Sorption Processes

by Dan Ferrante

Fall 1996

Introduction
Adsorption vs. Absorption
Key Properties of Contaminants
Characteristics of the Soil and Fluid
Sorption Isotherms
Retardation Factors
Sorption Kinetics
References

Introduction:

The sorption of a contaminant is one of the significant processes that can hinder the remediation of a ground water aquifer system. Sorption is defined as being the attraction of an aqueous species to the surface of a solid.(Alley, 1993). In ground water the sorbing species , usually an organic compound, is called the sorbate, and the solid media, usually soil, to which the sorbate is attracted is known as the sorbent.

The underlying principle behind this attraction results from some form of bonding between the contaminant and adsorption receptor sites on the solid. The amount of sorption that occurs in groundwater is dependent on particular characteristics of the sorbate and sorbent. The amount of sorption that takes place on organic matter also follows various isotherms or kinetic rates.

Sorption tends to cause contaminants to move more slowly than the groundwater, therefore the effects must be taken into consideration when calculating how far the contaminant has traveled in a given time period.

The following animation sequence displays how sorption can affect two separate particles' velocity. Basically, the animation shows a vertical cut from a soil column, interspersed particles of organic matter, and two contaminants that are moving through the soil.

Sorption Animation Sequence (1Mb)

Adsorption vs. Absorption

Sorption reactions generally occur over a short period of time, however if the adsorbed contaminant begins to be incorporated into the structure of the sorbent , a slow occurring reaction, known as absorption, begins to take place. To be more precise , the difference between adsorption and absorption is that adsorption is the attraction between the outer surface of a solid particle and a contaminant, whereas absorption is the uptake of the contaminant into the physical structure of the solid.

This figure shows the primary differences between intraparticle absorption versus surface adsorption. The main difference being that some contaminant particles are attracted to the outer surface of the soil particle, while another has been actually incorporated into the particle's structure.

Figure 1: Adsorption vs. Absorption

Key Properties of Contaminants:

Water Solubility
Polarity of the Compound
Kow (Octanol Water Partition Coefficient)

Solubility is defined as the maximum amount of a contaminant that can be dissolved in water at a specified temperature. The solubility of a compound tends to be inversely proportional to the amount of sorption that the contaminant can undergo.

The polarity of a compound plays a major role in the mobility of the compound. Polar substances tend to dissolve more readily in water than nonpolar substances and, therefore adsorb to soil particles less.

Benzene, toluene, and xylene are nonpolar constituents of gasoline that dissolve uniquely well in water. Interestingly, benzene is considered a carcinogen to humans, therefore great care must be taken to insure the safety of drinking water in the area of gasoline spills. Many of the polar contaminants found in soils are metabolites of pesticides and phenols.

The Kow, or Octanol - Water partition coefficient, is simply a measure of the hydrophobicity (water repulsing) of an organic compound. The more hydrophobic a compound, the less soluble it is, therefore the more likely it will adsorb to soil particles(Bedient 1994). Kow can be determined by adding a known amount of contaminant to a bottle consisting of equal volumes of Octanol and water. The coefficient is determined by calculating the concentration in the Octanol phase compared to the concentration in the water phase. The Kow of a compound can also be used to find the Koc of a particular contaminant. Koc is the partition coefficient of the contaminant in the organic fraction of the soil. Koc depends on the physico-chemical properties of the contaminant, not the percent of organic matter in the soil. One such relationship between Kow of aromatic compounds and Koc is:

			Log Koc = 1.00 (Log Kow) - 0.21

A separate equation is used for every class of compound to determine the organic partitioning coefficient from the octanol-water partitioning coefficient of the compound.

Characteristics of the Soil and Fluid:

Texture
Organic Carbon Content
Surface Charge
pH of the Fluid

The texture of a soil is extremely important in the sorption process. If a soil is made up of mostly clay and organic matter a significant amount of sorption will take place. Clay especially intermixed with organic particles, by far adsorbs the most out of the three main soil textures (clay, silt, and sand) because of its small particle size, high surface area, and high surface charge.

The pH of the fluid can affect sorption considerably because it can affect the solubility of a compound. Certain compounds dissolve better in fluids under certain pH's, for example organic acids tend to adsorb better under acidic conditions and amino compounds adsorb better under alkaline conditions (Cantor et al. 1985).

Sorption Isotherms

Kd Values
Freundlich Isotherms
Langmuir Isotherms

The sorption of pesticides to soil in the ground water system often needs to be quantified. A sorption or distribution coefficient ( Kd) is commonly used to accomplish this task. The Kd value is simply a ratio of the sorbed phase concentration to the solution phase concentration at equilibrium. (Alley 1993). The Kd value is related to the surface area of the soil, as well as the Kow value for the contaminant. The Kd value also can be equated to the more accurate Koc value in the cases of most organic contaminants through the relationship:

			Kd = Koc (%O.C)

This relationship shows that as the organic fraction of soils increase the distribution coefficient, Kd, increases. To use Koc in this relationship, the contaminant must be nonionic because sorption of ionic contaminants are affected by soil pH. Water flow must also be fairly slow to insure that sorption is occuring at equilibrium (Alley 1993).

