13

GES 166/266, Soil Chemistry

Winter 2000

Lecture Supplement 3

Solid-Water Interface

VI.1 SURFACE CHARGE

VI.1-1, Permanent Charge

There are two types of charge generally associated with mineral and organic surfaces: permanent and variable charge. As we have already stated, permanent charge arises from isomorphic substitution within a mineral. The substitution results in a charge deficiency which is delocalized so that we can think of this charged as distributed across a surface plane. In the phyllosilicate minerals it is the (001) plane that dominantly exhibits the permanent charge. An important point to consider is that although a charge was created from the ion substitution no bonds were altered; this means that the (001) plane, while having an excess charge, does NOT have a chemical affinity for solution ions. It only wishes to satisfy its electrical charge. As a consequence, ions will be bound only through electrostatic forces to the permanently charged (001) surfaces of the phyllosilicates. A few important points to remember about permanent charge:

1. it is pH independent

2. it is developed by isomorphic substitution

3. it is represented by the charge symbol so

VI.1-2, Variable Charge

Unsatisfied bonds at the terminal ends of minerals and organic matter result in a surface charge. These surfaces, however, have very different properties from those discussed for the permanently charged surfaces. Firstly, as the name implies, the charge can vary depending on the solution conditions; this charge is primarily a function of pH and is sometimes referred to as pH dependent charge. This results from the surface oxygens of the hydrous oxides and many organic functional groups, such as the carboxylic acid groups, having a high affinity for H+ ions. The addition or release of protons from the surface results in different charges. For example, consider the protonation reactions of an iron oxide surface functional group (surface site).

>Al-OH-1/2 + H+ <==> >Al-OH2+1/2

>Al2=O-1 + H+ <==> >Al2=OHo + H+ <==> >Al2=OH2+

In this reaction, the species to the left would represent a very high pH condition, the middle a moderate pH, and the right species a low pH. As you can see, the surface charge changes with pH not only in magnitude but also in sign. Note that at low pH values the surface is positively charged, as the pH increases the positive charge is reversed to a negative charge which increases with further increased pH.

Because the charge can change signs, we need to introduce another important term: The zero point of charge (ZPC). The ZPC is the solution pH at which the NET surface charge is zero. This does not mean that the surface has no charge, but rather that there are equal amounts of positive and negative charge. At pH values below the ZPC, the surface has a positive charge while it has a negative charge at pH values above the ZPC.

A second difference between permanently and variably charged surfaces is that on the latter the charge is localized at specific sites. In fact, adjacent sites may even have opposing charges. A third, and very important, difference is that variable charge surface sites are chemically reactive. In response, ions may be retained on the surface through electrostatic attractive forces or through a chemical bond. Summarizing important characteristics of variable charged surfaces:

1. they are pH dependent

2. developed from terminal bonds

3. represented by sH

4. surface groups do NOT develop a charge greater than |1|

5. dominant surfaces: hydrous metal-oxides, kaolinite, and organic matter

6. surface groups are weak acids and be defined by their ionization parameter(a)

Other Planes of Charge that can occur in the solid/water interface are attributed to: Inner-sphere ions (sis), outer-sphere ions (sos), and the diffuse Layer (sd). so and sH are the 'mineral' charge while the other 3 planes are due to reactions from the solution. It is important to note that the diffuse layer counter balances all the other planes of charge. The zero point of charge (ZPC) is defined as the pH where: so + sH + sis + sos = 0. This definition also means the ZPC is where the total particle charge is zero: (+) = (-). Some important points about the ZPC to remember are: (i) flocculation is greatest at pH = ZPC, (ii), mineral dissolution is at a minimum when pH = ZPC, and (iii) soils tend to weather toward a pH = ZPC.

VI.2 ION RETENTION

Unlike organic molecules, inorganic species cannot be degraded. They can, however, be retained on mineral surfaces or form discrete precipitates; in either case they are removed from the mobile aqueous phase and their bioavailability is consequently restricted. Retention processes thereby decrease the risk imposed by contaminants but they also can decrease the availability of needed plant nutrients. Accordingly, it is important for us to understand the processes by which ions are removed from solution and to have an idea of how strongly the ions will be immobilized. Or, in other words, what is the potential for the ions to be released back into solution?

