1. CE 510 Hazardous Waste Engineering
Department of Civil Engineering
Southern Illinois University Carbondale
Instructors: Jemil Yesuf
Dr. L.R. Chevalier
Lecture Series 6:
Volatilization

2. Course Goals
Review the history and impact of environmental laws in the United States
Understand the terminology, nomenclature, and significance of properties of hazardous wastes and hazardous materials
Develop strategies to find information of nomenclature, transport and behavior, and toxicity for hazardous compounds
Elucidate procedures for describing, assessing, and sampling hazardous wastes at industrial facilities and contaminated sites
Predict the behavior of hazardous chemicals in surface impoundments, soils, groundwater and treatment systems
Assess the toxicity and risk associated with exposure to hazardous chemicals
Apply scientific principles and process designs of hazardous wastes management, remediation and treatment

3. Volatilization Evaporation of solid or liquid into the gaseous phase
Air emissions from hazardous management facilities regulated under Clean Air Act.
May be considered Hazardous Air Pollutants (HAP)

4. Engineered Systems: Air Stripping
contaminated water in
clean water out
packing support
air blower
clean air in
packing material
vent
Diagram of an air stripping tower. Water is piped into the top, and pours over the packing. A Counter current of air is blown into the bottom of the tower, and blows by the water, removing the contamination.

5. Packing media: new and after four years
Source

6. Engineered Systems: Soil Vapor Extraction
vent to atmosphere
air vent or injection well
vacuum
vapor treatment
surface soil cap
unsaturated
LNAPL
LNAPL
C.F.
saturated
water table
residual NAPL
impermeable boundary

7. A typical vapor phase GAC system
A typical vapor phase GAC system. The grey cylinder on the right is an air/water separator. This unit is necessary to keep water from fouling the GAC vessels. Water removed from the soil gas is stored in the black tank on the right. The two white cylinders contain the GAC, which adsorbs organic contaminants from the soil gas. The grey unit on the left contains the blower that pulls the soil gas from the wells and through the vessels.
Source

8. Approach
Volatilization
from Open
Containers
Volatilization from
Deep Soil
Contamination
Volatilization
from Surface Soils
First we need to review the definition of vapor pressure and Henry’s Law

9. Vapor Pressure 10-10 mm Hg Range @ 20 C 760 mm Hg T  VP 
The temperature that causes the vapor pressure to reach 760 mm Hg is the boiling point of the compound.
See Appendix J p. 705
Table 6.1 p. 309
20 C
760 mm Hg
Higher driving force

10. Class Problem
Determine the vapor pressure for anthracene and carbon tetrachloride. Which is more volatile?

11. Henry’s Law For a closed system
Equilibrium between gaseous and aqueous phase
Dilute contaminant
P= partial pressure (atm)
H = Henry’s Law const. (atm-m3/mole)
X = concentration (mole/m3)
H<10-7 atm-m3/mole less volatile than water, conc. will increase.
H>10-3 atm-m3/mole volatilization is rapid

12. Henry’s Law
Henry’s Law constant may also be considered a partition coefficient between air and water, analogous to the octanol-water partition coefficient
Here, S is water solubility
High water solubilities and low vapor pressure tend to decrease the potential for volatilization of dilute species

13. Henry’s Law Correction for temperature See values Table 6.2 p. 310
Dimensionless quantity that may be used in design
the ratio of the mass of compound in the vapor phase to the mass of compound in the aqueous phase.

14. Class Problem
Estimate Henry’s Law constant for benzene at 30C.

15. Governing Equation

16. Solution From Table 6.2, A = 5.53 and B = 3190.
compare to H = at 25 C

17. Class Problem
Experimental determination and application of Henry’s constant
Air is comprised by 21% oxygen on a molar basis. If we bubble a large amount of air through a liter of water until it is saturated with the air, the amount of oxygen in the water at this condition of equilibrium is dictated by Henry’s law. If the amount of oxygen is measured using a DO probe and is 9.3 mg/L,
Calculate Henry’s law constant
Determine how much oxygen will be dissolved in water at 20 ºC if pure oxygen (PO2 = 1 atm) is bubbled through the water until it is saturated.
Density of air at 20 ºC = 1.2 g/L
Density of water at 20 ºC = 998 g/L

18. Solution a) Mass ratio of oxygen in the water:
= 9.3 mg/L X g/mg X 1L/998 g
= 9.3x10-6 g-O2/g-water
Mass ratio of oxygen in the air:
Remember that 1 mole of any gas occupies L of volume at 1 atm. pressure and 20ºC (Ideal Gas Law) and density of air is 1.2 g/L,
Number of moles of oxygen in 1 L of air is:
= 0.21 X (1/24.05) moles = moles
Mass ratio of oxygen in the air ( 1 liter basis) is then calculated as:
= ( moles X 32 g/mole)/(1.2 g) = g-O2/g-air

19. Solution
Henry’s constant in non-dimensional form (H’) is then: = (0.233 g-O2/g-air)/ 9.3x10-6 g-O2/g-water Converting g-air and g-water into volume basis, multiplying the above expression by, (1.2x103 g/m3)/(0.998x106 g/m3) H’ = (mol O2/m3-air)/(mol O2/m3-water) And from H’ = H/RT, H = H’RT = x 8.21x10-5 x 293 = atm-m3/mol

