Journal of Environmental Quality 30:1143-1149 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Ecological Risk Assessment
Factors Affecting the Ratio of Cation Exchange Capacity to Clay Content in Lignite Overburden
W.M. Stewart and
L.R. Hossner*
Department of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843
* Corresponding author (l-hossner{at}tamu.edu)
Received for publication April 21, 2000.
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ABSTRACT
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Unusually high cation exchange capacity (CEC) values relative to clay content are frequently reported for lignite overburden and minesoils. The CEC to percent clay ratio is commonly greater than one and would require that the average charge of the clay fraction be greater than 100 cmolc kg-1. A comparison of methods for particle-size distribution suggests that the major reason lignite overburden samples have CEC to percent clay ratios greater than one is incomplete dispersion of aggregates of clay minerals or shale fragments. Preliminary investigations revealed the presence of shale fragments, smectite, and partially weathered mica in the silt fraction. Methods commonly used in soil textural analysis underestimated clay content by approximately 24%. The silt fraction may, therefore, provide a "hidden" source of CEC. Another important factor influencing the CEC to percent clay ratio was the presence of organic materials (lignite) in the samples. Lignite may make a significant contribution to CEC in overburden materials. In a study designed to estimate the pH-dependent charge of both the mineral and organic fractions, the CEC of overburden organic constituents was determined to be approximately 158 cmolc kg-1 at pH 8.2. The high CEC to percent clay ratio in lignite overburden and minesoils may be resolved by adjusting methods for clay determination to optimize dispersion and by accounting for CEC due to organic materials. An alternative approach is to use existing methodology and use correction factors to account for incomplete dispersion of clay minerals and the charge contributions of organic materials.
Abbreviations: CEC, cation exchange capacity • EDS, energy-dispersive X-ray spectrometer • SEM, scanning electron microscope
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INTRODUCTION
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THE Railroad Commission of Texas uses the CEC to percent clay ratio of minesoils and overburden samples as one measure of the adequacy of laboratory data submitted by the mining companies. Cation exchange capacity to percent clay ratios greater than one are frequently reported for overburden and minesoils in the Gulf Coast lignite region. This suggests a clay fraction dominated by minerals with CEC greater than 100 cmolc kg-1 (vermiculite and/or high charge smectite), excessive levels of organic materials (lignite), or errors in the method of CEC, percent clay, or organic carbon determination.
The mineralogy of overburden and native soils overlying Texas lignite has been investigated and described by several researchers (Askenasy, 1977; Dixon et al., 1980; Senkayi et al., 1983; Arora et al., 1984). The most commonly occurring clay minerals in fluvial overburden sediments are smectite, kaolinite, mica, vermiculite, and chlorite (Dixon et al., 1980; Senkayi et al., 1983; Arora et al., 1984). Distribution of these minerals varies widely with depth and location.
Smectite is an important mineral in Wilcox Group overburden. Arora et al. (1984) found smectite to be the predominant clay mineral in the fine clay fraction of most east Texas lignite overburden samples with amounts ranging from 21 to 46%. High charge smectite has been identified in these overburden materials (Egashira et al., 1982; Senkayi et al., 1985). These researchers suggest that this mineral may be the result of mica weathering. High charge smectite has charge properties intermediate between those of smectite and vermiculite with a layer charge near that of vermiculite. Kaolinite is the most abundant mineral in the coarse clay (2–0.2 µm) size fraction and the second most abundant, after smectite, in the fine clay fraction of Wilcox Group overburden (Arora et al., 1984). Mica is common in the clay fraction of these overburden materials. Mica is generally present in lower quantities than either smectite or kaolinite (Dixon et al., 1982). Chlorite has been identified in the <2.0-µm clay fraction of Texas lignite overburden (Askenasy, 1977; McAllister, 1981; Senkayi et al., 1981, 1983; Arora et al., 1984) and occurs primarily in the reduced overburden zone (Senkayi et al., 1983). The sand and silt fractions of these materials consist primarily of quartz and feldspars with lesser amounts of mica, kaolinite, and smectite (Arora et al., 1984). Other minerals may include gypsum, jarosite, barite, calcite, siderite, magnesite, dolomite, rhodochrocite, and pyritic minerals (Dixon et al., 1982). Also siderite may weather to form goethite, hematite, and todorokite (Senkayi et al., 1986).
