Mineral Weathering and Mineral Surface Processes III - Sorption and Ion Exchange | GEOL 464, Study notes of Hydrogeology

Download Mineral Weathering and Mineral Surface Processes III - Sorption and Ion Exchange | GEOL 464 and more Study notes Hydrogeology in PDF only on Docsity! 1 1 THE GEOCHEMISTRY OF NATURAL WATERS MINERAL WEATHERING AND MINERAL SURFACE PROCESSES - III SORPTION AND ION EXCHANGE CHAPTER 4 - Kehew (2001) Pages 107-128 2 2 LEARNING OBJECTIVES z Learn about sorption; distinguish among adsorption, absorption and ion exchange. z Understand why minerals acquire surface charge and what the implications are. z Learn about sorption isotherms. z Learn to deal quantitatively with ion exchange. z Investigate the role of ion exchange in natural and contaminated waters. 5 5 ACQUISITION OF SURFACE CHARGE - I z In general, solutes interact with mineral surfaces because the latter have acquired electrical charge. z Two ways to acquire charge: y Substitution for a cation in a mineral by one of lesser positive charge. This type of charge is considered to be fixed. y 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. 6 6 ACQUISITION OF SURFACE CHARGE - II z Only 2:1 clay minerals (e.g., smectites, vermiculite) can acquire significant fixed charge through ionic substitutions. z 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. z 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. 7 7 ACQUISITION OF SURFACE CHARGE - III z 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). z Siloxane cavities occur at regular intervals on the surface and serve as reactive sites for the formation of surface complexes with cations. z 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. 10 10 ACQUISITION OF SURFACE CHARGE - V z Inner- and outer-sphere complexes help balance the excess negative charge. z An additional type of adsorption also helps balance the charge (diffuse double layer). y Involves presence of diffuse layer of cations (counter ions) near mineral surface. y Counter ions are not bonded to surface. y Counter ions are more abundant than diffuse anions (co-ions) . y Net positive charge balances remaining negative charge. To summarize then, the components of the variable charge on mineral surfaces include cations in inner- and outer-sphere surface complexes, and a layer of cations concentrated near the surface but in solution. Thus, mineral surfaces in contact with aqueous solutions can be modeled as two layers (double layer) - the layer consisting of surface complexes, and the diffuse ion layer. 11 11 ACQUISITION OF SURFACE CHARGE - VI z Minerals that do not have a fixed charge develop a surface charge by complexation reactions involving hydroxyl ions at the mineral surface. z This is variable charge; it depends on pH and other solution compositional parameters. z At low pH the surface is positive: S-OH + H+ ↔ S-OH2+ z At high pH the surface is negative: S-OH + OH- ↔ S-O- + H2O 12 12 The diffuse double layer: (a) note that counter ions (cations) in the diffuse layer tend to be concentrated close to the negatively charged ions on the surface. (b) σP represents the net surface charge and σD represents the diffuse ion charge. (c) distribution of electrical potential ψ. (d) concentrations of positive and negative charges with distance from the surface. (e) distribution of charge-density with distance. Both cations and anions exist in the diffuse layer next to the mineral surface. However, cations predominate; they are attracted by the residual negative charge on the surface. The above diagram depicts the diffuse double layer model and the variation of electrical properties near the surface. 15 15 ACQUISITION OF SURFACE CHARGE - VIII z The previous relationship is given mathematically by: σP + σD = 0 where σP = total net surface charge; σD = diffuse ion charge. z The total net surface charge is the sum of the following terms: σP = σ0 + σH + σIS + σOS where σ0 = fixed charge; σH = net proton charge; σIS = charge due to inner-sphere complexes; σOS = charge due to outer-sphere complexes. The first equation in this slide is just a charge-balance relationship. It simply states that the net surface charge is balanced by the diffuse ion charge. The second equation states that the net surface charge is the sum of contributions from fixed charge, proton charge, inner-sphere complexes and outer-sphere complexes. 