Environ. Sci. Technol. 1997, 31, 2707-2711
Carbonate Leaching of Uranium from Contaminated Soils C . F . V . M A S O N , * ,† W . R . J . R . T U R N E Y , † B. M. THOMSON,‡ N. LU,† P. A. LONGMIRE,† AND C. J. CHISHOLM-BRAUSE§ CST-7, Environmental Science and Waste Technology, Chemical Science & Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Civil Engineering, University of New Mexico, Albuquerque, New Mexico 87305, and Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, Virginia 23062
Uranium (U) was successfully removed from contaminated soils from the Fernald Environmental Management Project (FEMP) site near Fernald, Ohio. The laboratory column leach process, referred to as the simulated heap leach process, using 0.5 M sodium bicarbonate as the dominant reagent, was able to achieve uranium removals of 75-90%, corresponding approximately to the percentage of uranium in the oxidized state. Parametric optimization studies are reported. The dissolution of uranium took place in two stages: a rapid desorption associated with soil surfaces and a slow step associated with diffusion of uranium toward solid surfaces. In addition, use of the oxidizing agent, sodium peroxide, improved uranium removal due to oxidation of U(IV), enhancing the solubility of the uranium. The results suggest that the process will be effective for field scale remediation of uranium-contaminated soils because of the efficiency, mild complexing agent employed, lack of prescreening of the soil and the simple equipment necessary. Two relevant companion studies have recently been completed. The first, a scale-up demonstration [Turney, W. R. J. R.; Mason, C. F. V.; Lu, N.; Duff, M. C.; Dry, D. Pilot Treatment Project for the Remediation of Uranium-Contaminated Soil at a Former Nuclear Weapons Development Site at the LANL; Waste Management ’97, Tucson, 1997], using a Los Alamos site, confirms the approach to be effective up to 1 ton of soil and the second, a cost study, suggests this method is economically competitive for large soil volumes (>1000 cu yd) when combined with a radionuclide presort [Cummings, M.; Booth, S. R. Remediation of Uranium-Contaminated Soil using the Segmented Gate System and Containerized Vat Leaching Techniques: A Cost Effectiveness Study. Remediation 1996, 7, 1-14].
Introduction One of the legacies of the nuclear age is radioactive contamination of the environment surrounding facilities where radionuclides were processed, both for nuclear weapons and for nuclear power. Contaminated areas exist in all * Corresponding author telephone: 505-665-2422; fax: 505-6654955; e-mail:
[email protected]. † Los Alamos National Laboratory. ‡ University of New Mexico. § College of William and Mary.
S0013-936X(96)00843-7 CCC: $14.00
1997 American Chemical Society
countries where nuclear processes have been developed including the U.S., France, Britain, Russia, and Australia. The FEMP site was chosen by the Department of Energy (DOE) to evaluate technologies for large scale remediation of uranium-contaminated soils, referred to as Uranium in Soils Integrated Demonstration (USID) (1). This site is located at the Feed Materials Production Center (FMPC) where uranium metal and uranium tetrafluoride (UF4) were produced between 1951 and 1989. The resulting widespread uranium contamination is estimated at 2-4 million m3 of soil with uranium above regulatory standards of 1295 Bq/kg (2). Methods (3) used to extract uranium from its ores may potentially be adapted to achieve remediation of contaminated soils and water. Milling of uranium ore is accomplished by crushing the host rock and leaching with either strong acidic or strong alkaline solution. The acid leach process, using sulfuric acid, is the most common technology, but for carbonate-bearing soils, the only feasible process is the alkaline leach. The alkaline leach process is highly selective for uranium with the formation of the soluble uranyl tricarbonate complex (3, 4):
UO22+ + 2HCO3- + CO32- f UO2(CO3)34- + 2H+
(1)
However, this process only solubilizes uranium(VI). An additional step is needed to oxidize the U(IV) to U(VI). Uranium may then be recovered from solution, using ion exchange or other methodologies (5). The soil at the FEMP site has large amounts of naturally occurring calcite (CaCO3) (1, 6), making the alkaline leach process suitable for decontamination of uranium. In the commercial heap leach process, the leach solution is applied to soils at low application rates (∼0.01 cm/min) and percolates through the heap, extracting precious metals from low-grade ores down to levels of microgram per kilogram. The objectives of this research were (1) to examine the applicability of heap leach processes for remediation of uranium-contaminated soil from the FEMP site using carbonate or bicarbonate, (2) to determine the effects of various parameters including total carbonate concentration, temperature, solution application rate, bicarbonate to carbonate ratio, and selected oxidation reagents, and (3) to investigate the leachability of other metals present.
