CEE 680

25 November 1996

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.

A groundwater that is at pH 8.4 and has an alkalinity of 150 mg/L as CaCO3 is pumped to the surface. It is immediately mixed in equal proportions with a pH 7.0 surface water which has an alkalinity of 300 mg/L as CaCO3. What will the pH of this water be at the moment of mixing?


Problem #2: (60%) Complexation

Folic acid (also known at Vitamin B10) is an important vitamin found in Liver, green leafy vegetables, brewer's yeast, oranges, and whole grains. It is involved in the production of red blood cells, cell growth and reproduction, intestinal activity, nucleic acid formation, and protein metabolism.

This compound has 7 nitrogen atoms, two carboxyl groups and one phenolic-OH. As a result, it has as many as 10 sites where t could bind to metal atoms. Little complexation data exists for this compound, but at least one study was conducted with aluminum:

Complexation of Folic Acid by Aluminum (Al+3) Footnote1

Constants

Step-wise Constants

(Log K's)

Overall Constants

(Log Beta's)

1st

5.80

5.80

2nd

4.70

10.50

3rd

4.65

15.15

Part A.

Imagine you have a wastewater containing Folic Acid that is well buffered at pH 5.0. Furthermore, you wish to treat this wastewater by alum coagulation. In order for this process to be effective, you have to add enough alum to get the formation of an aluminum hydroxide precipitate. Aquatic chemistry calculations indicate that at the pH of this wastewater (5.0), precipitation occurs once the free Al+3 concentration reaches 10-6.5 M. To simplify this problem consider only the first complex from the above table (i.e., Al(FA)1).

A.1. Determine what fraction of the Folic Acid becomes bound to aluminum under these conditions.


A.2. Determine how much excess aluminum (in moles/liter) must be added to reach the onset of precipitation over that which would be required without the Folic Acid. In other words determine the amount of aluminum bound to the Folic Acid. Assume 0.01 M as a typical total folic acid concentration in this wastewater.


Part B.

The principals of your firm, Bob Stumm and Clarence "Bud" Morgan have taken issue with your analysis. They believe that the second complex must also be considered. Again assuming a 0.01 M total Folic Acid concentration, re-do your analysis as follows:

B.1. Determine what fraction of the Folic Acid becomes bound to aluminum under these conditions.

B.2. Determine how much excess aluminum (in moles/liter) must be added to reach the onset of precipitation over that which would be required without the Folic Acid. In other words determine the amount of aluminum bound to the Folic Acid. Assume 0.01 M as a typical total folic acid concentration in this wastewater.



Footnote1

From: Nayan, R., & Dey, A.K. (1970). Z. Naturforsch. Teil B, 25: 1453., as cited in: Nordstrom, D.K., & May, H.M. (1989). Aqueous Equilibrium Data for Mononuclear Aluminum Species. In G. Sposito (Editor), The Environmental Chemistry of Aluminum (pp. 29-53). Boca Raton, FL: CRC Press, Inc.