16 16 ACQUISITION OF SURFACE CHARGE - IX z All components of σP, except σ0, depend on pH. z Hydroxides, kaolinite and other 1:1 clay minerals develop only variable charge. y At low pH, the net surface charge is positive. y At high pH, the net surface charge is negative. y There must be some pH at which net surface charge is zero. z Point of zero charge (PZC) : The pH at which σP = 0. z Point of zero net proton charge (PZNPC) : The pH at which σH = 0. All the components of net charge, except the fixed charge, are functions of pH. For minerals that possess only variable charge, such as kaolinite, we would expect that, at low pH, where the activity of H+ in solution is high, the net surface charge will be positive, owing primarily to adsorption of protons. As pH increases and the activity of H+ in solution decreases, the net positive surface charge should decrease to zero, and then become negative. At high pH, the tendency is for protons to be removed into solution, leaving a negatively charged surface. The pH at which the net surface charge (σP) is zero is called the point of zero charge (PZC or pHpzc). The pH at which the net proton charge (σH) is zero is called the point of zero net proton charge (PZNPC or pHpznpc). The PZC and PZNPC are fundamental properties of minerals. They are important quantities because sorption of cations tends to be stronger when the surface charge is negative (i.e., pH > PZC), and sorption of anions tends to be stronger when the surface charge is positive (i.e., pH < PZC). The surface charge (and hence PZC and PZNPC) can be determined by conducting an acid-base titration on a slurry of the mineral of interest in water. The surface charge is calculated based on a charge-balance relationship: surface charge = Macid - MH+ - Mbase - MOH- where Macid and Mbase are the stoichiometric amounts of acid and base, respectively, added during the titration, and MH+ and MOH- are the actual measured H+ and OH- concentrations. The difference in the total concentration of protons added by the acid and those actually in solution is assumed to represent the protons on the mineral surface. Similar remarks apply for hydroxide. 17 17 No metal or anion Cation adsorption Anion adsorption SURFACE CHARGE IS DETERMINED BY SURFACE TITRATIONS These diagrams show surface titration curves (top row) and calculated surface charges (bottom row) for three cases. In the two diagrams on the left, a titration of the surface is conducted where neither foreign cations nor foreign anions are present. The net surface charge (denoted in the figure as ΓH+ - ΓOH-) is due only to protons or hydroxide ions and changes from positive to negative as pH increases. In the two diagrams in the middle, a titration of a surface is conducted in the presence of a cation (Mez+) that forms a surface complex. The dashed curves represent the titration curve and surface charge in the absence of the cation. The presence of the cation displaces the titration curve to lower pH; because of the positively charged cation on the surface, H+ ions are repulsed, and it takes less acid to attain a given pH. Because there are less protons on the surface, the net proton charge is less at any given pH than in the case on the left. The surface charge due to the metal is always positive and increases with increasing pH (more cations are sorbed to a negatively charged surface). The total charge is the sum of the proton and metal charges. The PZNPC occurs at a lower pH than the PZC. In the diagrams on the right, a titration of a surface is conducted in the presence of a cation (Az-) that also forms a surface complex. The dashed curves represent the titration curve and surface charge in the absence of the anion. The titration curve in the presence of a sorbed anion is displaced to higher pH; the anions on the surface tend to attract protons, so more acid is required to generate a given solution pH. In this case, the PZNPC occurs at a higher pH than the PZC. 20 20 POINT OF ZERO CHARGE CAUSED BY BINDING OR DISSOCIATION OF PROTONS Material pHpznpc Material pHpznpc Material pHpznpc α-Al2O3 9.1 α-Fe2O3 8.5 ZrSiO4 5 α-Al(OH)3 5.0 Fe(OH)3 8.5 Feldspars 2-2.4 γ-AlOOH 8.2 MgO 12.4 Kaolinite 4.6 CuO 9.5 δ-MnO2 2.8 Montmorillonite 2.5 Fe3O4 6.5 β-MnO2 7.2 Albite 2 α-FeOOH 7.8 SiO2 2 Chrysotile >10 As can be seen here, the pHpznpc of oxides and silicates varies over a wide range (at least from 2 to > 10). For minerals with very low pHpznpc values (e.g., quartz, feldspars), anion sorption is not likely to be very strong in most natural waters. This is because the pHpznpc value of about 2 is less than the common pH range of natural waters (5.5-8.5), so the surfaces of quartz and feldspars will be negatively charged in most natural waters. On the other hand, minerals with high pHpznpc values (e.g., corundum, Fe-oxides and chrysotile) will generally be more efficient sorbents for anions than for cations, because their surfaces will be positively charged over the pH range of most natural waters. 