Environ. Sci. Technol. 2003, 37, 2878-2882
Low-Field NMR Relaxometry: A Study of Interactions of Water with Water-Repellant Soils TIONA R. TODORUK,† MARINA LITVINA,† APOSTOLOS KANTZAS,‡ AND C O O P E R H . L A N G F O R D * ,† Department of Chemistry and Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
Petroleum-induced water repellency in soils is a problem that has been thought to develop randomly following contamination and then remediation of a site with petroleum. The emergence of the phenomenon can occur within months or years of original contamination and with seemingly no warning. Low-field NMR has been used to study these soils and, specifically, the processes of water uptake that occur in them. Critical aspects in the development of this phenomenon have been identified as wellsspecifically, a dependence on climatic events in the area and contamination levels that contribute are suggested.
Introduction Development of “water repellency” in soils is a widespread phenomenon that arises both naturally and as a result of contamination with hydrophobic pollutants. Water repellency is seen in a variety of circumstances worldwide including citrus groves, subalpine tundras, golf greens, and sandy soils of Australia and The Netherlands (1). As well, it arises in urban and cultivated soils that have been contaminated with hydrophobic contaminants, notably petroleum hydrocarbons in liquid or vapor phases (1). Of course, initial exposure to hydrocarbons can produce a simple form of water repellency that disappears after remediation. The puzzles arise, however, because of the irregular pattern of subsequent reoccurrence of water repellency and its variable reversibility (2). Remediation of petroleum-contaminated soils has proven successful in many instances. Bioremediation, for example “landfarming”, is one of the more common approaches because it is cost-effective and involves in situ exploitation of natural soil organisms (3, 4). Sometimes, it can be effective in mineralization of organic contaminants (2, 3), although it is not uncommon for hydrocarbons above C30 to remain in the soil as a result of incomplete degradation. In instances such as these, water repellency often reemerges, apparently “arbitrarily”, after a number of years. In this context, it is important to consider the detailed history of the site. It is noteworthy that the most common cases of reversible water repellency occur in hot dry climates such as those seen in Australia (2). Low-field NMR, or relaxometry, provides a unique and powerful method to characterize the movement and redistribution of water within a soil. It distinguishes among * Corresponding author phone: (403)220-3228; fax: (403)289-0344; e-mail:
[email protected]. † Department of Chemistry. ‡ Department of Chemical and Petroleum Engineering. 2878
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environments on the basis of the T2 relaxation times of the water proton signals. These times are controlled primarily by the motional freedom of the protons so that water bound to surfaces or restricted in narrow pores exhibits relaxation times greatly reduced from the 3-s relaxation time characteristic of pure bulk water. Of course, where water protons can exchange between environments on the time scale of NMR measurements, the signal observed will be the average of the T2 values of the environments weighted for the relative populations. Low-field NMR is widely applied to water distributions in porous media such as drill cores (5), reservoir fluids (6), rock (7), and food products (8, 9) but only recently to soils (10, 11). The capacity of T2 to identify pore size distributions by distinguishing the respective environments of water (12) makes this technique ideal for study of the dynamics of water redistribution in complex media such as soils. More details of this approach to monitoring soil water systems are given elsewhere (11). To understand how the phenomenon of water repellency develops, attention must be given to understanding water uptake and redistribution processes in clean soils. This has been reported elsewhere (11). Thus, the objectives of this study are to (i) understand water redistribution in contaminated soils and the potential for wetting to occur in apparently water repellent soils, (ii) to examine kinetic processes of water redistribution and the “apparent activation energies” that can be derived from these processes, and (iii) to examine other possible factors that may influence the development of this apparently random phenomenon.
