1. Particle Size Analysis of Gypsic Soils Justin Riggs 1, Curtis Monger 1, Mei Wang 1, Rebecca Kraimer 2 Abstract Gypsum is a common mineral in many dry land soils throughout the world. It is a relatively soft mineral that consists of calcium sulfate dihydrate (CaSO 4 ·2H 2 O). The chemical makeup of this mineral poses many problems that affect the chemical properties of soils (e.g., pH, EC., corrosiveness, etc.) and field/laboratory analysis (e.g. field texture, particle size analysis). The most problematic aspect of gypsum is its solubility. Because gypsum can be easily dissolved in water, the size of the particles become smaller during pipette and hydrometer analysis. This research effort will explore ways to prevent the dissolution of gypsum while conducting textural analysis. This investigation will test three hypotheses ( i.e. methods that prevent dissolution of gypsum grains): low viscosity oil, saturated NaCl solution, and saturated gypsum solution. Although the low viscosity oil prevented gypsum dissolution, it was unsuitable because of problems with the drying of the oil- saturated sample, disposal, and cost. The NaCl saturated solution was unsuitable because it did not prevent gypsum dissolution as a result of ion pairing. Preliminary data indicate that saturated gypsum solution prevents the dissolution of gypsum particles in the soil. Objective To maintain an efficient and economical method of particle size analysis for gypsic soils by preventing the dissolution of gypsum for particle size analysis Hypothesis # 2 Saturated NaCl solution Using a saturated NaCl solution, did not succeed in preventing gypsum from dissolving. This experiment consisted of dissolving NaCl into 100ml of water ranging in increments of 0g, 8.7g, 17.6g, 26.4g and 35.2 g, adding 1g of reagent gypsum to each 100ml beaker of solution, allowing each sample to sit for 24 hours, then analyzing each solution for Ca 2+ and SO 4 2-. Both Ca 2+ and SO 4 2- increased initially then decreased as concentration increased (Fig.#2A and Fig. #2B). This trend is the result of ion pairing. The Na is a monovalent ion and due to ion pairing with Ca a divalent ion the dissolution of gypsum was actually increased by the NaCl. References Soil survey staff. 2004 Soil survey laboratory methods manual. Version 4.0. USDA-NRCS. Soil Survey Investigations Report No. 42. U.S. Govt. Print. Office, Washington DC. Kilmer, V.J, and L.T. Alexander. 1949. Methods of making mechanical analyses of soils. Soil Sci. 68: 15-24 Nelson, R.E. 1982. Carbonate and gypsum. P. 181-197. In A.L. Page, R.H. Miller; and D.R. Keeny (eds.) Methods of soil analysis. Part 2. Chemical and microbiological properties. 2 nd ed. Agron. Mongr. 9. ASA and SSSA, Madison, WI. Nelson, R.E., L.C. Klamath, and W.D. Nettleton. 1978. Determining soil gypsum content and expressing properties of gypsiferous soils. Soil Sci. Soc. Am. J. 42: 659-661. Acknowledgements Funding for project provided by the Jornada Basin LTER program. Authors acknowledge, Ken Scheffe New Mexico State Soil Scientist, NRCS, Las Cruces Soil Survey Office, NRCS April Ulery Professor NMSU. Hypothesis # 1 : Use of a Low Viscosity oil Using the standard pipette method for particle size (Soil survey staff. 2004) analysis, with the replacement of deionized water with AW46 hydraulic oil, succeeded in preventing gypsum from dissolving. However, the heat needed to volatize the oil off of the soil particles during the drying step for the pipette method was well beyond the drying ovens capabilities. In addition, there were problems with disposal of the oil, and the cost of an individual test is much greater than the cost of a traditional analysis. *The analyzed result for diluted solution ( dilution with a factor of 10, and the diluted readings was multiply by 10) **The analyzed result for diluted solution ( dilution with a factor of 50, and the diluted readings was multiply by50) (Fig. 1A) PSA Setup for pipette method with AW 46 Hydraulic oil AB (Fig. 1B) Sample A gypsum soil after 24hr drying oven with DI water Sample B Gypsum soil after 24hr drying oven with AW46 oil Problem: Gypsum particles dissolve during standard particle size analysis allowing for inaccurate textural results. To show this, 10 beakers of 100ml of DI water was made and 1g of a gypsic soil from Culberson County, TX was added and then allowed to set for 24 hours (Soil survey staff. 2004). The data shows that an mean of 1.38g SO 4 2- /L is dissolved with a standard deviation of 0.01g SO 4 2- /L (Fig. A). So for every 10g sample of gypsic soil used for a PSA test, 1.38g of that sample will dissolve into solution, which presumably is the result of some particles dissolving from sand size to silt or clay size. Hypothesis # 3 Saturated Gypsum Solution Preliminary data is suggesting that the saturated gypsum solution is the most promising of the three hypotheses. This experiment consisted of three parts: first, figuring out how long it takes to saturate a solution, then determining how much gypsum is in a saturated solution, and last to determine if a gypsic soil dissolves during analysis. The first part involved placing 1g of gypsum in a series of 100ml beakers filled with DI water and allowed to stand for different increments of time: 15 minutes, 30 minutes, 1 hour, 12 hours, 24 hours, and 48 hours. The system saturated in less than 15 minutes, proving this to be an efficient and economical method. Part two showed how much gypsum is in a saturated solution. A reference concentration was determined using ten 100ml beakers of DI water with 1g of reagent gypsum and allowed to stand for 24hrs (red line in Fig. #3). The saturation point of the gypsum solution had a mean of 1.38 g SO 4 2- /L with a standard deviation of 0.02 g SO 4 2- /L (Nelson, R.E. 1978). Part three consisted of adding a gypsic soil to the saturated solution to see if any additional dissolution or if any precipitation occurred. 10 reps were created by adding 1g of gypsic soil to a saturated gypsum solution and allowed to set for 24 hours and then tested for SO 4 2-. The data showed a mean of 1.40 g SO 4 2- /L and a standard deviation of 0.01 g SO 4 2- /L, very close to the reference concentration (Fig #3). Conclusion Using gypsum-saturation method was the most effective of the three different hypothesizes. The use of oil may have prevented any gypsum from dissolving, but the cost of each sample and the disposal of the oil proved to be too difficult to be an efficient and economic method. The NaCl increased gypsum dissolution, proving this to be a poor method. The gypsum-saturation method is the best because it is very easy to acquire. Clean up is also very simple; all contents can be poured down the sink. Furthermore the data show that compared to the DI water the saturated gypsum solution prevents dissolution of gypsum particles in soil. Further experiments will involve running more reps for statistical validation and using microscopy and particle size analysis to determine if particles change in size. Fig. 1B Fig. 1A 1; New Mexico State University 2; New Mexico Natural Resource Conservation Service