THE GEOCHEMISTRY OF NATURAL WATERS

1. 1 THE GEOCHEMISTRY OF NATURAL WATERS MINERAL WEATHERING AND MINERAL SURFACE PROCESSES - III SORPTION AND ION EXCHANGE CHAPTER 4 - Kehew (2001) Sorption and surface charge

2. 2 LEARNING OBJECTIVES Learn about sorption; distinguish among adsorption, absorption and ion exchange. Understand why minerals acquire surface charge and what the implications are. Learn about sorption isotherms. Learn to deal quantitatively with ion exchange. Investigate the role of ion exchange in natural and contaminated waters.

3. 3 IS SOLUBILITY THE ONLY CONTROL ON SOLUTE CONCENTRATIONS? The answer is no! Solubility often controls the concentrations of major solutes such as Si, Ca, and Mg, and some minor or trace solutes such as Al and Fe. However, for many trace elements, sorption processes maintain concentrations below saturation with respect to minerals. In other words, sorption is a means to remove solutes even when the solution is undersaturated with any relevant solids. In the preceding two lectures, we have learned about solubility controls on natural water compositions. Both congruent and incongruent dissolution can exert control on the concentrations of major solutes, and some trace solutes as well. Nevertheless, the concentrations of many trace elements are controlled by a collection of processes collectively called sorption. Sorption processes involve the removal of solutes from solution into or onto a solid. These processes may occur even though the solution is not saturated with any mineral containing the solute of interest. For example, the concentration of a trace element such as Cd may be limited by sorption onto the surface of a clay or iron oxyhydroxide mineral, even though the solution is undersaturated with respect to all minerals of which Cd is an essential constituent. Sorption processes are important because they retard the movement of contaminants through aquifers. Sorption processes are expected to play a dominant role in retaining radionuclides near nuclear waste repositories, should the primary waste form be breached and come in contact with ground water. Most repository designs provide for backfilling of metal canisters (containing nuclear waste-bearing borosilicate glass) with clays. Sorption onto clay and other mineral surfaces should help retard the migration of radionuclides into the biosphere. Finally, sorption processes also are important in uncontaminated natural waters (recall the Madison Aquifer example in Lecture 4 where ion exchange occurred along flow path 2). Thus, an understanding of sorption processes is of paramount importance to aqueous geochemistry. In the preceding two lectures, we have learned about solubility controls on natural water compositions. Both congruent and incongruent dissolution can exert control on the concentrations of major solutes, and some trace solutes as well. Nevertheless, the concentrations of many trace elements are controlled by a collection of processes collectively called sorption. Sorption processes involve the removal of solutes from solution into or onto a solid. These processes may occur even though the solution is not saturated with any mineral containing the solute of interest. For example, the concentration of a trace element such as Cd may be limited by sorption onto the surface of a clay or iron oxyhydroxide mineral, even though the solution is undersaturated with respect to all minerals of which Cd is an essential constituent. Sorption processes are important because they retard the movement of contaminants through aquifers. Sorption processes are expected to play a dominant role in retaining radionuclides near nuclear waste repositories, should the primary waste form be breached and come in contact with ground water. Most repository designs provide for backfilling of metal canisters (containing nuclear waste-bearing borosilicate glass) with clays. Sorption onto clay and other mineral surfaces should help retard the migration of radionuclides into the biosphere. Finally, sorption processes also are important in uncontaminated natural waters (recall the Madison Aquifer example in Lecture 4 where ion exchange occurred along flow path 2). Thus, an understanding of sorption processes is of paramount importance to aqueous geochemistry.

4. 4 DEFINITIONS Sorption - removal of undersaturated solutes from solution onto minerals. Sorbate - the species removed from solution. Sorbent - the solid onto which solution species are sorbed. Three types of sorption: Adsorption - solutes held at the mineral surface as a hydrated species. Absorption - solute incorporated into the mineral structure at the surface. Ion exchange - when an ion becomes sorbed to a surface by changing places with a similarly charged ion previously residing on the sorbent. The three different types of sorption processes defined above cannot always be distinguished clearly in practice. However, it is useful to make these distinctions in theory. When it is not clear exactly which of these processes is occurring, the general term sorption should be used. It should also be kept in mind that not all authors define these processes in exactly the same way as Kehew (2001). Consult Figure 4-27 in Kehew (2001) to make the distinction between adsorption and absorption clearer. The three different types of sorption processes defined above cannot always be distinguished clearly in practice. However, it is useful to make these distinctions in theory. When it is not clear exactly which of these processes is occurring, the general term sorption should be used. It should also be kept in mind that not all authors define these processes in exactly the same way as Kehew (2001). Consult Figure 4-27 in Kehew (2001) to make the distinction between adsorption and absorption clearer.