Experimental Section Soils. Samples were acquired from the Ellerslie (ELL) and Devon (DEV) sites. Details on soil composition are given in the accompanying paper (22). The ELL site was an experimental petroleum spill site (24°51′25′′ NE, 4° W) about 18 km south of Edmonton and owned by the University of Alberta. The soil is an Eluviated Black Chernozem of the Malmo silty clay loam series (1). The contamination, conducted in 1973 with Redwater crude oil (RW), and subsequent remediation efforts are well documented. The time frame of development of water repellency is approximately known and it has been possible to search records for climatic conditions surrounding the period of phenomenon development (23). Our first set of samples are a nonwettable sample from the spill site designated ELL-NW and a control from the spill site that was contaminated but remained wettable after remediation designated ELL-CW. We also have data on a sample of the same soil type from near the contamination site, designated ELL-PW (for pristine wettable), that has been reported elsewhere (11). The Devon site (DEV) was the second site chosen. It is located approximately 15 km southwest of Edmonton, AB, at the legal coordinates of 23°50′26′′ NW, 4° W. The original soil is classified as a weakly Eluviated Black Chernozem of the Ponoka loam series. It developed on Alluvial Lacustrine, medium-textured materials. In the native state, the area was covered with a mixture of poplar groves and shrub patches. The present vegetation is characteristic of the Aspen Grove section. The topography is level to undulating with long and smooth slopes resulting in a naturally, well-drained soil (13). The site was first contaminated on March 8, 1948, when a well (Atlantic No. 3) drilled by The Atlantic Oil Company in 1947 blew (14). The pressure of the well was so extensive that oil bubbled through adjacent soil. When the well blew out, it required over 6 months (September 10, 1948) before the well was controlled. Overall production of the well before 10.1021/es026295t CCC: $25.00
2003 American Chemical Society Published on Web 05/31/2003
it was shut down is estimated at 1 407 000 barrels. The majority of these barrels were recovered after a short interval where the oil sat in the soil. However, losses to the soil as a result of spillage are crudely estimated at 165 000 barrels. As well as the impact of high levels of contamination, the soil was burned when an electrical spark ignited the gas pocket above the well on April 6, 1948 (14). The oil-soaked soil was also ignited. J. A. Rebus, current landowner, indicated that the field had displayed water-repellent patches for over 30 yr (13). This site, following cleanup, has residual contamination levels comparable to the ELL site. In addition to this, it is one of the better-documented sites as to the severity of the original contamination event. (Data on this site are found in the Supporting Information.) It should be noted that the terminology chosen to represent these soils is used to maintain consistency with other reports on these soils in the literature (13). Oil. RW is a light, conventional oil that is primarily parrafinic in composition. The specific gravity is approximately 0.85. The viscosity is 6.3 cP, as determined by lowfield NMR measurements. The oil contains hydrocarbons ranging from very small fragments to chains at least as large as C60. Residual hydrocarbons apparently >C19 in size were still observed in CH2Cl2 extracts from the contaminated soils using gas chromatography. Data are found in the accompanying paper (22). Low-Field NMR Relaxometry. Measurements were performed with a Corespec-1000 relaxometer at a frequency of 1 MHz that corresponds to a field strength of approximately 0.024 T. The Carr-Purcell-Meiboom-Gill pulse sequence was used (15). Measurements were recorded with interecho spacing of 0.3 ms with a series of 5000 pulses. The gain of the system was set to 8 in order to increase the signal-tonoise ratio. A set of 16 trains was used for measurement of the experimental samples to improve the confidence in the results. Post-train delay was set to 5500 ms in an attempt to allow all water to return to equilibrium. Initially, spectra were recorded every 30 min. After about 4 h, frequency was reduced to one per hour. When changes became small, spectra were recorded once daily until it was judged that no further significant changes in T2 distributions were occurring. Most samples were run at both 30 ( 1 and 40 ( 1 °C to characterize temperature dependence of kinetic process. Consideration was given to the fact that T2 can be affected by temperature ((5%), but in this range that effect is small (11). Samples were loosely packed in 3.5 cm by 5.0 cm glass vials. The height of the soil column in the vial was approximately 1.5 cm. Water was added dropwise at a rate of approximately one drop (∼0.05 mL) per second. Mass was monitored during water addition until the mass of water approximately equaled the mass of soil (1/1 w/w). Vials were sealed with Teflon tape to minimize evaporation. Samples were stored wrapped in Parafilm to further minimize evaporation. Mass was recorded daily to ensure that water loss was negligible. The mass of water used in the measurements was significantly higher than the minimum detection limits of the relaxometer. In some experiments, good signalto-noise (S/N) has been obtained with sample sizes containing as low as 0.5 g of water. In this system, quantities of water tended to exceed 10 g or more, eliminating concerns that measurements may not provide an adequate S/N ratio. Data Analysis. Data were analyzed using EchoFit v.3.02, a software package created by NUMAR Corporation, a Haliburton subsidiary. Fitting exploited the non-negative least-squares method to create T2 distributions out of a maximum of 5000 echoes. Resulting distributions were imported into Microsoft Excel. Data were normalized to show each spectrum on a scale of percent where 100% represents the integral under the total T2 distribution. It was found that almost no difference resulted in integrated absolute ampli-
