Solubility Equilibria and carbonate in solution

1
Solubility Equilibria (and carbonate in solution)

• Sparks Chapter 4 pp.128 to 131 Essington
  Chapter 6 up to 6.5.2 skip 6.2.1 and 6.3.2 and
  equations 6.10 to 6.16
• Additional Lindsay Chapter 6 and 12.1 - 12.5

2
Significance

• The maximum concentrations of many inorganic
  elements in soil solutions is controlled by the
  solubility of minerals or poorly ordered
  inorganic phases.
• Rate of dissolution and precipitation may be
  important in some cases but we will not consider
  this.
• Weathering of minerals and soil formation.
• Mobility and availability of plant nutrients
• E.g. Fe(III), Mn(IV), PO4, and Ca (in high pH
  soils)
• Mobility of some pollutant elements
• E.g. Pb, As, and Cr(III).

3

• Dissolution and precipitations of carbonates
  buffer pH in high pH soils.
• Calcite is significant in many soils

4
Solubility of Inorganic Solids

• At equilibrium the solubility product, Kso.
• CaSO42H2Os Ca2 aq SO42-aq 2H2Ol
• or
• Kso Ksp (Ca2)(SO42-)
• log Kso -4.62
• Kso 10-4.62
• Note this is written like a dissociation

5
In class exercise.

• Estimate the SO42- in equilibrium with gypsum
  if Ca2 0.1M
• In fact, formation of a CaSO40 complex in
  solution increases solubility

6
Answer

• Kso (Ca2)(SO42-)
• Kso 10-4.62
• 10-4.62   (10-1)(SO42-)
• (SO42-) 10-3.62
• (SO42-) 2.4 x10-4  M

7
Use VMINTEQ
8
Solubility of oxides and hydroxides is pH
dependent

• Gibbsite
• log K
• Al(OH)3 Al3 3OH- -34.0
• 3(H OH- H2O) 3 (14.0)
• --------------------------------------------------
  -----
• Al(OH)3 3H Al3 3 H2O 8.0
• (NOTE crystallinity is a factor in solubility.
  Poorly ordered (microcrystalline) gibbsite has a
  log K for dissolution in acid of 9.3. Amorphous
  Al(OH)3 has a log K greater than 10.

9

• The solubility equation is
• By taking the log, the dependence (Al3) on pH
  seen.
• log K 3pH log (Al3)
• log (Al3) log K - 3pH
• log K 8.0

10
In class exercise.

• Calculate (Al3) in pH 4 and pH 5 solutions.
• In fact, formation of hydrolysis complexes in
  solution increases solubility

11
Answer

• log (Al3) log K - 3pH
• log K 8.0
• At pH 4.0
• log (Al3) 8.0 - 3(4)
• log (Al3) -4.0
• (Al3) 1 x 10-4
• At pH 5.0
• log (Al3) 8.0 - 3(5)
• log (Al3) -7.0
• (Al3) 1 x 10-7

12
Important oxides and hydroxides

• Al
• Gibbsite and poorly ordered Al(OH)3
• Important for acid soil toxicity for plant growth
• Fe(III)
• Hematite
• Magnetite
• Goethite
• Ferrihydrite
• Soil Fe(OH)3

13
Ion activity product and over and under saturation

• Ion activity product (IAP)
• E.g. gypsum
• CaSO42H2O Ca2 SO42- 2H20
• IAP (Ca2)(SO42-)
• IAP is the measured value whether or not
  equilibrium exists
• At equilibrium IAP KS0

14
Evaluation of disequilibrium

• Saturation Index
• CaSO42H2O

15
Supersaturation or Undersaturation

• Supersaturation
• S.I. gt O, precipitation can occur
• Equilibrium
• S.I. O
• Undersaturation
• S.I. lt O, dissolution can occur

16
Gibbsite Formation and Dissolution

• For gibbsite
• log K
• Al(OH)3 Al3 3OH- -34.0
• 3H OH- H2O -3-14
• Al(OH)3 3H Al3 3H2O 8.0

17
Al(OH)3
18
Gibbsite Formation and Dissolution
19
In Class Exercise

• If pH 4.0 and (Al3 ) 10-3 what is SI with
  respect to gibbsite?

