Soil Colloids

Chapter 8

Ion exchangeCation exchange capacityAnion exchange capacity

Types of soil colloidsEmphasis on layer silicate mineralsTypes and properties of layer silicates

1:12:12:1:1

Types of electrostatic chargePermanent (isomorphic substitution)pH-dependent

Acidic and basic cations and soil acidity

Usually more

Small so

BIG SURFACE AREA / mass

Electrostatic charges (- / +) soAdsorb ions

Adsorbed cations and anions

Not stuck permanently, irreversibly and for ever and ever on colloids

Can trade places with other cationsand anions in solution

- charged sites of a colloid

Cation Exchange

Equilibrium between cations in solutionand adsorbed on colloids

Ca2+ + 2K+ Ca2+ + 2K+█Ca2+ +2K+ Ca2+ + █2K+

These equilibria are complex, involving all exchangeable species. The above isan example binary exchange reaction for which an equilibrium constant can bewritten 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.

+ charged sites

Anion Exchange

Like cation exchange

SO42- + 2Cl- SO4

2- + 2Cl-█SO42- +2Cl- SO4

2- + █2Cl-

Are adsorbed cations and anionsimportant to plant nutrition?

Does ion exchange replenish the soilsolution with nutrient ions?

Does ion exchange slow themovement of charged contaminants?

Since a decrease in solution concentration of a nutrient cation or anion by plantuptake or leaching tends to cause release of the same type ion into solution fromcolloids (this is accompanied by replacement on the colloid by a different type cationor anion), the adsorbed ions are a reservoir of nutrients. Much greater quantity soadsorbed 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.

Is a soil with a lot of adsorbed ionsmore fertile than a soil with very fewadsorbed ions?

Does a charged contaminant movemore slowly in a soil with a highcapcity to adsorb ions than in a soilwith a low capacity to adsorb ions?Yes to the first, assuming these were nutrient ions and certainly yes to the second.

Cation Exchange Capacity (CEC)

Milli-moles (+) charge / 100 g soil

Milli-equivalents (meq) + / 100 g soil

Is a centimole + charge / kg the same?

cmol (+) / kg

These are the units in which the concentration of exchangeable cations in asoil 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.

How do you measure CEC? Or AEC?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 cationsinto solution, separate solution from soil (like filter) and measure the concentrationof 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 theexample cation exchange reaction and notice that if the concentration of K+ wasvery 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 thisway. A portion of these cations is very strongly held by adsorption onto colloidsso that even a very high concentration of displacing cations won’t drive theexchange 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.

CEC is sum

Acidic + basic cation charges / kg

cmol (+) / kg

Depends on

Types of colloidsAmounts of colloidspH

It should make sense that differentcolloids likely have different CECs(and AECs). Thus, the relativeamounts of different colloids determinethe CEC. However, the charges oncolloidal particles partly depend onthe concentration of H+ in solution(i.e., pH, which is –log[H+])

Types of colloidsSources of charge

permanentpH-dependent

Types

Layer aluminosilicatesAmorphous aluminosilicatesAl and Fe oxidesOrganic (humic)

These are the general types of soil colloids. The layer aluminosilicates arecrystalline, however, amorphous ones have limited and interrupted crystal structure. Strictly, besides oxides there are related non-siliceous minerals, likehydroxides and oxy-hydroxides, including ones besides just Al and Fe forms.

Layer Aluminosilicates

Alternating sheets of Si tetrahedra and Al (or Mg) octahedra

Carry electrostatic charges due to

Isomorphic substitution

pH-dependent ionization or protonation

What charge (+ or -) does the crystal carry? Balanced by cations?

Is this source of charge permanent?

Isomorphic substitution

Al3+ for Si4+ in tetrahedral layer

Mg2+ for Al3+ the octahedral layer

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 negativecharge 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 chargethere are functional groupson the surfaces of colloidsthat can ionize or protonateto give rise to - / + charge.Here is a common example,surface Al–OH groups. Underconditions of higher soil pH(i.e., low concentration of H+),they tend to dissociate as inthe top reaction.