Experimental Procedures Soil Samples. Three soil samples were collected from the FEMP site and are shown in Table 1. The SP4(93) and SP9(93) (both received in 1993) were preprocessed at the FEMP site. Before the samples were shipped, the soils were tumbled in a mixer with just sufficient water to control dust emissions. During this tumbling process, the clays and silts agglomerated to form balls of soil 1-2 cm in diameter. The larger particles (>1.91 cm) and some soil organic matter (SOM) were removed. The SP9(94) soil (received in 1994) was not preprocessed and contained small amounts of organic matter including grass etc. Soil Preparation. The soils were tumbled in a rotational mixer with water to bring soil moisture to about 10% and rolled to form small homogenized agglomerated spheres (1-2 cm diameter). Agglomeration is used in the mining industry when a high clay content is present to increase hydraulic conductivity and eliminate preferential flow paths. All the soils used had high silt and clay content (∼80% of total weight) (Table 1). The soils were not separated into size fractions before being placed in the columns. Experiments. Details of the leach schemes of the six experiments are given in Table 2.
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TABLE 1. Description and Characterization of Soil Samples property
SP4(93) (screened at FEMP)
SP9(94) (unscreened)
origin major source of contamination total U mg/kg U (VI) %
storage pad (incoming scrap) liquid spills and airborneb
pH soil organic matter sand % silt % clay %
∼7.2 ∼2.3 13.9 52.6 32.9
incinerator areaa incinerated waste and airborne 1320 70-80 (varied with soil fraction) (8) 6.9 5.4 16.8 50.8 31.7
420 90 (8)
SP9(93) (screened at FEMP, removed particles > 1.9 cm) incinerator areaa incinerated waste and airborne 538 80-90 (varied with soil fraction) (8) 7.4 2.3 21 48.4 30.5
a The incinerator area is adjacent to the low level radioactive debris incinerator resulting in high-fired particles being deposited on the surrounding soils. b Airborne species are uranium dioxide (UO2), uranium sesquioxide (U3O8), and UF4.
TABLE 2. Experimental Parameters Used in the Six Experimentsa exp
T (°C)
amounts of soil used (g)
total CO32- (M)
chemical composition
HCO3-:CO32-
application rateb (cm/min)
1 2
25 25
1000 1000
KHCO3/K2CO3 NaHCO3/Na2CO3
1:1 1:1
0.012 0.012
no no
3c
150
NaHCO3
1:0
0.012
no
4
25 45 65 25
0.5 0.1 0.5 0.5
1000
0.5
NaHCO3/Na2CO3
1:1
no
5
25
1000
0.5
NaHCO3/Na2CO3
6d
25
80
0.5
NaHCO3/Na2CO3
1:0 2:1 1:1 1:1
0.004 0.012 0.024 0.012 0.012
30% H2O2 Na2O2 MgO2
oxidizing reagent
no
a All experiments were carried out in duplicate. b The application rate was obtained by dividing the flow rate (cm3/min) by the cross section of the column (cm2) and corresponds to the depth/min. c The temperature was controlled using jacketed (4.8 × 30 cm) columns containing circulating hot water from a thermostatically controlled water bath ((1 °C). d Sand (10%) was added, and smaller volumes were used in order to cut down on the time needed for leaching.
