Soil Colloids Definition of soil colloids: Size; smaller than 2 µm. Surface area; from ~10 m 2 g -1 to ~800 m 2 g -1 (surface area for sand is < 1 m 2 g -1 ). Surface charges; they could be negative or positive charges. Some charges are permanent while others are ph dependent. Types of colloids: inorganic (clays and oxides) and organic (humus). Main properties of colloids are related to adsorption of: Cations Water
A Mix of Clay and Silt/Sand Source: Lebron I, Suarez DL, Schaap MG. 2002. Soil pore size and geometry as a result of aggregate-size distribution and chemical composition. Soil Sci. 167 (3): 165-172.
Clay Content and Properties Generally less than 50% consists of clay having high swelling potential Unit contains abundant clay having high swelling potential Generally less than 50% consists of clay having slight to moderate swelling potential Unit contains little or no swelling clay Source: http://www.essc.psu.edu/soil_info/index.cgi Source: Olive, W. W.; Chleborad, A. F.; Frahme, C. W.; Schlocker, Julius; Schneider, R. R.; Schuster, R. L. 1989. Swelling Clays Map of the Conterminous United States. USGS, Report 1940.
Utilization of Clay by Humans Industrial applications: bricks, ceramics, paints, paper. Environmental and agricultural applications: Insulation of landfills (Na-bentonite). Sorbent of nutrient from water reservoirs. Clay as an amendment to reduce water repellency. Supplement in the diet of domestic animals.
Clay as Geophagic Material Geophagy: deliberate and regular consumption of earthy material. Widespread among humans and animals mainly (but not exclusively) in tropical regions. General properties of geophagic soils: High content of clay. Main colloids present in soil: clay, viruses, bacteria, fungi. Typically from the B horizon.
Hypotheses for Geophagy Detoxification and/or increase palatability. Mineral nutrient supplementation (Fe, Ca, Mg). Alleviating gastrointestinal upsets. Famine food. Testing of these hypotheses require an interdisciplinary approach.
Closer to Home
Kaolinite in Soils and Oceans Source: Thiry, M. 2000. Paleoclimatic interpretation of clay minerals in marine deposits: an outlook for the continental origin. Eart-Science Review 49: 201-221.
Average Mineral Composition of Rocks 50 45 40 % by Weight 35 30 25 20 15 10 5 0 O Si Al Fe Ca Na K Mg 75% Metals Bases
Soil and Crustal Rock Composition * * * * Source: Sposito, G. 1989. The Chemistry of Soils. Oxford.
Ionic Solids Ionic bond is more prominent than covalent bond in the structure of minerals (exceptions Si- O and Al-O): Ionic bond is relatively weak, the electrical configuration is unaltered by the presence of another ion. Covalent bond is relatively strong, sharing of electrons involves distortion of the ionic configuration. For ionic bond, radius and valence are the most important properties of ions.
Molecular Structure of Quartz Source: modified from http://www.soils.wisc.edu/virtual_museum/alpha-quartz/index.html
Weathering Rates of Minerals Source: Chadwick O, Chorover J. 2001. The chemistry of pedogenic threshold. Geoderma 100: 321-353.
Ionic Solids Mineral structures are largely determined by the way oxygen (anion) pack around available cations. To preserve electrical neutrality, the positive charge of a cation is divided equally among the surrounding anions (oxygen or hydroxyl). Typically, Si +4 occurs in fourfold and Al +3 in sixfold (tetrahedral and octahedral combination, respectively).
Basic Units The tetrahedral structure combines four O 2- with one Si 4+ resulting in a partial neutralization of the negative charges of oxygen. The octahedral structure consists of six OH - surrounding a central cation, usually Al 3+, Mg 2+, or Fe 2+.
Formation of Clay Layers
1:1 Polar Clay Type Kaolinite Hydrogen bonds keep layers together, which in turn reduces interlayer spacing. Source: modified from http://www.soils.wisc.edu/virtual_museum/kaolinite/kaolinite_ch.html
2:1 Swelling Clay Smectite Isomorphic substitution in 2:1 swelling clays occurs in the octahedral layer. The position of the charge (in this case created by the substitution of Al by Mg) in the center of the mineral creates a weak electrical force that allows for the interlayer space to shrink and swell in response to removal and additions of water, respectively (compare with the Biotite example). Source: modified from http://www.soils.wisc.edu/virtual_museum/soil_smectite/index.html
2:1 Non-Swelling Clay Biotite Isomorphic substitution in 2:1 non-swelling clays occurs in the tetrahedral layer. The proximity of the charge (in this case created by the substitution of Si by Al) to the surface of the mineral creates an strong electrical force that reduces the interlayer space (compare with the Smectite example). Source: modified from http://www.soils.wisc.edu/virtual_museum/biotite/index.html
Morphological Differences Between Clay Types Which one is a 1:1 clay?
