Adsorption And Ion Exchange

The behavior of elements present in large amounts in an aquatic system is controlled by the rules of solubility, acid-base equilibria, complexation, and so on, but the behavior of trace substances is largely controlled by adsorption on solid surfaces. Material will be partitioned between the surface and the bulk liquid, depending on the strength of the adsorption. Adsorption can be approximated by the equation

where Caq is the concentration in the aqueous phase, Cs is the quantity adsorbed per unit amount of solid, and Kd is the partition or distribution coefficient. If adsorption becomes more difficult as the amount already adsorbed increases (i.e., if the adsorption sites available vary in activity), the relation between Caq and Cs may not be linear. In addition, there is a limit to how much material can be adsorbed. Consequently, this expression is most applicable to low levels of adsorption.

The usual environmental particles in water systems, sediments, and soils are oxides of metals or metalloids. Since surface metal atom will adsorb OH4 or H2O from the water, the entire surface can be regarded as oxide. In a complex solid, all sites may not have the same affinity for the adsorbate, and thus adsorption may not show a linear relation between amount adsorbed and concentration in solution. At some point the surface will be saturated. Competition may occur between different adsorbates, and adsorption may be pH dependent.

A typical oxide surface site may be represented as follows

with an acid-base (and thus a pH-dependent) equilibrium (= X represents an atom to which a surface oxygen atom is attached). The surface may be positive, negative, or neutral, depending on the pH. The isoelectric point is the pH at which the surface is neutral. The isoelectric point for silica and many silicates is less than 3, so that under most conditions in natural waters and soils the particles carry a negative charge. Values for iron oxides tend to be near pH 7, while those for aluminum oxide and hydroxide materials are strongly basic. While actual values of neutrality will vary with electrolytes, these give some idea of the types of ions likely to be absorbed by a colloid of a given kind.

If a metal ion M2+ is present, possible interactions include the following.

The O—M linkage may be primarily electrostatic with an ion such as Na+, or it may have more covalency with transition metal ions. In any case, the strength of the interaction will be greater for smaller and more highly charged ions. A problem associated with this correlation, however, is knowing whether the free ion or the hydrated ion size is important. A small or highly charged ion may hold the water molecules that hydrate it strongly enough to ensure that they are retained upon adsorption, while a larger or lower charged ion may be bound without its waters of hydration and thus appear smaller.

The last of the foregoing structures illustrates the possibility that anions (OH4 in this case) can be adsorbed by an attached cation; more generally, = X—O—M—Y (Y is any anion). Another form of possible anion binding is

The = X—O group can also adsorb oxo anion species by replacing another group as follows, for example,

or ionized forms.

In any of these processes, one ion can be exchanged for another; for example, H+ is replaced by a metal ion, or M2+ can be replaced by another metal ion, or by Y That is, the adsorption of ions by the surface can lead to ion exchange behavior, an important process for nutrients in soils, for example (see Section 12.3.1). The pH behavior of adsorption is species specific, but in general, metal ions tend to be held most strongly at high pH and released at low pH, where H+ can replace them. Anions behave differently: Phosphate adsorbed by alumina, for example, is held most strongly at low and especially neutral pH, being released at high pH, where OH~ controls the behavior.

The ability of a material such as a clay to absorb ions is measured by its ion exchange capacity, determined by how much Na+ can be replaced by another ion, and is usually expressed as milliequivalents per 100 g. Values for mineral particles may exceed 100. A clay with an ion exchange capacity of only 10 mequiv/100g could absorb 1.04 g (5 mmol) of Pb per 100 g, for example. Thus retention capabilities may be quite significant. In a real situation, of course, there will be competition among the various ionic species present.

Neutral organic molecules, if polar, can be strongly adsorbed on oxide surfaces. In many natural water systems, the amount of dissolved organic carbon (e.g, soluble humic compounds) is sufficient to saturate the suspended inorganic particles and completely cover their surfaces. These organic surface films may still have exposed functional groups that can attract cations, but the organic layer may also adsorb hydrophobic organic materials. In this way, trace organic materials as well as heavy metals can be significantly concentrated in sediments as the suspended matter settles.

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