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

CEE 680 Lecture #38 4/3/2020 Print version Updated: 3 April 2020 Lecture #38 Precipitation and Dissolution: Metal Carbonates & Hydroxides (Stumm & Morgan, Chapt.7) Benjamin; Chapter 8.7 ‐ 8.15 David Reckhow CEE 680 #38 1 Metal Carbonates  Example of ligands that exist in different forms  Consider CaCO 3 in a closed system  Six species: Ca +2 , H + , OH ‐ ‐ 2 , HCO 3 ‐ , H 2 CO 3 * CO 3  Need six equations  K 1 , K 2 , K w CO 3 -2  K so Ca +2  ENE CaCO 3  MBE David Reckhow CEE 680 #38 2 1

CEE 680 Lecture #38 4/3/2020 Calcium Carbonate K  2  [ Ca ] so  K so  2 [ CO ] 3 K  so  C 2 T  MBE        [ Ca 2 ] C [ H CO * ] [ HCO ] [ CO 2 ] T 2 3 3 3  combining K  2  1 [ Ca ] so   2 [ Ca ]   2 2   [ H ] [ H ] 1 K K K 1 2 2 K   [ Ca 2 ] so  2 David Reckhow CEE 680 #38 3 0 -1 Ca+2 -2 - HCO 3 -3 +/-0.5 slope -4 +/-1 slope -5 -6 Log C -2 -7 CO 3 -8 * H 2 CO 3 -9 -10 -11  Dissolution of CaCO 3 in -12 pure water -13 -14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH David Reckhow CEE 680 #38 4 2

CEE 680 Lecture #38 4/3/2020 0 OH - -1 Problem - HCO 3 Ca+2 -2 -3 H+ -4  What is the pH of -5 CaCO3 in pure solution? -6 Log C -7 -2 CO 3 * H 2 CO 3 -8 -9 -10 -11 -12 -13 -14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH David Reckhow CEE 680 #38 5 0 OH - -1 Solution - HCO 3 Ca+2 -2 -3  Use ENE H+ -4 -5  2       2   2 [ Ca ] [ H ] [ HCO ] 2 [ CO ] [ OH ] 3 3 -6 Log C -7 -2 CO 3 * H 2 CO 3 -8 -9 -10 -11 -12 -13 -14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH David Reckhow CEE 680 #38 6 3

CEE 680 Lecture #38 4/3/2020 Analytical Solution  Start with ENE and substitute  2       2   2 [ Ca ] [ H ] [ HCO ] 2 [ CO ] [ OH ] 3 3 K K        2 so [ H ] C 2 C w  1 T 2 T  [ H ] 2 K K K K        2 so [ H ] so 2 so w  1  2   [ H ] 2 2 2 K K           [ H ] so 2 2 w 0  1 2  [ H ] 2 S&M, equation #30  For CaCO 3 : pH=9.91 Pg. 376 David Reckhow CEE 680 #38 7 With other acy/alk  CaCO 3 system with addition of:  Strong acid (C A )  Strong base (C B )   2       2    C 2 [ Ca ] [ H ] [ HCO ] 2 [ CO ] [ OH ] C B 3 3 A K K              so w [ H ] 2 2 C C  1 2  A B [ H ] 2 S&M, equation #31 Pg. 376 David Reckhow CEE 680 #38 8 4

CEE 680 Lecture #38 4/3/2020 K   [ Ca 2 ] so  [ CO 2 ] 3 CaCO 3 in open system K  so  C 2 T    K     [ Ca 2 ] so 0  Applying the open system C T :    K p   2 H CO 2  And substituting into the ENE:      2    2  2 [ Ca ] [ H ] [ HCO ] 2 [ CO ] [ OH ] 3 3  K p K p K K     H CO   H CO  2 so 0 [ H ] 2 w 2 2 1 2     K p [ H ] 2 H CO 0 0 2 K p K p  K K H CO H CO         2 so 0 [ H ] 2 w 0 2 2  1  2   K p [ H ] 2 H CO 0 0 2  The pH is calculated to be:  pH = 8.27 Quite similar to surface water processes David Reckhow CEE 680 #38 9 Open System  Assuming equilibrium with a constant partial pressure of CO 2 (10 ‐ 3.5 atm) Stumm & Morgan, 1996, Figure 7.10, pg. 379 David Reckhow CEE 680 #38 10 5

CEE 680 Lecture #38 4/3/2020 Open system with other acy/alk  CaCO 3 system with addition of:  Strong acid (C A )  Strong base (C B )   2       2    C 2 [ Ca ] [ H ] [ HCO ] 2 [ CO ] [ OH ] C B 3 3 A  K p K p K K       H CO   H CO    2 so 0 [ H ] 2 w C C 2 2  1  2   A B K p [ H ] 2 H CO 0 0 2 David Reckhow CEE 680 #38 11 Carbonate dissolution  Pathways  Not covered in class Stumm & Morgan, 1996, Figure 7.12, pg. 385 David Reckhow CEE 680 #38 12 6

CEE 680 Lecture #38 4/3/2020  To next lecture David Reckhow CEE 680 #38 13 7