This table gives Koc values for a few of the more popular pesticides used today.

Table 1: Pesticide Sorption Coefficients

Pesticide Name	Molecular Weight (g/mol)	Water Solubility (mg/L)	Sorption Coefficient, Koc (ml/g)
Atrazine (Herbicide)	215.69	33	163
Diazanon (Insecticide)	304.3	40	1000E
DDT (Insecticide)	354.5	0.001-0.004	24000
Carbofuran (Insecticide)	221.25	700	29
Cyanazine (Herbicide)	240.7	420	10000E

(Herner et al.)

Isotherms are graphical representations of the mass of contaminant adsorbed per unit dry mass of soil or organic matter (depending on whether Koc or Kd is being used) (S) versus the concentration (C) of the contaminant. In order to use isotherms to estimate the mass adsorbed, an instantaneous equilibrium must be reached between the sorbent and the sorbate, and the isotherm must be considered reversible. The Freundlich Isotherm is an equilibrium isotherm that is used most often in real world examples. The Freundlich normally results in a curved graph until the logs of both S and C are taken . By taking the log of these terms, a straight line develops making easier to obtain the slope and intercept of the line. The equation of the Freundlich isotherm is similar to the linear isotherm (S is directly related to Ce) , but a new exponential term (1/N) arises:

			S = KdC^1/N

Adapted from Bedient et al.

The Langmuir Isotherm contains two assumptions that make its use usually unfit for the real world, heterogeneous soil case. The assumptions are that the energy of adsorption is constant, and the number of binding sites are finite. The equation for the Langmuir isotherm is shown here:

			S = aBC/(1 + aC)

The Langmuir isotherm is shown here in the graph. The curvature at the end of the graph marks a point at which the receptor sites on the soil particle are full and there is no more room for additional adsorption.

Adapted from Bedient et al.

Retardation Factors in Ground Water

A correction factor known as a retardation factor(Rf) takes into account how much a contaminant's velocity is affected by sorption in the ground water system The overall correction factor is a measure of the bulk density of the media, the porosity, and the distribution coefficient (Kd). The retardation factor in ground water has been measured quite often by using nonsorbing tracers and highly sorbing contaminants in on site experiments. In one such experiment a chloride tracer, along with carbon tetrachloride (CTET) and tetrachloroethylene (PCE) were added to ground water. After a two year monitoring period the distance that the two organic contaminant plumes had traveled were measured against the distance that the chloride tracer had traveled and the difference in distances were used to calculate the retardation factor. It turned out that in the two year period the chloride tracer had moved 60 meters and the PCE had only traveled a distance of 10 meters (Masters 1991). The retardation factor figures into the advection - dispersion equation by dividing average linear velocity and any dispersion coefficients through by this retardation factor.

This image displays how a retardation factor can hinder the movement of a spill in the ground water:

Figure 2: Effect of R_f Value on Dispersion

Adapted from Bennett and Zheng

Sorption Kinetics

The isotherms described in detail above take the assumption that sorption is constantly occurring at equilibrium. This assumption is not valid at all times with most soil systems, therefore kinetics must be used to calculate the how fast concentrations of the compound are sorbing to the sorbate. In general, nonequilibrium sorption occurs when sorption is slow relative to the amount of time the soil particles and the compounds are in contact with each other. Rate constants determine how fast a given concentration of contaminant is sorbing onto a soil particle, but because of the difficulty in calculating these values, the equilibrium assumption is usually accepted.

References

Alley,W., Regional Groundwater Quality, Van Nostrand Reinhold, New York, New York, 1994.

Bedient, P.H., H.S. Rifai, and C.J. Newell. Ground Water Contamination: Transport and Remediation, Prentice Hall, Englewood Cliffs, NJ, 1994.

Bennett, G.D., C. Zheng. Applied Contaminant Transport: Theory and Practice, Van Nostrand Reinhold, New York, 1995.

Canter, L.W., R.C. Knox. Ground Water Pollution Control, Lewis Publishers, Michigan, 1985.

Herner, A.E., A.G. Hornsby, R.D. Wauchope. Pesticide Properties in the Environment, Springer Publishers, New York, 1996.

Lai, R., B.A. Stewert. Soil Processes and Water Quality, Lewis Publishers, Boca Raton, Fla, 1994.

Masters, G. M., Introduction to Environmental Engineering and Science, Prentice Hall, Englewood Cliffs, NJ, 1991.

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