tudes between peaks that were deconvoluted and peaks that were simply cut (80 kJ mol-1) and not those associated with diffusion. (The fast uptake of bulk water in PW and CW samples appears to require only a diffusion model since Eapp < 42 kJ mol-1 (20).) These diffusional processes do not require more than 24 h to reach a steady state. Comparable results are observed in the DEV-CW and DEV-NW soil systems (see Supporting Information). The question arises as to what is the nature of the “chemical” process with an activation energy above 80 kJ mol-1. The values are similar to activation energies for ester hydrolysis, and we cannot exclude the idea that as micropores close during air-drying some -COOH and -OH groups of soil organic matter (SOM) form ester linkages. However, it seems more likely that the large activation energies have a more general and complex origin involving more of the functionality of the SOM. An attractive picture is of a cooperative rearrangement in which the orientations of SOM components favoring hydrogen bonding with micropore water are turned around to maximize nonbonded van der Waals interactions, internal hydrogen bonds, and charge transfer interactions as water is lost. The result is a hydrophobic surface presented to entering molecules. Then re-wetting requires a major cooperative reorganization to restore the configurations favoring hydrogen bonds to water. That such substantial conformational reorganization of humic substances can occur with wetting is qualitatively supported by computational molecular modeling and quantum mechanics on a fulvic acid model dry and in water (21). Model for Water Repellency. The difference in water uptake by water-repellent soils is highlighted by the kinetic
k30 (d-1)
k40 (d-1)
Ea (kJ/mol)
ELL-NW micropore (1 ms) kf (growth) micropore (1 ms) ks (growth) micropore (10 ms) kf (growth) micropore (10 ms) ks (decay) mesopore (60-300 ms) kf (growth) mesopore (60-300 ms) ks (decay) bulk water (3000 ms) kf (decay) bulk water (3000 ms) ks (decay)
2.5616 0.1241 0.9391 0.2503 1.4896 0.1985 0.3912 0.1321
7.0174 0.3777 9.4768 5.582 4.5607 1.3738 4.771 1.8371
80 88 182 245 88.3 153 197 207
ELL-CW micropore (3 ms) kf (growth) micropore (3 ms) ks (growth) mesopore (60-300 ms) kf (growth) mesopore (60-300 ms) ks (decay) bulk water (1000 ms) kf (decay) bulk water (1000 ms) ks (decay)
1.1694 0.1161 0.8346 0.1135 0.3628 0.0854
2.0342 0.3871 1.6832 0.4487 0.5382 0.409
44 95 55 108 31 123
DEV-NW micropore (1 ms) kf (growth) micropore (1 ms) ks (growth) micropore (10 ms) kf (growth) micropore (10 ms) ks (decay) mesopore (60-300 ms) kf (growth) mesopore (60-300 ms) ks (decay) bulk water (3000 ms) kf (decay) bulk water (3000 ms) ks (decay)
0.4479 0.0966 1.5767 0.131 1.5197 0.179 2.3169 0.2706
0.7895 0.2093 3.0893 0.2969 3.1372 0.4348 6.1899 0.9322
44.8 61 53.1 64.6 57.2 70 77.8 97.9
DEV-CW micropore (3 ms) kf (growth) micropore (3 ms) ks (growth) mesopore (60-300 ms) kf (growth) mesopore (60-300 ms) ks (decay) bulk water (1000 ms) kf (decay) bulk water (1000 ms) ks (decay)
0.9254 0.2089 1.1086 0.2397 2.4255 0.2067
3.0174 0.7814 2.1021 0.5574 3.6215 0.4066
93.3 104.1 50.5 66.6 31.6 53.4
a
ELL-PW is reported elsewhere (11).
measurements. First, the large bulk water peak decreases but remains throughout the experiments. Over time, peaks associated with mesopore and micropore water develop. Eventually, mesopore water decreases in favor of increase of micopore water. However, these peaks never grow to the same amplitude as is observed in PW and CW samples. Large apparent activation energies are the rule. The implication is that significant changes in structure, especially of SOM, have taken place. An attractive picture related to the postulates of Roy and McGill (1) is one in which natural components of SOM form strong interactions with diagenetic products of the petroleum hydrocarbons during a drying event. The result is a strongly hydrophobic surface presented to entering water. Reaching the sites for the normal micropore hydrogen bonds requires extensive rupture of the complexes. The complexes must break up before micropores containing hydrogenbonded water can form. This proposal suggests two lines of further investigation: (i) Can the conditions for a special drying event be identified? (ii) Can chemical processing and spectroscopy of the organic matter elucidate the petroleumnatural SOM complex? The first of these is taken up below. The second is treated in an accompanying paper (22). The detailed information on the Ellerslie experimental spill site is very helpful. The specific sites developing the “arbitrary” water-repellent character were parts of areas most heavily contaminated (50-90 gal/150 ft2 plot) and least vigorously remediated (only barley or fallow followed by normal cultivation). Comparable phenomena are observed at the DEV site where all the most highly contaminated sites tend to develop water repellency. There is one exception to this, a low-lying area near the wellhead. The pitch of the land, however, is such that the entire site would naturally VOL. 37, NO. 13, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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drain to this location, and as such, the evacuation of all residual water (as will be discussed later) becomes substantially more difficult. This evidence suggest that there is a critical level of residual hydrophobic contaminants. As to timing of emergence, comparison with another site known as the Redwater site (RWS) is helpful. This underwent the same treatment as ELL. In both it is known that water repellency developed between 1977 and 1982. Inspection of aerial photographs of the RWS shows development between 1981 and 1985. This narrows the timeline to 1981 and 1982. Comparison of climate records from the nearby Edmonton International Airport shows that both years had significantly hotter (2-3 °C) and dryer summers (80-110 mm of rain less) than preceding years (23). Similarly, the landowner of the DEV site refers to water repellency developing sometime throughout the mid-1960s (13), and a search of climatic records indicates hot, dry years prior to his recalled time of development. Thus, the second factor suggested is unusual drying that eliminates water from more strongly held sites and forces the rearrangement of the natural organic matter in these sites. The formation of the hydrophobic complexes then seems to require a critical concentration of hydrophobic hydrocarbons (possibly partially oxidized to provide some groups for hydrogen bonding to natural organic components) and a drying event sufficient to remove water from strongly hydrogen-bonding sites. Preliminary experiments agree well with this paper.