Reasonable hypotheses for the rather high CEC to percent clay ratios include incomplete dispersion of aggregates of clay minerals in the determination of clay content, the presence of lignite or other organic material not readily oxidized by the conventional method of organic carbon (C) determination, the presence of significant concentrations of high charge smectite, or a combination of these factors.
The primary objective of this study was to provide an explanation for the occurrence of relatively high CEC to percent clay ratios in lignite overburden samples. This involved evaluation of (i) the methods for the determination of percent clay and percent organic carbon, (ii) the pH dependence of CEC and quantifying the contributions of both mineral and organic components, and (iii) the mineralogy of selected samples including X-ray diffraction and electron microscopic analyses.
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MATERIALS AND METHODS
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Fifty overburden samples were obtained from Texas Utilities Mining Company operations (Dallas, TX) through Intermountain Laboratories (College Station, TX). The samples were from various overburden depths from three Texas Utilities mines: Big Brown, Monticello, and Martin Lake.
Laboratory Analysis
All samples had been air-dried, crushed, and passed through a 2-mm sieve and had been previously analyzed for several parameters by standard methods stipulated by the Railroad Commission of Texas. The commercial laboratory determined CEC by the NaOAc method (U.S. Salinity Laboratory Staff, 1954) at pH 7.0, percent organic C was determined by dichromate oxidation (Nelson and Sommers, 1982), and percent clay by the hydrometer method after shaking overnight in sodium hexametaphosphate solution (Gee and Bauder, 1986).
Additional Laboratory Analysis
Fifty samples were selected represent a range of chemical and physical properties (Table 1) and on the basis of CEC to percent clay ratio. Half of the samples had CEC to percent clay ratios greater than one and the other half had ratios less than one. Data in Fig. 1 illustrate the wide and relatively even distribution of samples above and below the line representing a CEC to percent clay ratio of one (1:1 line).
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Table 1. Selected chemical and physical properties of all samples (50 total) used in the study and of the samples used in the cation exchange capacity (CEC) study (16 total).
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Fig. 1. Distribution of cation exchange capacity (CEC) and percent clay values reported by a commercial laboratory for 50 overburden samples selected for general study.
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Chemical parameters were determined in duplicate for each sample. Sample pH was determined in a 1:1 soil to water suspension. The suspensions were stirred intermittently and pH was determined after 0.5 h equilibration time. Cation exchange capacity was determined by homoionic saturation with sodium (Na) and displacement with NH4 ions (U.S. Salinity Laboratory Staff, 1954) at pH 7.0, the same method used by the commercial laboratory. The Na concentration of the final solutions was determined with a PerkinElmer (Norwalk, CT) Model 3100 atomic absorption spectrophotometer. Total C was determined by dry combustion (Nelson and Sommers, 1982). Inorganic C was determined by the method of Bundy and Bremner (1972). This method involves the measurement of CO2 evolved after reaction between a carbonate-containing sample and hydrochloric acid (HCl) in an air-tight container. The procedure suggests a reaction time between sample and HCl of 16 to 24 h; however, 48 h were allowed to ensure complete reaction of HCl with any siderite (FeCO3) that may have been present (Frisbee and Hossner, 1995). Organic C was calculated as the difference between total C and inorganic C. Moisture corrections were made on all determinations.
Particle-size distribution was determined in duplicate by the pipet method (Gee and Bauder, 1986) after removal of flocculating and secondary cementing agents, dispersion by ultrasonic vibration, and shaking overnight in sodium hexametaphosphate solution. Carbonates, soluble salts, iron oxides, and organic constituents were removed from samples by the method described by Gee and Bauder (1986). The removal of organic materials was slightly modified by using 5 M sodium acetate (NaOAc), buffered at pH 5, in the destruction of organic matter by hydrogen peroxide (H2O2).
Samples were ultrasonically dispersed for approximately 60 s in a minimum suspension volume using pulsed sonication from an ultrasonic probe (Model H-1A; Heat Systems–Ultrasonics, Farmingdale, NY). Samples were then passed through a 0.05-mm sieve to remove sand.
Sixteen of the original 50 overburden samples were selected for a study designed to evaluate the pH dependence of CEC of both the mineral and organic fractions of these materials. The samples were selected on the basis of CEC to percent clay ratio. Eight of the 16 samples had ratios greater than one, the remainder had ratios less than one. The method of Helling et al. (1964) was used to determine the pH dependence of CEC for both the mineral and organic fractions of the selected samples. This procedure used barium (Ba) as the saturating cation in solutions buffered at various pH values with p-nitrophenol and monochloroacetic acid. Final adjustments in the pH of saturating solutions were made with Ba(OH)2 and HCl. The final pH values for the saturating solutions used in this study were 3.0, 4.0, 5.0, 6.0, 7.0, and 8.2.