But when the pH is low, theO tends to be protonated byH+ from solution, giving a +site.

There are lots of functionalgroups, both on mineral andorganic colloids that do this.

Three typesof layer silicates

1:1

2:1

2:1:1

Tetrahedral sheetOctahedral sheet

Having said a bit about electrostaticcharges, let’s look at the commonlayer aluminosilicate minerals. Theseare they.

2:1 layer silicates

Unit consists 1 octahedral sheet between 2Si tetrahedral sheets

Certain types expand

2:1 Types

This is a cutawayshowing interlayerspace betweentwo units of a2:1 type mineral.In this case, thestack of crystalunits are shownto be able to expand, imbibingwater in betweenadjacent crystals.

Some 2:1 do this,others don’t. Those can areresponsible for macroscopicshrinking and swelling behavior.

Three types of 2:1 minerals

Smectite

Vermiculite

Illite

Smectite

Units weakly held togetherby cations

Expand whenadsorb waterbetween units

2:1 Types

Big CECHighly plastic and swelling

Does this soil have a lotof smectite in it?

2:1 Types, Smectite

Vermiculite

Even bigger CECMore isomorphic substitution

2:1 Types

Most of the isomorphic substitution insmectite is in the octahedral layer andthese expand.

The CEC of vermiculite is bigger and alot of it is due to substitution in thetetrahedral layers.

Does vermiculite expand as much assmectite?

Very little, in fact. Apparently, the higher density of negative charge located verynear the surface of the crystal face (tetrahedral sheet) leads to higher electrostaticattraction for cations in the interlayer space. The mutually strong attraction by twoadjacent crystals for these cations greatly limits the extent to which water entersthe interlayer space and causes expansion. Make sense?

No because

Strong affinity for cations that bridgetetrahedral layers

Limited-expansion

2:1 Types, Vermiculite

Illite

Isomorphic substitution in Si tetrahedral sheet

Geometry favors adsorption of K+ at interlayer positions

Holds units tightly together This is much the same thing as withvermiculite, however, the presence ofK+ leads to especially strong bridgingof adjacent crystals. See next slide.

These are different representations of the silica tetrahedral sheet. Noticethe hole-like features that some call siloxane cavities. K+ has just theright 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 itsneighbor. Thus, illite does not expand.

Nonexpanding

Smaller surface area than smectite orvermiculite

CEC much less than other 2:1 minerals

2:1 Types, Illite

By the way, surface area is measuredfrom gas adsorption.

Whereas smectite has open interlayerspace, illite does not. Thus, much of the planar area of the tetrahedral layers in illiteis not exposed to the gas.

Further, the K+ in the interlayer space is notexchangeable. Thus, the high amount of negative charge (high extent of isomorphic substitution) cannot be measured bysumming the charge of cations releasedby CEC determination.

1:1 layer silicates

1 Si tetrahedral sheet1 Al octahedral sheet

Adjacent units H-bonded together

Os from the tetrahedral sheet ofone crystal H-bond with the –OHsof the octahedral sheet of theneighboring crystal.

If adjacent crystal units are H-bondedtogether, do 1:1 minerals expand?

Little plasticity or swelling

Small CEC

Little isomorphic substitution

And since there is little isomorphic substitution, most of the CEC isdue to pH-dependent charge that arises from ionization of edge –OHs.

2:1:1 minerals

Additional octahedral sheet (2:1:1) contains Mg

Nonexpanding and fairly low CEC

Less common than the others.

