Soil Colloids, Ion Exchange and Redox Potentials PDF

SOIL COLLOIDS, ION EXCHANGE AND REDOX POTENTIALS Soil particles especially which are very small exhibit greater chemical reactivity. Colloidal particles of soil are chemically active portion of the soil. Its activity is mainly attributed to colloidal nature. Soil particles which are less than...

SOIL COLLOIDS, ION EXCHANGE AND REDOX POTENTIALS Soil particles especially which are very small exhibit greater chemical reactivity. Colloidal particles of soil are chemically active portion of the soil. Its activity is mainly attributed to colloidal nature. Soil particles which are less than 0.001 mm in size possess colloidal properties and are known as soil colloids. The colloidal state refers to a two- phase system in which very finely divided state of matter is dispersed through a second. Because of extremely low size colloidal materials float in a medium and do not tend to settle. In natural environment soil particles gets dispersed in water which is the most common solvent or dispersion medium.  PROPERTIES OF SOIL COLLOIDS Adsorption: The surface adsorption is very large when small sized colloidal material is present in the system due to higher surface area of the colloidal materials. In soil colloidal particles adsorb nutrient ions on their surfaces. Brownian movement: Colloidal particles show continuous random movement or motion. This movement is responsible for the coagulation or flocculation of soil colloidal material. Electrical charge: Colloidal particles usually have and electrical charge- some positive and some negative. Hence soil colloids can attract both positive and negatively charged ions in soil water.  Flocculation: The colloidal particles are coagulated by adding an oppositely charged ion. This process of formation of flocs is known as flocculation. For example clay is coagulated by the use of Al3+ or Ca2+. In the soils, these cations play significant role in the formation of flocs which is a first stage in the aggregate development. So the phenomenon of flocculation is quite important in the field of agriculture by improving the physical conditions. Plasticity: It is the ability to mould the material in to desired shape. Soils containing more than 15% colloidal material exhibit plastic properties. Plasticity occurs when the soils are moist or wet. Plasticity is important because it encourages a change in soil structure during tillage operation.  Cohesion and adhesion: Colloidal clay particles exhibit the properties of cohesion and adhesion. Due to cohesion force clay particles are able to form aggregates and also due to adhesive forces clay particles envelope sand particles. Both the forces develop in the presence of water and these two forces help in the retention of water in the soil as well as absorption to the plants and micro organisms. Swelling and shrinkage: Contraction and expansion in volume of soil with the variation in moisture are influenced by the presence of colloidal particles in soil. Swelling occurs when colloidal clay particles are allowed to be placed in contact with moisture due to imbibing water. This phenomenon depends on type of clay and in soils this plays a crucial role in development of structure.  Non permeability: Colloids are unable to pass through a semi permeable membrane and this membrane allows passing water and other dissolved substances but retains colloidal materials. Surface area: Because of extremely small size offers enormous surface area  Chemical composition of soils Solid phase The major elements in soils are those with concentrations that exceed 100 mg/kg, all others being termed trace elements. The major elements include O, Si, Al, Fe, C, K, Ca, Na, Mg, Ti, N, S, Ba, Mn, P, and perhaps Sr and Zr, in decreasing order of concentration. TYPES OF SOIL COLLOIDS  Inorganic fraction-Soil minerals Name Chemical formula Importance Quartz SiO2 Abundant in sand and silt Feldspar (Na, K)AlO2[SiO2]3 Abundant in soil and is not leached CaAl2O4[SiO2]2 readily Mica K2Al2O5[Si2O5]3Al4(OH)4 Source of K in most temperate K2Al2O5[Si2O5]3(Mg, zone soils Fe)6(OH)4 Amphibole (Ca, Na, K)2,3(Mg, Fe, Easily weathered to clay minerals Al)5(OH)2 and [(Si, Al)4O11]2 oxides Pyroxene (Ca,Mg, Fe, Ti, Al)2(Si, Easily weathered Al)2O6 Olivine (Mg, Fe)2SiO4 Easily