Energy & Fuels 1991,5, 568-573
568
here may be lost after extended exposure to the atmosphere, either by oxidation or loss of humidity. For example, fairly mild heating of FeOOH (at 200 "C)produces cu-Fe20, (hematite). Three of the samples examined here have not had the same rigorous protection from the atmosphere: the SIU sample of Illinois No. 6 coal and the Beulah and Big Brown Lignites. The SIU Illinois No. 6 coal is conspicuous in that it is the only material in which the iron pair signal is not observed; prominent signals are present in both of the lignites. The present observations have important implications for the study of carbon radicals in coals. The presence of mineral matter can significantly alter the EPR properties of carbon radicals, particularly in the higher rank coals. These results suggest that a significant fraction of the
radicals may not be observable in coal. Furthermore, the radicals that are observable may not be representative of the coal, but may be a subset which are less influenced by the magnetic impurities. We suggest that such factors be carefully considered when analyzing the EPR properties of coals and their products.
Acknowledgment. We are particularly grateful to A. R. Garcia for his assistance in preparation of the samples and for the use of wideline NMR techniques to determine the level of water in the EPR samples. We have benefited from a number of stimulating discussions with G. N. George, M. L. Gorbaty, and J. C. Scanlon. Ragistry No. Citric acid, 11-92-9;hydrofluoric acid, 1664-39-3; hydrochloric acid, 7647-01-0.
Speciation of Uranium in a South Texas Lignite: Additional Evidence for a Mixed Mode of Occurrence Mysore S. Mohan,* J. Drew Ilger, and Ralph A. Zingaro Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 Received December 21, 1990. Revised Manuscript Received March 20, 1991 New experimental evidence indicates that uranium in a Jackson lignite (Karnes County, South Texas) occurs principally (70-90%) in the form of uranyl humates and the rest in the form of poorly crystallized mineral(s). The low-temperature ash (LTA) of the lignite was separated by density gradient fractionation. Examination of the fractions by scanning electron microscopy-energy-dispersive X-ray spectrometry showed the presence of grains (10-30 pm) of uranium minerals only in the highest density (>2.90 g/mL) fraction. The X-ray spectral data indicate that the uraniferous grains are composites of coffinite (U(Si04)l-JOH)~Jand possibly its alteration product uraninite (UO,). Minor amounts also appear to be present. Much of the uranium (-90%) of autunite (Ca(U02)2(P04)2-10-12Hz0) which is extremely fine-grained is recovered in the lower density fractions and probably originates from the destruction of uranyl humates during the ashing. Extractions of the ground lignite with HC1 solutions of varying pH (0.7-3.0) have been carried out. The experimentally obtained uranium extraction efficiencies were compared with corresponding values calculated assuming a predominantly humic mode of occurrence for uranium. If suitable simplifications are made, the agreement is quite good.
Introduction In our earlier publications'J we reported the results of some detailed investigations on the modes of occurrence of uranium in a South Texas (eocene) lignite. A variety of experimental strategies which included float-ink separations, solvent extractions, and characterization of lignite-derived humic fractions strongly indicated a mixed mode of occurrence of uranium. Approximately 60-80% of the total uranium appears to be bound to the humic fraction, probably in the form of uranyl humates. The remainder appears to be present as fine-grained minerals. As has been noted in the earlier a precise differentiation between uranium which is truly organically associated, e.g., as uranyl humates, and fine-grained uranium minerals intimately associated with the organic matrix is difficult to establish unequivocally. Even the comparatively mild techniques used to isolate humic ma(1) Mohan, M. s.; Zingaro,R. A.; Macfarlane,R. D.; Irgolic, K. J. Fuel
1982,61,853-858.
(2) Ilger, J. D.; Ilger, W. A.; Zingaro, R. A,; M o b , M. S.Chem. Geol.
1987,63, 197-216.