21 21 Figure 4-31 from Kehew (2001). FIXED CHARGE IS ALWAYS GREATER THAN VARIABLE CHARGE: Surface charge of three clays as a function of pH. From Langmuir and Mahoney (1984). Variable charge only Fixed and variable charge Fixed and variable charge The above figure demonstrates that fixed charge on clay minerals is always a larger contributor to total surface charge than variable charge. Kaolinite, which has no fixed charge, has a near zero surface charge at low pH, but the surface becomes slightly negative with increasing pH as protons dissociate from surface hydroxyls. Illite has a moderately large fixed charge, and bentonite (montmorillonite) has a rather large fixed charge. Thus, clay minerals will primarily be sorbents for cations. 22 22 SORPTION ISOTHERMS - I z The capacity for a soil or mineral to adsorb a solute from solution can be determined by an experiment called a batch test. z In a batch test, a known mass of solid (S m) is mixed and allowed to equilibrate with a known volume of solution (V ) containing a known initial concentration of a solute (C i). The solid and solution are then separated and the concentration (C ) of the solute remaining is measured. The difference C i - C is the concentration of solute adsorbed. 25 25 LANGMUIR ISOTHERM The Langmuir isotherm describes the situation where the number of sorption sites is limited, so a maximum sorptive capacity (S max) is reached. C (mg L-1) 0 10 20 30 40 S ( m g g- 1 ) 0 10 20 30 40 LANGMUIR ISOTHERMS C C S 1.01 1.030 + × = C C S 5.11 5.130 + × = The governing equation for Langmuir isotherms is: KC KCS S + = 1 max The Langmuir isotherm describes cases in which there are a limited number of sites available for sorption, so the sorption sites become saturated. Note that the Langmuir isotherm has a form very similar to the Michaelis-Menton equation used to describe the kinetics of enzyme-mediated reactions (hyperbolic kinetics). Recall that the Michaelis-Menton equation results from the possibility that saturation of the enzyme may limit the rate of reaction. 26 26 ADSORPTION OF METAL CATIONS - I z In a natural solution, many metal cations compete for the available sorption sites. z Experiments show some metals have greater adsorption affinities than others. What factors determine this selectivity? z Ionic potential: defined as the charge over the radius (Z/r). z Cations with low Z/r release their waters of hydration more easily and can form inner- sphere surface complexes. Cations with low charge to radius ratios (ionic potentials) are not strongly hydrated. These cations can easily shed their waters of hydration to participate in inner-sphere surface complexes. Cations with high ionic potentials are strongly hydrated; they do not surrender their waters of hydration easily, and so are more likely to form outer-sphere surface complexes. Because inner- sphere complexes are stronger than outer-sphere complexes, we would expect that cations with low ionic potentials would sorb more strongly to surfaces than cations with high ionic potentials. 27 27 ADSORPTION OF METAL CATIONS - II z Many isovalent series cations exhibit decreasing sorption affinity with decreasing ionic radius: Cs+ > Rb+ > K+ > Na+ > Li+ Ba2+ > Sr2+ > Ca2+ > Mg2+ Hg2+ > Cd2+ > Zn2+ z For transition metals, electron configuration becomes more important than ionic radius: Cu2+ > Ni2+ > Co2+ > Fe2+ > Mn2+ Experimental results confirm our suspicions: for series of cations with the same oxidation state (isovalent series), larger cations have greater sorption affinities than smaller cations. Because the charge is constant, larger cations have lower ionic potentials than smaller cations. Thus, as expected, lower ionic potentials correlate with higher sorption affinities. For transition metals, there are additional complications. Transition metals, by definition, differ in the number of d-electrons in their valence shells. These different electronic configurations give rise to something called ligand field effects. Ligand field effects are more important than ionic size in determining sorption affinities, resulting in the order given above. 30 30 CATION EXCHANGE CAPACITIES OF MINERALS AND SOILS Mineral CEC (meq/100 g) Mineral CEC (meq/100 g) Chlorite 10-40 Soil organic matter >200 Illite 10-40 Sand 2-7 Kaolinite 3-15 Sandy loam 2-18 Montmorillonite 80-150 Loam 8-22 Vermiculite 100-150 Silt loam 9-27 Oxides and hydroxides 2-6 Clay loam 4-32 Clay 5-60 Note that cation exchange capacities are greatest for 2:1 clay minerals (montmorillonite and vermiculite) and soil organic matter. Oxides, hydroxides and silica sand have the lowest CEC values. 