weathered Tourmaline NaMg3Al6B3Si6O27 Highly resistant to chemical (OH, F)4 weathering Name Chemical formula Importance Kaolinite Si4Al4O10(OH)8 Abundant in sand and silt Smectite Mx (Si, Al)8(Al, Fe, Abundant in soil clay Mg)4O20(OH)4 M = interlayer fractions as products Illite cation, 0.4 ≤ x ≤ 2.0 = layer of weathering Vermiculite charge Chlorite Allophane SiyAl4O6+2y · nH2O, Abundant in soils derived from 1.6 ≤ y ≤ 4, n ≥ 5 volcanic ash deposits Imogolite Si2Al4O10 · 5 H2O Abundant in soils derived from volcanic ash deposits  Silicate Clays Layer silicates with 1 Si tetrahedra and 1 Al octahedral sheets-1:1 type Layer silicates with 2 Si tetrahedra and 1 Al octahedral sheets Minerals Typ Layer C axis CEC pH Specific shape Size (μ) e charg spacin me/10 depende Surface e g A0 0g nt area charge m2/g Kaolinite 1:1 0 7.2 1-10 high 5- 20 Hexagonal 0.1- 5.0 crystals Mica (Illite) 2:1 1.0 10 20- 40 low 100-120 Irregular 0.1-2.0 flakes Vermiculite 2:1 0.8 10-15 120- low 300-500 Irregular - 150 flakes Montmorillo 2:1 0.4 12-18 80-120 low 700-800 - 0.01-1.0 nite/smectit e Chlorite 2:1:1 1.0 14 20-40 high - - - Organic - - - 100- high - - - matter 300  ION EXCHANGE In an agricultural soil, seat of various chemical reactions including ion exchange is clay sized particles or colloids with little contribution from silt size fractions and organic matter. The soil colloids are amphoteric in nature hence have the power to hold and exchange them for cations and anions. The charge density and potentials are higher on edges, corners, furrows and cavities as compared to flat (plane) surfaces and consequently ion exchange phenomenon are probably concentrated more in these sites.  Ion exchange is defined as reversible process by which cations and anions are exchanged between solid and liquid phases, and between solid phases it is in close contact with each other. Depending on the type of ion involved in the exchange reactions, both cation and anion exchange reactions occur naturally in soils. Of the two reactions cation exchange reaction is considered to be most important, since anion exchange capacity of most agricultural soils is much smaller than the cation exchange capacity.  Charges on clays Permanent Charge- Isomorphous substitution Clay (layer silicates) Minerals Variable Charge (pH dependent) Mineral Edges Oxides and Hydroxides of Fe and Al Organic Matter Distribution of charge CATION EXCHANGE CAPACITY It is the sum total of exchangeable cations that can be adsorbed by a soil and is expressed in c mol kg-1 dry soil or m eq.100 g-1 dry soil. It can be even estimated for different soil materials like clay or organic colloid. It is one of the important chemical properties of soils and is usually closely related to soil fertility. 1 meq/100 g soil = 1 cmol /kg (SI unit)  For example: A soil has high amounts of adsorbed Ca on the clay colloid. Now if it is desired to replace all Ca from colloid, the soil is leached with NaCl solution so that Ca from the exchange complex is constantly being replaced by Na until equilibrium is attained and thereby the clay would be saturated with Na. Cation exchange phenomenon is chemically equivalent and reversible. Colloid - Ca + 2 NaCl ----- >Colloid – Na + CaCl2  The most important cations in the soils are calcium (Ca2+), magnesium (Mg2+), hydrogen (H+), sodium (Na+), potassium (K+) and aluminium (Al3+). In acidic soils the principal cations are Al3+, H+, Ca2+, Mg2+ and K+. In neutral and alkaline soils the predominant cations are Ca2+, Mg2+, K+ and Na+. FACTORS AFFECTING CEC Texture of the soil Organic matter content Amount and kind of clay Exchangeable Al and polymeric hydroxy Al etc PERCENTAGE BASE SATURATION The percentage of the total CEC satisfied with basic cations is termed percent base saturation. It is the extent to which the exchange complex is saturated with exchangeable cations other than Al and H and it is expressed as a percentage of the total cation exchange capacity. S % BS= ------- X 100 T Where BS = base saturation, S = meq of basic cations per 100 g soil, T= total exchange capacity meq/100g soil.  