Transcript Sorption - Western Carolina University

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).
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.
Equilibrium Constants
Barite, 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
a = 10-4.99
Ion Activity Product
• IAP = (a Ba2+ )(a SO42-) = 10-9.98
– Chemical Activities are based on
solution concentration data obtained
from water samples
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.
Chemical Activity
• ai = γici
• I = 0.5 ∑ ci (zi)2
• ai = chemical
activity
• ci = total
concentration
• γi = activity
coefficient
• Zi = valence
• I = Ionic Strength
Once ionic strength is known, use one of several
equations to determine activity coefficient
Aqueous Complexes
• CaCO3 ↔ Ca2+ + CO32• HCO3• H2CO3
Other possible
complexes
• Mg HCO3Total CO32- concentration – concentration of
complexes to get concentration of uncomplexed
carbonate. Use in activity equation to get activity of CO32-
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.
From Forstner and Wittmann, 1979
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.
Charge on Mineral Surface
• σ min. surface = σpsc + σreaction
Charge on
Mineral Surface
Permanent
Structural Charge
Charge due to
Chemical Reactions
Internal Structure of Halite
(NaCl)
Geometric Packing
Silicate Tetrahedron
Silicate Structures
Clay Mineral Structure
Fig. from Birkland, 1999; originally from Grim, 1968
Clay Mineral Structure
Fig. from Birkland, 1999; originally from Jackson, 1964
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
Figure from Deutsch, 1997
M-OH + H+ ↔ MOH2+
M-OH + OH- ↔ M-O- + H2O
M-OH ↔ M++ OH-
From Deutsch, 1997
From Langmuir, 1997; based on Healy, 1974
From Hochella
and White, 1990
Electric Double Layer
From Krauskopf and
Bird, 1995
pH edge
From Deutsch, 1997
From Langmuir, 1997
Distribution Coefficient
• 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
Isotherms
From Deutsch, 1997
From Kay et al, 2001; ES&T, v. 35,
p. 4719-4725