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Sorption

Sorption. Precipitation of surface coating that is compositionally different from that of the underlying host grain (referred to as epitaxial overgrowth in petrology).

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Sorption

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  1. Sorption • Precipitation of surface coating that is compositionally different from that of the underlying host grain (referred to as epitaxial overgrowth in petrology). • Incorporation of ions into crystal structure of the mineral by processes of diffusion or dissolution and reprecipitation (collectively called absorption). • The removal of dissolved species from solution and their accumulation on the particle surface without the formation of a distinct 3-D molecular structure (Adsorption).

  2. Precipitation - Dissolution • Solid Phases in river consist of minerals, noncrystalline (amorphous) solids, and organic matter. • Aqueous phase (water and dissolved constituents) • Gases • Mineral Solubility: amount of material that dissolves in a solution. • All minerals are soluble to some degree in pure water.

  3. a = 10-4.99 Equilibrium ConstantsBarite, pure water at 25 C • BaSO4 ↔ Ba2+ + SO42- • K = (a Ba2+ )(a SO42-) = 10-9.98 (aBa SO4) • K = (a Ba2+ )(a SO42-) = 10-9.98

  4. Ion Activity Product • IAP = (a Ba2+ )(a SO42-) = 10-9.98 • Chemical Activities are based on solution concentration data obtained from water samples

  5. Saturation Index SI = log10 IAP Kmineral • When, • SI = 0, the solution is saturated and in equilibrium with the mineral of interest; • SI < 0, the solution is undersaturated with respect to the mineral; • SI > 0, the solution is oversaturated with respect to the mineral, and the mineral is non-reactive, perhaps because reaction rates are too slow to limit dissolved ion concentrations within the water.

  6. ai = γici I = 0.5 ∑ ci (zi)2 ai = chemical activity ci = total concentration γi = activity coefficient Zi = valence I = Ionic Strength Chemical Activity Once ionic strength is known, use one of several equations to determine activity coefficient

  7. Aqueous Complexes • CaCO3 ↔ Ca2+ + CO32- • HCO3- • H2CO3 • Mg HCO3- Other possible complexes Total CO32- concentration – concentration of complexes to get concentration of uncomplexed carbonate. Use in activity equation to get activity of CO32-

  8. Oxidation-Reduction Reactions • Involve the transfer of electrons from one element to another. • Result in change in the charge (valence) of the primary constituents. • Some elements are the relatively more soluble in one valence state than another (e.g., Fe+2 vs Fe+3) • Eh (redox potential) dictates relative proportions of the two forms in the system and, thus, the mobility within the aquatic environment. • Eh more positive, system is more oxidizing. Note that what is oxidizing for one element may not be for another.

  9. From Forstner and Wittmann, 1979

  10. Adsorption-Desorption • Adsorption: removal of dissolved species from solution and their accumulation on a particle surface without the formation of a distinct, 3-D molecular structure associated with a mineral • Desorption: release of the adsorbed materials back into solution.

  11. Charge on Mineral Surface • σ min. surface = σpsc + σreaction Charge due to Chemical Reactions Charge on Mineral Surface Permanent Structural Charge

  12. Internal Structure of Halite (NaCl)

  13. Geometric Packing

  14. Silicate Tetrahedron

  15. Silicate Structures

  16. Clay Mineral Structure Fig. from Birkland, 1999; originally from Grim, 1968

  17. Clay Mineral Structure Fig. from Birkland, 1999; originally from Jackson, 1964

  18. Low-coordinated metal ions Water molecules attracted to metal cations Water dissociates and forms a hydroyxlated surface which is very reactive Figure from Anderson and Rubin, 1981

  19. Figure from Deutsch, 1997 M-OH + H+ ↔ MOH2+ M-OH ↔ M++ OH- M-OH + OH- ↔ M-O- + H2O

  20. From Deutsch, 1997

  21. From Langmuir, 1997; based on Healy, 1974

  22. From Hochella and White, 1990

  23. Electric Double Layer From Krauskopf and Bird, 1995

  24. pH edge From Deutsch, 1997

  25. From Langmuir, 1997

  26. Caq + X  Cab Kd = (Cab) (X)(Caq) Kd = (Cab) (Caq) X = weight of absorbent Caq = concentration of metal in solution Cab = concentration of metal on X Distribution Coefficient

  27. Isotherms From Deutsch, 1997

  28. From Kay et al, 2001; ES&T, v. 35, p. 4719-4725

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