Acknowledgments The authors would like to acknowledge an NSERC strategic grant and Imperial Oil Ltd. for funding, Florence Manalo for her tremendous effort with processing NMR files and her intellectual input, and Dr. Julie Roy for her continuing advice and guidance.
Supporting Information Available Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Roy, J. L.; McGill, W. B. Can. J. Soil Sci. 1998, 78, 331.
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(2) Dekker, L. W.; Ritsema, C. J.; Ostindie, K.; Boersma, O. H. Soil Sci. 1998, 163, 780. (3) McNicoll, D. M.; Baweja, A. S. Environment Canada Report EN40-491; Environment Canada: Ottawa, 1995. (4) St.-Cyr, M.; Nelson, C. H.; Hawke, C. T. Oil Gas J. 1992, 2, 68. (5) Straley, C.; Rossini, D.; Vinegar, H.; Tutunjian, P.; Morriss, C. Log Anal. 1997, 2, 84. (6) Kleinberg, R. L.; Vinegar, H. J. Log Anal. 1996, 7, 20. (7) Akselrod, S.; Mirotchnik, K.; Kantzas, A.; Allsopp, K. Annual Meeting Conference Proceedings; Society of Core Analysts: 2000. (8) Thybo, A. K.; Bechmann, I. E.; Martens, M.; Engelsen, S. B. Lebensm.-Wiss. Technol. 2000, 33, 103. (9) Signe, J. M.; Pedersen, H. T.; Soren, E. B. J. Sci. Food Agric. 1999, 79, 1793. (10) Kantzas, A.; Todoruk, T.; Manalo, F.; Langford, C. H. Annual Meeting Conference Proceedings; Society of Core Analysts: September 2001. (11) Todoruk, T. R.; Langford, C. H.; Kantzas, A. Environ. Sci. Technol. 2003, 37, 2707-2713. (12) Chen, S.; Liaw, H. K.; Watson, A. T. J. Appl. Phys. 1993, 74, 1473. (13) Roy, J. L. Soil Water Repellency at Old Crude Oil Spill Sites; University of Alberta: Edmonton, AB, Canada, 1997. (14) Kerr, A. Atlantic 1948 No. 3; Friesen Printers: Altona, MB, Canada, 1986. (15) Brey, W. S., Ed. Pulse Methods in 1D and 2D Liquid-Phase NMR; Academic Press Inc.: San Diego, 1988. (16) Frost, A. A.; Pearson, R. G. Kinetics and Mechanism; John Wiley and Sons: New York, 1953. (17) Xu, X.; Davis, L. A. SPE On-Line J. 1999, Paper SPE 56800. (18) Huang, W.; Young, T. M.; Schlautman, M. A.; Yu, H.; Weber, W. J., Jr. Environ. Sci. Technol. 1997, 31, 1703. (19) Belliveau, S. M.; Henselwood, T. L.; Langford, C. H. Environ. Sci. Technol. 2000, 34, 2439. (20) Eyring, H.; Polanyi, M. Z. Phys. Chem. 1931, B12, 279. (21) Bruccoleri, A. G.; Sorenson, B. T.; Langford, C. H. In Humic Substances, Structure, Models and Functions; Ghabbour, E. A., Davies, G., Eds.; Special Publication 273; Royal Society of Chemistry: Cambridge, 2001; pp 193-208. (22) Litvina, M.; Todoruk, T. R.; Langford, C. H. Environ. Sci. Technol. 2003, 37, 2883-2888. (23) Access to records available through http://www.ec.gc.ca/ pands_e.html, last accessed March 2003.
Received for review November 2, 2002. Revised manuscript received April 9, 2003. Accepted April 29, 2003. ES026295T
