pE 0 Values of Redox Reactions Important in Natural Waters (at 25°C)
5.10 PHASE INTERACTIONS IN CHEMICAL FATE AND TRANSPORT
In Section 1.8, the topic of environmental chemical fate and transport was introduced. The hydro-sphere is arguably the most important environmental hydro-sphere with respect to chemical fate and transport processes, many of which involve distribution of species between phases. It is appropriate at this point to consider chemical fate and transport in the hydrosphere. This is an environmentally important topic that increasingly utilizes sophisticated mathematical models and calculations that are beyond the scope of this book. For more details and example calculations, the reader is referred to a comprehensive reference work on the subject.5
5.10.1 RIVERS
The physical movement of chemical species in a river is largely by advection (see Section 1.8) due to the gravitational movement of masses of water downstream. This results in relatively rapid mixing and dilution because of velocity shear in which parcels of water move at different rates in a river where water in contact with the banks and bottom moves more slowly than water in the middle of the stream. Also contributing to this mixing are turbulent mixing and diffusive transport of dis-solved species and colloidal particles. The net result of these processes is that a pollutant introduced into the river in a relatively concentrated “plug” becomes spread out as it fl ows downstream.
5.10.2 LAKESAND RESERVOIRS
Lakes and reservoirs are relatively quiet compared with rivers, but there is still movement within such bodies of water. Water enters by stream infl ow, infl ow from underground springs and aquifers, and directly by rainfall. It exits by outfl ow, such as over a spillway in a reservoir, infi ltration into ground, and evaporation. The balance between input and loss of water in a body of water means that the water has a hydraulic residence time. A major factor in mixing processes in lakes is the infl uence of wind.
This occurs because windblown surface water moves at a rate that is 2–3% of the wind velocity, a phenomenon called wind drift. Water moved toward one side of a lake by wind drift must return some distance below the water surface as a return current. In a shallow, unstratifi ed body of water, the return current fl ows along the bottom where it contacts and may agitate bottom sediments. In stratifi ed lakes (see Figure 3.6) the circulation of water occurs in the upper epilimnion layer as shown in Figure 5.8.
5.10.3 EXCHANGEWITHTHE ATMOSPHERE
Exchange of chemical species between water and overlying air is an important process. It is the means by which atmospheric oxygen enters water to provide oxygen needed by fi sh. Carbon dioxide required for algal growth may come from the air. Air pollutants such as acid gases may enter water from the
Wind drift Return current
Disturbed sediment Wind flow
Wind drift Return current epilimnion layer
Quiescent hypolimnion
Undisturbed sediment Wind flow
FIGURE 5.8 Mixing due to wind in a shallow unstratifi ed lake (left) and in a stratifi ed lake (right). In the former case in which the return current contacts the sediment, the sediment may be stirred, enabling release of substances to the water.
Phase Interactions in Aquatic Chemistry 123 atmosphere. Under circumstances of high algal photosynthetic activity, oxygen produced by algae is released to the air. Decay of organic matter can oversaturate water with carbon dioxide, requiring its release to the air. Anoxic microbial processes in sediments can produce hydrogen sulfi de and methane that are released to air. Volatile organic water pollutants may move from water to the atmosphere.
The air–water interface is the boundary across which species move and is crucial in determining the rate of exchange of materials. Current models of these processes assume that there is a thin, stationary liquid fi lm on the surface of the water in direct contact with a thin, stationary layer of air.
Both of these layers are thin—as little as a few micrometers—and molecular diffusion is the only mechanism for movement of solute species in them. Immediately below the thin surface fi lm of water, turbulent diffusion mixes water solutes, and immediately above the thin air fi lm, turbulent diffusion mixes the species in air.