Column Leaching Procedure. Laboratory-scale column systems were used to simulate an unsaturated heap leach operation. Three different size columns were used: (1) 8.6 cm internal diameter (i.d.) and 15 cm height to leach 1.0 kg of soil, (2) 4.8 cm i.d. and 30 cm for the temperature experiments to leach 150 g of soil, and (3) 4.5 cm i.d. and 5 cm height to leach 80 g of soil. The columns had a layer of Ottawa sand (white quartz sand, mesh size from -50 to +70) on top of the fiberglass screen and paper filter (pore size, 8 µm) at the bottom to support the soil. The pretreated soils were loaded into the columns with no compaction. Bicarbonate/carbonate leach solutions were prepared by dissolving appropriate amounts of (ACS reagent grade) sodium (or potassium) bicarbonate (NaHCO3 or KHCO3) and sodium (or potassium) carbonate (Na2CO3 or K2CO3) into double-deionized water. A large excess of bicarbonate/ carbonate was used throughout. A peristaltic pump transferred leach solution from the reservoir to the top of the column. Effluent samples were collected from the bottom of the column at different time intervals. The influent and effluent pH were measured. At the conclusion of the experiment, the leached soils were analyzed for uranium. Several different factors influenced the size of the column used. In early experiments, 1 kg of soil was used. This is a compromise that provides sufficient sample to compensate for local inhomogeneities, an adequate leach volume with respect to the porosity of the soil, and gives data in a reasonable amount of time. Smaller weights (150 g) were necessary in order to use available jacketted columns. Time
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constraints dictated that the oxidative experiments employed 80 g of soil, which however, gave reproducible results. Analyses. The concentrations of uranium and other metals in the effluent were analyzed using a Varian Liberty 200 inductively coupled plasma atomic emission spectrophotometer (ICP-AES) or a kinetic phosphorescence analyzer (KPA). Total uranium content in pre- or postleached soils was determined by neutron activation analysis (NAA) performed by Activation Labs, Ancaster, Ontario, Canada. Several postleached samples were taken at different radial and vertical locations to establish the distribution of residual uranium and to determine if any column wall effects or preferential flow paths existed. A mass balance for uranium was calculated for each experiment. The efficiency of uranium removal from the soil was defined as the ratio of the uranium remaining to the initial levels, expressed as a percentage. The total quantities of uranium (in each successive effluent sample as a function of time) were integrated to calculate the total amounts of uranium removed from the soil, as a function of time.
Results Characterization. The SP4(93) soil was used only in experiment (exp) 1 to examine the initial hypothesis that carbonate leaching scheme would be applicable. Due both to availability and to the perception that uranium would be easier to remove from the SP4 soils than from the SP9 soils due to the latter’s prior heat treatment in an incinerator (Table 1), the SP9 soils were used in all subsequent experiments. High-fired particles
FIGURE 3. Removal of uranium as a function of solution application rate from homogenized agglomerates of the SP9(94) soil. FIGURE 1. Removal of uranium as a function of time, leaching 1.0 kg of SP4(93) soil with 0.5 M total carbonate, 1:1 K2CO3:KHCO3 at an application rate of 0.012 cm/min.
FIGURE 2. Removal of uranium as a function of carbonate concentration from homogenized agglomerates of the SP9(93) soil. are, in general, more resistant to solubilization because the process of firing makes the crystal structure more defined and less readily solubilized. Thus, all the parametric studies were completed on the soils considered more resistant to remediation. The SP9(94) soil was expected to be a replica of the SP9(93) soil, but was found to have a higher uranium concentration, thought to be due to the processing carried out at the FEMP site on SP9(93), which removed both large particles and organic matter and thus the uranium attached to them (Table 1). KHCO3/K2CO3 Leach. The solution effectively removed 80% of uranium from the SP4(93) soil within 48 h and an additional 5% over the next 288 h (Figure 1). Duplicate experiments gave results so similar that both results fall within the thickness of the points on the graph (this was found for all experiments). The potassium salt was used to prevent decrease of permeability associated with clay swelling caused by sodium ions. However, preliminary tests showed no loss of permeability using sodium salts. Therefore, successive experiments were conducted using leach solutions of NaHCO3/Na2CO3. Carbonate Concentration. Uranium removal was independent of carbonate concentration (0.5 M versus 0.1 M) over the initial 6 h (Figure 2). After that time, there is a noticeable dependence for the next 48 h. After 48 h, the residual U was 50% higher using the less concentrated solution, and for longer times, uranium removal did not increase significantly. Temperature. Increasing temperature often increases reaction rate for solution reactions. We have shown, using 0.5 M HCO3-, that there was no appreciable change in the removal rates at temperatures of 25, 45, and 65 °C (data not shown).
