Laboratory Methods

Laboratory Methods PowerPoint PPT Presentation


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Laboratory Methods. Direct methods ($$$$$$)XRD (all crystalline minerals)Infrared spectroscopy (mainly kaolinite)Thermal analysis (DTA) ? mainly kaolinite, gibbsite)Indirect methodsAtterberg TestsCOLEBulk Density/KsatCEC. DIRECT METHODS. XRD. Describe the testDefine the resultsPicture of DiffractogramMineral IDQuantify resultsPros and ConsDirect measurement of mineralsCostTimeSensitivity (approx 10%) ? exactly where cut is in taxonomySemi-quantitative (approx. 10-20%).

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Laboratory Methods

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1. Laboratory Methods

2. Laboratory Methods Direct methods ($$$$$$) XRD (all crystalline minerals) Infrared spectroscopy (mainly kaolinite) Thermal analysis (DTA) – mainly kaolinite, gibbsite) Indirect methods Atterberg Tests COLE Bulk Density/Ksat CEC Summary – of the direct and indirect lab methods; of direct methods, elaborate only on x-ray diffraction.Summary – of the direct and indirect lab methods; of direct methods, elaborate only on x-ray diffraction.

3. DIRECT METHODS

4. XRD Describe the test Define the results Picture of Diffractogram Mineral ID Quantify results Pros and Cons Direct measurement of minerals Cost Time Sensitivity (approx 10%) – exactly where cut is in taxonomy Semi-quantitative (approx. 10-20%) DELETEDELETE

5. X-ray Diffraction (XRD) Identifies minerals based on their crystal structure (repeating planes of atoms), and expansion and contraction of structure following chemical and heat treatments. PrinciplePrinciple

6. Up front cost of x-ray diffractometer is between $80,000 to $130,000 or greater.Up front cost of x-ray diffractometer is between $80,000 to $130,000 or greater.

7. Soil sample collected, mineral fractions separated (SEMs show minerals), x-ray diffraction patterns for different treatments. This one shows smectite, mica, and kaolinite in XRD patter.Soil sample collected, mineral fractions separated (SEMs show minerals), x-ray diffraction patterns for different treatments. This one shows smectite, mica, and kaolinite in XRD patter.

8. X-ray Diffraction (XRD) Pros and Cons Direct measurement of minerals Cost ($175 - $300 for XRD + cost for time-consuming clay separation) Time (2-3 day turnaround for XRD at commercial lab, excluding separation time) Detection limit (approx 5-10%) – exactly where cutoff is in taxonomy Semi-quantitative (approx. ±10-20%) I got the cost and time numbers from H&M Analytical services in Allentown, NJ. However, the owner would not give me a cost for separating the clay fraction, and he did not seem real keen on getting a bunch of soil samples needing fractionation. He would rather just do the XRD for the $175-$300 cost for treated samples (Mg, Mg-EG, K, K-350, K550)I got the cost and time numbers from H&M Analytical services in Allentown, NJ. However, the owner would not give me a cost for separating the clay fraction, and he did not seem real keen on getting a bunch of soil samples needing fractionation. He would rather just do the XRD for the $175-$300 cost for treated samples (Mg, Mg-EG, K, K-350, K550)

9. Indirect Methods

10. Atterberg Limits Demonstrate the test Define the results Plastic limit Liquid limit Plasticity index Pros and Cons

11. Liquid Limit The liquid limit (LL) is the water content where a soil changes from plastic to liquid behavior. Uses a Casagrande device. Soil is placed into the cup portion of the device and a groove is made down its center. The cup is repeatedly dropped until the 13 mm (½ inch) groove is closed. The moisture content at which it takes 25 drops of the cup to cause the groove to close is defined as the liquid limit.

12. Casagrande Device

13. Plastic Limit The plastic limit (PL) is the water content where soil starts to exhibit plastic behavior. A thread of soil is at its plastic limit when it is rolled to a diameter of 3 mm and crumbles.

14. Plasticity Index The plasticity index (PI) is a measure of the plasticity of a soil. The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. The PI is difference between the liquid limit and the plastic limit (PI = LL-PL). Soils with a high PI tend to be clay, Soils with a lower PI tend to be silt, Soils with a PI of 0 tend to have little or no silt or clay.