5.10.4 EXCHANGEWITH SEDIMENTS
Sediments are very important in chemical fate and transport in the hydrosphere. This is because substances, including pollutants such as heavy metals or hydrophobic organics, bind with particles as they settle through the water column and are incorporated into sediments. The settling fl ux den-sity, J, is equal to the mass of a substance transported across an area through which the settling occurs per unit time and is given by the product of the rate at which sedimentary materials settle and the concentration of the substance in question in the settling materials:
J = (sediment deposition rate) ¥ (substance concentration in particles) (5.40) Incorporation of pollutants by settling from the water column into sediments is a particularly important mode of chemical fate and transport under quiescent conditions during which very small (colloidal) particles are aggregating together (fl occulation). If undisturbed, the layers of sediments can provide a record of pollution. A hypothetical, though typical, such plot is shown for lead in Figure 5.9.
Although sediments are normally repositories of pollutants and reduce their environmental harm, they can also provide sources of pollutants that can be mobilized by physical, chemical, or biochemical processes. For example, mercury precipitated in sediments can be mobilized as soluble methylmer-cury species by the action of anoxic bacteria in the oxygen-defi cient sediments (see Section 7.3).
Replacement of leaded gasoline with unleaded
1860 1880 1900 1920 1940 1960 1980 2000
70 60 50 40 30 20 10 0
0
Lead concentration in sediment (mg/kg)
Year of deposition
Depth into sediment (cm)
Gradually increased industrial use, lead paint
Rise in use of leaded gasoline
100 200 300 400 500 600 700 800
FIGURE 5.9 Typical sediment record for lead deposition refl ecting the sharp increase in lead from gasoline and a drop of lead after phaseout of leaded gasoline.
124 Environmental Chemistry
LITERATURE CITED
1. Ghosh, U., The role of black carbon in infl uencing availability of PAHs in sediments, Human and Ecological Risk Assessment, 13, 276–285, 2007.
2. Massoudieh, A. and T. R. Ginn, Modeling colloid-facilitated transport of multi-species contaminants in unsaturated porous media, Journal of Contaminant Hydrology, 92, 162–183, 2007.
3. Huang, T.-L., X.-C. Ma, C. Hai-Bing, and B.-B. Chai, Microbial effects on phosphorus release in aquatic sediments, Water Science and Technology, 58, 1285–1289, 2008.
4. Schwarzbauer, J., M. Ricking, B. Gieren, and R. Keller, Anthropogenic organic contaminants incorporated into the non-extractable particulate matter of riverine sediments from the Teltow Canal (Berlin). In Eric Lichtfouse, Jan Schwarzbauer, and Robert Didier, Eds, Environmental Chemistry, Springer, Berlin, 329–
352, 2005.
5. Gulliver, J. S., Introduction to Chemical Transport in the Environment, Cambridge University Press, New York, 2007.
SUPPLEMENTARY REFERENCES
Allen, H. E., Ed., Metal Contaminated Aquatic Sediments, Ann Arbor Press, Chelsea, MI, 1995.
Barnes, G. and I. Gentle, Interfacial Science: An Introduction, Oxford University Press, New York, 2005.
Beckett, R., Ed., Surface and Colloid Chemistry in Natural Waters and Water Treatment, Plenum, New York, 1990.
Berkowitz, B., Contaminant Geochemistry, Springer, New York, 2007.
Bianchi, T. S., Biogeochemistry of Estuaries, Oxford University Press, New York, 2007.
Birdi, K. S., Ed., Handbook of Surface and Colloid Chemistry, 2nd ed., CRC Press, Boca Raton, FL, 2003.
Burdige, D. J., Geochemistry of Marine Sediments, Princeton University Press, Princeton, NJ, 2006.
Calabrese, E. J., P. T. Kostecki, and J. Dragun, Eds, Contaminated Soils, Sediments, and Water, Volume 10, Successes and Challenges, Springer, New York, 2006.
Eby, G. N., Principles of Environmental Geochemistry, Thomson-Brooks/Cole, Pacifi c Grove, CA, 2004.
Evans, R. D., J. Wisniewski, and J. R. Wisniewski, Eds, The Interactions Between Sediments and Water, Kluwer, Dordrecht, The Netherlands, 1997.
Golterman, H. L., Ed., Sediment–Water Interaction 6, Kluwer, Dordrecht, The Netherlands, 1996.