FIGURE 4. Removal of uranium as a function of varying stoichiometric ratios of solid Na2O2 rate from homogenized agglomerates of the SP9(94) soil. Solution Application Rate. Changing the application rate, while maintaining the overall carbonate concentration, changes the rate of uranium removal (Figure 3). The removal rate decreased from 0.36 mM/h to 0.12 mM/h when the application rate decreased from 0.012 cm/min to 0.004 cm/min, showing a linear dependency over this range. Doubling the rate to 0.024 cm/min marginally increased uranium removal. The results show the maximum rate for 1 kg soil columns was achieved with an application rate g 0.012 cm/min. Bicarbonate/Carbonate Ratio. Varying the HCO3-:CO32ratio without changing total molar concentration changes the solution pH. The solutions with HCO3-:CO32- ratios of 1:0, 2:1, and 1:1 have pH values of 8.3, 9.9, and 10.3, respectively. Our results indicate that the ratio does not significantly influence the uranium removal in the pH range 8.3-10.3 (data not shown). Oxidizing Reagent. Uranium(IV) species, which are not dissolved by carbonate, are amenable to oxidation by various oxidizing agents, including potassium permanganate and peroxides (7). When 30% hydrogen peroxide (H2O2) was added prior to the carbonate solution, no increase in the removal of uranium was detected (data not shown) due to effervescence with heating, liberating carbon dioxide, and thus preventing uniform distribution of H2O2. This may be due to organic components or to reduced metal species. A different method was used to introduce solid peroxide (sodium or magnesium), which consisted of mixing solid peroxide with the soil prior to loading the columns. Comparative studies were undertaken (Figure 4) using different mole ratios of sodium peroxide to initial uranium concentra-
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roles (10), the tricarbonate species being dominant throughout the range. In addition, the high concentration of HCO3and/or CO32 prevents precipitation of uranium as hydroxide. The initial fluctuations in pH, as indicated in eqs 2 and 3, show only a transitory affect on the buffering capacity of the system. During the uranium leaching, some SOM was also removed. About 30% of SOM was extracted by 0.5 M CO32as organic matter (13). When the pH is greater than 8.3, the solution dissolves structural components from fresh organic matter resulting in the effluent became highly colored with time. Chemical oxidizing reagents, such as peroxides, can be used to enhance oxidizing uranium(IV) in the leach processes. The overall reaction is FIGURE 5. The concentrations of selected elements in the carbonate leach solution from homogenized agglomerates of the SP9(94) soil. tion. Sodium peroxide addition had a marked effect on initial U removal unlike solid MgO2 which had no effect (data not shown). Removal of Other Constituents. Few metals (other than uranium) form strong soluble complexes with CO32-. For exp 4, other metal concentrations were determined in the effluent (Figure 5). The concentrations of calcium, magnesium, potassium, and iron changed little with time after the first 24 h. The concentrations of iron and aluminum in the effluent were low (5 ppm of iron and 0.5 ppm of aluminum). Post Leached Soils. Analyses of the post leached soil samples (from exp 4) showed uniform uranium removal with no evidence of incomplete leaching due to wall affects or preferential pathways.
Discussion Leaching of uranium from soil is controlled by both dissolution kinetics and transport processes in addition to equilibrium formation of carbonate species. Many factors complicate the interpretation of kinetic data including surface reaction on the soil particles, unsaturated flow hydrodynamics, solid diffusion, different uranium species, and heterogeneity of contaminants (8-10). The results suggest that leaching of uranium by HCO3-/ CO32- follows a two-stage kinetic process: a rapid initial stage followed by a slower process as has been found in desorption of uranium and thorium from the mineral monazite (a crystalline orthophosphate mineral) by HCO3-/CO32- (10). In our experiments, it was observed that about 50-80% of uranium was leached in the initial stage (24 h) while up to 35% of uranium was desorbed in the second stage (24-240 h). The initial stage may be governed by a reaction ratecontrolled removal of uranium from the surface of the soil particles (10). In this stage, the reaction rate is influenced by the amount of uranium per unit surface area of the soil particles, by sorption site variability and by soil mineralogy. The subsequent slower process may be controlled by the diffusion of uranium toward the solid surface and solid/liquid reactions, but probably not by solution chemistry as there is no temperature dependence. During the leach processes, UO22+ reacts with CO32- or HCO3- to form U(VI)-dicarbonate [UO2(CO3)22-] at a relatively slow rate followed by rapid reaction with HCO3- to form soluble uranyl-tricarbonate (3, 9), [the stable form in this pH range (11, 12)]:
UO22+ + 2 HCO3- f UO2(CO3)22- + 2H+ (slow) (2) UO2(CO3)22- + HCO3- f UO2(CO3)34- + H+ (fast)
(3)
Generally, within the pH range from 8.3 to 10.3, the rate of dissolution is independent of the HCO3-/CO32- ratio because the carbonate and bicarbonate ions play equivalent
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U4+ + H2O2(aq) f UO22+ + 2H+
(4)
which significantly increases the first fast stage with little effect on the second stage. A competing reaction for the peroxide consumption is with the soil’s organic component. Characterization (6) of the initial bulk soil samples showed that approximately 75-95% of the uranium is in the hexavalent state. Thus, in a macroscopic scale, we have removed most of the U(VI). In the final bulk samples (14), the U(IV): U(VI) ratio varied with the position of the sample in the column and also with the density and size fractions. The experimental system used here did not specifically examine possible microbial-mediated reactions such as oxidation of soil organic matter or iron compounds. The latter is of considerable relevance as the presence of iron(III) compounds such as a goethite is an indication of oxidizing conditions in the soil that are consistent with the thermodynamic stability of U(VI) species (6). Uranium in the FEMP soils has been characterized by previous researchers (2, 6, 14, 15) using a variety of techniques, X-ray absorption spectroscopy (XAS) for the determination of the bulk uranium oxidation state in the soils, a combination of scanning electron microscopy (SEM) and analytical transmission electron microscopy (AEM) for the characterization of the individual uranium phases and optical luminescence, and Raman vibrational spectroscopy for fingerprinting uranium phases in the bulk soil samples (15). These various techniques complemented each other so that a complete picture of the uranium speciation in the FEMP soils was obtained. The XAS work indicated that 75 -90% of the uranium in the FEMP soils was present as the uranyl species (15), and the AEM/SEM investigations determined that the major uranium phase was meta-autunite, a calcium uranyl phosphate [Ca(UO2) 2(PO4)2‚H2O] (17). The other important uranium phases were identified with AEM as uranium metaphosphate [U(PO3)4] and uranium oxide (UO2). In addition, uranium silicide, uranium silicate (soddyite), uranium associated with fluorite, and uranium associated with iron oxide hydroxides was found during AEM studies (16). Uranyl oxide hydrate (possibly schoepite) and a uranyl organic complex were identified with optical luminescence (15). The possible presence of schoepite was also reported (15) in treated soils. Possible causes of the predominance of phosphate forms are the use of tributylphosphate during uranium processing and high levels of phosphate in the background soils, enhanced by use of fertilizers. The solubility of carbonate species suggests that any such species will have already migrated off-site leaving insoluble phosphate minerals in predominance. Characterization studies (14) of the postleached soil show different uranium speciation relative to the initial samples as well as decreased average size of soil particulates (although this varies with position of sample in the column) and detached particles from the soil matrix. The major identified uranium phases were schoepite-like (14) probably resulting
from altered schoepite due to heat treatment in the incinerator rendering it insoluble. The overall predominant reaction is
Ca(UO2)3(PO4)2 + 9HCO3- f 3(UO2)(CO3)34- + Ca2+ + 2PO43- + 9H+ (5) During the leach process, calcium was not found to be leached in significant amounts or in amounts that tracked the release of uranyl ions, suggesting it is unlikely that the solubility of a calcium-uranium phase (i.e., autunite) is controlling the concentration of calcium in the leachate (17). The leachate is oversaturated with respect to several carbonate minerals, such as calcite, resulting in the precipitation of calcite encouraged by the slow flow of carbonate solution. In addition, the dissolution reaction releases phosphate with subsequent precipitation of calcium phosphate. The aluminum and silicon concentrations in the leachate are low (∼4-30 µM and 250-420 µM, respectively). Under alkaline conditions, the low solubilities of aluminum and silicon are controlled by anionic insoluble mineral species. The low concentration of iron (0.5-140 µM) in the effluent may indicate that the solubility of ferric hydroxide Fe(OH)3 and siderite (FeCO3) control the concentrations of Fe3+ or Fe2+ (17). The low concentrations of these elements in the effluents suggest that carbonate leach is selective for removal of U, and leaves the soil material relatively unaffected (18).