15. Atterberg Limits Plastic limit – moisture content at which a soil loses plasticity and behaves as a solid Liquid limit – moisture content at which soil begins to flow Plasticity index – difference between LL and PL (range of moisture the soil behaves in a plastic state) If… Liquid limit > 50% and Plasticity limit > 30% Soil has expansive clay mineralogy

16. Atterberg Limits Pros and Cons Pros Relatively cheap Easy to perform In the rules Cons Operator dependent Designed for geotechnical engineering – foundations etc. Not designed for clay mineralogy Unclear relation to clay mineralogy

17. COLE (Coefficient of Linear Extensibility) Describe the test Define the results Discuss Pros and Cons

18. COLE (Coefficient of Linear Extensibility) Modified Bulk Density Analysis Volume change of clod between moist and dry conditions

19. COLE (Coefficient of Linear Extensibility) “Field” Quick Test Uses a rod

20. COLE Results 0.00-0.03 = slight 0.03-0.06 = moderate 0.06-0.09 = severe >0.09 = very severe What does it mean?

21. Bulk Density/Ksat Describe the test Define the results Discuss Pros and Cons

22. Bulk Density/Ksat Standard bulk density analysis Standard laboratory textural analysis Nomographs produced by NRCS Use nomographs to determine Ksat and maximum LTAR

24. Bulk Density/Ksat Pros and Cons Pros Moderate cost Theoretically OK Cons Difficult test Operator error Based on theory not real measurements Relation to field and wastewater unknown No defined parameters in Rules

25. APPARENT CEC DELETEDELETE

26. Apparent CEC Cation Exchange Capacity (CEC) definition: Sum of exchangeable bases (Na+, K+, Ca2+, Mg2+) plus acidity (H+, Al3+) at a specific pH. (from Soil Sci. Soc. Am., 1997). However, direct extraction of soil cations cannot distinguish between exchangeable cations and cations dissolved from soluble salts. This is the main definition for CEC (not specifically apparent CEC) given by the Glossary of Soil Science terms published by SSSA (I added the cations given in parentheses). Emphasize that direct extraction can introduce errors, and represents the CEC at the soil pH (we had a discussion about this with Steve Bristow. CEC definition differs from how we actually measure CEC.This is the main definition for CEC (not specifically apparent CEC) given by the Glossary of Soil Science terms published by SSSA (I added the cations given in parentheses). Emphasize that direct extraction can introduce errors, and represents the CEC at the soil pH (we had a discussion about this with Steve Bristow. CEC definition differs from how we actually measure CEC.

27. Apparent CEC (cont’d) More accurate to first saturate soil with a single “index” cation (NH4+, Na+, or Ba2+), then displace and measure the amount of that cation adsorbed). In practice, CEC is operationally defined by the procedure used and tailored to the purpose for which the results will be applied (measured CEC depends on the method used). This slide should emphasize that there are a number of different CEC methods used, and the resulting value obtained depends on the method used.This slide should emphasize that there are a number of different CEC methods used, and the resulting value obtained depends on the method used.

28. First (animated) slide in series illustrating the CEC measurement process – displacement of native cations by ammonium at pH 7. First (animated) slide in series illustrating the CEC measurement process – displacement of native cations by ammonium at pH 7.

29. Slide shows that the soil clay particles are now saturated with ammonium and native cations are in the water. To be able to measure adsorbed (exchangeable) ammonium, the excess in water must washed out.Slide shows that the soil clay particles are now saturated with ammonium and native cations are in the water. To be able to measure adsorbed (exchangeable) ammonium, the excess in water must washed out.

30. Excess ammonium from solution is removed. Now displace exchangeable ammonium with Na+. Excess ammonium from solution is removed. Now displace exchangeable ammonium with Na+.

31. Ammonium displaced into solution (can be done with NaHCO3/acid for aeration method; or displace by leaching with acid and NaCl and measure ammonium with Kjeldahl (like EPA 9080 essentially).Ammonium displaced into solution (can be done with NaHCO3/acid for aeration method; or displace by leaching with acid and NaCl and measure ammonium with Kjeldahl (like EPA 9080 essentially).

32. Apparent CEC Methods Wash soil with pH-7 NH4OAc solution to NH4+ saturate the exchanger phase. Emphasize the point that the EPA method uses pH 8.2 Na-saturation (this will likely inflate the measured CEC). Buol and I question 8.2. CEC also depends on saturating cation (NH4 vs Na) because NH4 can be fixed by any vermiculite and high-charge smectite.Emphasize the point that the EPA method uses pH 8.2 Na-saturation (this will likely inflate the measured CEC). Buol and I question 8.2. CEC also depends on saturating cation (NH4 vs Na) because NH4 can be fixed by any vermiculite and high-charge smectite.