terial, such as extraction with dilute alkali followed by precipitation, inevitably causes significant disruption of the original matrix and the information obtained may not be certain with respect to elucidating the original mode of occurrence. Also, the identification and characterization of the accessory minerals (such as the U-containing minerals) in the unmodified coal or even in its low-temperature ash (LTA) will be difficult because of their (comparatively) low concentrations. In the investigation reported in this paper we have attempted to further characterize the uranium by methods which should result in a minimal disruption of the sample matrix under study. A U-containing accessory mineral has been characterized by scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDS) after preconcentration by density-gradient separation of the low-temperature ash (LTA). The uranium-humic acid association has been investigated by extracting the ground coal with dilute hydrochloric acid solutions at different pH values. The experimentally obtained uranium extraction efficiencies have been compared with those that would be expected if the
0881-0624/91/2505-0568$02.50/00 1991 American Chemical Society
Energy & Fuels, Vol. 5, No. 4,1991
Speciation of Uranium
I
I
Crush lignite to pass100mesh
I=&-
I
I
569
Gray-Brown Solid Brown Suspended Material
fractionation
I
SEM-EDS
1 I
Analyze for
Black Suspended Material Brown Suspended Material
Filter
I
Filtrate: Analyze for U
1 I
Residue: Analyze for U
1
Extract with HCI
I
Filter
Figure 2. Diagram of centrifuge tube showing relative positions of LTA fractions.
I
at varying pH
1
Filter
rlrz Filtrate: Analyze for U
1
Residue, Analyze for U
Figure 1. Flow chart of sequential steps in the speciation of uranium.
uranium were associated predominantly with the humic acids. A flow chart illustrating the sequence of steps involved in the experimental methodology is given in Figure 1.
Material and Methods Coal Sample. The lignite sample was obtained from the Pawelek mine (Karnes County, TX) at the top of the Tordilla Sand Member of the Whitaett Formation of the Jackson Group. The sample was received as a gift from Conoco Corp. which operated the mine as part of the Conquista Uranium Project. A representative sample was extracted from the lumps (1-10 cm) and disaggregatedby using a jaw crusher and a Shatterbox to pam U.S. 100 mesh. The proximate and ultimate analyses of the sample have been previously reporteda2 Uranium Analysis. The uranium contents of the solid samples and the HC1 extracts were determined by delayed neutron activation analysis according to the prooedure described by Walker and Rowe? The samples were irradiated at a thermal neutron flux of 2 X 10l2n/(cm2 a) for 15 s and the fast neutron emission was counted for 60 s after a delay of 40 s to allow the short-lived reactions to cease. The detection limit is approximately 0.1 pg of U/g of sample.‘ Procedure for HCl Extractions. Solutions having the desired pH values (range pH 3.0-0.7) were prepared by dilution of concentrated HCl with distilled deionized water. Lignite samples of between 2.5 and 3.5 g were weighed to 0.1 mg into 250-mL Erlenmeyer flasks sealable with 24/40 ground glass stoppers and 150 mL of the HCl solution of the appropriate pH was added to each sample. The mixtures were stirred magnetically and the pH was monitored with a glass electrode. In cases where a small increase in pH was observed it was compensated for by adding small volumes of 2 M HCl. The extractions were carried out for 24 h. The extract was separated from the residue by using Whatman No. 42 filter paper (retention rating of 2.5 pm) and the extract saved. Each residue was reextraded twice more with HCl solutions of the same pH as the first extraction under identical conditions. Each of the three sequential extracts was concentrated separately by evaporation at reduced pressure and prepared for ~~
(3)Walker, K.L.;Rowe, M. W. Radiochem. Radioanal. Lett. 1980, 45(5),331-340. (4) Ledger, E. B.; Tieh, T. T.; Rowe, M. W. Ceostandards Newslett. 1980,4(2), 153-166.
Table I. Uranium Distribution in Density Gradient Fractionation of LTA separate %LTA density, d m L recovered U, U ~ / R % U recovered >2.9 4.2 4050 8.2 2.7-2.43 79.5 2090 78.8 2.43-2.40 13.7 1130 7.3 2.40-2.0 2.6 4610 5.7 U analysis. The residues were dried to a constant weight at 40 OC under reduced pressure before analysis. Preparation and Density Gradient Separation of the LTA Sample. The low-temperature ash of the lignite was obtained in a radio-frequency-generated oxygen plasma by using a Branson/IPC Model 4005-448 AN Automatic Plasma Systemwith four 4 in. X 8 in. Pyrex reaction chambers. It was found that unless the lignite samples were thoroughly dried before ashing they tended to deposit a tarry material during the ashing procedure. Hence, the lignite samples were dried in a microwave drying oven prior to their introduction into the reaction chambers. The samples were ashed to a constant