Chemistry of Freshwaters - Principles of Environmental Chemistry - page 120

Fe(OH) 2 þ e þ 2H 1 Fe 21 þ 2H 2 O K 3 ¼ 10 18.7 "

Fe(OH) 3(s) þ e þ 3H 1 Fe 21 þ 3H 2 O K 4 ¼ 10 17.9 "

Fe(OH) 3(s) þ e þ H 1 K 5 ¼ 10 5.5 " Fe(OH) 2(s) þ H 2 O

Fe(OH) 4 þ e þ 2H 1 K 6 ¼ 10 22.2 " Fe(OH) 2(s) þ 2H 2 O

Reduction of Fe II to Fe 0 :
Fe 21 þ 2e

K 7 ¼ 10 13.8 " Fe (s)

Fe(OH) 2(s) þ 2e þ 2H 1 K 8 ¼ 10 1.4 " Fe (s) þ 2H 2 O

Hydrolysis of Fe III :
Fe 31 þ H 2 O Fe(OH) 21 þ H 1 K 9 ¼ 10 2.2 "

Fe(OH) 21 þ H 2 O Fe(OH) 2 þ H 1 K 10 ¼ 10 3.5 "

Fe(OH) 2 þ H 2 O Fe(OH) 3(s) þ H 1 K 11 ¼ 10 0.8

"

Fe(OH) 4 þ H 1 K 12 ¼ 10 16.7

Fe(OH) 3(s) þ H 2 O "

Hydrolysis of Fe II :
Fe 21 þ 2H 2 O K 13 ¼ 10 12.4 Fe(OH) 2(s) þ 2H 1 "

Carbonate complexation of Fe II and additional reduction reactions:
FeCO 3(s) þ H 1 Fe 21 þ HCO 3 K 14 ¼ 10 5.8 "

Fe(OH) 3(s) þ HCO 3 þ 2H 1 þ e FeCO 3(s) þ 3H 2 O K 15 ¼ 10 12.1 "

Fe(OH) 4 þ HCO 3 þ 3H 1 þ e FeCO 3(s) þ 4H 2 O K 16 ¼ 10 32.4 "

In the stability field of sulfate:
SO 2 4 þ Fe 21 þ 16H 1 þ 14e FeS 2(s) þ 8H 2 O K 17 ¼ 10 86.8 "

FeCO 3(s) þ 2SO 2 4 þ 14e þ 18H 1 " FeS 2(s) þ H 2 CO 3 þ 8H 2 O

K 18 ¼ 10 85.4

At lower pe values (in the H 2 S and HS stability fields):
FeS 2(s) þ 4H 1 þ 2e

Fe 21 þ 2H 2 S (aq) K 19 ¼ 10 5.0

"

FeS 2(s) þ H 1 þ 2e FeS (s) þ HS K 20 ¼ 10 14.6

"