[Daumic (OP)]

The oxidation of Fe2+ by water

The Pourbaix diagram (1) defines the fields of existence of chemical compounds according to the pH and the redox potential of an environment. The lines on a Pourbaix diagram define the reactions of transformation of a compound in the other. An area delimited by these lines thus represents the field of existence of a compound.

The following web page (2) shows the Pourbaix diagram of iron and its hydroxides in water. One can see on this diagram that the field of stability of Fe2+ in basic environment is particularly narrow. In addition, this field overlaps the limit of stability of water. This situation shows that the Fe2+ reduces the water and produces hydrogen in basic environment. Only the slow kinetic of the decomposition of water moderates this reaction.

(1)https://en.wikipedia.org/wiki/Pourbaix_diagram
(2)https://chem.libretexts.org/Textbook_Maps/Inorganic_Chemistry_Textbook_Maps/Map%3A_Inorganic_Chemistry_(Wikibook)/Chapter_04%3A_Redox_Stability_and_Redox_Reactions/4.5%3A_Pourbaix_diagrams

Comparison basalts/peridotites

The fragility of Fe2+ in basic environment appears in nature by the reaction of serpentinization. This reaction is the oxidation of Fe2+ contained in a rock in contact with water. The peridotites are particularly sensitive to this reaction because of their content very high of MgO, about 30 %. This high percentage of Mg2+ makes water in contact with these rocks sufficiently basic to allow the oxidation of Fe2+ by the water and the production of hydrogen.

According to the web page (3), the average content of FeO of the peridotites is slightly lower than that of basalts: 6.6 % for 7.1 %.

One can see on this web page which the composition of basalts is not sufficiently basic to cause the reaction of serpentinization. The Fe2+ of basalts is stable in contact with natural water.

(3)http://www.geolalg.com/chabou/cours4.pdf

Production of hydrogen with basalts

The thesis of Benjamin Malvoisin (4) studies the oxidation of the Fe2+ by water in natural environment. It also shows the possibility of producing hydrogen using steel slags. Hydrogen is obtained by the oxidation of the Fe2+ contained in the slags by water. The high pH necessary to the reaction is due to the high content of ions calcium of the slags.

I propose to take the parameters of this experiment and to apply them to another source of Fe2+: basalts. 

To exploit Fe2+ in basalts, the sequence could be:
-   vertical drilling to reach an underground layer of basalt,
-   horizontal drilling in the layer of basalt followed by a hydraulic fracturing,
-   sending in the well of a limewater.

The limewater is the cheaper base and confers on water in contact with basalt a pH in the order of 12 (5). It should be a sufficient pH to allow the oxidation of Fe2+ of the rock. Ions hydroxyl brought by the limewater are not consumed, so the reaction should be maintained by a regular addition of water in the well.

The other parameter controlling the reaction, the temperature, is determined by the depth of the well. If the layer of basalt is at depths greater than 4000 meters, the temperature of the rock should be higher than 150°C. According to the kinetic established in the thesis of Mr. Malvoisin, this temperature should allow a sufficient speed reaction.

Another parameter could be favourable to the reaction in the well: the oxidation of Fe2+ transforms the olivine contained in rock into serpentine. This passage from one mineral to another produces a swelling and generates a pressure of crystallization of 300 MPa (4). This pressure is higher than the lithostatic pressure at depths of 4000 meters. This situation could make the well autofracturing. 

The production curve of hydrogen could be similar to that of a shale gas well and thus last several years. After the end of the production, the well remains usable for the CO2 sequestration or geothermic energy.

(4)https://tel.archives-ouvertes.fr/file/index/docid/934238/filename/33513_MALVOISIN_2013_archivage.pdf
(5)https://en.wikipedia.org/wiki/Limewater

[Bored chemist]

The reaction of Fe(OH)2 with water to produce hydrogen is slower than a very slow thing.

[syhprum]

If the infrastructure you are building is suitable for geothermal energy harvesting I would have thought that the generation of Hydrogen would be more a nuisance than an asset.

[Daumic (OP)]

The reaction of Fe(OH)2 with water to produce hydrogen is slower than a very slow thing.

Yes, this reaction is slow. For this reason it should be accelerated by a high temperature and a basic environment.

[chiralSPO]

It's an interesting approach. The chemistry is reasonable, but I suspect it would be an engineering nightmare...

Hydrogen has its drawbacks, but even if that is the fuel of the future, my (biased) gut feeling is that solar-powered water splitting will be the most economical approach. The energy of visible photons is well-matched for this type of chemistry, and it is easier to find access to sunlight than it is to locate and reach subterranean mineral deposits (plus you can never use up the sunlight!). Additionally, engineering for large-scale liquid-to-gas reactions is much easier than solid-to-gas (not to say that the latter is impossible, by any means).

That said, I also think that all avenues should be explored. I don't see any obvious fatal flaws in the approach of industrial serpentinization, and think it could be worth further investigation. Ultimately, though, we must remember that economics and policy (not science) will determine which technologies are adopted. I suspect that cost may be the biggest hurdle...

[Daumic (OP)]

If the infrastructure you are building is suitable for geothermal energy harvesting I would have thought that the generation of Hydrogen would be more a nuisance than an asset.