weight with an ashing period that was typically 4-5 h between each weight determination. The surface of the sample was regenerated by stirring after each weighing. The total ashing procedure for a given sample lasted between 60 and 72 h. The density gradient separation of the minerals in the LTA was performed by using the method of differential density centrifugation. The procedure followed was that of Saether et alp A similar procedure has been described by Russel and Ri”er.6 Solutions having densities ranging from 2.9 to 2.0 g/mL in steps of 0.1 g/mL were prepared from 1,1,2,2-tetrabromoethaneand ethanoL The density gradient was made by layering 1mL of each density solution into each of two polypropylene centrifuge tubes with a Finn pipet, starting with the heaviest solution at the bottom. Approximately 0.25 g of LTA was accurately weighed and dispersed into 7 mL of ethanol which contained 10.2% (w/v) of poly(viny1pyrrolidone) (av MW 40000) as dispersant by sonication for 15 min. Three milliliters of the LTA “solution”was layered on top of each of the two density gradient solutions. The samples were spun at loo00 rpm in a Sorval RC-5B centrifuge thermostated at 25 OC by using an 99-34 rotor head which gave g values ranging from 7.5 x 109 at the top to 1.2 X lo‘ at the bottom. After centrifugation,four distinct bands separated. The distance of each band from the bottom of the tube was measured and the density estimated. The bands were carefully removed with disposable pipettes and filtered through preweighed, 0.02-rcm Teflon filters. The residues on the filters were washed with ethanol and dried to constant weight at 40 “C in a vacuum oven. SEM-EDSMeasurements. The SEM-EDS measurements were made with a JEOL35 CF Scanning electron microscope with a Tracor-Northern multichannel analyzer at the Electron Microscopy Center, Texas A&M University. The sample preparation consisted of the following steps: Epoxy pellets 1cm in diameter were prepared. In each pellet several holes 1mm in diameter were drilled. The holes were packed with the LTA sample and the (5) Saether, 0. M.; Runnels, D. D.; Meglen, R. R. Chem. Ceol.
1984/1985,47, 1-14.
(6) Russel, S. J.; Rimmer, S. M. In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.;Academic Press: New York, 1979;Vol. 3, pp 13f3-139.
Mohan et al.
570 Energy & Fuels, Vol. 5, No. 4, 1991 80-
5oooj
70-
Density (g/ml)
Figure 3. Hist~gramof the concentrations of uranium in the LTA density gradient fractions.
surface of the pellet w&s reset with epoxy. The pellets were then polished to a grit of 0.3pm and coated with carbon (2.5 nm) before examination.
Results and Discussion Distribution of Uranium in the LTA Density Fractions. A diagram of the centrifuge tube and the relative positions of the LTA fractions separated is shown in Figure 2. The percent (of total)LTA recovered in each fraction along with the uranium concentration are given in Table I and also represented as histograms in Figures 3 and 4. The highest uranium concentrations were observed in the highest P2.9 g/mL) and lowest (2.0-2.4 g/mL) density fractions. However, most of the LTA (80% of the total) and uranium (-79% of the total) was recovered in the fraction with an intermediate (2.43-2.7 g/mL) density. As discussed in the next section, all the density gradient fractions were examined by SEM-EDS and discrete uranium-containing grains were found to be present only in the highest density fraction (>2.9 g/mL). We are of the opinion that most of the uranium which is recovered in the two lower density fractions (-90%) originates from the decomposition of the humically associated uranium during low-temperature ashing. Using scanning transmission electron microscopy (STEM), Allen et al.' have demonstrated that the inorganic elements chemically bound in the organic matrix of coal form a 'gauzy" film (network) of ash particles as a result of the low-temperature ashing. The individual ash particles in this gauzy material were found to be about 30 nm across and unstable under the electron beam. I t is reasonable to expect that when a lignite is ashed at low temperature there will occur an extensive formation of this fine-grained ash if high concentrations of metals are present as humates. If a significant fraction of the uranium is humically associated the presence of corresponding quantities of extremely fine-grained uranium in the LTA follows. If, as Allen et al. observed, the fine-grained ash envelops the larger mineral particles, the presence of small amounts of uranium in the envelope will not significantly change the densities of the larger mineral particles. The high recovery of uranium in the 2.43-2.7 g/mL fraction is therefore not surprising. Even if the fine-grained particles are detached and mobilized during centrifugation, their sedimentation velocities are expected to be low which results in their reporting with the lower density fractions. Although this (7) Allen, R. M.;Carling, R. W.; VanderSande, J. B. Fuel 1986, 65,
321-326.
