In the Classroom
Bubble Stripping To Determine Hydrogen Concentrations in Ground Water: A Practical Application of Henry’s Law Daniel M. McInnes* Department of Chemistry, East Central University, Ada, OK 74820; *
[email protected] Don Kampbell U.S. EPA, Office of Research and Development, National Risk Management Laboratory, Subsurface Protection and Remediation Division, 919 Kerr Research Drive, Ada, OK 74820
Henry’s law states that at constant temperature, the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid. Assuming ideal behavior, Henry’s law can be written, P = H S where P is the pressure of the gas above the solution (atm), S is the solubility of the gas in solution (expressed as a mole fraction), and H is the Henry’s law constant (atm). The mole fraction of gas in solution is given the variable X, and can be approximated: X = mol gas兾mol solution. Generally, P ≤ 1 atm and X ≤ 10᎑3 are conditions under which ideal behavior can be assumed (1). The Bubble Stripping Method is a chemical testing method that involves applying Henry’s law to determine concentrations of dissolved hydrogen (H2) in ground water (2). The method’s role in achieving restoration of sites that are polluted with chlorinated organic solvents is described. Theory and sample calculations are presented that may be of use to general chemistry lecturers who want to illustrate how a basic chemical principle can find application in the increasingly important field of environmental science. Chlorinated Organic Contaminants in Ground Water Ground water flows by way of underground aquifers; that is, water-bearing strata of permeable rock, sand, or gravel. It supplies water for farms, industry, and city residential neighborhoods. Pollutants that are poured onto the ground can reach ground water by migrating through porous soil and rock. Any contaminants that dissolve in ground water can be transported over long distances as the ground water flows. As a result, there have been many cases in which improper waste disposal has caused contamination of the public water supply. To make matters worse, pollutants that are only slightly water-soluble can migrate from their sources into ground water over lengthy periods of time, resulting in longterm exposure to unacceptable levels of toxic compounds. Chlorinated organic chemicals have many uses in industrial applications. In recent years, numerous sites have been identified as being contaminated with chlorinated organic solvents (3). These solvents are toxic and tend to partition slowly into ground water, causing a long-term threat to the environment. Much attention, therefore, is directed at restoring sites polluted with chlorinated solvents (4). Trichloroethylene, TCE, is an example of a chlorinated 516
organic solvent that is the focus of many clean-up efforts. TCE is used in dry cleaning operations and as a solvent for degreasing metals. It is carcinogenic, as are the products of its degradation under anaerobic conditions: dichloroethylene, DCE, and vinyl chloride, VC (Figure 1). Anaerobic dechlorination is the predominant process occurring. As a result of its improper disposal, TCE and its daughter products have been found as contaminants in ground water in the vicinity of landfills and U.S. Air Force fire-training sites. Monitored Natural Attenuation as a Strategy for Remediation of Contaminants At certain contaminated field sites, natural processes cause a decrease in contaminant concentration with time; that is, the pollutants dispersed naturally, without the need for any clean up effort. Relying on nature in this manner is a strategy known as monitored natural attenuation (MNA). Natural attenuation of contaminants occurs through a variety of physical, chemical, and biological processes. Some of these processes serve to transport or dilute the contaminants,
Cl
Cl
Cl
Cl
Cl
H
H
H2
H
H
Cl
Cl
H
Cl
H
Cl
H
HCl
TCE
DCE
H2
HCl
H
Cl
H
H2
H
H
H
H
H
ethane H
H VC
HCl
H
H
H
H
ethylene
Figure 1. The pathway of biodegradation of TCE under anaerobic conditions.