A Clay Domain Source: Lebron I, Suarez DL, Schaap MG. 2002. Soil pore size and geometry as a result of aggregate-size distribution and chemical composition. Soil Sci. 167 (3): 165-172.
Micrographs of Mica (1:1) Transmission electron micrographs performed on: (a) mica pseudo-crystal; (b) details of the previous micrograph showing swelling interlayers (S) which occur at a given location and when prolonged give way to voids (V); (c) beginning of crystal separation at the edge of the particle by increasing void size growing from the interlayer spaces. Source: H ardy, M, M. Jamagne, F. Elsass, M. Robert& D. Chesneau. 1999. Mineralogical development of the silt fractions of a Podzoluvisol on loess in the Paris Basin (France). European J. Soil Sci. 50: 443-456.
Figure 8.7 Schematic drawing illustrating the organization of tetrahedral and octahedral sheets in one 1:1-type mineral (kaolinite) and four 2:1-type minerals. The octahedral sheets in each of the 2:1-type clays can be either aluminum dominated or magnesium dominated. Note that kaolinite is nonexpanding, the layers being held together by hydrogen bonds. Maximum interlayer expansion is found in smectite, with somewhat less expansion in vermiculite because of the moderate binding power of numerous Mg2+ ions. Fine-grained mica and chlorite do not expand because K+ ions (fine-grained mica) or an octahedral-like sheet of hydroxides of Al, Mg, Fe, and so forth (chlorite) tightly bind the 2:1 layers together.
Drying a Clay Source: Tessier D. 1990. Behaviour and microstructure of clay minerals. P. 387-416. In: De Boodt, MF, MHB Hayes, A Herbillon (eds.) Soil Colloids and their Association in Aggregates. NATO ASI Series: Physics Vol. 215.
Source of Charges in Clays Isomorphic substitution results in permanent charges, with a net negative charge. Edge charges are created when the continuity of the crystal is broken. Charges could be or + (ph dependent charges).
Distribution of Ions Around a Charged Surface Typically, high concentrations of cations and a deficit of anions are found in the proximity of clay particles. The region with anomalous distributions of ions is known as the double layer. The region outside the influence of the charged particle is the soil solution. Cations can be exchanged between the soil solution and the double layer.
Figure 8.1 Simplified representation of a silicate clay crystal (micelle), its complement of adsorbed cations, and ions in the surrounding soil solution. The enlarged view (right) shows that the clay comprises sheetlike layers with both external and internal negatively charged surfaces. The negatively charged micelle acts as a huge anion and a swarm of positively charged cations is adsorbed to the micelle because of attraction between charges of opposite sign. Cation concentration decreases with distance from the clay. Anions (such as Cl2, NO32, and SO422), which are repulsed by the negative charges, can be found in the bulk soil solution farthest from the clay (far right). Some clays (not shown) also exhibit positive charges that can attract anions.
Cation Selectivity Cations are selectively adsorbed based on their charge and size. Selectivity increases with increasing charge (+++>++>+) decreases with increasing hydrated radius, R H Negatively charged surface R H water cation (+ or ++) Negatively charged surface R H water cation (+ or ++) Large R H : held less strongly Small R H : held more strongly
Dehydrated Radii, RA
Cation Selectivity The relative preference of clays for cations (considering only charge and size) is: R A (Å) 3+ 2+ 2+ Al > Ca > Mg > NH4 K > Na Li 0.51 0.99 0.66 1.43 1.38 0.97 0.68 + + + + Hydrogen is a special case: it is a monovalent that is adsorbed as strongly as a trivalent.
The Thickness of the Double Layer Imagine clay saturated with Ca 2+ and with Na + : Negatively charged surface Negatively charged surface It takes twice as many + ions than ++ ions to balance a surface charge: + ions result in thicker double layers. Hydrated radius of Ca 2+ ~9.6 Å Hydrated radius of Na + ~7.9 Å The concentration of the soil solution is another factor determining the thickness of the double layer: greater concentration results in thinner double layers.