'IwI 20
22
2
26
n 28
30
Density (g/ml)
Figure 4. Histogram of the recovery of uranium in the LTA density gradient fractions.
mechanism of the origin of finely divided uranium in the middle- and low-density LTA fractions from the humically associated uranium is very plausible, an alternative mode of occurrence also needs to be considered. This alternative is that the uranium may be present in an adsorbed form on clinoptilolite and clay minerals present in these fractions. However, if one considers the distribution coefficients (Kd) for the sorption of uranium on these materials, it appears unlikely that this mode of occurrence accounts for a significant amount of uranium. Ames et al.8 studied the sorption of uranium from solutions which were 0.01 M NaCl or NaHC03 on clinoptilolite and other clay minerals. The distribution coefficient, Kd, of uranium, calculated from their data, ranged from 212 to 66 mL/g in the NaCl solution and from 0.77 to 0.54 in the NaHC03 solution at 25 "C. This is much less than the Kd value of 2000 mL/g for the sorption of uranium from ground water on lignite obtained from the data of S ~ a l a y . ~A comparison of the data of Szalay with that of Ames et al.8 lends support to the conclusion that sorption of uranium on clinoptilolite and other clay minerals is not a principal mechanism of uranium concentration is this deposit. SEM-EDS Studies. While samples of all four density gradient fractions were examined in detail by SEM-EDS, discrete grains containing uranium could be identified only with the highest density fraction (>2.9 g/mL). These grains ranged in size from approximately 3 X 5 pm to 12 X 30 pm. An electron micrograph of one the larger Ucontaining grains and the corresponding energy dispersive spectrum are given in Figures 5 and 6. The nature of the energy-dispersivespectrum of the uranium mineral makes its identification difficult. The presence of a strong Si peak supports the conclusion that this is a U(1V) silicate, cofA perhaps less likely alternative finite, U(SiO,),,(OH),. is that the uranium is present as an oxide, uraninite, or pitch blende and the Si signal arises from the surrounding silicate minerals. Also, the significant intensities of the phosphorus and calcium peaks suggests that lesser amounts of an oxidized uranium mineral, autunite, Ca(U02)2(P04)2.10-12H20may be present. I t is pertinent to (8)Ames, L. L.; McGarrah, J. E.; Walker, B. A. Clays Clay Miner.
1983, 31(5), 321-334. (9) Szalay, A. Geochim. Cosmochim. Acta 1964,28, 1605-1614.
Speciation of Uranium
Energy & Fuels, Vol. 5, No. 4, 1991 571 Table 11. Uranium Distribution in Hydrochloric Acid Extraction Experimento w t of
pH of HCl soln
starting lignite, g
3.0 2.5 2.0 1.5 1.0 0.7
2.425 3.307 3.760 2.649 2.889 2.779
U in 1.R.) 3170 4020 3390 1060 440 300
pg
U in extract. up! 298 849 2110 2810 3690 3710
OThe concentration of uranium in the starting material is 1520 pg/g. * Insoluble residue.
Figure 5. Electron micrograph of a uranium-rich particle in the >2.90 g/mL LTA fraction.
7000
m 2000
1000
'.. OG
5.00
10.00
ENERGY
15.00
keV
Figure 6. ED spectrum of the uranium-rich particle in Figure 5.
mention that autunite has been identified as an important uranium mineral in the Karnes area oxidized ore deposits.gJO The implications of some of these possibilities as they relate to the uranium depositional environment will be discussed later. The presence of the Fe and S peaks are evidently due to the associated pyrite and/or its alteration products. The Cu and Zn peaks are due to instrumental factors. A uranium M a (3.17 keV) X-ray dot map of the particle in Figure 5 is given in Figure 7. Several of the U-containing grains were found to be associated with phyllosilicates (kaolinite?). This leads us to the suggestion that the uranium mineral(s) may have authigenically precipitated on the silicate. The powder X-ray diffractogram of the >2.9 g/mL fraction (which was noisy) contained only lines corresponding to quartz. On the basis of powder X-ray diffraction and EDS evidence, clinoptilolite was identified as the major mineral constituent of the 2.70-2.43 g/mL fraction. Since the 2.70-2.43 g/mL fraction is also the major fraction of the LTA (78%) this is consistent with the earlier observation that clinoptilolite is the major mineral component of the unmodified LTA.2 HCI Extractions. The results of the HCl extraction experiments are given in Table 11. It can be seen that the fraction of uranium extracted increases regularly with decreasing pH and ranges from 8.1% of the total extracted at pH 3.0 to 87.7% at pH 0.7. Since there exists sub(10) Schnitzer, M.; Khan,S. V. Humic Substances in the Enuironment; Marcel Dekker: New York, 1972.