Multiple linear regression equations were formulated to describe the relationship between CEC (dependent variable) and the independent variables of organic carbon and clay content at each pH value. From the multiple regression coefficients of these six equations, simple linear regression equations relating CEC (dependent variable) of both the organic and clay fractions to pH (independent variable) were formulated. The experimental results were analyzed using Statistical Analysis Systems (SAS Institute, 1990).
Four samples were selected for a detailed mineralogical investigation. The samples were selected based on the CEC to percent clay ratio with two samples having a CEC to percent clay ratio less than one and two samples having a CEC to percent clay ratio greater than one. The overburden samples were separated into sand, silt, fine clay, and coarse clay fractions. The fractionation was achieved by sieving and centrifugation after removal of organic C, carbonates, and their residues (Dixon and White, 1995).
Cation exchange capacity was determined for each size fraction by the method outlined by the U.S. Salinity Laboratory Staff (1954), the same method used by the commercial laboratory. Total potassium (K) was determined on both fine and coarse clay fractions (Jackson, 1969). Determination of Na for CEC and K for total K was with a PerkinElmer Model 3100 atomic absorption spectrophotometer.
Oriented slides of clay and silt fractions were prepared for X-ray diffraction analysis. Slides were prepared for each sample that was K saturated, magnesium (Mg) saturated, or Mg saturated–glycerol solvated (Dixon and White, 1995). Clays and silts were analyzed by X-ray diffraction on an X-ray diffractometer (XRG 3000; Philips Analytical X-ray North America, Mahwah, NJ) with a curved graphite monochrometer and a theta compensating slit using Cu-K
radiation at 30 kv and 18 ma. Magnesium- or K-saturated, air-dried slides were scanned from 2° to 32° 2
in 0.05° steps with 5-s counts per step. Magnesium saturated–glycerol solvated and heat treated (300° and 550°C) K-saturated samples were scanned from 2° to 15° 2.
Electron microscopic analysis of the silt fraction was performed with a scanning electron microscope (JMS 6400; JEOL, Peabody, MA) equipped with an energy-dispersive X-ray spectrometer (SEM–EDS). Two sets of samples, each consisting of two samples with CEC to percent clay ratios greater than one and two samples with a CEC to percent clay ratios less than one, were prepared for examination by SEM. One set of samples selected for mineralogical investigation was shaken overnight in sodium hexametaphosphate solution. The silt fraction was then separated for study. The second set of samples was prepared by removal of carbonates, organic constituents, and iron oxides. The sample was then subjected to ultrasonic dispersion and shaking overnight in sodium hexametaphosphate solution. The silt fraction was then separated for study. Samples were prepared for SEM by sprinkling the silts on C-covered sample stubs followed by sputter-coating with gold.
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RESULTS AND DISCUSSION
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The organic C values determined by difference between total and inorganic C are generally higher than those reported by the commercial laboratory (Fig. 2). The discrepancy in organic C values suggests that the wet oxidation method used by the commercial laboratories does not completely oxidize organic materials in these samples. The simple linear regression equation relating these two methods of organic C determination further suggests that oxidation by the rapid dichromate oxidation method (Nelson and Sommers, 1982) underestimates organic C by about 14%. Lignite in these overburden samples is evidently rather resistant to dichromate oxidation. Jackson (1958) has stated that C sources such as coal are only partly attacked by wet oxidation methods.
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Fig. 2. Comparison of percent organic C determined by a commercial laboratory (organic Cc) and percent organic C determined in the present study by difference in total C and inorganic C.
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Comparison of percent clay determined after overnight shaking in hexametaphosphate compared with those determined following destruction of cementing agents, physical and chemical dispersion, and pipet analysis reveal disagreement in these values (Fig. 3). The values for percent clay determined by the intensive dispersion method are generally higher than those determined following shaking in sodium hexametaphosphate. The simple linear regression equation relating these two sets of data indicates that the method currently in use is 76% as efficient as intensified dispersion, thus, clay is underestimated by approximately 24%.
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Fig. 3. Comparison of percent clay determined by a commercial laboratory (clayc) and percent clay determined after intensive dispersion.
