phase and hydrophobic organic phase:
Kow = \|F(Co,Cw) = \|F(fw,fo) = b\|F(Vw,Vo)
where o and w denote the octanol and aqueous phases. b is a unit adjustment constant. Co and Cw are mole fractions in each phase.
Koc = \|F(fo,foc) Kow
Thus, a correlation line can be drawn:
log Koc = a log Kow + b
a and b are curve-fitting coefficients
The result is a somewhat divergent group of relationships. A number of factors contribute to this variability.
I. Hydrophobic Contribution to Sorption
The thermodynamic rationale for relating Koc to Kow is predicated on:
a)predominance of organic carbon as the key sorbent component of soil and sediment
b)the predominance of hydrophobic sorption
c)sorption linearity
II. Kinetic or Steric Inhibition of Sorption


Koc and Liquid Chromatography Retention

Similar co-relations have also been drawn between liquid chromatography retention and Koc.
In any chromatographic process, the retention volume can be expressed as:

VR = Vm + KVs
where K is the equilibrium
distribution coefficient = \|F(concentration in stationary phase,concentration in mobile phase)
Vm = volume of mobile phase
Vs = volume of stationary phase
The expression can be written as net retention volume:
Vm = VR ­ Vm = KVs
The last parameter is an important parameter in chromatography, and it is referred to as capacity factor K'.
K' = K\|F(Vs,Vm) = \|F(amount of solute in stationary phase,amount of solute in mobile phase)
VR = Vm (1 + K') K' = \|F(VR ­ Vm,Vm)
When K' is zero, VR = Vm.
Since, in chromatography, R is expressed in terms of tR (retention time) of contaminant, assuming constant flow rate:
VR = tR * flow rate
Vm = to * flow rate
K' = \|F(tR ­ to,to)
in a column of length L and mobile phase velocity of v:
tR = \|F(L,v) (1 + K')



\|
F(log Koc = a log tR + b,r2 @ 0.96)
Bioconcentration
Pollutant uptake by higher order aquatic animals can be viewed in a similar manner. That is, fish will accumulate or discharge chemicals in accordance with the pollutant fugacity differential within the organism relative to the external environment.
Kfi = \|F(Cf,Ci)
where Kfi is the equilibrium distribution coefficient
Cf and Ci are pollutant concentrations in fish and pollutant source
Thus, equilibrium ratio of concentration of a pollutant in fish and the concentration in the surrounding water is defined as the Biological Concentration Factor (BCF). It can be expressed as:
Kfi = \|F(1,Ci) Sj Vj Cfj
where Cfi are pollutant concentrations in individual tissue
Vj are the corresponding component weight factors.