Metal Carbonates Example of ligands that exist in different forms - - PDF document

CEE 680 Lecture #38 4/3/2020 1

Lecture #38 Precipitation and Dissolution: Metal Carbonates & Hydroxides

(Stumm & Morgan, Chapt.7)

Benjamin; Chapter 8.7‐8.15

David Reckhow CEE 680 #38 1

Updated: 3 April 2020

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Metal Carbonates

 Example of ligands that exist in different forms  Consider CaCO3 in a closed system

 Six species: Ca+2, H+, OH‐

CO3

‐2, HCO3 ‐, H2CO3 *

 Need six equations

 K1, K2, Kw  Kso  ENE  MBE

David Reckhow CEE 680 #38 2

CaCO3 Ca+2 CO3

• 2

CEE 680 Lecture #38 4/3/2020 2

Calcium Carbonate

Kso MBE combining

David Reckhow CEE 680 #38 3 T so so

C K CO K Ca

2 2 3 2

] [ ] [   

 

] [ ] [ ] [ ] [

2 3 3 * 3 2 2   

    CO HCO CO H C Ca

T

] [ ] [

2 2 2  

 Ca K Ca

so



2 2]

[ 

so

K Ca 



1 1

2 2 1 2

] [ ] [

 

 

K H K K H

 Dissolution of CaCO3 in

pure water

David Reckhow CEE 680 #38 4

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

Ca+2 H2CO3

*

CO3

• 2

HCO3

• +/-0.5 slope

+/-1 slope

CEE 680 Lecture #38 4/3/2020 3

David Reckhow CEE 680 #38 5

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

OH-

Ca+2 H2CO3

*

CO3

• 2

HCO3

• H+

 What is the pH of

CaCO3 in pure solution?

Problem

 Use ENE

David Reckhow CEE 680 #38 6

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

OH-

Ca+2 H2CO3

*

CO3

• 2

HCO3

• H+

] [ ] [ 2 ] [ ] [ ] [ 2

2 3 3 2     

    OH CO HCO H Ca

Solution

CEE 680 Lecture #38 4/3/2020 4

Analytical Solution

Start with ENE and substitute

For CaCO3: pH=9.91

David Reckhow CEE 680 #38 7

] [ ] [ 2 ] [ ] [ ] [ 2

2 3 3 2     

    OH CO HCO H Ca ] [ 2 ] [ 2

2 1 2  

    H K C C H K

w T T so

  

 

] [ 2 2 ] [

2 1 2

    

 

H K K H

w so

   ] [ 2 ] [ 2

2 2 2 1 2  

    H K K K H K

w so so so

     S&M, equation #30

• Pg. 376

With other acy/alk

 CaCO3 system with addition of:

 Strong acid (CA)  Strong base (CB)

David Reckhow CEE 680 #38 8 A B

C OH CO HCO H Ca C      

    

] [ ] [ 2 ] [ ] [ ] [ 2

2 3 3 2

   

B A w so

C C H K K H      

 

] [ 2 2 ] [

2 1 2

   S&M, equation #31

• Pg. 376

CEE 680 Lecture #38 4/3/2020 5

CaCO3 in open system

 Applying the open system CT:  And substituting into the ENE:  The pH is calculated to be:

 pH = 8.27

David Reckhow CEE 680 #38 9 T so so

C K CO K Ca

2 2 3 2

] [ ] [   

 

        



2

2 2]

[

CO H so

p K K Ca   ] [ ] [ 2 ] [ ] [ ] [ 2

2 3 3 2     

    OH CO HCO H Ca ] [ 2 ] [ 2

2 1 2

2 2 2

 

    H K p K p K H p K K

w CO H CO H CO H so

      ] [ 2 ] [ 2

2 1 2

2 2 2

    

 

H K p K p K H p K K

w CO H CO H CO H so

     

Quite similar to surface water processes

Open System

 Assuming

equilibrium with a constant partial pressure of CO2 (10‐3.5 atm)

David Reckhow CEE 680 #38 10

Stumm & Morgan, 1996, Figure 7.10, pg. 379

CEE 680 Lecture #38 4/3/2020 6

Open system with other acy/alk

 CaCO3 system with addition of:

 Strong acid (CA)  Strong base (CB)

David Reckhow CEE 680 #38 11 A B

C OH CO HCO H Ca C      

    

] [ ] [ 2 ] [ ] [ ] [ 2

2 3 3 2

 

B A w CO H CO H CO H so

C C H K p K p K H p K K      

 

] [ 2 ] [ 2

2 1 2

2 2 2

     

Carbonate dissolution

 Pathways

 Not covered in class

David Reckhow CEE 680 #38 12

Stumm & Morgan, 1996, Figure 7.12, pg. 385

CEE 680 Lecture #38 4/3/2020 7

To next lecture

David Reckhow CEE 680 #38 13