20
Answer

• SI log10 1

21
Plot Al3 vs. pH at equilibrium

• log K 3pH log (Al3)
• If log K 8.0
• log (Al3) 8 - 3pH

22
Influence of pH Al3 in solution (see also the
hydrolysis species, McBride Fig 5.3)
23
Soil Fe(OH)3 solubility (Lindsay Fig. 10.3)
24
Mulitistep hydrolysis of metal cations e.g. Al3

• Al3 H2O AlOH2 H - 5.0
• AlOH2 H2O Al(OH)2 H - 4.9
• Al(OH)2 H2O Al(OH)3o H - 5.7
• Al(OH)3o H2O Al(OH)4- H - 7.4
• These are different than used by some text books

25
Combine solubility equation with hydrolysis
equation

• Al(OH)3(gibbsite) 3H Al3 3H2O 8.0
• Al3 H2O AlOH2 H -
  5.0
• _________________________________________________
• Al(OH)3(gibbsite) 2H Al(OH)2 2 H2O
  3.0
• log K log (Al(OH)2) 2pH
• log (Al(OH)2) log K - 2pH

26
Al(OH)3 solubility
27
Process of precipitation of new phases

• With sufficient supersaturation, nuclei of a new
  phase can form.
• If sufficiently oversaturated precipitation can
  go forward.
• The greater the over saturation, the faster the
  precipitation.

28
Carbonate Chemistry

• Carbonates are important in the chemistry of most
  high pH soils
• Bicarbonate is a major anion in soil solution

29
Equilibrium Solubility for the Carbonates of 2
Metals

• This involves solid--solution--gas phase
  equilibria.
• Equilibrium is generally considered to be quite
  rapid relative to the alumino-silicates and
  hydrous oxides but slower than for gypsum and
  other evaporite minerals.

30
Crystalline forms

• Calcite CaCO3
• Aragonite CaCO3
• A biogenic form slightly more soluble than
  calcite
• Dolomite CaMg(CO3)2
• Forms very slowly in geological sediments
• Magnesite MgCO3
• More soluble than calcite
• Siderite FeCO3
• flooded soils
• Rhodochrosite MnCO3
• flooded soils - solid solution with siderite

31
Carbonate species in water

• CO2 in water CO2 H2O H2CO3 (aq)
• Is a function of the partial pressure of CO2
  (Pco2) only. It is independent of pH.

32
Carbonate species in water (cont.)

• In ambient air, CO2 is 0.00038 atm or 0.038, or
  380 ppm by volume.
• Soil Pco2 is elevated due to respiration by roots
  and microbes.
• 0.003 to 0.2 atm
• Highest in flooded soils.
• The rate of movement of CO2 (or any gas) through
  water is about 0.00001x that in the air.
• When soils very wet gas exchange with the
  ambient air is slow

33
CO2 in corn field soil Rosemount MN in July
(D.Chen et al., 2005)
34
CO2 data for a whole season plotted with
rainfall, 2006
35
Calculation of H2CO3

• CO2 H2O H2CO3 log K -1.46
• log (H2CO3) -1.46 log (Pco2)
• If Pco2 .0003 atm then
• log (H2CO3) - 5.0
• (increases linearly with Pco2)

36
Basic Equations

• log K
• 1. H2O CO2 H2CO3 - 1.46
• 2. H2CO3 H HCO3- - 6.35
• 3. HCO3- H CO32- -10.33
• 4. H2O CO2 H HCO3- - 7.81
• (equation 1 and 2)
• 5. H2O CO2 2H CO32- -18.14 (equations
  1,2, and 3)
• 6. H2O H OH- -14.00 (Kw)
• 7. CaCO3 Ca2 CO32- - 8.48 to -8.35

37
Equilibrium Equations (cont.)

• These equations can be used to express species in
  terms of other species e.g. H2CO3 and CO32- can
  be expressed in terms of HCO3- and H.
• In soils and natural waters with pH 5.5 - 9.5,
  HCO3- is a very important anion.
• In low pH soils, organic anions and SO42- become
  relatively more important.
• CO32- is important only in very alkaline soils.

38
Activity of dissolved inorganic C species at log
PCO2 -3.5 and -2.0 (McBride Fig 8.2)
39
Effect of pH and PCO2 on HCO3- and CO32-

• From equations 1 2 we get equation
• H2O CO2 H HCO3- log K -7.81
• log (HCO3-) pH log Pco2 - 7.81

40
Effect of pH and PCO2 on HCO3- and CO32-
(cont.)

• On a log (HCO3-) vs. pH plot the slope 1
• From equations 1, 2, and 3 we get equation 5 (see
  McBride Fig. 8.2)
• H2O CO2 2H CO32- log K -18.14
• From the equilibrium constant expressions
• log (CO32-) 2pH log Pco2 - 18.14
• On a log (CO32-) vs.. pH plot the slope 2

41
Effect of pH and PCO2 on HCO3- and CO32-
(cont.)

• Example pH 7.0, Pco2 0.0050 atm
• log (HCO3-) 7.0 - 2.3 - 7.81
• log (HCO3-) -3.1
• A significant anion
• log (CO32-) 2(7.0) - 2.3 - 18.14
• Log (CO32-) -6.44

