Print version Updated: 26 March 2020 Lecture #34 Precipitation and - - PowerPoint PPT Presentation

Lecture #34 Precipitation and Dissolution: Basics and metal solubility

(Stumm & Morgan, Chapt.7)

Benjamin; Chapter 8.7-8.15

David Reckhow CEE 680 #34 1

Updated: 26 March 2020

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Topics

 Hydrolysis

 Aquo metal ion gives rise to hydroxo complexes

 Magnesium and Iron Hydroxide solubility

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Precipitation and Dissolution

 Environmental Significance

 Engineered systems

 coagulation, softening, removal of heavy metals

 Natural systems

 composition of natural waters  formation and composition of aquatic sediments  global cycling of elements

 Composition of natural waters

 S&M, 3rd ed., figure 15.1 (pg. 873)  Benjamin, 2nd ed., figure 1.1

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Elemental Abundance in Crust

 ad

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 dfs

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From: USGS, 2005 Top 8

IN EARTH’S CRUST

Abundance of inorganics in fresh water and in earth’s crust

David Reckhow CEE 680 #35 6 From: Stumm & Morgan, 1996; Benjamin, 2002; fig 1.1

Terrestrial water

Solubility Products

 General Equilibrium

 AmBn(s) ↔ mA+n + nB-m

 Solubility Product Equation

 Kso = [A+n]m[B-m]n

 also sometimes written: Ksp

 Example

 Calcium Carbonate

 sources: Smith & Martell; S&M, table 7.1 (pg.362-364)

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Solid Cation Anion

Kso and Q

 Reaction Quotient (Q)

 computed value from actual measurements

 may not be at thermodynamic equilibrium

 comparison with Ksp will tell you about tendency

toward dissolution or precipitation

 Q>Kso, then water will precipitate solid phase  Q<Kso, then water will dissolve solid phase

 Example: Calcium Carbonate solubility

 Ca+2 = 40 mg/L and CO3

• 2 = 100 mg/L as CaCO3

 what is Q?  if Kso is 10-8.34, what does this tell us?

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Solubility of some simple salts

 Barium sulfate

 BaSO4 = Ba+2 + SO4

• 2

 Kso = 10-9.96 = [Ba+2][SO4

• 2]

 How much will dissolve, and

what will the barium and sulfate concentrations be?

 How much will dissolve in a

1mM solution of Na2SO4 ?

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98 . 4 2 96 . 9

10 10

− −

= = x x

( )

96 . 6 7 96 . 9 6 3 3 2 3 96 . 9

10 10 097 . 1 2 10 4 10 10 10 10 10

− − − − − − − −

= = − ± − = + = + = x x x x x x x

Solubility of “simple” salts

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Stumm & Morgan, 1996, Figure 7.1, pg. 354

Solubility of oxides & hydroxides

 Does not consider the hydroxometal complexes

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Stumm & Morgan, 1996, Figure 7.3, pg. 365

Solubility of metal hydroxides

 Adds complexity

 hydroxide concentration is controlled by pH and

therefore affected by buffering

 many “hydrolyzing” metals have soluble hydroxide

species too

 Example: Magnesium Hydroxide

 Weakly hydrolyzes  Only one soluble hydroxide species  Practical: we remove Mg by precipitative softening

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Magnesium Hydroxide

 Thermodynamics

 Mg(OH)2 (s) = Mg+2 + 2OH-

Kso = 10-11.16

 Mg+2 + OH- = MgOH+

K1 = 102.6

 Mass Balance

 MgT = [Mg+2] + [MgOH+]

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Total dissolved concentration: does not include precipitated Mg

2.56 Stumm 2.12 Benjamin 2.6 Morel 2.58 Smith

• 11.79 Benjamin
• 11.1 Morel
• 10.74 Butler
• 12.9 SM&P
• 11.16 Brezonik
• 11.15 Smith

Mg(OH)2 (s) is crystalline Brucite

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 Smith &

Martell

Mg(OH)2 solid

Magnesium Hydroxide

 Tableau

 [Mg+2] = 1016.84[H+]2  same as:

 [Mg+2] = 10-11.16/[OH+]2

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Components MgOH2 (Brucite H+ Mg+2 1 2 16.84 MgOH+ 1 1 5.42 H+ 1 Reactants Log K

Magnesium Hydroxide II

 From the Kso and Kw

develop an equation for the free metal in terms of H+

 Then use the K1 to get an

equation for the soluble hydroxide species

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pH Mg Log H Mg H K K Mg OH K Mg OH Mg K

W so so so

2 84 . 16 ] [ ] [ 10 ] [ ] [ ] [ ] [ ] [ ] ][ [

2 2 84 . 16 2 2 2 2 2 2 2 2

− = = = = =

+ + + + + − + − +

{ }

pH MgOH Log H H Mg K OH Mg K MgOH OH Mg MgOH K

H H KW

− = = = = = =

+ + + + − + + − + +

+ − +

44 . 5 ] [ ] [ 10 ] [ 10 10 ] [ ] ][ [ ] [ ] ][ [ ] [

44 . 5 ] [ 10 2 84 . 16 6 . 2 ] [ 2 1 2 1 2 1

14

Magnesium Hydroxide III

 Total Magnesium

 MgT

= [Mg+2] + [MgOH+]

 Follows upper line

where lines are well separated

 Falls 0.3 log units above

intersection of any two major species

 Applications

 Mg is a hardness cation  Solubility is controlled

by hydroxide precipitate

 Easily removed by

softening at high pH

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pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Log C

• 14
• 13
• 12
• 11
• 10
• 9
• 8
• 7
• 6
• 5
• 4
• 3
• 2
• 1

H+

OH-

Mg(OH)- Mg+2 MgTotal

To next lecture

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Calcium Phosphate

 Providence, RI example

 See Edwards & Giammar manuscripts

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