Journal of Chemical Education • Vol. 80 No. 5 May 2003 • JChemEd.chem.wisc.edu
In the Classroom
while others involve the degradation or decomposition of the contaminants. When MNA is employed as a remediation strategy, ground water in the vicinity of the contamination is monitored over time. Monitoring is accomplished by drawing samples from wells that tap into the ground water. Providing that contaminant concentration decreases at an acceptable rate, the natural attenuation process is allowed to continue until remediation is complete. MNA is considered a viable remedy for a site when it will meet the site cleanup goals and offer favorable cost benefits as compared to the available alternatives. Hydrogen Concentration in Ground Water as Evidence of Reductive Dechlorination of Chlorinated Hydrocarbons Chlorinated hydrocarbons undergo biodegradation by microorganisms under anaerobic conditions via the mechanism of reductive dechlorination. Reductive dechlorination requires the presence of dissolved hydrogen, which acts as a reducing agent (5, 6). Molecular hydrogen is produced in ground water as a byproduct of the microbial degradation of naturally occurring organic matter under anaerobic conditions. At sites contaminated with chlorinated hydrocarbons, analysis of dissolved hydrogen concentration in the ground water can provide evidence that reductive dechlorination is occurring. A dissolved hydrogen concentration equal to or greater than 1 nM will promote reductive dechlorination in ground water contaminated with chlorinated organic compounds, while dissolved hydrogen concentrations of less than 0.1 nM are most likely too low for reductive dechlorination to occur (4). Hydrogen analysis can therefore be used as a stand-alone method of determining the likelihood that reductive dechlorination is taking place at sites contaminated with chlorinated hydrocarbons. It should be noted that other degradation processes (which are also dependant on dissolved hydrogen concentration) can take place at a given site at the same time as reductive dechlorination (7–11), but the process of reductive dechlorination is the main concern at sites contaminated with chlorinated organics. Taking several factors into account, such as cost and rate of the dispersion of contaminants, hydrogen analysis can aid in determining the viability of implementing MNA as a strategy for restoring a site contaminated with chlorinated organic compounds.
mal. This process is continued for 30 minutes, allowing equilibrium to be established between the aqueous phase and the headspace. Hydrogen concentration in the headspace is then determined by gas chromatographic analysis, after which hydrogen concentration in solution is calculated using Henry’s law. Hydrogen concentrations in well water on the order of nanomoles per liter (nM) can be determined using the Bubble Stripping Method.
Instrumentation Analysis of headspace samples is accomplished using a Trace Analytical RGA3 Reduction Gas Analyzer, a gas chromatograph with a detector containing a heated bed of mercuric oxide (HgO). Hydrogen flows over the hot HgO, reducing Hg(II) to Hg(0) H2 + HgO → H2O + Hg The mercury vapor is quantitatively determined by an ultraviolet photometer. Sample injection volume is 2 mL, the column temperature is set at 100 ⬚C, and the detector temperature is set at 265 ⬚C. The column used to separate hydrogen is 1/8 in. × 30 1/4 in. with Molecular Sieve 5A 60兾80 packing. The retention time of hydrogen is 0.50 minute with a carrier gas (high purity nitrogen) flow rate of 22 mL兾min. Hazards The mercury vapor scrubber on the RGA3 should be replaced after two years of use. This scrubber is hazardous and should be given to a laboratory Environmental Compliance Manager when replaced. The mercury oxide bed is hazardous and must be treated as hazardous waste upon replacement. Material must be given to a laboratory Environmental Compliance Manager for proper disposal. The RGA3 needs to warm up for 24 hours prior to use, otherwise analytical precision and accuracy will be unacceptable. Make sure the carrier gas is flowing prior to turning on the detector.
Experimental Section
Bubble Stripping To Determine Dissolved Hydrogen Concentration in Ground Water Ground water is accessible through wells that access the aquifer system. The Bubble Stripping Method is used to analyze well water for dissolved hydrogen. The bubble stripping procedure involves pumping water from the well into a sample cell, and then injecting 20 mL of air to produce a headspace (a “bubble”) over the water (Figure 2). Applying this method will cause any gases dissolved in solution to be “stripped” into the headspace, hence the name “bubble stripping method.” A peristaltic pump is used to pump the well water through the cell such that a stream of water flows into the headspace, producing agitation in the aqueous phase. A solution flow rate of 300 mL兾min through the cell is opti-
Figure 2. A schematic of the bubble stripping cell. After stripping, a needle is inserted through a sampling port in the rubber stopper to withdraw headspace samples.
