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Hydrophobic Organic Chemicals and Total Organic Carbon: The Ideal and Reality of Site Clean-Up

An On-Line Version of a Column First Published in:
The National Environmental Journal May./June 1993 Vol. 3 No. 3 Pages 16-17

by: David B. Vance

The transport of Hydrophobic Organic Chemicals (HOCs) in groundwater is first dependent upon the hydrodynamic properties of the soil matrix supporting the advective and diffusional flow system. Other dominant considerations include physical/chemical interactions between HOC and the soil matrix.

The mechanisms and conditions governing those physical/chemical interactions are complex and include (among others) the following:

  • The chemical properties of the exposed surfaces of soil particles.
  • Soil particle size distribution.
  • Type of HOC exposure, free phase or dissolved; and the duration of exposure.
  • The chemical nature of the HOC and interactions from concurrent exposure to multiple HOCs.
  • Geochemistry of the groundwater.
  • Overall rate of groundwater flow.

 Now, carbonaceous materials are common components of soils and have particular impact with regards to contaminant adsorption. Mineral soil constituents may also be adsorptive, in some clay rich soils 30 to 50 percent of the bulk HOC adsorptive capacity may be associated with clays. However, the remainder of this column will focus on the carbonaceous component, termed Total Organic Carbon (TOC).

The affinity of a HOC for soil is symbolized by the solid-water partition coefficient, Kd. The value of Kd is expressed by the following relationship:

Kd = Koc x foc (Equation 1)

The term Koc is the organic carbon-water partition coefficient and foc is the fraction of organic carbon in the soil. The adsorptive property defined by Koc is very similar to that exhibited by activated carbon. The value of Koc for different TOC fractions in soils have been measured and do not vary by a large factor over a wide range of soil types. However, the physical behavior of individual HOCs exposed to the TOC is a significant variable.

The degree of hydrophobicity of a specific HOC is represented by the Octanol Water Partition Coefficient (Kow). The higher Kow, the greater affinity a particular HOC will have for TOC. Values for log Kow are dependent upon the chemical properties of a particular HOC. Examples for log Kow values follow:

  • Benzene 2.13;
  • Tetrachloroethylene 2.60;
  • 1,2 DCA 1.45;
  • Heptachlor 5.36;
  • Naphthalene 3.28;
  • Pentachlorophenol 5.27.

 The value for Koc in Equation 1 is more accurately portrayed by its relation to Kow through the following equation:

log Koc = a log Kow+ b (Equation 2)

The two constants have values empirically determined and reported in the literature. The value of a ranges from 1 to 0.54, b ranges from 1.32 to -0.21 (The values tend to stay consistent within major classes of organic compounds).

These equations should be viewed as approximations (as the range for the above constants indicate). This discussion is of value because it will illuminate the general process and provide reasonable approximations. Other factors (as you will read) tend to overwhelm an overly specific analysis.

The second component in Equation 1 is foc, which is TOC expressed as a decimal rather than percent. TOC is a parameter that is one of the most important to have, but is seldom evaluated on a regular basis during site assessments. The normal range for TOC in soils is from 0.5% to 5% (foc = 0.005 to 0.05), examples of measured TOC concentrations include:

  • Coarse soil - 4.2%
  • Clayey silty loam - 0.4%
  • Silty Loam - 1.6%
  • Silty Clayey Loam - 2.95%
  • Silty Loam - 5.2%
  • Clayey Loam - 0.38%
  • Glaciofluvial - 0.02% to 1.0%

The value of Kd obtained after the application of Equations 1 and 2 is useful for two purposes. First, it is an indication for the strength of adsorptive reactions between the soil matrix and impacting HOCs.

Secondly, the retardation factor (R) can be calculated using the following equation:

R = 1 + (Rho / Theta)Kd (Equation 3)

Where Rho is the dry bulk density of the soil (1.5 to 1.9 g/cm3) and Theta is the saturated pore volume of the soil (0.35 to 0.55).

As an example:

  • 1,2 DCA in a soil with 1% TOC; foc = 0.01; log kow = 1.45; assume that Rho = 1.70; Theta = 0.40; a = 0.80; and b = 0.70
  • log Koc = (0.80 x 1.45) + 0.7 = 1.86 (Reference Equation 2)
  • Koc = 72.4
  • Kd = 72.4 x 0.01 = 0.724 (Reference Equation 1)
  • R = 1 + (1.70/.40)0.724 (Reference Equation 3)
  • R = 3.1

 Dividing the groundwater velocity and dispersion coefficient by R, will give an estimate of the rate of transport of a given contaminant. Ideally, it also represents the number of groundwater pore volumes that should be flushed through a contaminated zone to desorb HOCs from impacted soils (i.e. 3 pore volumes from above).

Unfortunately, the process of desorption by flushing is not ideal. Thus the general failure of pump and treat as a clean-up technology.

There are two principal reasons for this problem. The first involves the physical nature of TOC and how HOCs interact with it. A key interaction (aside from adsorption) is the migration of the HOC through TOC particles. This process has been termed "intraorganic matter diffusion". In this process the effect of time is critically important.

At many sites HOC impact has been chronic over years. Ideally, desorption and removal of adsorbed HOCs from within TOC particles would take an amount of time equal to the chronic exposure interval. Even under ideal conditions the problem lies in the stimulation of an artificial hydraulic gradient and treatment of recovered groundwater at considerable capital and operating expense over a period equal to the chronic exposure time. Simply flushing with the appropriate number of pore volumes (as indicated by the retardation factor) will not accelerate this intraorganic diffusion process and lessen operating duration and expenses.

Now, the second and more profound reason for departure from ideal desorption. Bench and field scale studies have repeatedly demonstrated that some fraction of HOC adsorbed by TOC is irreversibly bound. Using pump and treat soil flushing methodologies, the HOC will never totally desorb (or at best desorb at an imperceptible rate)! Other mechanisms may make alternate approaches such as in-situ biodegradation equally ineffective (this issue will be addressed in the next column.)

The key point is the following: The in-situ clean-up levels possible at any site are limited by physical/chemical interactions between the impacting HOC and TOC within the soil matrix. These limits will not be changed be spending more money for a "better" remediation system, nor will they be changed by regulating agencies setting arbitrary clean-up standards.

Once exposed to HOCs, some level of residual impact in soils is unavoidable. On a positive note, if HOCs are irrevocably bound up in the soil matrix, then they are not available to present a hazard to the public. There lies the value and extreme importance of risk assessment as a critical component of the remediation process.

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