Review

- charged sites of a colloid

Cation Exchange

Equilibrium between cations in solutionand adsorbed on colloids

Ca2+ + 2K+ Ca2+ + 2K+█Ca2+ +2K+ Ca2+ + █2K+

CEC is sum

Acidic + basic cation charges / kg

cmol (+) / kg

Depends on

Types of colloidsAmounts of colloidspH

Types

Layer aluminosilicatesAmorphous aluminosilicatesAl and Fe oxidesOrganic (humic)

Three typesof layer silicates

1:1

2:1

2:1:1

Tetrahedral sheetOctahedral sheet

Three types of 2:1 minerals

Smectite

Vermiculite

Illite

expanding, high CEC

limited expansion, higher CEC

not expanding, trapped K+

1:1 layer silicates

1 Si tetrahedral sheet1 Al octahedral sheet

Adjacent units H-bonded together

2:1:1 minerals

Additional octahedral sheet (2:1:1) contains Mg

Nonexpanding and fairly low CEC

Formation and stability of mineral colloids

Primary minerals weather to2:1 clays which weather to

1:1 clays which weather toOxides

Thus, soils in mildly weathering climates tend to have minerals towardsthe top of this sequence, and soils in harshly weathering climates (lotsof water and high temperatures), tend to have minerals towards the bottom.For edification, check out Jackson-Sherman weathering sequence.

More on Electrostatic Charges

Permanent

You know negative charges come fromisomorphic substitution, like Al3+ for Si4+

or Mg2+ for Al3+

But what if Al3+ substitutes for Mg2+?What do you get?Refer to next slide. There are types of octahedral sheets that contain Mg2+ asthe central cation. These are called trioctahedral and those with Al3+ are calleddioctahedral. Basically, the ideally electro-neutral structure in a trioctahedralsheet requires 1½ times as many Mg2+ as there are Al3+ in a dioctahedral sheet.

Thus, isomorphicsubstitution of the higher valence Al3+

for Mg2+ results inan excess of + chargein the crystal lattice,which must bebalanced by adsorptionof 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+

Does CEC increase or decrease as pHincreases?

What about AEC? Think of it this way –permanent charge is unaffected, right, but as the concentrationof H+ in solution decreases (i.e., pH increases), whatever ionizable H there is oncolloidal surfaces tends to ionize, creating negative sites and making the colloidmore 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 solutionincreases, more and more sites become protonated, increasing the positive chargeon the colloid and its capacity to adsorb anions from solution.

Charge at pH 7

Type Perm pH-dep Total

Humus 20 180 200Vermiculite 140 10 150 Smectite 95 5 100 lllite 20 6 301:1 minerals 0.4 7.6 8Oxides 0 4 4

This is somebody’s breakdown of CEC into permanent and pH-dependentcomponents. The notion that organic colloids (humus) have permanent chargemakes no sense since isomorphic substitution is not applicable. What, however, makes sense is that even at very low pHs (not to be encountered except in somedrained wetlands or contaminated sites) some of the acidic functional groups on soil organic matter are sufficiently acidic to be ionized.

More on CEC

Ca2+, Mg2+, K+ and Na+ are basic cations

H+ and Al3+ are acidic cations

Percentage of CEC that is made up of basiccations is called

Percentage base saturation

%BS

Here’s an example calculation: Extractable cations

Ca2+ Mg2+ K+ H+

----------- cmol(+) kg-1----------- 2 1 1 1

For this case, CEC = 5 cmol(+) kg-1 and there are 4 cmol(+) kg-1 due to the basesso the %BS = 4 / 5 x 100% = 80%

True or False

As pH ↑ %BS ↑

True. If the pH increases, there is less acidity in the system (H+ and otheracidic cations, e.g., Al3+, both in solution and adsorbed on colloidal surfaces).Thus, since the negative charge on colloids must be satisfied by adsorbedcations, decreased concentration of acidic cations means increasedconcentration of basic cations. Also, with increasing pH the negative charge on colloids increases, compounding the effect of increased concentrationof adsorbed bases.

Soil A Soil B cmol (+) / kg

Basic cations 90 5Acidic cations 10 5

Which soil has the lower pH?Which soil is more fertile?

Let’s just say likely lower pH. A has a %BS = 90 and B, 50. Thus, B likelyhas the lower pH. The matter of fertility is clearer since most basic cationsare nutrients –A has 18x as many.