5. 5 ACQUISITION OF SURFACE CHARGE - I In general, solutes interact with mineral surfaces because the latter have acquired electrical charge. Two ways to acquire charge: Substitution for a cation in a mineral by one of lesser positive charge. This type of charge is considered to be fixed. Reactions involving functional groups on the mineral surface and ions in solution (surface complexation). This type of charge is variable and dependent on solution pH. The main reason that ions are attracted to mineral surfaces is because these surfaces generally have an excess charge that is acquired in one of two ways. In the first way of acquiring charge, a less highly charged ion, e.g., Al3+, substitutes in the crystal lattice of a mineral for a more highly charged ion, e.g., Si4+. Such substitution leads to an excess negative charge on the mineral surface. Because the ionic substitution causing the charge imbalance takes place within the mineral structure, the charge imbalance is permanent, or fixed. The second way of acquiring charge is via the formation of surface complexes, i.e., the formation of a bond between reactive atoms on the mineral surface and ions in solution. In ensuing slides, we will investigate these mechanisms further. The main reason that ions are attracted to mineral surfaces is because these surfaces generally have an excess charge that is acquired in one of two ways. In the first way of acquiring charge, a less highly charged ion, e.g., Al3+, substitutes in the crystal lattice of a mineral for a more highly charged ion, e.g., Si4+. Such substitution leads to an excess negative charge on the mineral surface. Because the ionic substitution causing the charge imbalance takes place within the mineral structure, the charge imbalance is permanent, or fixed. The second way of acquiring charge is via the formation of surface complexes, i.e., the formation of a bond between reactive atoms on the mineral surface and ions in solution. In ensuing slides, we will investigate these mechanisms further.

6. 6 ACQUISITION OF SURFACE CHARGE - II Only 2:1 clay minerals (e.g., smectites, vermiculite) can acquire significant fixed charge through ionic substitutions. Substitution of divalent cations for trivalent cations in octahedral sites, and of trivalent cations for tetravalent cations in tetrahedral sites, results in a deficiency of positive charge, or a net negative fixed charge on the surface. This negative charge can be balanced by the sorption of cations from solution. We saw in Lecture 4 that 1:1 clay minerals such as kaolinite do not generally exhibit much ionic substitution. Thus, 1:1 clay minerals do not possess fixed surface charge, but as we will see, they may acquire variable surface charge through surface complexation reactions. On the other hand, many 2:1 clay minerals, such as smectite and vermiculite, do exhibit extensive ionic substitutions, and these can lead to charge imbalance. For example, if divalent ions (Mg2+, Mn2+, Fe2+) substitute for trivalent ions (Fe3+, Al3+) in the octahedral layers or if trivalent ions (Al3+) substitute for Si4+ in the tetrahedral layers, a fixed excess negative charge will exist on the surfaces of these clays. The negative charge is balanced by sorption of cations. We saw in Lecture 4 that 1:1 clay minerals such as kaolinite do not generally exhibit much ionic substitution. Thus, 1:1 clay minerals do not possess fixed surface charge, but as we will see, they may acquire variable surface charge through surface complexation reactions. On the other hand, many 2:1 clay minerals, such as smectite and vermiculite, do exhibit extensive ionic substitutions, and these can lead to charge imbalance. For example, if divalent ions (Mg2+, Mn2+, Fe2+) substitute for trivalent ions (Fe3+, Al3+) in the octahedral layers or if trivalent ions (Al3+) substitute for Si4+ in the tetrahedral layers, a fixed excess negative charge will exist on the surfaces of these clays. The negative charge is balanced by sorption of cations.

7. 7 ACQUISITION OF SURFACE CHARGE - III Silica tetrahedra near the outer surface of a 2:1 clay mineral are arranged in such a way to present a plane of oxygen atoms (siloxane surface). Siloxane cavities occur at regular intervals on the surface and serve as reactive sites for the formation of surface complexes with cations. Complexes can be formed with either hydrated or dehydrated cations. An alternative way to acquire surface charge is through surface complexation reactions. Surface complexes are analogous to aqueous complexes. The reactive sites for complex formation on silicate surfaces are cavities formed by oxygen atoms and hydroxyl groups. An alternative way to acquire surface charge is through surface complexation reactions. Surface complexes are analogous to aqueous complexes. The reactive sites for complex formation on silicate surfaces are cavities formed by oxygen atoms and hydroxyl groups.

8. 8