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Soil Colloids. Chapter 8. █Ca 2+ +2K +  Ca 2+ + █2K +. These equilibria are complex, involving all exchangeable species. The above is an example binary exchange reaction for which an equilibrium constant can be
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Soil Colloids Chapter 8
█Ca2+ +2K+ Ca2+ + █2K+ These equilibria are complex, involving all exchangeable species. The above is an example binary exchange reaction for which an equilibrium constant can be written as KK-Ca = [Ca2+][K+ad]2 / [K+]2[Ca2+ad]. If you’ve had 2nd semester chemistry or remember high school chemistry it should make sense.
Since a decrease in solution concentration of a nutrient cation or anion by plant uptake or leaching tends to cause release of the same type ion into solution from colloids (this is accompanied by replacement on the colloid by a different type cation or anion), the adsorbed ions are a reservoir of nutrients. Much greater quantity so adsorbed than in the soil solution. If a portion of a substance in the soil is distributed between solution and solid (adsorbed to) phases, its mobility must be less than if it were all in solution.
Yes to the first, assuming these were nutrient ions and certainly yes to the second.
These are the units in which the concentration of exchangeable cations in a soil are expressed, particularly cmol(+) kg-1. The others are probably archaic but notice that they are numerically the same. An equivalent is a mole of reactive units, in this case, charge.
In principal, this is a straightforward matter but there are complications in practice. The basic idea is that you use cation exchange to force all initially adsorbed cations into solution, separate solution from soil (like filter) and measure the concentration of all cations in solution. This requires use of a cation in solution that is not very common in the soil and it requires a high concentration of it. Look back at the example cation exchange reaction and notice that if the concentration of K+ was very high, the extent to which Ca2+ would be displaced into solution would be greater than if the concentration of K+ were modest. However, there is a problem with determining acidic cations like H+ and Al3+ in this way. A portion of these cations is very strongly held by adsorption onto colloids so that even a very high concentration of displacing cations won’t drive the exchange reaction to completion. However, alternatives exist to deal with this. For base cations, ammonium, NH4+, is the typically cation used to displace them. AEC is done the same way but with a displacing anion, of course.
It should make sense that different colloids likely have different CECs (and AECs). Thus, the relative amounts of different colloids determine the CEC. However, the charges on colloidal particles partly depend on the concentration of H+ in solution (i.e., pH, which is –log[H+])
These are the general types of soil colloids. The layer aluminosilicates are crystalline, however, amorphous ones have limited and interrupted crystal structure. Strictly, besides oxides there are related non-siliceous minerals, like hydroxides and oxy-hydroxides, including ones besides just Al and Fe forms.
Substitution of a lower valence cation for a higher valence cation during the formation of the crystal results in a deficit of positive charge relative to negative charge carried by the O and OH in the structure. Thus, the charge is – and it is permanent to the crystal structure.
\ \ Al – OH Al – O- + H+ / / \ \ Al – OH + H+ Al – OH2+ / / pH dependent charges Besides permanent charge there are functional groups on the surfaces of colloids that can ionize or protonate to give rise to - / + charge. Here is a common example, surface Al–OH groups. Under conditions of higher soil pH (i.e., low concentration of H+), they tend to dissociate as in the top reaction. But when the pH is low, the O tends to be protonated by H+ from solution, giving a + site. There are lots of functional groups, both on mineral and organic colloids that do this.
Having said a bit about electrostatic charges, let’s look at the common layer aluminosilicate minerals. These are they.
2:1 Types This is a cutaway showing interlayer space between two units of a 2:1 type mineral. In this case, the stack of crystal units are shown to be able to expand, imbibing water in between adjacent crystals. Some 2:1 do this, others don’t. Those can are responsible for macroscopic shrinking and swelling behavior.
Very little, in fact. Apparently, the higher density of negative charge located very near the surface of the crystal face (tetrahedral sheet) leads to higher electrostatic attraction for cations in the interlayer space. The mutually strong attraction by two adjacent crystals for these cations greatly limits the extent to which water enters the interlayer space and causes expansion. Make sense?
This is much the same thing as with vermiculite, however, the presence of K+ leads to especially strong bridging of adjacent crystals. See next slide.
These are different representations of the silica tetrahedral sheet. Notice the hole-like features that some call siloxane cavities. K+ has just the right ionic radius to fit into these. Thus, electrostatic attraction between it and the isomorphic negative charge (much of it in the tetrahedral sheet) leads to very strong bridging between one crystal unit of illite and its neighbor. Thus, illite does not expand.
2:1 Types, Illite By the way, surface area is measured from gas adsorption. Whereas smectite has open interlayer space, illite does not. Thus, much of the planar area of the tetrahedral layers in illite is not exposed to the gas. Further, the K+ in the interlayer space is not exchangeable. Thus, the high amount of negative charge (high extent of isomorphic substitution) cannot be measured by summing the charge of cations released by CEC determination.
Os from the tetrahedral sheet of one crystal H-bond with the –OHs of the octahedral sheet of the neighboring crystal.
And since there is little isomorphic substitution, most of the CEC is due to pH-dependent charge that arises from ionization of edge –OHs.
Review █Ca2+ +2K+ Ca2+ + █2K+
expanding, high CEC limited expansion, higher CEC not expanding, trapped K+
Formation and stability of mineral colloids Primary mineralsweather to 2:1 clayswhich weather to 1:1 clayswhich weather to Oxides Thus, soils in mildly weathering climates tend to have minerals towards the top of this sequence, and soils in harshly weathering climates (lots of water and high temperatures), tend to have minerals towards the bottom. For edification, check out Jackson-Sherman weathering sequence.
Refer to next slide. There are types of octahedral sheets that contain Mg2+ as the central cation. These are called trioctahedral and those with Al3+ are called dioctahedral. Basically, the ideally electro-neutral structure in a trioctahedral sheet requires 1½ times as many Mg2+ as there are Al3+ in a dioctahedral sheet.
Thus, isomorphic substitution of the higher valence Al3+ for Mg2+ results in an excess of + charge in the crystal lattice, which must be balanced by adsorption of anions from solution.
pH-dependent Negative charge Ionization of H from –OH on surface of oxides and edges of silicate clays Al—OH → Al—O- + H+
Ionization of –OH and –COOH on humic colloids O O ║ ║ --C—OH → --C—O- + H+
Positive charge Protonation of –OH to give OH2+ Oxide surfaces and silicate clay edges Al—OH + H+ → Al—OH2+
Think of it this way –permanent charge is unaffected, right, but as the concentration of H+ in solution decreases (i.e., pH increases), whatever ionizable H there is on colloidal surfaces tends to ionize, creating negative sites and making the colloid more negative. So, the capacity of the colloid to adsorb cations increases, i.e., the CEC increases. The AEC is opposite. As the concentration of H+ in solution increases, more and more sites become protonated, increasing the positive charge on the colloid and its capacity to adsorb anions from solution.