RELATION-PBS & SOIL pH There is a definite relationship between pH and PBS Soil pH determination is easy than PBS. pH can help to determine PBS A low PBS means acidity, where as a PBS approaching 100 will result in neutrality or alkalinity. As the BS is reduced due to leaching of Ca, the pH is also decreased in definite proportion. Within the range of soil pH 5-6, the ratio for humid temperate mineral soils is roughly at 5% BS change for every 0.1 unit change in pH. Thus if the PBS is 50% at pH 5.5, it should be 25% and 75% at pH 5 and 6 respectively.  CATION EXCHANGE & PLANT NUTRITION Important reaction in soil fertility, in correcting soil acidity and alkalinity, in altering soil physical properties, and as a mechanism in detoxification of waters percolating through. Plant roots acquire exchangeable cations directly by interchange or contact exchange between the root hairs and colloidal complex. Exchangeable K content-soil test for available K Soil acts as a large cation exchanger. K, NH4+, and Ca2+ will not move far in soil before they are adsorbed to exchange sites. Thus mobility and leaching are reduced in soil. Cleansing action. Pollutants Cd2+ or Pb2+ on soil are not leached easily. Adsorbed and form insoluble hydroxides, carbonates etc which are immobile. Nature of exchgl cations influences physical, chemical and biological properties. ANION EXCHANGE CAPACITY It is defined as the capacity of a soil to adsorb and release anions under normal conditions and is expressed as meq per 100g soil. Anion retention capacity of a soil increases with a decrease in soil pH. Anion exchange is much greater in soil high in 1:1 clays and those containing hydrous oxides of iron and aluminium as compared to soils with predominantly 2:1 clays. OXIDATION-REDUCTION POTENTIALS The oxidation potential of a chemical system is a measure of the tendency for oxidation reactions to occur in that system. Oxidation of an element involves the loss of an electron. oxidation Reduced state ------> oxidized state + n electrons The more + the oxidation potential is, the greater the tendency for oxidation to occur. The more – negative the oxidation potential the less is the tendency for oxidation to occur. Highly reduced substances such as H2S or Na2S2O4 have high oxidation potentials (around +0.6 to +1.0 volts).  Thus highly reduced soils also have fairly high oxidation potentials about +0.3 to +0.5V. They have high tendency to oxidize, a low tendency to be reduced since they are already highly reduced. Highly oxidized substances such as KMnO4 in H2SO4 have low oxidation potential (- 1.5 V). They have a high tendency to undergo reduction since they are already highly oxidized. Well oxidized soils thus also have low oxidation potentials of -0.3 to -0.5V.  Soil redox potential (Eh): is a measure of the intensity of the reducing or oxidizing conditions; that is the tendency of the soil solution to donate electrons to or accept electrons from a chemical species or electrode introduced in to the solution. It is generally expressed in millivolts (mV). Soil redox potential measurements are made using a platinum electrode and the potential developed shall be read against a reference electrode (either Ag/AgCl or calomel electrode) with the help of a voltameter. “Redox indicators” can be used for semi quantitative determination of Eh. IMPLICATIONS OF Eh MEASUREMENT Redox systems are helpful in the buffer of soil pH/ reaction. Redox status is an indicator of soil aeration status. Presence and supply of O2 to plant roots is critical and we can get an estimate of root conditions with Eh measurement. Redox measurement helps to know presence or absence of nitrate, toxicities of certain elements in soils. Microbiological activities of soil can be assessed. The redox conditions govern solubility and availability of several nutrient elements.  Reduced conditions in the soil can increase availability of P, Fe and Mn. Conversely availability of S, Cu and Zn can be lessened due to continuous flooding The redox systems govern loss of nutrients from the soil especially N and S Redox systems play important roles in the formation of soil. Such reactions are involved in the chemical weathering of rocks and minerals Prolonged reduced conditions in certain soils help us in soil grouping. Presence of mottling or gley horizons in soil gives bluish- green color to the soil Affects the germination and seedling emergence Continuous low redox values may result in toxicity of certain nutrient elements to plant growth.