42
Review of Basic Equations

• log K
• 1. H2O CO2 H2CO3 - 1.46
• 2. H2CO3 H HCO3- - 6.35
• 3. HCO3- H CO32- -10.33
• 4. H2O CO2 H HCO3- - 7.81
• (equation 1 and 2)
• 5. H2O CO2 2H CO32- -18.14 (equations
  1,2, and 3)
• 6. H2O H OH- -14.00 (Kw)
• 7. CaCO3 Ca2 CO32- - 8.48 to -8.35

43
Dissolution of Calcite

• Combining eqn. 7 with eqn. 5.
• CaCO3 2H H2O CO2 Ca2 log K 9.66
  (11)
• Assume Kso -8.48
• Write the equilibrium constant then take the log
  of both sides.
• 9.66 log Pco2 log Ca2 2 pH (12)
• log Ca2 9.66 - log Pco2 - 2pH
• Fixed Ca2
• e.g. Fix(Ca2) at 0.010 M, Pco2 10-3.5
• pH 7.6

44
SOLUBILTIY OF COMMON CARBONATES, HYDROXIDES AND
SULFIDES (McBride Table 4.4)
45
Extraction of precipitated trace metal ions

• For metal ions that might be controlled by
  precipitated phases what extraction might you use
  to judge toxicity, mobility risk and, and or
  plant availability ?

46
Pure or Mixed Phases

• Pure solids are possible but isomorphic
  substitution is common
• Examples
• Al3(0-20) in goethite (FeOOH)
• Mg2 (0-10) in calcite (CaCO3)
• Minor elements
• More likely than not to be controlled by
  isomorphic substitution

47
Pure or Mixed Phases (Continued)

• Examples
• Cr3 in Fe(OH)3
• Cd2 in CaCO3
• Fe2 and Mn2 carbonates form in flooded soils.
  These ions have similar radii and form mixed
  carbonates.
• Sulfides
• Hg2, Cd 2, Pb2, Fe2, Zn2 can form mixed
  phase sulfides in flooded soils.
• Phosphates
• Cd2 substitutes in calcium phosphates.

48
Coprecipitation of Trace Elements

• Sometimes very difficult to distinguish from
  surface adsorption
• Example CdCO3 in CaCO3
• CdCO3 has a calcite structure, similar radius.
• In soils calcite surface sorption and/or
  incorporation in the calcite structure controls
  Cd2 activity.

49

• Cd2 (and Pb2) are found in Ca-phosphates
• Mn(IV) oxides trap many ions including Cd2

50
Phosphorus and solubility equilibrium plots
51
Phosphate Precipitation in Acid Soils (competes
with surface sorption, discussed later)

• Assume Al3 is controlled by Al(OH), and
  phosphate is controlled by variscite
  (AlPO42H2O) log K
• Al3 3H2O Al(OH)3 3H
  -(8.0)
• AlPO42H2O Al3 H2PO4- 2 OH-
  -30.50
• 2(OH- H H2O)
  -2(-14)
• AlPO42H2O H2O Al(OH)3 H H2PO4- -10.5
• However, Al3 in acid surface soils can be
  influenced by SOM. Might be undersaturated with
  respect to A1(OH)3 The above argument works best
  for subsoils.

52

• K 10-10.5 (H)(H2PO4-)
• Log K -10.5 log (H) log (H2PO4-)
• -pH log (H2PO4)
• log (H2PO4-) pH -10.5
• Similarly for FePO4-H2O (strengite)

53
Possible Phosphate Precipitation

• Soils at higher pH high in Ca2
• Ca2 phosphates can form
• CaHPO4 2H2O dical (brushite or monetite)
• Ca8H2(PO4)6 5H2O octacal
• Ca5OH(PO4)3 hydroxyapatite
• Ca5F(PO4)3 fluoroapatite
• Phosphate minerals lower on the list are less
  soluble and form very slowly especially in the
  presence of organic matter.

54
Reactions

• Ca5OH(PO4)3 7H 5Ca2 3H2PO4- H2O Log K
  14.46
• Log K (Ca2)5(H2PO4-)3/(H)7
• Log K 5log(Ca2) 3log(H2PO4-) 7pH
• If (Ca2) 0.005M, log(Ca2) -2.5
• 3log(H2PO4-) 14.46 12.5 - 7pH
• log(H2PO4-) 9.0 - 7/3pH

55
Solubility of P phases (Lindsay Fig. 12.8)
56
In class activity

• What extractants might be reasonable for
  correlation with bioavailability of soil P?

57
Short Summary

• Adsorption and precipitation can take place
  simultaneously.
• Adsorption next section.
• Precipitation equilibrium can control the
  concentration of many important metals and some
  important anions like arsenate and phosphate
• Use Kso to predict solubility equilibrium
• Precipitation can occur when the
• S.I gt 0.
• Solubility equilibrium plots can be useful.
• Precipitation is complicated by formation of
  mixed solids.

58

• P Precipitation occurs in several forms
• Precipitation and dissolution are generally
  slower than adsorption and desorption.