Ions can bind by different mechanisms (reactions); the retention strength is dependent on mechanism. Additionally, models predicting ion sorption will differ depending on the mechanism. Possible retention mechanisms include adsorption and surface precipitation.

Before proceeding further we should define some terms so that we will all be speaking the same language. The terms sorption, adsorption, absorption, precipitation, surface precipitation, retention, and others are all used to refer to the loss of a species from solution. They differ, though, in their implicit meaning. The definitions we will use are as follows.

VI.2-1, Sorption Terms

Sorption: The retention of a species without implication to its retention mechanism. This term is inclusive of adsorption, absorption, precipitation, and surface precipitation.

Adsorption: The binding of an ion or small molecule to a surface at an isolated site--a 2-D surface complex. There is no interaction (or at least only minimal interaction) between adsorbed species.

Absorption: The uptake of a species WITHIN another material. This mechanisms is somewhat analogous to water uptake into a sponge.

Surface precipitation: A 3-dimensional growth mechanism of a species on a surface. This mechanism differs from adsorption in that the retained species directly interact with each other on the surface and can even have the solid structure grow away from the original substrate.

Precipitation: The formation of a 3-D structure without the association of a substrate (sorbent) material. This process occurs in solution directly and leads to discrete particles (it is also refereed to as a 'homogeneous precipitate').

Sorptive: A species in solution that may undergo sorption.

Sorbate: A species retain on another material.

Sorbent: The substrate material responsible for the retention of a solution species.

As these definitions should imply, there are many different processes responsible for the removal of a species from solution. We now need to look at these various mechanisms in more detail to gain an understanding of their retention strengths.

VI.2-2, Adsorption Mechanisms

The energy of adsorption can be divided into two components, that from the electrostatic interaction and that from the chemical: Adsorption Energy = Eelectrostatic + Echemical. It is important to note that even if the electrostatic component is negative, the chemical affinity of an ion for the surface can override it. That means ions can adsorb against an electrostatic gradient, e.g., transition metal cations bind to goethite at pH < 8.

We can separate possible adsorption mechanisms into three classes:

1) Inner-sphere complexation (chemical reaction)

- a chemical reaction between the surface and the ion

- 'specific adsorption'

- very strong association

- exchangeable

2) Outer-sphere complexation (electrostatic reaction)

- localized electrostatic charge neutralization

- 'non-specific' adsorption

- exchangeable

- analogous to ion pairs

3) Retention in the Diffuse Swarm (electrostatic attraction)

- delocalized electrostatic attraction

- 'swarm' neutralizes remaining surface charge

- exchangeable

VI.2-3, Chemical Surface Reactions:

When we discussed the charge developed from broken bonds we recognized an imbalance in charge that resulted at a surface. In addition to the charge imbalance, these surface functional groups are also coordinately unsaturated and would therefore like to satisfy their bonding environments. We should realize then that these groups or surface sites may incorporate other ions from solution into their structure. When this happens the adsorbed ion will loose its hydration sheath and a chemical bond (covalent or ionic) will result. As might be expected, these bonds are much stronger than those of an outer-sphere complex and we generally do not consider such sorbates as exchangeable. Inner-sphere complexes do not form on the (001) plane of the phyllosilicates but are readily formed on the (010) and (100) plane of the phyllosilicates; in fact, all of the variable charged surfaces allow inner-sphere complex formation.

In addition to considering the surface we should also discuss the ions which may form inner-sphere surface complexes. It is beyond the scope of this course to present a rigorous discussion about electron characteristics of an ion which dictates the type of complex it forms, but there are some general rules we can assign to qualitatively assess this phenomena. Generally, the alkaline earth ions will form outer-sphere complexes only, while the transition metals have the capacity to form inner-sphere complexes. We can not generalize about the ions from the right side of the periodic chart as this is partially dependent on their molecular arrangement; here is a list of their capacity to form inner-sphere complexes (realize, of course, that any charged ion has the capability to be retained due to electrostatic forces as well).

