CE 420/520 Environmental Engineering Chemistry Problem Set No. 5 Dr. S.K. Ong

CE 420/520 Environmental Engineering Chemistry
Problem Set No. 5
Dr. S.K. Ong
Due Date: Nov. 19, 2004
Problem 1, page 313
Problem 3, page 313
Problem 7, page 313
Problem 13, page 314
Problem 14, page 314
As primer to using MINTEQA2, we will start with a simple system that you have a good understanding.
(i) Use MINTEQA2 to determine the ionic strength and pH of a 0.001 M HCl solution. Specify H + = 10-3
and Cl- = 10-3 for the different components (under component name and click add).
(ii) What is the pH for 10-3 CaCO3 solution?
(iii) What will be the pH if you add 10-3 HCl to the solution in (ii)?
(iv) What will be the pH if you add 2 x 10 -3 HCl to solution in (ii)?
(For (ii), (iii) and (iv) you may want to check your answer against what we have done before by graphical
method or calculations)
Zinc in wastewater may be precipitated out using NaOH. Determine the pH range for best precipitation of
zinc as Zn(OH)2 (amorph.). Use the solubility constants for Zn in Table 5.1 in your notes (do not use ZnO
in your problem)
With MINTEQA2 prepare a pC-pH diagram as before. To do this you need to specify Zn(OH) 2 as an
infinite solid (look for the menu bar on top of the page, i.e. solid phases and excluded species). Run the
program at two fixed pH, i.e. 4.0 and 10.0. If the program refuses to run, you may have to specify a low
concentration of an indifferent salt such as 10-7 Na+ and 10-7 Cl-. Search the output file for the various
activity of Zn2+ and their hydrolysis species and list them as shown in the Table below. Plot the points on a
pC-pH diagram. Attach a printout of your output file. Compare your results with your hand calculation
pH 4
pH 10
For CE 520 Students
A metal plating wastewater contains cadmium. You wish to precipitate Cd with NaOH. Estimate the pH
range where formation of a precipitate (Cd(OH) 2) is most favored to ensure minimum Cd in solution? See
Table 5.1 (hand out in class) for stability constants. The solid phase formed is -Cd(OH)2(s).
In an anaerobic environment, ferrous iron (Fe2+) may precipitate with carbonate,
phosphate or hydroxide. Determine which mineral forms in an anaerobic system with the following
characteristics (Use calculations or pC-pH diagram):
pH = 6
CT,P = 10-5 moles/liter (total phosphorus)
PCO2 = 10-3.5 atm
H2CO3 system
H3PO4 system
pKso = 10.5
pKso = 14.5
pKso = 29.9
pK1 = 6.3, pK2 = 10.3, pKH = 1.5
pK1 = 2.3, pK2 = 7.2, pK3 = 12.3