Q1. Solutions of Calcium Chloride (CaCl2) are often sprayed on dirt roads at large construction projects to reduce dust associated with construction vehicle traffic.
Consider the problem created by runoff of CaCl2 into a recreational/irrigation pond on your property from a neighboring construction project. Over time, the concentration of CaCl2 in your pond reaches 0.05M.
Equations to use related to the carbonate species in water.
KH = 10-1.5
Ka1 = 10-6.3
Ka2 = 10-10.3
a. Using the Davies equation, calculate the activity coefficient (γ) for any monovalent and divalent ionic species in the pond.
b. Calculate the carbonate [CO32-] concentration as activity at pH of 7.5 taking the activity coefficients into account (remember, pH is measured as activity) assuming equilibrium with air having a PCO2 of 400 ppm.
c. Calculate the Ion Activity Product (IAP) for CaCO3 in the pond water (again taking activity into account).
Q2. You are asked to treat a mixed acidic plating aqueous waste having a pH of 4 to reduce the concentrations of Co2+ and Cu2+.
Your first approach is to raise the pH by bubbling ammonia gas through the waste solution to precipitate the associated metal hydroxides.
Given Ks for Co(OH)2(s) is 1015.9 and Ks for Cu(OH)2(s) is 10-19.4.
a. What is the minimum pH necessary to bring the concentration of both the free Co2+ and free Cu2+ down to concentrations of 100PPB or below? Report the concentrations of each phase and the pH you choose.
b. What will be the NH4+ concentration required to get to the solution above pH 7?
Q3. When treating the waste in problem 2 above, you find that once you have raised the pH high enough to meet the target for free metal concentration, you find the total metal for at least one species (either Co or Cu) exceeds the limit of 100 PPB.
a. Given that the Ka for ammonia is 10-9.3, calculate the dissolved ammonia (NH3) at the pH you calculated for part 2a above?
b. Calculate the Tot Co and Tot Cu in solution when you calculate the appropriate complexes in the treated system. Use formation constants from tables 9-3 and 9-4 to consider complexes that might explain the results. (just use the β1 for each ligand). Report the concentrations of each complex as well.
c. Will just raising the pH higher solve reduce the metal concentrations to the level required, explain?
Q4. Now consider treating the same waste by adding Na2CO3 to precipitate the metals as their carbonate solid phases. Given Ks for CoCO3(s) is 10-9.98 and Ks for CuCO3(s) is 10-11.5.
a. What will be the HCO3- concentration required to get to the solution above pH 7?
b. What concentration of CO32- would be necessary to reach the target metal concentration of 100PPB as the free metal (Me2+ form) for both metals?
c. What would the pH be at the required carbonate concentration?
d. Would carbonate precipitation be a reasonable method of getting the total metal concentrations to the target of 100PPB, explain?
Q5. You are interested in seeing if the occurrence of Harmful Algal Blooms (and their toxins) in Coastal SC has changed with changing climate conditions. You collect a sediment core from a coastal location and measure 210Pb activity in the sediments to measure the age of sediment as a function of depth in the cores. You then measure the amount of toxin per gram of sediment in each horizon and see clear evidence of higher toxin levels in younger (more recent sediments). You need to correct for any change in concentration of the toxin that is due to decay of the toxin over time. Only then can you reconstruct the accumulation history of the toxin.
a. Your first task is to measure the decay rate of the toxin in these sediments. You know the rate of decay is slow because you have measured toxins in sediments that are tens of years old. You run decay experiments at 70oC to be able to obtain reasonable rates over a 40 hour experiment. Treat this as a first order decay and plot the data you collected (below) and determine the decay rate.
|
Time (hours)
|
Conc. nM/g
|
|
0
|
285
|
|
2.5
|
263
|
|
5.0
|
236
|
|
7.5
|
218
|
|
10.0
|
201
|
|
20.0
|
138
|
|
30.0
|
99
|
|
40.0
|
66
|
b. In a separate experiment the activation energy for the first order decay is found to be 124kJ/mole. Given that the temperature of the sediment has been a constant 5oC over time, find the appropriate decay rate for reconstructing the toxin deposition history in the sediments.
c. In order to reconstruct the decay history, you need to know the age at each sediment horizon (depth). Use the 210Pb data to determine the age of each horizon, assuming that the activity of 210Pb at the surface represents a constant input or time (i.e. is A0 for all depths). Fill in the ages below.
d. Using the measured toxin data for each horizon, reconstruct the accumulation history by corrected each value for decay and report in the table below.
|
Depth (cm)
|
210Pb (dpm/g)
|
Age (years)
|
Toxin (PPB)
|
Decay Corrected Toxin
|
|
0 (surface)
|
156
|
|
124
|
|
|
10
|
95.8
|
|
89.5
|
|
|
20
|
64.2
|
|
65.2
|
|
|
30
|
39.4
|
|
33.3
|
|
|
40
|
25.3
|
|
15.2
|
|