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In the Classroom
Calculations and Discussion
Calibration Curves A calibration curve is prepared using a 100 ppm mixture of hydrogen in nitrogen (purchased from Scott Specialty Gases) as a working standard. Calibration standards are made by injecting the appropriate amount of working standard into 160-mL glass serum bottles, which have been filled with highpurity nitrogen by water displacement and sealed with rubber septa (Wheaton, 13 mm × 20 mm gray butyl Teflon faced, straight plug style) and tear away aluminum seals (Supelco). Calibration standards are allowed to stand for 1 h before analysis to ensure complete mixing of gases (Table 1). Calibration standards are analyzed in duplicate and average area counts are plotted against hydrogen concentration. All area counts, AC, are corrected to standard atmospheric pressure (29.92 in. Hg at 0 ⬚C) to eliminate atmospheric pressure, Patm, as a variable, ACcorrected = (ACuncorrected)(29.92 in. Hg兾Patm) where the pressure is in units of in. Hg. The relationship between area count and concentration is approximately linear over a range of 0 ppm to 20 ppm, and a linear curve fit can be performed to yield an equation in the form y = mx + b, where y = [H2], and x = ACcorrected (Figure 3).
Gas Chromatographic Analysis of the Sample Cell Headspace After applying the stripping procedure to well water, the headspace in the sample cell is analyzed, and the area count is corrected to standard atmospheric pressure. The corrected area count is then used to determine the hydrogen concentration in the headspace, [H2]hs, which is multiplied by another correction factor to yield [H2]hs, corr in parts per million:
[H2 ]hs,corr
= [H2 ]hs
std-atm-pressure + over-pressure std-atm-pressure
Concentration H2 (ppm)
12
y = 0.568 x R 2 = 0.968
8
Volume of 100 ppm H2 in N2 /mL
0.0
0.0
0.5
0.8
1.25
2.0
2.5
4.0
5.0
8.0
10.0
16.0
20.0
32.0
equipped with a needle that can be inserted into the sample cell headspace and is typically on the order of 0.01 atm to 0.04 atm.
Using Henry’s Law To Calculate [H2] in Solution from [H2] in the Headspace The mole fraction of dissolved hydrogen in solution can be calculated using the following equation, X (H2 )soln =
P (H2 )hs H
where X(H2)soln is the mole fraction of dissolved hydrogen in solution (≈ mol H2兾mol H2O), P(H2)hs is the partial pressure of H2 in the bubble (atm), and H is the Henry’s law constant (atm). Using headspace concentration data derived from GC analysis,
=
mol H2 mol H2 O
[H2 ]hs,corr (ppm)
× 10−6 × Ptotal,hs (atm)
H (atm)
where [H2]hs, corr is the concentration of hydrogen in the headspace (corrected to standard atmospheric pressure and for over-pressure), Ptotal,hs is the total pressure over the solution (the value 1.00 atm is used here since area counts were corrected to 1 atm), the factor 10᎑6 accounts for [H2] being expressed in parts per million, and H is Henry’s law constant.
Table 2. Derivation of Water mol/L Ratio from Density
6
T /⬚C
d /g cm᎑3
d /g L᎑1
mol H2O/L H2Oa
0
0.99987
999.87
55.501
5
0.99999
999.99
55.508
10
0.99973
999.73
55.493
15
0.99913
999.13
55.460
20
0.99823
998.23
55.410
25
0.99707
997.07
55.346
30
0.99567
995.67
55.268
4 2 0 0
2
4
6
8
10
12
14
16
18
6
Area Count /10
Figure 3. An example of a calibration curve relating area counts to H2 concentration.
518
H2 (ppm)
NOTE: Standards prepared in 160mL serum bottles.
