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Vol. 8  No. 8

VOLATILITZATION
&
RISC

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Volatilization

Volatilization plays an important role in both groundwater, soil and atmospheric contamination.  It can be a driving force behind a remediation technique, or the cause behind poor and possibly dangerous indoor air quality.  This newsletter will describe some of the properties of volatilization, and will provide you with a calculator to determine a key parameter in calculating how volatile a contaminant may be.

• What is volatilization?

• What parameters is volatilization dependent upon?

• How do we calculate volatilization?

What is volatilization?

Volatilization is the process by which a contaminant, or compound, evaporates in the vapour phase to the atmosphere from another environmental compartment (soil, surface water, groundwater, etc.). Volatilization can be an important mechanism for the loss of a contaminant from the soil or water to the air.  Remediation techniques such as air sparging depend upon volatilization for the removal of a contaminant from water to the atmosphere, where dilution will reduce harmful effects.

What parameters is volatilization dependent upon?

The volatilization of contaminants from waters and soils depends on the chemical and physical properties of the contaminant, interactions with other media (i.e. suspended sediment in water, or water in soil), and properties of the water-atmosphere interface.  Some of the more important parameters include:

Vapour Density - This is essentially the concentration of the contaminant in the air.  The vapour density ultimately determines the volatilization rate because there exists a maximum concentration in the air (saturated vapour), and the closer the vapour density is to this value, the less contaminant can volatilize into the air.

Temperature - While temperature does effect the volatilization rate, it is unpredictable.  An increase in temperature normally increases the equilibrium vapour density, which in turn increase the volatilization rate.

Aqueous Concentration - Much like how the vapour density limits how much the system can volatilize, the aqueous concentration also regulates it.  The higher the aqueous concentration, the higher the concentration can also be in the air before it reaches equilibrium.

How do we calculate volatilization?

Volatilization can be determined by a mathematical description of the physical process of volatilization based on Henry's Law.  It does not provide a single number or parameter that represents a volatilization rate.  Henry's Law estimates the tendency of a chemical to partition between water and its gas or vapour phase.  Generally, when the Henry's Law constant (H) of a chemical is greater than approximately 10^-4 atm-m³/mol, then volatilization is considered to be an important mechanism.

The equation for Henry's Law is:

A calculator for Henry's law is given below.

Henry's Law Constant Calculator

Vp(t):    atm

S(t):  mol/m³

H:   atm m³/mol

Example Calculation

Nitrogen gas (N2) has a vapour pressure of 3.77 atm, and a solubility of 2.3 mol/m³.

Vp(t) = 3.77 atm

S(t) = 2.3 mol/m³

H = 1.639 atm m³/mol

 

There are many software products available to calculate the rate of volatilization, or the concentrations of indoor and outdoor air from groundwater or soil sources.  One of these programs is RISC.

 

References

Petruzzelli, D. and Helfferich, F.G. (1993). Migration and Fate of Pollutants in Soils and Subsoils. Published by Springer-Verlag, New York, NY.

Pierzynski, G.M., Sims, J.T. and Vance, G.F. (2005). Soils and Environmental Quality. Published by CRC Press, New York, NY.

Yong, R.N., Mohamed, A.M.O. and Warkentin, B.P. (1992). Principles of Contaminant Transport in Soils. Published by Elsevier, New York, NY.

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RISC

RISC is a software package for performing fate and transport modeling and human health risk assessments for contaminated sites. A unique feature of RISC is its ability to perform a backward risk calculation as well as the conventional forward risk calculation. The backward risk calculation refers to calculating a cleanup level for an input value of risk. Fate and transport models are available to estimate receptor point concentrations in groundwater and indoor and outdoor air. No other risk package offers all this.  RISC can be used to estimate the potential for adverse human health impacts (both carcinogenic and non-carcinogenic) from up to nine exposure pathways. These exposure pathways include:

• Ingestion of soil
• Dermal contact with soil
• Ingestion of groundwater
• Dermal contact with groundwater
• Inhalation in the shower
• Inhalation of outdoor air
• Inhalation of indoor air
• Ingestion of surface water
• Dermal contact with surface water

Additional pathways and other non-human health impacts may be considered in future revisions.

 

For more info click here:  RISC
For demo download click here: Demo Download

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