Acknowledgments This research was supported by Mike Malone, program manager of the USID program, EM 50, DOE. Thanks to Charles Cotter, Angela Chacon, David Dander, Kara Johnson, Jason Kitten, David Morris, Brad Schake, Ine´s Triay, and Don York for technical help.
Literature Cited (1) Lee, S. Y.; Marsh, J. D., Jr. Characterization of Uranium Contaminated Soils from DOE Fernald Environmental Management Project Site: Results of Phase I Characterization; Report ORNL/TM-11980, Oak Ridge, TN, 1992. (2) Radiological Assessments Corporation (RAC). The Fernald Dosimetry Reconstruction Project; RAC Report CDC-5, November 1993.
(3) Merritt, R. C. In The Extractive Metallurgy of Uranium; Merritt, R. C., Ed.; Colorado School of Mines Research Institute, 1971. (4) Clark, D. L.; Hobart, D. E.; Neu, M. P. Chem. Rev. 1995, 95, 2548. (5) Lu, N.; Mason, C. F. V.; Marsh, S. F.; Turney, W. R. J. R. Evaluation of Ion Exchange for Recovering Uranium from Bicarbonate Leach Liquors from Los Alamos National Laboratory Technical Area Site Soil (LA-UR-96-3923); 18th U.S. DOE Low Level Radioactive Waste Management; Salt Lake City, Utah, May 1997. (6) Morris, D. E.; Allen, P. G.; Berg, J. M.; Chisholm-Brause, C. J.; Conradson, S. D.; Donohoe, R. J.; Hess, N. J.; Musgrave, J. A.; Tait, C. D. Environ. Sci. Technol. 1996, 30, 2322-2331. (7) Francis, C. W. Removal of Uranium from Uranium Contaminated Soils. Phase I: Bench-Scale Testing, Report ORNL/TM-6762; Oak Ridge National Laboratory: Oak Ridge, TN, 1993. (8) Bertsch, P. M.; Hunter, D. B.; Sutton, S. R.; Bajt, S.; Rivers, M. L. Environ. Sci. Technol. 1994, 28, 980. (9) Schortmann, W. E.; DeSesa, M. A. Kinetics of the Dissolution of Uranium Dioxide in Carbonate-Bicarbonate Solution. In Proc. Second United Nations Int. Conf. Peaceful Uses of Atomic Energy; Geneva, 3, 333-341, 1958. (10) Eyal, Y.; Olander, D. R. Geochim. Cosmochim. Acta 1990, 54, 1867-1877. (11) Brookins, D. G. Eh-pH Diagrams for Geochemistry; Brookins, D. G., Ed.; Springer-Verlag: NY, 1988; pp 73-81, 151-157, and 163. (12) Langmuir, D. Geochim. Cosmochim. Acta 1978, 42, 547-569. (13) Stevenson, F. J. In Humus Chemistry-genesis, composition, reactions; Stevenson, F. J., Ed.; John Wiley & Sons, Inc., 1994; 24-30. (14) Morris, D. E.; Donohoe, R. J.; Tait, C. D.; Ewing, R. C.; Finch, R. J.; Chisholm-Brause, C. J.; Musgrave, J. A. Manuscript in preparation. (15) Buck, E. C.; Brown, N. R.; Dietz, N. L. Environ. Sci. Technol. 1996, 30, 81-88. (16) Buck, E. C.; Brown, N. R.; Dietz, N. L. Mater. Res. Soc. Symp. Proc. 1994, 333, 437-444. (17) Longmire, P. A.; Turney, W. R. J. R.; Mason, C. F. V.; York, D.; Dander, D. Predictive Geochemical Modeling of Uranium and Other Contaminants in Laboratory Columns in Relatively Oxidizing, Carbonate-Rich Solutions. (LA-UR-94-401) Waste Management ’94, Tucson, 1994. (18) Bohn, H. L.; McNeal, B. L.; O’Connor, G. A. In Soil Chemistry; Bohn, H. L., et al., Eds.; John Wiley & Sons: New York, 1985; pp 262-276.
Received for review September 30, 1996. Revised manuscript received May 29, 1997. Accepted June 4, 1997.X ES960843J X
Abstract published in Advance ACS Abstracts, August 1, 1997.
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