X (H2 )soln ≈
The correction factor corrects for a slight over-pressure in the system during stripping, owing to resistance in the tubing through which the water is pumped. Over-pressure is measured with a digital manometer (Dwyer Series 475 Mark III)
10
Table 1. Volume of 100 ppm H2 in N2 Needed To Prepare Hydrogen Standards
mol H2O/L H2O = d, H2O (g L᎑1)/18.02 (g mol᎑1)
a
Journal of Chemical Education • Vol. 80 No. 5 May 2003 • JChemEd.chem.wisc.edu
In the Classroom
Table 3. Henry’s Law Constants at Various Temperatures T /C
H /atm
0
56883
5
60277
10
63412
15
66225
20
68681
25
70771
30
72516
The concentration of H2 in solution can then be determined using X(H2)soln as follows
[H2 ]soln
mol H2 mol H2 O mol = X (H2 )soln ≈ L mol H2 O L H2O
order of nanomoles per liter. The method provides information that is used in determining whether or not monitored natural attenuation is a viable strategy for restoration of sites contaminated with chlorinated solvents. With simple procedures and calculations, hydrogen concentrations in well water can be determined at a field site in less than an hour. Acknowledgments The authors would like to acknowledge funding support from the Air Force Center for Environmental Excellence at Brooks AFB, Texas (IAG-RW57938631). Gratitude is also extended to John T. Wilson (EPA) for suggestions and comments, to Mark Blankenship (ManTech) for providing procedures for using the Reduction Gas Analyzer, and to William G. Lyon (ManTech) for providing us with access to unpublished derivations and discussions relating to the equations used in applying Henry’s law. Notice
where [H2]soln is the concentration of dissolved hydrogen in the aqueous solution in moles兾liter, and mol H2O兾L H2O represents the number of moles of water in 1 liter of water, which depends on temperature (Table 2; ref 12). Selected Henry’s law constants for hydrogen gas at temperatures ranging from 0 ⬚C to 30 ⬚C are tabulated in Table 3 (1). Solubility data for many other gases are also readily available in ref 1.
The U.S. Environmental Protection Agency through its Office of Research and Development funded and managed the research described here under Contract No. 68-C-98-138 to ManTech Environmental Technology, Inc., and IAG No. RW57938631. It has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Sample Calculation
Literature Cited
For a corrected headspace hydrogen concentration of 0.50 ppm and a water temperature of 25 ⬚C, the mole fraction of hydrogen in solution can be determined using the following equation;
1. Wilhelm, E.; Battino, R.; Wilcock, R. J. Chem. Rev. 1977, 77, 219–262. 2. Chapelle, F. H.; Vroblesky, D. A.; Woodward, J. C.; Lovley, D. R. Environ. Sci. Technol. 1997, 31 (10), 2873–2877. 3. Avon, L.; Bredehoeft, J. D. J. Hydrol. 1989, 110, 23–50. 4. Wiedemeier, T. H.; Swanson, M. A.; Moutoux, D. E.; Gordon, E. K.; Wilson, J. T.; Wilson, B. H.; Kampbell, D. H.; Haas, P. E.; Miller, R. N.; Hanson, J. E.; Chapelle, F. H. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water; EPA Report EPA/600/R-98/128. 5. Wiedemeier, T. H.; Swanson, M. A.; Wilson, J. T.; Kampbell, D. H.; Miller, R. N.; Hansen, J. E. Ground Water Mon. Remed. 1996, 16 (3), 186–194. 6. Kampbell, D. H.; Wiedemeier, T. H.; Hansen, J. E. J. Haz. Mater. 1996, 49, 197–204. 7. Chapelle, F. H.; McMahon, P. B.; Dubrovsky, N. M.; Fujii, R. F.; Oaksford, E. T.; Vroblesky, D. A. Water Resour. Res. 1995, 31 (2), 359–371. 8. Chapelle, F. H.; Haack, S. K.; Adriaens, P.; Henry, M. A.; Bradley, P. M. Environ. Sci. Technol. 1996, 30 (12), 3565–3569. 9. Lovley, D. R.; Goodwin, S. Geochim. Cosmochim. Acta. 1988, 52, 2993–3003. 10. Lovley, D. R.; Chapelle, F. H.; Woodward, J. C. Environ. Sci. Technol. 1994, 28 (7), 1205–1210. 11. Vroblesky, D. A.; Chapelle, F. H. Water Resour. Res. 1994, 30 (5), 1561–1570. 12. The CRC Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1974.
X (H2 )soln ≈
0.50 ppm × 10-6 atm 70771atm
At 25 ⬚C, mol H2O兾L H2O ratio is equal to 55.346, therefore:
[H2 ]soln
0.50 ppm × 10 −6 atm 55.346 mol H2 O = 70771atm L H2O =
3.9 × 10 −10 mol H2 = 0.39nM L H2 O
A hydrogen concentration of 0.50 ppm in the sample cell headspace with a water temperature of 25 ⬚C corresponds to a hydrogen concentration of 0.39 nM in solution at equilibrium. Conclusions The Bubble Stripping Method is a chemical testing method that operates on the principle of Henry’s law. It is useful for determining concentrations of hydrogen in well water, and it is capable of detecting concentrations on the
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