stantial indirect evidence that uranium is associated with the humic component in this it would be interesting to determine whether the results of the HCl extraction experiments, limited as they are in scope, would provide additional evidence with regard to this association. During the past two decades there has been an increasing interest in the study of the interactions of metal ions with humic materials.lOJ1 It is now well recognized that, because of their ubiquitous nature and strong tendency to associate with metal ions, the humic materials strongly influence the species distribution of metals in the environment. The humic substances are polydisperse materials consisting of a t least two distinct classes of ligands (carboxylic and phenolic) with which association with protons or metal ions could take place. In the literature, the potentiometric and other experimental data have often been fitted to one- or two-parameter Scatchard12plots to obtain the "intrinsic" association constants for each class of sites and the number(s) of sites of each type.13-15 However, in the recent literature, several authors have considered this approach to be totally empirical.16 The mixture of ligands present in the humic acids represents an extremely complex situation and the usual methods which are successful in describing the association between the small or well-characterized ligands and metal ions are inappropriate. Realistically, humic substances are best represented as polymers containing a continuous distribution of nonidentical functional groups which can bind protons and metal ions. As a result, the equilibrium functions vary continuously with the solution ~omposition'~ and it is possible to extract reasonably constant values for the association "constants" only when the solution is invariant with respect to total metal and total ligand concentrations. From the foregoing discussion it is apparent that even when the reactants are present in true solution, a quantitative description of the equilibria involving humic substances and metal ions is difficult, if not intractable. The extraction of lignite with a dilute acid solution represents a heterogeneous system and hence a quantitative treatment would be even more complex. Given the complexity of the system and limited data available, it is realistic to utilize the data only to determine, semiquantitatively, whether they are consistent with a model based on pre(11) Aquatic and Terrestrial Humic Materials; Christman,R. F., k i n g , E. T., We.; Ann Arbor Science: Ann Arbor, MI, 1982. (12) Scatchard, G.; Coleman, J. S.; Shen, A. L. J. Am. Chem. Soc. 1957,
79,12-20. (13) Mantoura, R. F. C.; Riley, J. P. Anal. Chem. Acta 1975, 78, 193-200. (14) Bresnahan, W. T.; Grant,C.L.;Weber, J. H.Anal. Chem. 1978, 50,1675-1679. (15) Spsito, G.Enuiron. Sei. Technol. 1980, 15, 396-403. (16) Perdue, E. M.; Lytle, C. R. Enuiron. Sci. Technol. 1983, 27, 654-660. (17) Gamble, D. S.; Underdown, A. W.; Langford, C.H. Anal. Chem. 1980,52,1901-1908.
Mohan et al.
572 Energy & Fuels, Vol. 5, No.4,1991
Figure 7. X-ray dot map of the uranium-rich particle in Figure 5.
dominant uranyl-humic association. In order to perform these calculations the following conditions and/or simplifications were assumed to be valid: (1)The system is treated as being homogeneous with all the binding sites being freely accessible to the ions. (2) Because the weight of the lignite and the volume of the extracting solutions are essentially invariant, the total uranium (TM)and total ligand (Tdconcentrations are also essentially unchanged. Under these conditions, we can expect the relevant equilibrium conatants also to remain invariant." (3) In the experimental pH range the uranyl ion-proton exchange only at the carboxylic sites need be considered; Le., all the phenolic sites remain protonated. (4) No activity corrections are made. (5) Only the uranyl ion-proton exchange is considered. Multiple metal ionexchange equilibria18 are not considered. Using the material balance equations
and the expressions for the equilibria
it is easy to show that
The left-hand side of eq 5 which is the extraction efficiency, is experimentally determined. The other literature and experimentally determined quantities used in determining the two sides of eq 5 are the following: TM, total uranium concentration; calculated from U concentration in the starting lignite (1500 pg/g), weight in grams of the (18) Gamble, D. S.; Schnitzer, M.; Kerndorff, H.; Langford, C. H.
Geochim. Cosmochim. Acta 1983,47,1311-1323.
w
0
1
2
3
4
PH
Figure 8. Experimental and calculated uranium extraction efficiencies for the first HC1 extraction step: ( 0 )calculated; ( 0 ) experimental.
lignite, and the volume of the extracting solution; TL,total concentration of the carboxylic groups, calculated h m the experimentally determined concentration in the lignite' (4.3 mequiv/g), weight in grams of the lignite and volume of the extracting solution; K,, the pKa value of 4.19 f 0.06 reported by Shanbhag and Choppinlgfor the dissociation of the carboxylic groups of purified Aldrich humic acid was used. The use of one of the other pKa values available in the literature such as that of Perdue and Lytle16 (PKa = 3.62) will not significantly change the results of our calculation. Theoretical percent extraction efficiency (rightrhand side of eq 5) at each pH value was calculated for varying values of K w The best fit with the experimental values was obtained at a log KM value of 4.8 (Figure 8). This value may be compared with the correspondingvalue (log &) of 5.11 obtained by Shanbhag and Choppinlgfor the binding of uranyl ion by humic acid. (19) Shanbhag,P. M.; Choppin, G. R J. Inorg. N u t . Chem. 1981,43, 3369-3372.