20. Solution
b) From Henry’s law equation: PO2= H.X, And for pure oxygen, PO2 = 1 atm; thus X = PO2 /H = 1 atm/(0.725 atm-m3/mol) = 1.38 mol/m3 = 44.1 g/m3 = 44.1 mg/L
......end of example

21. Estimation of Flux from an Open Container
This term represents the driving force
Q is the evaporation rate (mass/time)
VP is the vapor pressure (atm)
P is the partial pressure of the compound above the liquid
If open, P = 0

22. Estimation of Flux from an Open Container
where
Q = mass flux (evaporation rate)
M = molecular weight
K = mass transfer coefficient per area

23. Estimation of Flux from an Open Container
The mass transfer coefficient K can be estimated using water as a reference
Kw = cm/s
Molecular weight of water = 18 g/m

24. Class Problem
A container of benzene has been left open. Estimate the rate of volatilization across the surface of the container. The dimensions of the container are 1.25m x 0.75m x 0.3 m deep. The temperature is 20C.

25. Approach
Governing Equation

26. Solution Need to solve for K.
Molecular weight of benzene is 6(12)+6 = 78 g/mol

27. Solution
3. Area = 1.25 m x 0.75 m = 0.94 m2
4. Vapor pressure = 76 mm Hg = 0.1 atm
Compare to Ex. 6.1 p. 313

28. Saturation in an Enclosed Area
Spills in
waste transfer areas
drum storage areas
Need to assess the saturated vapor concentration in order to assess toxicity or explosivity of the vapor
Factors to consider:
volume of enclosed space
contaminant flow rate out of enclosed space
ventilation rate
contaminant volatilization rate

29. Saturation in an Enclosed Area
where
Qm = volatilization rate of the compound [M/T] (g/s)
Qv = ventilation rate of the enclosed area [M/T] (m3/s)
k = factor for incomplete mixing ( )
M = molecular weight (g/mol)

30. Class
A container of benzene has been left open in a warehouse. The dimensions of the container are 1.25 m x 0.75 m x 0.3 m deep. The temperature is 20C and pressure is 0.1 atm. The facility is 220 m3 in volume. Ventilation is 12 changes of air per hour. Using k=0.2, determine the steady state benzene concentration in the warehouse.

31. Governing Equation

32. Solution From previous problem Qm = 1.55 g/s
Calculate the ventilation rate
Qv = (12 changes of air/hr)(220 m3/change)(1 hr/3600 sec)
= 0.73 m3/s

33. Solution Continued
3. Determine the steady-state benzene concentration

34. Volatilization from Soils
Soil particles
air
diffusion
diffusion
water
Sorption/desorption

35. Volatilization from Soils
Soil particles
diffusion
air
diffusion
water
sorption

36. Volatilization from Soils
Soil particles
DOW researchers determined an empirical relationship for a first-order decay rate for contaminant loss from a surface spill, kv
where
VP = vapor pressure (mm Hg)
Koc = soil adsorption coeff. (mL/g)
S = solubility (mg/L)
diffusion
air
diffusion
water
sorption

37. Example
A carrier has spilled lindane on soil. Estimate the time required for 70% volatilization.

38. Governing Equations

39. Solution From reference tables in the appendix S = 7.3 mg/L
Koc = 1995 mL/g
VP = 9.4 x 10-6 mm Hg

40. Solution

41. Volatilization in Deep Soils
More complex
Models are mostly specific to matrix
Hamaker Equation
one of the better generalized models and assumes that the contaminant zone is semi-infinite
i.e. the contaminant zone extend into the aquifer and the source is large

42. Volatilization in deep soils
Qt = volatilization of compound per unit surface area (g/cm2)
Co = initial concentration (g/cm3)
D = diffusion coefficient of vapor through coils (cm2/s)
t = time (sec)
Very few data are available for diffusion coefficients. This equation allows a prediction of D based on the known value of 0.01 cm2/s for ethylene dibromide and cm2/s for ethanol.

43. Problem
Estimate the flux of benzene from a deeply contaminated soil over 1 day with a concentration of 500 ppm and a bulk density of 1.50 g/cm3.

44. Data and Governing Equations
MW Benzene 78 g/mol
MW Ethanol 46 g/mol
MW Ethylene dibromide 188 g/mol
D for ethanol cm2/s
D for ethylene dibromide 0.01 cm2/s

45. Solution Daverage = 0.024 cm2/s
1. Determine D using both equations and take the average
Daverage = cm2/s

46. Solution
2. Convert 500 ppm benzene to g/cm3

47. Solution
3. Determine Q

48. Summary of Important Points and Concepts
The two most important parameters for assessing volatilization are vapor pressure and Henry’s Law constant
Vapor pressure of hazardous waste range from essentially nonvolatile to those that rapidly evaporate
Henry’s Law constant is a useful predictor of volatilization from water
Henry’s Law constant is analogous to the octanol-water partition

49. Summary of Important Points and Concepts
Vapor pressure and the mass transfer coefficient K are needed to determine the flux across an open container
For enclosed areas, an equation based on mass balance can be used to determine the concentration of contaminant in the air
Equations for determining the volatilization for soils are more complex because of variability in characteristics (e.g. sorption, water content, diffusion)

50. Summary of Important Points and Concepts
Researchers at Dow developed an empirical equation for estimating the first order decay rate for surface soils
Hamaker’s equation works reasonable well for a range of problems involving the volatilization of contaminants in deep aquifers