Gustafsson, O. and P. M. Gschwend, Aquatic colloids: Concepts, defi nitions, and current challenges, Limnology and Oceanography, 42, 519–528, 1997.
Holland, H. D., and K K. Turekian, Eds, Treatise on Geochemistry, Elsevier/Pergamon, Amsterdam, 2004.
Holmberg, K., D. O. Shah, and M. J. Schwuger, Eds, Handbook of Applied Surface and Colloid Chemistry, Wiley, New York, 2002.
Hunter, R. J., Foundations of Colloid Science, 2nd ed., Oxford University Press, New York, 2001.
John V. W., Essentials of Geochemistry, 2nd ed., Jones and Bartlett Publishers, Sudbury, MA, 2009.
Jones, M. N. and N. D. Bryan, Colloidal properties of humic substances, Advances in Colloid and Interfacial Science, 78, 1–48, 1998.
Jones, S. J. and L. E. Frostick, Eds, Sediment Flux to Basins: Causes, Controls and Consequences, Geological Society, London, 2002.
McSween, H. Y., S. M. Richardson, and M. E. Uhle, Geochemistry: Pathways and Processes, 2nd ed., Columbia University Press, New York, 2003.
Mudroch, A., J. M. Mudroch, and P. Mudroch, Eds, Manual of Physico-Chemical Analysis of Aquatic Sediments, CRC Press, Boca Raton, FL, 1997.
Myers, D., Surfaces, Interfaces, and Colloids: Principles and Applications, 2nd ed., Wiley, New York, 1999.
Shchukin, E. D., Colloid and Surface Chemistry, Elsevier, Amsterdam, 2001.
Stumm, W., L. Sigg, and B. Sulzberger, Chemistry of the Solid–Water Interface, Wiley, New York, 1992.
Stumm, W. and J. J. Morgan, Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, 3rd ed., Wiley, New York, 1995.
U.S. Environmental Protection Agency, website on contaminated sediments, 2003, available at http://www.epa.
gov/waterscience/cs/
Wilkinson, K. J. and J. R. Lead, Eds, Environmental Colloids and Particles: Behaviour, Separation and Characterisation, Wiley, Hoboken, NJ, 2007.
Phase Interactions in Aquatic Chemistry 125
QUESTIONS AND PROBLEMS
The use of internet resources is assumed in answering any of the questions. These resources would include such things as constants and conversion factors as well as additional information needed to complete an answer.
1. A sediment sample was taken from a lignite strip-mine pit containing highly alkaline (pH 10) water. Cations were displaced from the sediment by treatment with HCl. A total analysis of cations in the leachate yielded, on the basis of millimoles per 100 g of dry sedi-ment, 150 mmol of Na+, 5 mmol of K+, 20 mmol of Mg2+, and 75 mmol of Ca2+. What is the CEC of the sediment in milliequivalents per 100 g of dry sediment? Why does H+ not have to be considered in this case?
2. What is the value of [O2(aq)] for water saturated with a mixture of 50% O2, and 50% N2 by volume at 25∞C and a total pressure of 1.00 atm?
3. Of the following, the least likely mode of transport of iron(III) in a normal stream is:
(a) bound to suspended humic material, (b) bound to clay particles by cation-exchange processes, (c) as suspended Fe2O3, (d) as soluble Fe3+ ion, and (e) bound to colloidal clay–
humic substance complexes.
4. How does freshly precipitated colloidal iron(III) hydroxide interact with many divalent metal ions in solution?
5. What stabilizes colloids composed of bacterial cells in water?
6. The solubility of oxygen in water is 14.74 mg/L at 0∞C and 7.03 mg/L at 35∞C. Estimate the solubility at 50∞C.
7. What is thought to be the mechanism by which bacterial cells aggregate?
8. What is a good method for the production of freshly precipitated MnO2?
9. A sediment sample was equilibrated with a solution of NH4+ ion, and the NH4+ was later displaced by Na+ for analysis. A total of 33.8 milliequivalents of NH4+ were bound to the sediment and later displaced by Na+. After drying, the sediment weighed 87.2 g. What was its CEC in milliequivalents per 100 g?