Energy & Fuels, Vol. 5, No. 4, 1991 673
Speciation of Uranium l
o
0
1
o
2
k
3
4
PH
Figure 9. Experimental and calculated uranium extraction efficiencies for the second and third extractions with HCk (upper curve) calculated; (middle) second; (lower) third.
Taking into account the complexity of the system, the agreement is perhaps an indication that there is a gross justification for the simplifications made in order to calculate the theoretical extraction efficiency. In the pH range studied the calculated extraction efficiencies were not significantly changed by postulating the presence, also, of an ML,species. Hence these species were not included in the calculation. The experimental uranium extraction efficiencies observed in the case of the second and the third extraction steps were also compared with the calculated values. In these calculations, the total uranium concentration, TM, was based on the uranium content of the lignite remaining after the previous extraction. For a given value of KM,the calculated extraction efficiency at a given pH remains the same for each of the extractions steps, since the value of TLis the same. The results illustrated in Figure 9 show that the model breaks down in the case of the second and third extractions. These results are consistent with the explanation that uranium extracted in the first extraction represents the easily exchangeable, h u m i d l y associated uranium (-70% of total).
Concluding Remarks To summarize our experimental results, approximately 10% of the uranium originally present in the lignite is recovered in the form of large-grained (1630pm) uranium minerals in the highest density (>2.90 g/mL) fraction of the LTA. Examination of these grains by SEM-EDS indicates that they are composites of coffinite or its alteration products. The uranium recovered in the lower density fractions is extremely fine-grained. In the HC1 lignite extractions, the experimental uranium extraction efficiencies at varying pH values show good agreement with
values calculated assuming that uranium is associated with the humic fraction of the lignite. The best fit is obtained when the value of the association constant (log KM)is adjusted to 4.8 which can be compared with the corresponding literature value of 5.11. The experimental extraction efficiencies in the second and third extraction steps show very poor agreement with the calculated values. It is interesting to examine the results of our study with the current theories view of the mechanism of uranium deposition in the lignite-sandstone ore bodies of South Texas.2 Lithologic evidence indicates that these deposits are of the roll-front or roll type with the uranium originating from the diagenetic alteration of the source rock (Catahoula Tuff). Investigators are generally in agreement that the formation of a roll-front deposit is initiated when oxygenated U-bearing groundwater advances through the porous sandstone and contacts reduced (unaltered) sediments. The reduced sediments usually contain, pyrite, H2S, and carbonaceous materials. When the EH-pH conditions resulting from the presence of these components are favorable, the emplacement of uranium, often in the form of U(1V) minerals uraninite or coffinite, is believed to occur. The co-occurrence of a U(IV) mineral and uranyl humates in this sample suggests that the emplacement of uranium may involve the following mechanisms: (i) Uranium is transported via the groundwater in the form of U02(C03)22-.This assumption is predicated on the slightly alkaline nature (pH range 7.4-8.3) of the present day natural waters in Karnes County20and the abundance of calcareous minerals. When this groundwater enters an acidic environment perhaps caused by the oxidation of pyrite, the resulting uranyl ions are scavenged by the humic matter or are precipitated as oxidized uranium minerals, e.g., autunite. If the contact zone is inhomogeneous, it is likely that there will be microenvironments where more negative EH values permit the precipitation of U(1V) minerals. (ii) The U(1V) minerals initially precipitated in the contact zone (reduced zone) are oxidatively recycled and the released uranyl ions are trapped by humic substances.
Acknowledgment. Financial assistance furnished by the Robert A. Welch Foundation of Houston, Texas, in support of this research is gratefully acknowledged. We thank Ms. Lisa Donaghe and Mr. Jim Ehrman of the Electron Microscopy Center, Texas A&M University, for their expert assistance. Registry No. U,7440-61-1; coffinite, 14485-40-6; uraninite, 12143-25-8;autunite, 12333-86-7. (20) Texas Department of Health. "Chemical Analysis of Public Water Systems";1972; 95 pp.