33. Apparent CEC (cont’d) Because EPA Method 9081 is a pH 8.2 method instead of a pH 7.0 method, it is NOT appropriate for apparent CEC measurements tied a cutoff of 16.3 meq/100 g as used in soil taxonomy. This is a key point – the EPA Method saturates the soil at pH 8.2, so the measured CEC may be too high for 16.3 comparison. Next slide shows pH effect.This is a key point – the EPA Method saturates the soil at pH 8.2, so the measured CEC may be too high for 16.3 comparison. Next slide shows pH effect.

34. Apparent CEC Calculation Apparent CEC = (meq CEC7/ 100g soil) / (% clay/ 100g soil) Apparent CEC is a weighted test based on the clay fraction. CEC at pH 7 and Particle Size Analysis must be known. Note: Must use the CEC measured at pH 7 not 8.2.

35. CEC increases with increasing pH (Data compiled by S. W. Buol) In all cases shown, the CEC increases between the pH 7 and pH 8.2 methods. Explains why it is important to use pH 7 saturation method. The saturation step dictates how much of the index cation present – if at pH 8.2, then there is more index ion ending up in the exchanger phase.In all cases shown, the CEC increases between the pH 7 and pH 8.2 methods. Explains why it is important to use pH 7 saturation method. The saturation step dictates how much of the index cation present – if at pH 8.2, then there is more index ion ending up in the exchanger phase.

36. Soil minerals have a range of CEC (chemical compositions vary) kaolinite 2 – 15 smectite 70 – 120 vermiculite 100 – 200 mica 10 – 40 (organic matter) 150 – 300 (Sparks, 1995) Emphasize that there is no single CEC value for a given mineral – need to take an average for a cutoff value. OM is not a mineral, so given in parentheses.Emphasize that there is no single CEC value for a given mineral – need to take an average for a cutoff value. OM is not a mineral, so given in parentheses.

37. Natural variation in chemical composition and CEC of 47 soil smectites (Borchardt, 1989) These data show that properties of soil smectites vary along a continuum – THERE IS NO ONE CHEM. FORMULA OR CEC VALUE; Ideal montmorillonite, beidellite and nontronite fall on the top left, top right, and bottom right corners, respectively.These data show that properties of soil smectites vary along a continuum – THERE IS NO ONE CHEM. FORMULA OR CEC VALUE; Ideal montmorillonite, beidellite and nontronite fall on the top left, top right, and bottom right corners, respectively.

38. So why 16.3? The AG office wants a single number!

39. Basis for apparent CEC cutoff of 16.3 meq / 100 g for 10% smectite Based in taxonomy to define kandic horizons Assumes kaolinite and smectite are dominant clay minerals Average CEC values used kaolinite – 7 meq / 100 g smectite – 100 meq / 100 g (90% kaol. x 7) + (10% smect. x 100) = 16.3 meq/100g Shows the calculation (and assumptions) leading to the 16.3 value. Note that other minerals such as mica and hydroxyinterlayered 2:1 minerals are ignored.Shows the calculation (and assumptions) leading to the 16.3 value. Note that other minerals such as mica and hydroxyinterlayered 2:1 minerals are ignored.

40. Range of apparent CEC for range of mineral CEC Low end (kaolinite 2; smectite 70 meq/100 g) (90% kaol. x 2) + (10% smect. x 70) = 9.8 meq/100g High end (kaolinite 14; smectite 120 meq/100g) (90% kaol. x 15) + (10% smect. x 120) = 25.5 meq/100g Shows the extremes in calculated CEC cutoffs based on high and low values of kaolinite and smectite.Shows the extremes in calculated CEC cutoffs based on high and low values of kaolinite and smectite.

41. Pros and Cons PROS Reproducible and precise Relatively cheap ($50) Based on the actual mineral properties that relate to water movement. CONS Should not be used in soils < 35% clay Should not be used on soils >0.5% OC 16.3 meq/100g ACEC is based on ideal soil sample containing only smectite and kaolinite Most soil labs are not set up to run the correct test Modification of original slide.Modification of original slide.

42. Summary of methods in relation to rules Only when a soil is determined unsuitable will any alternative methods be used No single method (field or lab) will be fool proof Of the available lab methods ACEC is the most accurate, cost effective and time efficient method for estimating mineralogy If the soil is still unsuitable for mineralogy there is always .1948d or DWQ

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