Does brewing tea with hard water reduce the amount of bioavailable fluorine?

Question

I read that brewed tea contains a relatively large concentration of Fluorine.

Fluorine can have negative health effects such as Skeletal Fluorosis. EDIT: I understand some Fluorine is necessary and has benefits. This is just to inform why I have structured this hypothesis. I am interested in the chemistry not the health effects as that is a different topic.

Tap water and especially hard water contains lots of Calcium ions.

Would Calcium and Fluorine bond to form Calcium Fluoride? I'm obviously not a chemist, but I assume it would be a lower energy state than the soluble ion pair in the tea, correct? It forms Fluorapatite on tooth enamel.

This Calcium Fluoride SDS indicates that eating solid CaF2 straight won't immediately kill you, so I assume Calcium Fluoride does not decompose in HCl stomach acid, correct?

So with that logic would the following be true?

Mol bioavilable F- = Mol F- from brewed tea - 2*(Mol of Ca2+ in hard water)

If that is the case, it brings up some other interesting questions, like if moderately hard municipal water actually has any Fluorine in solution or not...

EDIT: To select a best answer I need to know:

1. if Ca ions bond with F ions in solution to form CaF2
2. and to know the percent or time-rate solubility of CaF2 in stomach acid.

Thanks!

Answer 1

$\ce{CaF2}$ has a relatively low solubility in water, about 15 mg/L, with a $K_{sp}$ of about $4\times 10^{-11}$. According to the paper you linked, the concentrations of fluoride in tea are around 5 mg/L, which corresponds to about $1.3\times 10^{-4}$ M. Thus, the concentration of $\ce{Ca^2+}$ required to precipitate the fluoride out is about 2.5 mM, which corresponds to 0.1 g/L. Hard water is defined as having a calcium concentration above about 0.6 mM, with very hard water being 1.8 mM calcium. Thus, it is unlikely that there would be high enough concentrations for $\ce{CaF2}$ to precipitate out of solution.

Because of the large solvation energy of fluoride ion, the molecules in solution are likely to be nearly completely ionized. That is, the fluoride anions and the calcium cation will each have their own solvation shell of water molecules. That means that they will not interfere with each other until the concentrations are high enough that solid $\ce{CaF2}$ begins to precipitate.

As long as the concentrations are below the solubility limit, the presence of calcium ions will therefore not affect the availability of the fluoride ions.

Furthermore, since HF is a weak acid, in the acidic environment of the stomach some of the fluoride ions will be protonated to HF, decreasing the concentration of free fluoride and increasing the apparent solubility of $\ce{CaF2}$.

Answer 2

What Atharva and Ericmuch say is sometimes contrary to the truth. Calcium fluoride cannot be dangerous, because it is insoluble in water, and in diluted acidic solutions. It can produce negative effects such as skeletal fluorosis, but this happens only in cases where it is taken in huge amounts. Fluorides have no negative effects, if taken in small amounts.

It is even the contrary. Adding sodium fluoride in small amounts (90 - 250 mg/kg) to the usual salt NaCl has had a remarkable effect on the dental health of the general population. I can give you some values measured before and after the introduction of NaF in the salt (or in the tap water). This introduction has been introduced in USA during the war 1939 - 1945. It was adopted much later in Europe, where it was accepted with hesitation and reluctance.

I can give the results I know from my own country, Switzerland. In 1961, before the fluoration in NaCl, only 25.2% of the 7y children had no dental caries (tooth decay). And only 1.2% of the teenagers (15 y) had no dental caries. And an average teenager had 7.4 teeth with caries.

The fluoration of the salt was introduced in 1962. Fifteen years later, in 1977, 76.6% of the young children (7 y) had no dental caries at all. The tendency has been reversed. And 15,1% of the teenagers (15 y) had no dental caries, instead of 1.2% earlier. And the average teenager had 1.2 teeth with caries. It is a huge progress. It was not perfect, of course, but it shows that adding a small amount of fluorine improves the dental health.

The same measurements have been made in many if not all European and American countries. See the following references.

H. Dean, Fluorine in the Control of Dental Caries, Intern. Dent. Journal 4: 311-337,, 1954. K. Binder, W.S. Driscoll, Caries-Preventive Fluoride Tablets Programs, Car. Res. 12 (Suppl. 1):22-30, 1978. F.B. Glenn, The Rationale for the Administration of a NaF supplement, J. Dent. Child, 48:118-122, 1981. I. Hellsröm, Y. Ericsson, Fluoride Reactions with Dental Enamel Following Different Forms of Fluoride Supply, Scand. J. Dent. Res. 84:225-267, 1976. H.R. Mühlemann, K.G. König, T.M. Marthaler, Anleitung zur praktischen individuellen Kariesprophylaxie, Zahnärtl. Mitt, 16, 741 -743 1966. H.S. Horowitz et al., Perspectives on the use of Prenatal fluorides : A Symposium, J. Dent. Child 48, 102-134, 1981. T.M. Marthaler, Ph. de Crousaz, Médecine dentaire préventive, en particulier dans les écoles, Schweiz Mntschr. Zahnheilk., 89: 619-628, 1979. K. Toth, Ten Years of Domestic Salt Fluoridation in Hungary, Caries Res. 13:101, 1979.

Answer 3

Even I am not a chemist, but according to my textbook there is already a counter ion present in hard water to the $\ce{Ca^2+}$ and $\ce{Mg^2+},$ which are mostly bicarbonates (temporary hardness) or fluorides, chlorides and sulfates (permanent hardness). These counter-ions would by themselves prevent any appreciable amount of fluoride forming a bond with the $\ce{Ca^2+}.$

Also, even though $\ce{CaF2}$ probably won't decompose fully in stomach acid, it would decompose enough to cause the negative effects of fluorides to come into effect, according to my professor. My professor also says that this the reason $\ce{EDTA^4-}$ is used to complex with $\ce{Ca^2+}$ and $\ce{Mg^2+}$ in hard water: since coordinate compounds don't dissociate in water, the $\ce{Ca^2+}$ and $\ce{Mg^2+}$ are not able to make the water hard. The anions are removed using organic ion exchangers.

Thus I believe making tea in hard water won't reduce the fluoride concentration very much, and in any case drinking hard water is generally a bad idea because it is not pure enough to be drunk.