Lab 6 Properties of Soil Clay Minerals

Lab 6 Properties of Soil Clay Minerals OBJECTIVES In this lab, you will learn how clay structure and saturating cations determine the physical propert...
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Lab 6 Properties of Soil Clay Minerals OBJECTIVES In this lab, you will learn how clay structure and saturating cations determine the physical properties of clays. You will be determining which kinds of clay swell and why. You will also explore one of the most important functions of clay minerals: cation exchange capacity. It is strongly recommended that you review the material covered in Lab. 4 (Computer-Based Learning Aids for Soil Science). INTRODUCTION Phyllosilicate clay minerals have particular properties and reactions related to, and controlled by, their small size and net electrical charge. The sheet-like crystallographic structure along with the small size give these soil clays a very high surface area in relation to their mass. The excess negative charge on many of these clay surfaces results in the attraction of water and ions (charges atoms and molecules) to the clay minerals. The Properties and Reactions of Clays 1) Adsorption: The negative electrostatic charge of clays attracts cations from the surrounding soil solution, which can be retained, or adsorbed, at the mineral surface. Other cations can compete with and replace adsorbed cations. This results in the property known as the cation exchange property (CEC). Exchange reactions result in a) retention of certain plant nutrients in a plant available form in soils; b) control and buffering of soil pH (acidity); c) modification of the chemical composition of water as it passes through soil. CEC can also result in the adsorption of organic compounds if they are charged. Exchange reactions are usually rapid, reversible and stoichiometric. a) Cation Exchange Capacity: The cation exchange capacity is defined as the sum of exchangeable cation charge that a soil can adsorb. The ability to exchange cations is an important property of clay minerals; however, this phenomenon is not exclusive to phyllosilicate clay minerals. Other components of soil, such as organic matter and soil oxides have cation exchange properties as well. In this laboratory we will determine the total exchange capacity of soils which includes contributions from phyllosilicate clays, organic matter and oxides. b) Anion Exchange Capacity: Clay minerals also have positively charged sites, which attract anions. Phyllosilicate clays usually have fewer anion exchange sites than cation exchange sites. Sesquioxide clays (Al and Fe oxides) may have net positive charge at low soil pH. The subject of pH dependent charge will be discussed in lecture. c) Adsorption of Organic Substances: Clays that exhibit a net negative charge due to isomorphic substitution (such as montmorillonite) will attract water and cations. Small, nonpolar molecules such as benzene, unable to displace either water or a cation, will not adsorb. Organic cations, such as some herbicides, will be adsorbed through cation exchange. Large molecules, even if uncharged, can be adsorbed on to some clays due to a hydrophobic effect (they would rather be on the clay than in the water). Negatively charged molecules may 27

either bridge to the clay via a multivalent cation, or adsorb to broken edges of the clay particles. Association with clay surfaces is believed to protect organic molecules from biodegradation. 2) Dispersion/Flocculation: Dispersion is the condition of each individual clay particle acting independently. Dispersion occurs as a result of the net electrical charge on the clay causing the particles to repel each other. Aggregates are separated into individual particles that are distributed through soil or suspended in a medium such as water. A dispersed condition results in the disruption of soil structure and production of a "massive" condition, with loss of the desired soil structure, reduced soil space, decreased aeration, and restricted water movement. Flocculation is the clumping together of individual particles into small clumps or floccules. Flocculation is the opposite of dispersion and results from the neutralization of the net electrical charge on a clay. Clay particles can only continue to approach each other when the attractive forces of the cations in the "cation swarm" layer overcome the repulsion of the like charges. Flocculation promotes the formation of stable aggregates or granules (peds), and of a favorable soil structure. The charge or valence of the adsorbed ions, and the ionic strength of the soil solution are two of the most important factors determining dispersion/flocculation of soil clays. 3) Swelling/Shrinking: Water molecules may move into the space between clay platelets. kaolinite already has one layer of water molecules between the plates; due to lack of attraction for cations, there is no driving force to allow further water into this space (also termed the ‘interlamellar area’). Clays with negatively charged basal plates attract cations, which are surrounded by water. As they are pulled into the interlamellar area to neutralize the negative charge, they ‘drag’ their water molecules in with them, forcing the clay particles apart. As clays dry, the plates are attracted by the positive charges between their layers, which is stronger due to the loss of water. The plates will be pulled closer together. In some cases, the clays can collapse upon themselves, preventing rehydration. 4) Plasticity: Plasticity describes the pliability of clays. Clay can be molded and shaped. The smectites (2:1 clays) exhibit the greatest plasticity. 5) Cohesion: this is the tendency of clay particles to stick together. It is due to the attraction of clay particles for water and occurs over a certain range of water contents. Cohesion results in the formation of clods in a tilled field. Cohesion is greatest for smectite type clays, and may be reduced by increased humus content. 6) Stickiness: This property is related to the cohesive ability of clay minerals. It is greatest for smectites. Cation Exchange Capacity All of the above information comes together in the property to be measured in this experiment, the cation exchange capacity. Only the readily exchangeable ions, that is, the ions that are fully solvated on the surface of the clay, or those in the cation swarm, are the ions which exchange for another electrolyte. Any cation in direct contact with the clay surface (for example, 28