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The increase in clay content with intensive dispersion suggests the presence of relatively stable aggregates of clay-sized particles. Overburden materials are, by and large, subjected to significant compaction forces and may therefore be more consolidated or "shaley" than more highly weathered soils. Adams and Stewart (1969) found that silt and clay increased and sand-sized particles decreased following ultrasonic dispersion of Silurian shale in an aqueous suspension.
Overburden CEC varied with pH of the saturating solution. Multiple linear regression equations that related CEC to organic C and clay content reveal that CEC increased with pH of the saturating solution (Table 2). Cation exchange capacity of both the mineral and the organic fractions of these overburden materials was estimated by manipulating the equations in Table 2. The CEC of the clay fraction at each pH was estimated by setting the partial regression coefficient for organic C (ßoc) at zero and that for clay (ßcl) at 100 and solving each equation for CEC. Estimates of CEC of the organic fraction were calculated by setting ßcl at zero and ßoc at 100 and solving for CEC.
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Table 2. Multiple regression equations relating cation exchange capacity (CEC) to clay and organic C content of lignite overburden at pH 3.0, 4.0, 5.0, 6.0, 7.0, and 8.2. ßcl = percent clay, ßoc = percent organic C.
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Simple linear regression equations relating pH of the saturating solution to CEC were formulated for the clay and organic fractions. These data indicate that the pH dependence of CEC for the organic fraction is much higher than for the clay fraction (Fig. 4). The estimated CEC of the organic fraction of these overburden materials increased from 23.0 cmolc kg-1 at pH 3 to 158.2 cmolc kg-1 at 8.2 based on linear regression equations, an almost sevenfold increase in CEC. Helling et al. (1964) observed a sixfold increase in CEC of the organic fraction of 60 Wisconsin soils as pH was increased from 2.5 to 8.0. The estimated CEC of the organic fraction at pH 2.1 is 0 cmolc kg-1 of organic C.
The pH dependence of the mineral fraction CEC in overburden materials was found to be 0.63 cmolc kg-1 per unit of pH (Fig. 4). Helling et al. (1964) reported an increase in CEC for 60 Wisconsin soils collected from the Ap horizon of 4.4 cmolc kg-1 per unit of pH. The pH-dependent charge was largely attributed to the presence of allophane in the clay fraction. The dominance of smectite, a mineral with relatively low pH-dependent charge, in the clay fraction of the Wilcox Group overburden samples explains the rather low pH-dependent charge. The materials were only slightly weathered and most had limited amounts of kaolinite. Each point on the graph represents an average value for 16 samples.
Estimating the CEC of both the organic and clay fractions of these overburden materials made it possible to estimate the contributions of each of these fractions to the CEC of individual samples. By subtracting the estimated CEC due to organic C from the whole soil CEC, an estimate of CEC due to the mineral or clay fraction could be calculated:
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where CECclay = estimated CEC clay fraction (cmolc kg-1), CECsoil = CEC of whole soil (cmolc kg-1), OC = organic carbon, and 158.2 = CEC of organic C at pH 8.2 (cmolc kg-1).
A plot of CECclay against percent clay (Fig. 5) reveals that after intensive dispersion of clay minerals and accounting for CEC due to organic C, the number of samples having CEC to percent clay ratios equal to or greater than one decreases from 25 to 5, an 80% decrease. Therefore, by accounting for these factors, 80% of the samples that previously had a CEC to percent clay ratio greater than one now had ratios less than one.
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Fig. 5. The relationship between the estimated cation exchange capacity (CEC) of the clay fraction after accounting for CEC due to organic C and percent clay following intensive dispersion.
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After removal of organic matter, dispersion of clay minerals, and separation of size fractions, CEC and total K were determined on both the coarse (2.0–0.2 µm) and fine (<0.2 µm) clay fractions (Table 3). X-ray diffraction analysis was used to identify minerals in the clay fraction of each sample (Fig. 6 and 7). The clay mineralogy of the two samples having a CEC to percent clay ratio greater than one was dominated by smectite, while the clay mineralogy of the two samples with ratios less than one was dominated by kaolinite. This is supported by the CEC values and the K content for fine and coarse clay (Table 3). The CEC of both the fine and coarse clay fractions of the two samples having CEC to percent clay ratios greater than one are greater than 80 cmolc kg-1 and the clay fractions have low total K content, suggesting weathering of mica and the dominance of smectitic minerals. The silt fraction of the two samples with high CEC to percent clay ratios had CEC values of 27.1 and 31.2 cmolc kg-1.
