[hockey1212]

I need help with a chemistry project from my Equilibrium Systems class. I think I've got a good idea where to start but I'm not sure where to go.

Essentially the problem is, you need to design a cost-effective solution to removing the fluoride found in the surface waters near a local village because large corporations are unwilling to help.

I have the data for three specific wells: such as pH, total dissolved solid, and the relative concentrations of fluoride and other ions in the water. The pH's are 7.7, 8.2, and 7.9 and F- concentrations in mg/L are 3.06, 3.12, and 13.1, respectively. TDS's are 579, 548, and 975.

Using this I need to answer the questions below.

1. What forms of fluoride are found in the aquatic environment (there may be only one)
2. How does pH alter the related fluoride compounds in terms of solubility.
3. How does ionic strength play a role in predicting solubility.
4. What level of fluoride is safe in drinking water?
5. What possible compounds could be added to the water supply to lower the fluoride level?

This is what I have so far, any help would be great.

1. F- ion
2. I found some research suggesting that using activated carbon works well for removing fluoride, but only at pH levels of 3 or less, so this wouldnt work without dropping the pH, treating it, and then raising the pH. Related fluoride compounds in the water are CaF2, NaF, MgF2, and some others.
3. As ionic strength decreases, solubility increases.
4. 0.7-1.2 mg/L is recommended by the WHO
5. Some other research I found shows that using activated alumina will remove flouride by binding it to the filter, but at pHs above 6 the hydroxyl ions become competitors so it's not as effective. 


The only hint we were given was look at saturation (Q vs. K) and fractional composition of the bicarbonate/carbonate system since it controls pH.

The only thing I can come up with is trying to precipitate it out but I'm not sure where to go from here. Each well has very calcium concentrations so would adding more help to precipitate out CaF2?

Sorry for the length, but I had to explain what was happening.

[Nobby]

Flouride can be lowered by adding of lime Calciumcarbonate/hydroxide to precitipate CaF₂.

[hockey1212]

So by adding CaO₂, Calcium fluoride would precipitate and the O₂ would just be a gas that bubbled up and out?

Also, could I use Calcium chloride instead? or would I end up with too much chlorine in the water to be safe?

[Nobby]

It is not CaO₂. It is CaCO₃ or Ca(OH)₂. Calciumchloride would rise the Chloride not the Chlorine content.

[AWK]

3. As ionic strength decreases, solubility increases.

This statement is not true.

Bone ash or (chloro,hydroxy)apatite can be used for removing fluoride.
Ca₅(PO₄)₃(Cl,OH) +NaF = Ca₅(PO₄)₃F + Na(Cl,OH)