CEE 680

 

16 November 1999

SECOND EXAM

Closed book, two pages of notes allowed.

Answer all questions. Please state any additional assumptions you made, and show all work.

 

Problem #1: (40%) Carbonate System.

Two raw drinking waters are mixed in equal proportions as they enter the headworks of a water treatment plant. The two are characterized as follows:

Water

Alkalinity (mg/L as CaCO3)

pH

A

70

7.31

B

220

9.12

What will the pH of the blended water be?

 

 

 

Problem #2: (60%) Precipitation/Dissolution

Aluminum Solubility

  1. Prepare a solubility diagram for Aluminum Hydroxide (amorphous) in water. Present it in the usual form (log C vs pH). Outline the zone of precipitation.
  2. Prepare a solubility diagram as above in Part 1, but this time assume that 0.1 mM total phosphate is present. Outline the zones of precipitation and mark the identity of the precipitates.

 

Equilibria

Log K

AlOH3(s) (amorphous) + 3H+ = Al+3 + 3 H2O

10.8

Al+3 + H2O = AlOH+2 + H+

-4.97

Al+3 + 2H2O = Al(OH)2+ + 2H+

-9.3

Al+3 + 3H2O = Al(OH)3(aq) + 3H+

-15.0

Al+3 + 4H2O = Al(OH)4- + 4H+

-23.0

AlPO42H2O(s) (variscite) = Al+3 + PO4-3 + 2H2O

-21

H3PO4 = H+ + H2PO4-

-2.15

H2PO4- = H+ + HPO4-2

-7.2

HPO4-2 = H+ + PO4-3

-12.3