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Table 3. Cation exchange capacity (CEC) to percent clay ratio, whole clay CEC, CEC, and total K for the fine and coarse clay fractions, and CEC of the silt fraction of four samples selected for detailed mineralogical investigation.
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Fig. 6. X-ray tracing of the fine (<0.2 µm) and coarse (0.2–2.0 µm) clay fractions from a sample dominated by smectitic clay.
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Fig. 7. X-ray tracing of the fine (<0.2 µm) and coarse (0.2–2.0 µm) clay fractions from a sample dominated by kaolinitic clay.
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X-ray diffraction was used to identify minerals in the silt fraction (data not shown). The silt fraction of the high CEC to percent clay material contained smectite, mica, kaolinite, quartz, and feldspar. The low CEC to percent clay materials consisted of the same minerals; however, smectite was present in very limited quantities as indicated by the near absence of smectite peaks. These results suggested that smectite was present in the silt fraction of the high CEC to percent clay samples and was responsible for the CEC that was not accounted for by organic C or the incomplete dispersion of the high CEC to percent clay samples. Dixon et al. (1982) has also reported the presence of smectite in the silt fraction of Texas lignite overburden.
The CEC of samples with a high CEC to percent clay ratio that was not accounted for in the clay or organic fractions arises from expandable 2:1 phyllosilicates in the silt fraction. These phyllosilicates may occur in shale fragments. Scanning electron microscopy images show that shale fragments remained after intensive dispersion treatments (Fig. 8). These fragments consisted of a mixture of mica, smectite, and quartz. Some qualitative evidence of mica weathering to smectite at the crystal edges was also found through SEM–EDS. A depletion of interlayer K at the edge of mica crystals was confirmed with EDS. In addition, large aggregates of smectite and kaolinite were found in the silt fraction. Thus, the results of the SEM investigation support data that indicate incomplete dispersion of the silt fraction and data generated by X-ray diffraction analyses.
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Fig. 8. Scanning electron micrographs of shale fragments from the silt fraction of high cation exchange capacity (CEC) to percent clay samples of lignite overburden. (a) Shale fragment following overnight shaking in sodium hexametaphosphate. (b) Shale fragment following overnight shaking in sodium hexametaphosphate. (c) Shale fragment following destruction of CaCO3, organic C, Fe oxides, sonication, and shaking overnight in sodium hexametaphosphate. (d) Shale fragment following destruction of CaCO3, organic C, Fe oxides, sonication, and shaking overnight in sodium hexametaphosphate.
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CONCLUSIONS
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Methodology currently employed in the characterization of overburden samples results in a number of the samples having CEC to percent clay ratios greater than one. The unusually high CEC values relative to clay content are due primarily to incomplete dispersion of clay minerals. Silt-sized aggregates or shale fragments consisting of variable amounts of smectite, kaolinite, mica, quartz, and feldspar in these materials are not completely dispersed by the standard method currently employed. According to data collected in this study, incomplete dispersion of aggregates containing clay minerals from larger size fractions is responsible for approximately 48% of overburden samples having CEC to percent clay ratios greater than one.
Another factor contributing to the high CEC to percent clay ratios in these overburden materials is organic C (lignite). When organic C is present largely as lignite, CEC from this source may cause CEC to percent clay ratios to be greater than one. Based on estimates of the CEC of organic C, ratios greater than one in 14% of the samples can be explained in terms of organic C content. The CEC of overburden organic constituents at pH 8.2 is 158.2 cmolc kg-1 organic C.
Several samples had CEC to percent clay ratios greater than one after intensive dispersion of clay minerals and accounting for CEC due to organic C. These samples contained silt-sized aggregates that were resistant to dispersion according to SEM–EDS investigations. Also, large silt-size aggregates of smectite and crystals of mica apparently weathering to smectite were observed. Thus, the silt fraction of these overburden materials can provide a source of CEC not accounted for in the soil CEC to percent clay ratio.
Reasonable options for resolution of the analytical discrepancies can be time consuming and costly, but include (i) the adoption of methodology by commercial laboratories designed to optimize dispersion of clay minerals and oxidation of organic C, or (ii) the use of correction factors, based on this study, to account for incomplete dispersion of clay minerals and oxidation of organic C by current methodology.
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ACKNOWLEDGMENTS
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This research was supported with funding provided by Texas Utilities Mining Company. The assistance of Intermountain Laboratories in providing samples and sample analyses is greatly appreciated.
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REFERENCES
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ABSTRACT
INTRODUCTION