Soil Dispersion and Flocculation Fig. 9.23
Cation Exchange Process Exchange Reaction: Soil _ B + + + + A Soil _ A + B + When the exchange surfaces are accessible, the process of cation exchange can take from µs to h. When diffusion rate is the limiting factor, the process can take months or years. The total amount of cations per unit mass is the Cation Exchange Capacity or CEC.
Cation Exchange Process There is equilibrium between the compositions of the soil solution and the double layer (stoichiometric reaction) : Soil _ B K + A/ B + + + A Soil _ A + B = Soil _ A Soil _ B + + B A + + + The composition of the exchange complex is a function of the composition of the soil solution, which in turn is affected by climate and soil management.
Example of Cationic Composition Soils ph CEC Exchangeable cations (% of CEC) mmol C /100 g Ca 2+ Mg 2+ K + Na + H + +Al 3+ Average Agricultural soil (Holland) 1 7.0 38.3 79.0 13.0 2.0 6.0 Mollisol (Russia) 1 7.0 56.1 84.3 11.0 1.6 3.0 Sodic (USA) 1 10.0 18.9 0.0 0.0 5.0 95.0 0.0 Spodosol (Holland) 2 4.6 8.8 35.6 6.0 3.3 1.4 53.7 Oxisol (Puerto Rico) 2 4.1 30.4 1.6 2.6 0.3 0.6 94.9 1 Bohn, H, McNeal B, O Connor G. 1985. Soil Chemistry. Wiley; 2 Bolt, GH and MGM Bruggenwert. 1978. Soil Chemistry. Elsevier
Percent Base Saturation H + and Al 3+ are known as acidic cations Ca 2+, Mg 2+, K +, Na +, and NH 4+ are known as basic cations. Percent base saturation reflects the proportion of the CEC that is occupied by basic cations: % base saturation = base cations CEC 100
A Model of SOM Source: modified from http://www.soils.wisc.edu/virtual_museum/som/index.html
Functional Groups in SOM Carboxyl groups: -COOH Phenolic groups: -ArOH Proteinaceous material Alcoholic groups: -ROH Saccharides (sugars) Water Source: modified from http://www.soils.wisc.edu/virtual_museum/som/index.html
Surface Charges in SOM Charges in SOM are mainly due to protonation of surface oxygen or dissociation of hydroxyl groups. As such, SOM could be positively charged (low ph s) or negatively charged (high ph s).
Figure 9.4 General relationship between soil ph and cations held in exchangeable form or tightly bound to colloids in two representative soils. Note that any particular soil would give somewhat different distributions. (Upper) A mineral soil with mixed mineralogy and a moderate organic matter level exhibits a moderate decrease in effective cation exchange capacity as ph is lowered, suggesting that ph-dependent charges and permanent charges (see Section 8.6 for explanation of these terms) each account for about half of the maximum CEC. At ph values above 5.5, the concentrations of exchangeable aluminum and H+ ions are too low to show in the diagram, and the effective CEC is essentially 100% saturated with exchangeable nonacid cations (Ca2+, Mg2+, K+, and Na+, the socalled base cations). As ph drops from 7.0 to about 5.5, the effective CEC is reduced because H+ ions and Al(OH)xy2 ions [which may include AlOH2+, Al(OH)2+, etc.] are tightly bound to some of the ph-dependent charge sites. As ph is further reduced from 5.5 to 4.0, aluminum ions (especially Al3+), along with some H+ ions, occupy an increasing portion of the remaining exchange sites. Exchangeable H+ ions occupy a major portion of the exchange complex only at ph levels below 4.0. (Lower) The CEC of an organic soil is dominated by ph-dependent (variable) charges with only a small amount of permanent charge. Therefore, as ph is lowered, the effective CEC of the organic soil declines more dramatically than the effective CEC of the mineral soil. At low ph levels, exchangeable H+ ions are more prominent and Al3+ less prominent on the organic soil than on the mineral soil.
Prediction of CEC for NJ Soils Relative Contribution, % Geomorphic Region R 2 Regression Equation Clay(X1) OM(X2) Appalachian 0.76 CEC = 3.4 + 0.28X1 + 2.42X2 51.4 48.6 Piedmont 0.21 CEC = 8.0 + 0.21X1+ 1.30X2 82.9 19.1 Coastal Plain 0.65 CEC = 1.0 + 0.39X1+ 2.04X2 73.6 26.4 As expected CEC can be predicted from information on clay and organic matter content, but the prediction is dependent on the geomorphic region. Source: Drake, E.H., and Motto, H.L. 1982. Analysis of the effects of clay and organic matter content on the cation exchange capacity of N.J. soils. Soil Sci. 133: 281-287.