31 31 ION EXCHANGE REACTIONS - II z Exchange reactions involving common, major cations are treated as equilibrium processes. z The general form of a cation exchange reaction is: nAm+ + mBX ↔ mBn+ + nAX z The equilibrium constant for this reaction is given by: m B n A n A m B N N a aK = In the ion-exchange reaction given above, the X represents the solid surface on which ion exchange occurs. 32 32 ION EXCHANGE REACTIONS - III z In the preceding equation, the a i are the usual activities of dissolved species, but N i is the equivalent fraction, defined as: z If An and B are the only sorbed cations: z The equilibrium constant for exchange reactions is known as the selectivity coefficient. CEC sediment of g 100per of meq ANA = BA ANA meq meq meq + = 35 35 A NATURAL WATER-SOFTENING REACTION z A common ion-exchange reaction in natural waters is exchange of adsorbed Na for Ca and Mg in solution. z Required conditions: presence of carbonate minerals, high PCO2’s and clays with abundant exchangeable Na. z The reaction is CaCO3 + H2CO30 + 2NaX ↔ CaX + 2Na+ + HCO3- z Produces waters with high Na and bicarbonate concentrations. 36 36 ION EXCHANGE IN THE KOOTENAI FORMATION - I z Ion exchange occurs in the Lower Cretaceous Kootenai Formation in the Judith Basin of Montana. z Recharge occurs where the sandstone aquifer units of the formation crop out; flow trends north-northwest. z The groundwater evolves by Ca/Na ion exchange. z Values of change from positive to negative along the flow path.         + + ++ 2 22 log Na MgCa M MM 37 37 Figure 4-35 from Kehew (2001). Potentiometric contours and inferred flow paths for the Kootenai Formation, eastern Judith Basin, Montana. 40 40 A FIELD EXPERIMENT - I z Compared to natural ion-exchange reactions, those induced by the introduction of contaminant solutions can produce drastic changes in ion concentrations over very short flow paths. z Dance and Reardon (1983) injected a simulated contaminant solution into a sand aquifer and monitored the downgradient ground water both spatially and temporally. z Calcium was the dominant cation in the natural ground water. 41 41 A FIELD EXPERIMENT - II z The injected solution had much higher cation concentrations than natural ground water, and Na, K and Mg had roughly the same concentrations as Ca. z The aquifer comprised carbonate-rich sand with a low CEC of ~0.5 meq/100 g. The dominant exchangeable cation on aquifer solids was Ca. z Breakthrough curves were obtained at a distance 0.75 m from the injection point. z Chloride is assumed to be a conservative ion. 42 42 Figure 4-42 from Kehew (2001). Breakthrough curves for various ions at a distance of 0.75 m from injections wells in tracer experiments by Dance and Reardon (1983). 45 45 CONTINUOUS CONTAMINANT SOURCE z If the contaminant source is continuous, cation exchange processes produce a chromatographic pattern. z The cation of lowest adsorption affinity travels at velocities approaching average ground water flow velocity. z The other cations are distributed according to their selectivity coefficients. z Each separate cation plume has a sharp front that moves more slowly than average flow velocity. A chromatographic pattern is one in which ions with different retentivities are separated from one another. In a ground water flow system, different retentivities result from different selectivity coefficients for the exchange of the various ions on the solids in the aquifer. Ions with high selectivity coefficients tend to spend more time on the mineral surfaces than in solution, and their migration is retarded. In essence, a given cation cannot move forward until all cation exchange sites on aquifer solids are saturated with respect to that cation. In fact, an analytical instrument commonly employed to determine anion concentrations in natural waters, the ion chromatograph, is based on a similar principle. 46 46 Figure 4-43 from Kehew (2001) Contours of Ca and NH4+ in ground water around waste stabilization cells at McVille, North Dakota. Black dots are locations of monitor wells. Concentrations shown in Cell I are from waste water. Higher concentrations of ammonium in ground water are caused by mixing and reduction from nitrate derived from the landfill at upper left. Note the high Ca downgradient, and the significant retardation of the ammonium. At this site, NH4+ is being exchanged for Ca2+ on solids (mostly clays) existing in the aquifer. A contaminant plume extends in a south-south easterly direction. High Ca2+ concentrations are at the head of the plume, because contaminant cations such as NH4+ are displacing Ca from the solids. On the other hand NH4+ is retarded because it is being retained by the solids.