Fractionation of Clays for Mineralogical Analysis

One hundred gram portions of soil sample were suspended in 9.5 L glass containers in distilled water by shaking end-over-end for 30 seconds. Suspensions were allowed to stand for 24 hours until the silt had settled below the sampling point and clays were siphoned off. Containers were refilled with distilled water and the extraction was repeated two times. If less than 5.5 gm of sample was obtained, a second 100 gm portion of the sample was extracted. Extracted clay suspensions lay extracts were flocculated by addition of NaCl.

Na₂CO₃ Dispersion for Coarse- and Fine-Clay Fractions

One hundred gram portions of the samples were suspended in 1L containers with 400 mL of distilled water and 100 mL of saturated NaCl. Suspensions were shaken for 30 minutes and allowed to stand over night before the clear supernatant was discarded. Samples were transferred to 9.5L containers and filled pH 10 water. The dispersing solution was prepared by addition of 100mL of saturated Na₂CO₃ to 1 L of distilled water. Suspensions were allowed to stand for 24 hours until the silt had settled below the sampling point. Suspensions were refilled with dispersing solution and the process was repeated until the solution was clear. Coarse (2.0-0.2µm) and fine clays (<0.2µm) were separated using super centrifugation, and the resulting suspensions were flocculated with NaCl.

Primary Soil Characterization

Detailed descriptions of individual methods can be found in: Soil Survey Staff. 1992. Soil Survey Laboratory Methods Manual. Soil Survey Investigations Report No. 42, Version 2.0. 400 pages.

• Particle Size Analysis (3A1) - Pipet method using sodium hexametaphosphate as a dispersing agent; sands were sieved (Table 2).
• Soil pH (8C1f) - 1:1 soil water and 1:2 soil-0.01 M CaCl₂; equilibrated for one hour (Table 3).
• CaCO₃ (6E1g) - samples were treated with HCl; evolved CO₂ was measured manometrically (Table 3).
• Organic Carbon (6A1c) - Walkley-Black modified acid-dichromate FeSO₄ titration (Table 3).
• Total Nitrogen (6B4a) - Kjeldahl digestion (Table 3).
• Dithionite Citrate Extractable Fe, Al, and Mn (6C2b, 6G7a, 6D2a) - Samples were extracted with sodium dithionite and sodium citrate and analyzed by atomic absorption (Table 3).
• Acid Oxalate Extractable Fe, Si, and Al (6C9a, 6V2, 6G12) - Samples were extracted with a 0.2 M acid oxalate solution buffered at pH 3.0 under darkness. Solutions were analyzed by ICP (Table 3).
• Water Content - 15 Bar (4B2a) - Pressure desorption procedure (Table 3).
• Ammonium Acetate Extractable Bases (5B5a) - Samples were extracted with 1 M NH₄OAc, buffered at pH 7.0 (Table 4).
• Acidity (6H5a) - Samples were leached with a BaCl₂-TEA solution buffered at pH 8.2. The extract is back titrated with HCl (Table 4).
• Extractable Aluminum (6G9b) - Sample is leached with 1 M KCl (Table 4).
• CEC Sum of Cations (5A3a) - Sum of NH₄OAc extractable bases plus the BaCl₂ extractable acidity.
• CEC NH₄OAc (5A8b) - Exchange sites were saturated with NH₄⁺. Free NH₄⁺ is removed with ethanol. Adsorbed NH₄⁺ is determined by steam distillation and titration.
• Base Saturation Sum of Cations (5C3) - Divide the sum of NH₄OAc extractable bases by the sum of cations CEC (Table 4).
• Base Saturation NH₄OAc (5C1) - Divide the sum of NH₄OAc extractable bases by the NH₄OAc CEC (Table 4).
• Exchangeable Na (5B5b) - Divide the NH₄OAc extractable Na by the Sum CEC (Table 4).
• Saturated Paste Extracts - Ca, Mg, Na, K (6N1b, 6O1b, 6Q1b) - atomic absorption spectroscopy. HCO₃ (6J1b) - acid titration. Cl, SO₄, NO₃ (6K1c, 6L1c, 6M1c) - ion chromatography (Table 5). Conductivity (8A3a) - conductivity cell (Table 5)

Elemental Composition of Soil and Clay Fractions

The three clay fractions, water-dispersible clay (WDC), coarse clay (CC), and fine clay (FC), were rinsed with ethanol to remove excess NaCl from the fractionation procedure, powdered using an agate mortar and pestle, and digested in Teflon bombs using HF and Aqua Regia (Bernas, 1968). Digests were analyzed for elemental composition by ICP with results reported on an oxide basis. Elements considered for this analysis were Si, Al, Fe, Mg, K, Ca, Mn, and Na. Sodium concentrations were assumed to represent the exchangeable phase of the clays as it would be expected to be the saturating cation as a result of dispersion and subsequent flocculating process for concentrating the sample prior to analysis.

X-Ray Diffraction and Thermal Analysis

X-ray diffraction, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) were performed on the water- and chemically-dispersed clay fractions, the latter of which was further divided into coarse (2-0.2 µm) and fine (< 0.2 µm) clay fractions, to quantify smectite, hydroxy-interlayered smectite and vermiculite, gibbsite, goethite, kaolinite, and chlorite contents.

Neutron Activation Analysis of WDC

Approximately 100 to 800 mg of each sample were encapsulated in polyethylene vials and irradiated together with standards and flux wires with a thermal neutron flux of 7X10¹² n/cm²s at the McMaster Nuclear Reactor. After a seven day decay, activities of the samples were measured on a high purity Ge detector with a resolution of 1.63 KeV for the ⁶⁰Co photopeak. Two batches of Kirkland Bt clay sample were received from two different shipments and the samples were analyzed separately on different dates. In general, analytical errors would not be expected to exceed 15% on the basis of the results at the Oak Ridge National Laboratory Neutron Activation Facility. Detection limits of most rare-earth and actinide elements analyzed were less than 10% of the actual concentrations of the elements presented in Table 18. The less than symbol ì<ì in Table 18 indicates the analytical results were below detection limits for the elements.

Statistical Analysis