THE GEOCHEMISTRY OF NATURAL WATERS

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
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
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.

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.

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.

6
ACQUISITION OF SURFACE CHARGE - II

• Only 21 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.

7
ACQUISITION OF SURFACE CHARGE - III

• Silica tetrahedra near the outer surface of a 21
  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.

8
Siloxane cavity formed by surface oxygens and
hydroxyls in silicate minerals. From Sposito
(1989) The Chemistry of Soils. Oxford University
Press.
9
ACQUISITION OF SURFACE CHARGE - IV

• Inner-sphere surface complex a surface complex
  formed directly between a dehydrated cation (no
  surrounding water molecules) and the siloxane
  cavity. The bond formed is very strong.
• Outer-sphere surface complex a surface complex
  formed between a hydrated cation and the siloxane
  cavity. The bond formed here is much weaker than
  in the outer-sphere case. Outer-sphere complex
  ions exchange more readily with ions in solution.

10
ACQUISITION OF SURFACE CHARGE - V

• Inner- and outer-sphere complexes help balance
  the excess negative charge.
• An additional type of adsorption also helps
  balance the charge (diffuse double layer).
• Involves presence of diffuse layer of cations
  (counter ions) near mineral surface.
• Counter ions are not bonded to surface.
• Counter ions are more abundant than diffuse
  anions (co-ions) .
• Net positive charge balances remaining negative
  charge.

11
ACQUISITION OF SURFACE CHARGE - VI

• Minerals that do not have a fixed charge develop
  a surface charge by complexation reactions
  involving hydroxyl ions at the mineral surface.
• This is variable charge it depends on pH and
  other solution compositional parameters.
• At low pH the surface is positive
• S-OH H ? S-OH2
• At high pH the surface is negative
• S-OH OH- ? S-O- H2O

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) ?P represents the net surface
charge and ?D represents the diffuse ion charge.
(c) distribution of electrical potential ?. (d)
concentrations of positive and negative charges
with distance from the surface. (e) distribution
of charge-density with distance.
13
GOUY-CHAPMAN DOUBLE-LAYER MODEL
STERN-GRAHAME TRIPLE-LAYER MODEL
14
ACQUISITION OF SURFACE CHARGE - VII

• Other surface complexation reactions can also
  affect the surface charge.
• Adsorption of a cation
• S-OH Zn2 ? S-OZn H
• Adsorption of an anion
• Mn-OH2 CH3COO- ? Mn-OCH3CO0 H2O
• Net surface charge is the algebraic sum of fixed
  charge and all surface complexes.
• If the net surface charge is not zero, it is
  balanced by the diffuse-ion layer adjacent to the
  surface.

15
ACQUISITION OF SURFACE CHARGE - VIII

• The previous relationship is given mathematically
  by ?P ?D 0
• where ?P total net surface charge ?D diffuse
  ion charge.
• The total net surface charge is the sum of the
  following terms
• ?P ?0 ?H ?IS ?OS
• where ?0 fixed charge ?H net proton charge
  ?IS charge due to inner-sphere complexes ?OS
  charge due to outer-sphere complexes.

16
ACQUISITION OF SURFACE CHARGE - IX

• All components of ?P, except ?0, depend on pH.
• Hydroxides, kaolinite and other 11 clay minerals
  develop only variable charge.
• At low pH, the net surface charge is positive.
• At high pH, the net surface charge is negative.
• There must be some pH at which net surface charge
  is zero.
• Point of zero charge (PZC) The pH at which ?P
  0.
• Point of zero net proton charge (PZNPC) The pH
  at which ?H 0.

17
SURFACE CHARGE IS DETERMINED BY SURFACE
TITRATIONS
No metal or anion
Cation adsorption
Anion adsorption
18
Determination of the surface charge on
rhodochrosite (MnCO3) via surface titration.
Surface charge is shown as a function of pH and
pCO2. From Charlet et al. (1990) Geochim.
Cosmochim. Acta 54, 2329-2336.
19
Surface titration of rutile (TiO2) in the
presence of various concen-trations of KNO3
circles, 0.001 M squares, 0.01 M triangles, 0.1
M. Data from Yates (1975) Ph.D. Univ. Melbourne,
Australia and Wiese and Healy (1975) J. Colloid.
Interf. Sci. 51, 427-433.
20
POINT OF ZERO CHARGE CAUSED BY BINDING OR
DISSOCIATION OF PROTONS
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