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It's been documented that NASA hope to capture an asteroid in 2025, and have subsequent aims to mine that asteroid. If if this is successful, we would expect other asteroids to be mined in the future.

A consequence of this is that relative atomic masses of elements mined—those with two or more stable isotopes—will no longer be faithful to our current periodic table. Ruthenium alone, a high-value rare earth, has five stable isotopes alone, ranging from $^{98}\ce{Ru}$ to $^{102}\ce{Ru}$. The relative abundances of these isotopes are bound to differ in other places other than Earth, and so would the weighted average.

Say after mining our ruthenium, we use it in an electronic device and consequently that device is thrown away. For argument's sake, also say that this ruthenium—of an isotopic distribution never before seen—leaches into the environment and later comes into contact with biological systems. Could it be possible that compounds hitherto considered non-toxic on Earth become toxic to biological systems by virtue of a newly realised isotopic discrimination?

As an example, $^{13}\ce{CO2}$ is effectively discriminated against in uptake by plants in comparison to $^{12}\ce{CO2}$ (due to it being a diffusion limited reaction). Ruthenium is purported to be carcinogenic, but the most abundant isotope on Earth of Ruthenium is $^{102}\ce{Ru}$ which could be effectively non-toxic by virtue of the fact that compounds containing these atoms takes so long to diffuse, they don't diffuse across biological membranes at all.

If a sample of Ruthenium from the moon found its most abundant isotope in $^{98}\ce{Ru}$, then this would be expected to diffuse much faster than its heavier counterparts. Therefore could it be possible that a compound previously thought of as non-toxic could become toxic via this effect. Does this sound plausible?

NB: I've deliberately missed out other routes of exposure such as ingestion to only consider diffusion controlled reactions.

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Can you provide a source for plant's not using C13? Because I know they use at least some C14, that's how radiocarbon dating works. And by your argument, C14 should diffuse even more slowly than C13. –  user137 Aug 19 at 20:06
    
I didn't say that they don't use it, but that there is an active discrimination. And yes C14 will diffuse even more slowly. Radiocarbon dating relies on absolutely minute quantities of C14. As for evidence, the δ13C isotope signature is well known in biogeochemistry for example. –  TheJsyChemist Aug 19 at 20:09
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Also, to allude to C3 and C4 plants, carbon dioxide uptake is different in the different species. In C4 it occurs via the Hatch-Slack pathway which is less diffusion controlled than C3's Calvin-Benson. –  TheJsyChemist Aug 19 at 20:27
    
I like the question, but my guess is that every answer would boil down to "we don't know, here are some proposed (or: performed) tests about it [refs 1, 2, 15 and 42]" ... –  tschoppi Aug 19 at 20:44
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The largest biological consequences of isotopes are found, of course, with hydrogen. Both deuterium and tritium are toxic. Since many biological processes rely on resonance effects, too much deuterium messes up metabolism. Tritium, while having the same effect as deuterium, adds the beta decay radiation dose. –  Jon Custer Aug 19 at 22:06

3 Answers 3

up vote 6 down vote accepted

This is an interesting question and you raise a number of points, let's step through them.

A consequence of this is that relative atomic masses of elements mined—those with two or more stable isotopes—will no longer be faithful to our current periodic table.

But this is already happening. $\ce{^235U}$ constitutes 0.72% of uranium found on earth and decays to the stable isotope $\ce{^207Pb}$, which is found in a natural abundance of 22.1%. Before we get to the asteroid the abundance of various isotopes and their weighted average mass is already shifting.

98 Ru , then this would be expected to diffuse much faster than its heavier counterparts

Diffuse much faster? First off, we're talking about diffusion, not chemical reaction. Elements or compounds usually enter the body by ingestion of one sort or another (eating, breathing) rather than diffusion, and these ingestion pathways wouldn't involve isotopic discrimination. That is, if we breath air that contains isotopes in a certain ratio, then that's the ratio that will initially appear in our lungs. Nonetheless if diffusion were found to be an issue, my guess is that isotopic discrimination by diffusion would be a very small effect. For a gas the maximum separation of two isotopes is given by $$\ce{\sqrt{\frac[MW~of~compound ~with ~isotope~1][MW~of~compound ~with ~isotope~2]}}$$ In the case of separating $\ce{^235UF6/^238UF6}$ this amounts to a fractionation ratio of 1.0043 after 1 pass. Ruthenium is lighter, so the effect would be larger, about 1.02 ($\ce{^102Ru/^98Ru}$) after 1 pass, even less if it is in a molecule.

Chemical reaction within our body of ingested isotopic compounds would also show discrimination due to primary kinetic isotope effects. The maximum primary kinetic isotope effect is proportional to the reduced mass as follows: $$\ce{k~ \thicksim ~\sqrt{\frac[m_1 + m_2][m_1 * m_2]}}$$ Applying this to $\ce{^102Ru}$ and $\ce{^98Ru}$ yields a primary kinetic isotope effect of 1.020, again, even less if the element is incorporated into a compound.

To me, the effects look small and since shifts in natural abundance have been occurring for a long time here on earth, with no one raising a flag, my guess (and that's all it is, a guess) is that it's not something to worry about. Still, I'd feel better if a NASA toxicologist looked at the question and said "no problem"

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For the second point I realised that talking about diffusion relating to human health is probably a bit daft, although there's no reason why it couldn't happen in plants such that the effect is passed down a food chain. If we apply your formula for gaseous separation to carbon dioxide as mentioned, then this amounts to a fractionation ratio of 1.011, which is less than atomic ruthenium! –  TheJsyChemist Aug 20 at 7:17
    
If gas centrifuges can sort out U238 and U235, which have a tiny percent difference, could they be used to remove CO2 from air? CO2 is about 40g/mol, and O2 seems like the next heaviest at 32g/mol, so the percent difference is larger and separation should be more efficient. But how much energy would it take to run the centrifuges? –  user137 Aug 20 at 14:10
    
@user137 Interesting thought. Sounds like it should work, I know that mean free path and membrane porosity are also design factors, and as you point out, energy considerations. –  ron Aug 20 at 14:17

Whatever the isotopes are for asteroidal material (and they are mostly close to those seen down here on Earth), they are contained in the 5 to 100 tons of meteoritic material that falls onto the Earth's atmosphere (and thus filters down to us on the surface) every day. It will be a long time before the cumulative pollution from asteroid (or lunar) mining can match the meteoritic rate.

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Also, if you follow what @ron said, for large isotope effect you generally need lighter elements. So the isotope effect for deuterium and hydrogen can be large. But even in these cases biological effects can be mostly observed only when one use high concentrations of D2O.

I don't think that diffusion control would play any role in the biology of metal ions. If we are talking about transition metals with biological role, nature is generally relies on transport enzymes and similar to carry around them. Isotope effects are most pronounced in chemical reactions with relatively high activation barrier and strong vibrational coupling with the given atom. In practice it means enzymatic reactions with active centers where there is a significant role of the given metal.

Do we even know any enzymes in which's active center Ru plays important role? You can ask this for any elements that we expect to mine, In practice, isotope effects hardly can seen even for iron or cobalt atoms, which are pretty common active centers compared to most other metals. If you study natural iron enzymes, the isotopic ratio of 56/57 iron is not anomalous. So I wouldn't expect much biological effect to anything that heavy or heavier.

If we are going beyond biological function, and talking about metals and toxicity, most of the time the complex formation between e.g. proteins and the given metal ions is the important question to study. I don't think is it much influenced by isotope effects, and I definitely would not expect that a toxic heavy metal became harmless or similar drastic change.

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