Inner-sphere

Ion Capacity

Cl- no

SO42- partially

NO3- no

F- yes

PO43- yes

SeO32- yes

SeO42- partially

AsO43- yes

AsO33- partially

CrO42- yes

VI.2-4, Exchange Reactions

A significant means by which ions adsorb is due to an electrostatic attraction between an ion and a surface of opposite charge. Electrostatic reactions occur between any ion and surface of opposite charge. Such electrostatic forces may arise from the permanent negative charge of a phyllosilicate clay mineral and a cation such as Na+, Ca2+, or many others. Remember that a variable charged surface may have either a positive or negative charge (or both) depending on the solution conditions. Electrostatic interactions between a surface and an ion are analogous to ion pairs that we discussed in the Aqueous Chemistry portion of the course. As such, you should also remember that this would be an outer-sphere complex where the surface and the ion maintain their hydration sheath. These outer-sphere complexes are rather weak compared to chemical forces and result in an exchangeable sorbate. In fact, it is predominantly the electrostatically bound ions that make up the cation exchange or anion exchange capacity. Electrostatically bound ions can be displaced by other ions or displaced simply due to a diffusion gradient. Exchangeable ions are essential for maintaining plant nutrient levels, but are not strong enough to immobilize environmental pollutants.

6.2-4.1, Cation Exchange Capacity

The CEC is usually dominated by Ca, Mg, Na, K, and Al; thus,

CEC (mmol charge) ≈ 2[Ca] + 2[Mg] + [K] + [Na] + 3[Al]

The selectivity of cation by exchanger is based on the ion's charge/size

1. SIZE: The smaller the hydrated radius the greater the affinity

(note: ions with small dehydrated radius have large hydrated)

2. VALENCE: This is the dominant factor influencing adsorption. The higher the valence the greater the exchanger preference: 4+ > 3+ > 2+ > 1+.

The preference of an ion for a surface is summarized by the Lytropic Series (strength of retention):

Th4+ > Al3+ > La3+ > Ba2+ ≈ Sr2+ > Ca2+ > Mg2+ ≈ Cs+ > Rb+ > NH4+ > K+ > Na+ ≈ Li+

However, deviations from Lytropic series occur if specific chemical affinity occurs. Examples of such deviations include:

1. K+ on vermiculite

2. Increased affinity of highly charged surfaces for highly charged ions

3. Vermiculite also has an unusually high affinity for Mg

VI.2-5, Precipitation Mechanisms

Precipitation reactions result from a solution being oversaturated with respect to a mineral phase. Solubility constants for precipitation in bulk solution are tabulated in many text books. Using these constants, one can use the saturation index we discussed previously to determine if a solution is undersaturated (SI < 0) , oversaturated (SI > 0), or in equilibrium (SI = 0) with a solid,

SI = log (IAP / Ksp)

where IAP is the ion activity product for the specific reaction and Ksp is the solubility constant for this reaction. Remember that IAP is the measured activity values for the reaction while Ksp is the value representative of an equilibrium situation; they are only equal if the system is at equilibrium (SI = 0).

While the SI gives a convenient means for assessing the thermodynamic possibility of precipitation, it does not tell us whether the reaction will actually happen--only if it is possible. Kinetic factors usually govern the phase we can expect to form over a short period of time. This is primarily dictated by the activation energy, or energy barrier, of a reaction. Generally, large, well-crystallized particles have a lower Ksp but higher activation energy. Consequently, we frequently find many amorphous particles in soils due to their meta-stable conditions. Given sufficient time, these amorphous phases will transform into more crystalline solids, which are thermodynamically more stable.

All this is applicable for the bulk solution, but what about the mineral/solution interface? Here, there are additionally forces that must be considered. Unfortunately, it is not yet possible to provide a quantitative value for most of these factors, so we must restrict or discussion to a qualitative one. Due to electrostatic and chemical forces, Ksp values are always lower in the interfacial area relative to the bulk solution. That is, surfaces will catalyze the precipitation of solids. They will also partially influence the mineral phase that forms. Consequently, although IAP values may indicate that the bulk solution is undersaturated with respect to a mineral, such a mineral may form at the solid/solution interface. Since this phenomena can not be quantitatively describe, it is simply your job to remember that their is a potential for such a reaction.

Precipitation is modified relative to solution precipitation in 2 ways: (i) surface lower activation energy and catalyzes precipitation (ii) electrostatic charge decreases Ksp (thermodynamic change, no violation)