adsorbed in the siloxane cavity) will not be rinsed off by the exchange solution. It will remain on the clay. By using a vast excess of a solution containing monovalent cations, we are assured of an exchange reaction which removes the cations in those positions. PROCEDURES CEC Determination The purpose of this extraction is to compare the CEC of a sandy soil to that of a loamy soil. (Recall the soil textural triangle. Which soil do you suppose will have more clay: the sandy soil or the loamy soil?). You will perform separate extractions on two different types of soil and compare the CEC results. WARNING!!! The purpose of this experiment is to compare the results from two different natural soils. DO NOT use the same soil for different extractions. DO NOT use the pure clay minerals for this part of the laboratory. DO NOT combine the extracts of the different soils. At the conclusion of this lab, you will have two separate extracts: one of a coarse textured soil, one of a fine textured soil. IF YOU DO NOT FOLLOW THESE INSTRUCTIONS, YOU WILL BE REQUIRED TO REPEAT THE EXTRACTIONS. Materials 1) Choose two soils of radically different texture. Determine the texture by the “Texture by Feel” procedure and make a note of your textural determination. 2) DI water; 1 N sodium acetate; 1 N ammonium acetate 3) Centrifuge tubes Procedure 1) Take 2 centrifuge tubes. Weigh 1.0 g of a fine textured soil and place in one tube. Label this tube as “Fine” and your initials. Weigh out 1.0 g of the sandy soil, place in the second tube, and label as “Sandy” and your initials. 2) Add as much sodium acetate solution to the tubes as they will hold. Cover securely with Parafilm and shake manually for 10 minutes. 3) Place tubes in centrifuge and spin for 5 minutes. Make sure the centrifuge is filled with tubes before beginning. This ensures that the centrifuge is balanced; an unbalanced centrifuge will ‘walk’ across a bench and fall off. 4) After spinning down the solids, carefully pour off the liquid portion (‘supernatant’). Do not discard any soil! Discard the liquid. This part of the procedure exchanges all the cations for sodium. 5) Add as much DI water to the centrifuge tubes as they will hold. Cap with Parafilm and shake for 5 minutes. Centrifuge as in Step 3. 6) Discard liquid. This is a rinsing procedure to ensure that all excess (still in solution, not exchanged on to the clay) sodium cations are removed. 7) Repeat Steps 5 and 6, each time discarding the liquid, and making sure no soil is lost. 8) Add as much ammonium acetate to the centrifuge tubes as they will hold. Cap with fresh Parafilm and shake 5 minutes. This part of the procedure rinses off the sodium ions, which are being exchanged by the ammonium. You will be measuring the 29

concentration of sodium ions in the solution to calculate the CEC of the soil. SAVE this liquid! 9) Centrifuge as in Step 3, but place the supernatants into beakers, labeled “Fine” and “Sandy”, corresponding to your soils. 10) Repeat the ammonium acetate extraction two more times, each time centrifuging and pouring off the supernatant into the appropriate beaker. 11) Bring the final volume to 45 mL in each beaker. You will end with two beakers, one “Fine” and one “Sandy”. Place your initials and section number on the beakers and cover with Parafilm. Adsorption of Indicators As discussed in the review of the physical properties of clays, a molecule can be attracted or repelled from the surface of the clay due to its charge. Although in this procedure you will be using indicators (large organic molecules which can change color at different pH values), the results model the behavior of molecules which can be pollutants. Materials 1) Loamy sand soil 2) Two indicators of different charge Procedure 1) Prepare two separate leaching columns by placing a piece of filter paper at the bottom of the leaching apparatus and moisten with a small amount of water. 2) Place about ½ inch of loamy sand in the columns and wet with water, making sure the soil is level. 3) Place a beaker under each column to collect runoff. 4) Add the cationic indicator to one column and the anionic indicator to the other (about 1 mL of indicator). 5) Add additional DI water and note the color of the leachate solution.

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Flocculation and Dispersion of Natural Soil WARNING!!! Use a natural soil, NOT the pure clay minerals for this procedure. Materials 1) 2) 3) 4)

Loamy sand soil; 3 test tubes; 3 funnels lined with filter paper DI water 0.5% sodium hydroxide (Corrosive. Avoid contact with skin) 0.5% calcium hydroxide (Corrosive. Avoid contact with skin)

Procedure 1) Set up 3 funnels with folded cones of filter paper. Moisten paper after placing in funnel to ensure contact with funnel. Place a beaker under each funnel. Label the systems “Control”, “Sodium”, and “Calcium”. 2) Add a large scoop of soil into each test tube. Label the tubes like the funnels. 3) Add 15 mLs of DI water to the “Control” test tube; add 15 mLs of the appropriate hydroxide solutions to the tubes labeled “Calcium” and “Sodium”. Cover each tube with Parafilm and shake well. 4) Empty each tube into the corresponding funnel apparatus, rinsing out any large amounts of soil remaining in the test tube with DI water. 5) Observe the appearance of the soils on each of the filters. Note the appearance of any leachate, and the approximate time it takes for the water to filter through each soil system.

Comparison of Two Pure Clay Minerals Materials 1) Samples of two different pure clay minerals 2) Two test tubes Procedure 1) Place about a ½ inch layer of the different clay minerals in to separate test tubes. 2) Add about 1 inch of distilled water and allow to sit for several minutes. 3) Note changes in the appearance of the two different clays.

Calculation of CEC You will receive the results of the CEC extract analysis from your lab instructor in approximately one week. These results will be the concentration of Na (Sodium) in your samples. Calculate the CEC of your two soils using the following equation: CEC (cmole/kg) = Conc. of Na (mg/L) x cmole/0.23 g Na x 1g Na/1000 mg Na x 0.045L/1 g soil x 1000 g/kg Present the results for both soils in your lab report. 31