10. A sediment sample with a CEC of 67.4 milliequivalents per 100 g was found to contain the following exchangeable cations in milliequivalents per 100 g: Ca2+, 21.3; Mg2+, 5.2; Na+, 4.4; K+, 0.7. The quantity of hydrogen ion, H+, was not measured directly. What was the ECS of H+ in milliequivalents per 100 g?
11. What is the meaning of ZPC as applied to colloids? Is the surface of a colloidal particle totally without charged groups at the ZPC?
12. The concentration of methane in an interstitial water sample was found to be 150 mL/L at standard temperature and pressure of 0∞C and 1 atm (STP). Assuming that the methane was produced by the fermentation of organic matter, {CH2O}, what mass of organic matter was required to produce the methane in a liter of the interstitial water?
13. What is the difference between CEC and ECS?
14. Match the sedimentary mineral on the left with its conditions of formation on the right:
a. FeS(s) 1. May be formed when anoxic water is exposed to O2. b. Ca5OH(PO4)3 2. May be formed when oxic water becomes anoxic.
c. Fe(OH)3 3. Photosynthesis by-product.
d. CaCO3 4. May be formed when wastewater containing a particular kind of contaminant fl ows into a body of very hard water.
15. In terms of their potential for reactions with species in solution, how might metal atoms, M, on the surface of a metal oxide, MO, be described?
16. Air is 20.95% oxygen by volume. If air at 1.0000 atm pressure is bubbled through water at 25∞C, what is the partial pressure of O2 in the water?
126 Environmental Chemistry 17. The volume percentage of CO2 in a mixture of that gas with N2 was determined by bubbling
the mixture at 1.00 atm and 25∞C through a solution of 0.0100 M NaHCO3 and measuring the pH. If the equilibrium pH was 6.50, what was the volume percentage of CO2?
18. For what purpose is a polymer with the following general formula used?
n SO3–
C C
H
H H
19. Of the following statements, the one that is true regarding colloids is: (A) Hydrophilic col-loids consist of aggregates of relatively small molecules; (B) hydrophobic colcol-loids do not have electrical charges; (C) hydrophilic colloids are those formed by clusters of species, such as H3C(CH2)16CO2-,; (D) association colloids form micelles; (E) the electrical charges of hydrophobic colloids are insignifi cant.
20. For a slightly soluble divalent metal sulfate, MSO4, Ksp = 9.00 ¥ 10-14. An excess of pure solid MSO4 was equilibrated with pure water to give a solution that contained 6.45 ¥ 10-7 mol/L of dissolved M. Considering these observations the true statement is:
(A) MSO4 has a signifi cant degree of intrinsic solubility; (B) the solubility product, alone, accurately predicts solubility; (C) the value of the solubility product is in error; (D) the concentration of “M” in water must have been in error; (E) the only explanation for the observations is formation of HSO42-.
21. Of the following, the incorrect statement regarding sediments and their formation is:
(A) Physical, chemical, and, biological processes may all result in the deposition of sedi-ments in the bottom regions of bodies of water; (B) indirectly, photosynthesis can result in formation of CaCO3 sediment; (C) oxidation of Fe2+ ion can result in formation of an insol-uble species that can be incorporated into sediment; (D) sediments typically consist of mixtures of clay, silt, sand, organic matter, and various minerals, and may vary in compo-sition from pure mineral matter to predominantly organic matter; (E) FeS that gets into sediment tends to form at the surface of water in contact with O2.
22. Given that at 25∞C, the Henry’s law constant for oxygen is 1.28 ¥ 10-3mol/L/atm and the partial pressure of water vapor is 0.0313 atm, what is the value of [O2(aq)] for water satu-rated with a mixture of 33.3% O2 and 66.7% N2 by volume at 25∞C and a total pressure of 1.00 atm in units of mol/L?
23. Match the following regarding colloids:
A. Hydrophilic colloids 1. CH3CO2-Na+
B. Association colloids 2. Macromolecular proteins C. Hydrophobic colloids 3. Often removed by addition of salt D. Noncolloidal 4. CH3(CH2)16CO2-Na+
127