Carbon sequestration

This is about opportunities for carbon dioxide removal from the atmosphere
using advanced gemstone-based atomic precision manufacturing (APM),
in comparison with approaches accessible by today's (2024) pre-APM technology.

Element pairings for carbon sequestration

Silicon Si (& Titanium Ti)

Sequestration in the form of silicon carbide (SiC)

• exceptional stability (chemical inertness, water insolubility)
• incombustible i.e. self-extinguishing
• current day production limitations (high temperature, poor carbon efficiency, 2/3 of carbon goes back to atmosphere right away)
• APM advantages (room temperature production & high efficiency)

(TODO: Investigate energy efficiency comparisons.)

Overall perhaps the best possible carbon sequestration target possible thus
it'd be a primary target for APM-based carbon sequestration.
Construction blocks and blocks optimized for stable long term carbon storage.
(TODO: Are there any other promising silicon based (single stoichometry) compounds that locking carbon away in a extremely long term stable fashion?)

Titanium Ti:
Sequestration properties of titanium carbide TiC may be similarly good compared to silicon carbide SiC.
While titanium is still abundant it is much less so that silicon and not too concentrated in occurrence.
TiC forms simple cubic rock salt structure and has ultra hard ceramic properties. It is not water soluble.

Calcium Ca & Magnesium Mg (& Iron Fe)

• Carbonate formation (CaCO₃, MgCO₃)
  
• Moderate water solubility somewhat limits long-term stability.
  
• Abundance patterns (CaCO₃ more common than MgCO₃).
  

Some considerations for the case that these elements are sourced from sea water:
Ca & Mg are only dilute in concentration compared to Na (sodium).
Removal of Cl counter-ions would pose the problem of chlorine sequestration.
And there are no good elements other than (earth) alkali metals to pair it with to make it harmless.
Thus extracting Ca & Mg it'd be better to balance it out with removal of other (more dilute) conter ions.
I.e. extracting sulfates, phosphates, and nitrates. This may lead to depletion of them though. While initial beneficial in nutrient-excess zones there may emerge nutrient depletion at full CO₂ removal to pre-indurstial levels scale. Depletion of the Ca & Mg itself may also be problematic for sea life that depends on it for shell formation.

Some considerations for the case of formation of such minerals by ultramafic (i.e. metal rich) silicate based rock:
Mining crushing transport and adding to agricultural fields adds energy needed and
reduces long term stability of the sequestration.

These compounds can server as biodegradable structural materials of mid strength,
though degradation re-releases the CO₂ so in the context of carbon sequestration (as is the focus here)
this would defeat the purpose.

Iron Fe:

• Iron carbonate (FeCO₃)
  
• Limited natural abundance despite iron availability
  
• Solubility issues limit long-term stability
  
• Iron is a main component in ultramafic rock (alongside magnesium Mg).
  

Less suitable elements

These elements, while abundant, face limitations for carbon sequestration.

Sodium Na & Potassium K

High water solubility makes carbonates rather unsuitable.
Thus this would require storage as well long term stable sealed packages.
Carbonates if these elements are not good as structural materials either as they are very soft.
Extraction rapidly in quantity from sea water may force extracting chlorine as the counter-ions.
That (as already mention) themselves may pose a problem in sequestration of this chlorine.

Sodium is natural to consider due to it's ease of accessibility from sea water.
There is the chlorine counter ion issue though thwarting that accessibility.

Aluminum Al

As typical for aluminum salts carbonates too have severe water solubility/reactivity issues.
It' just natural to take a look as aluminum is in abundance at place three right after silicon (and oxygen).
Earth surface of course. Carbon sequestration only makes sense here on our planet where we face that problem.

Others elements

Li, Be, B are non-volatile but all lack abundance. Li salts are highly water soluble/reactive too.
P, S are not too abundant and combinations form some oddball compounds. CS₂ is a liquid e.g.
Of the transition metals of their first period only Fe & Ti have decent abundance compared to the CO₂ problem.
With higher periods abundance only goes further downhill.

Economic viability - with gem based APM

With high throughput advanced gem based APM using air as a resource via
mechanosynthetic carbon dioxide splitting should become highly energy efficient.

As a feedstock the silicon for silicon carbide might actually be the
harder part than carbon for SiC formation that sequestrates carbon.
Procurement of silicon involves hard rock mining on the ground rather than just sucking in ambient air.
Still silicon is the most abundant element in the lithosphere (not counting oxygen).
It's usually readily accessible unless one is situated in a pure carbonate mountain range.
Or situated atop deep organic soil (as e.g. in the case of permafrost). Or atop an ocean.

Unless enormous scale building projects emerge
orders of magnitude more carbon might get purely into stable sequestration storage rather than
into products fulfilling a function other than that.
Sequestration could happen in the context of solar energy harvesting across the pacific.
See: Carbon dioxide collector & Carbon capture buoy scenario.

Related

• Air as a resource
• Carbon dioxide collector & Carbon capture buoy scenario – Pacific solar harvesting.
• Abundant elements
• Mechanosynthetic carbon dioxide splitting
• Climate crisis

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Relevant for products beside pure storage:
Mechanical metamaterial & Emulated elasticity