There is currently, in the United States and in Iceland, some tests of CO2 sequestration in underground layers of basalt. The results are promising but the cost compromises the future of the process in the absence of a carbon tax.

If the production of hydrogen from basalt is economically viable, that could contribute to the development of the sequestration of CO2 and that without tax incentive.

[evan_au]

I suspect that this will be the subject of protests, in the same way that coal fracking is today.
- Access roads to injection/extraction points
- increase in minor earthquakes, due to lubrication of pre-existing fault lines
- pollution of underground and surface waters
- injection pressures will be much higher, as basalt is a much tougher rock than coal
- but the biggest hurdle in both cases is providing reasonable remuneration to those affected - in my country, landowners have no rights to the resources under their feet, and the government can award these resources to mining companies with little payment to the current landowners.

[Daumic (OP)]

It's an interesting approach. The chemistry is reasonable, but I suspect it would be an engineering nightmare...

Hydrogen has its drawbacks, but even if that is the fuel of the future, my (biased) gut feeling is that solar-powered water splitting will be the most economical approach. The energy of visible photons is well-matched for this type of chemistry, and it is easier to find access to sunlight than it is to locate and reach subterranean mineral deposits (plus you can never use up the sunlight!). Additionally, engineering for large-scale liquid-to-gas reactions is much easier than solid-to-gas (not to say that the latter is impossible, by any means).

That said, I also think that all avenues should be explored. I don't see any obvious fatal flaws in the approach of industrial serpentinization, and think it could be worth further investigation. Ultimately, though, we must remember that economics and policy (not science) will determine which technologies are adopted. I suspect that cost may be the biggest hurdle...

About the economy of serpentinization of basalts:

To calculate the quantity of hydrogen economically recoverable by oxidation of Fe2+ contained in basalts, one can make comparisons with the exploitation of shale gas in the United States. The web page (1) brings elements of calculation. The following list is made with the data of this web page. The methane concentrations estimated in several deposits of shale in the United States are expressed in scf/ton (standard cubic feet/ton):
-   concentration of CH4 in Barnett: 325 scf/ton,
-   concentration of CH4 in Ohio: 80 scf/ton,
-   concentration of CH4 in Antrim: 70 scf/ton,
-   concentration of CH4 in  New Albany: 60 scf/ton,
-   concentration of CH4 in Lewis: 30 scf/ton.

I converted the unit of these data into kg/ton:
-   concentration of CH4 in Barnett: 6.5 kg/ton,
-   concentration of CH4 in Ohio: 1.6 kg/ton,
-   concentration of CH4 in Antrim: 1.4 kg/ton,
-   concentration of CH4 in  New Albany:  1.2 kg/ton,
-   concentration of CH4 in Lewis: 0.6 kg/ton.

The exploitation of this type of deposit by hydraulic fracturing has caused a fall of the price of gas in the United States. Most of this gas is used to produce electricity. The essential property of gas for this production is its heat of combustion.

The oxidation of FeO by water produces magnetite according to the reaction:

3 FeO + H2O > Fe3O4 + H2

One needs 216 g of FeO to produce 2 g of H2. The average content of basalts of FeO is 7 %. A ton of basalt thus contains 70 kg of FeO. The oxidation of this Fe2+ by water can release 0.6 kg of H2/ton of basalt.

This value is small in regard of the concentrations of shale gas given higher:
-   concentration of CH4 in Barnett: 6.5 kg/ton,
-   concentration of CH4 in Ohio: 1.6 kg/ton,
-   concentration of CH4 in Antrim: 1.4 kg/ton,
-   concentration of CH4 in  New Albany:  1.2 kg/ton,
-   concentration of CH4 in Lewis: 0.6 kg/ton,
-   possible production of H2 in basalt: 0.6 kg/ton.

But several data relativise this low value: the heat of combustion and adsorption. The heat of combustion of hydrogen is more twice the higher than that of methane:
-   heat of combustion of CH4 in MJ/kg: 50.01,
-   heat of combustion of H2 in MJ/kg: 120.5.

The following list presents the methane concentrations and the possible production of hydrogen expressed in MJ/ton:
-   concentration of CH4 in Barnett: 328.5 MJ/ton,
-   concentration of CH4 in Ohio: 80.9 MJ/ton,
-   concentration of CH4 in Antrim: 70.8 MJ/ton,
-   concentration of CH4 in  New Albany:  60.7 MJ/ton,
-   concentration of CH4 in Lewis: 30.3 MJ/ton,
-   possible production of H2 in basalt: 78.1 MJ/ton.

The data are more favourable to basaltic hydrogen. If the hydraulic fracturing of basalt has the same performances as the fracturing of the shale, hydrogen could replace natural gas for the electric production.

In addition, as the web page (1) indicates it, part of methane present in the shale is in adsorbed form, which makes it difficult to extract:
-   adsorbed gas in Barnett: 20 %,
-   adsorbed gas in Ohio: 50 %,
-   adsorbed gas in Antrim: 70 %,
-   adsorbed gas in New Albany: 50 %,
-   adsorbed gas in Lewis: 70 %.

Because of its lightness, the hydrogen molecule is much less prone to this phenomenon of adsorption. Most of produced hydrogen should be recoverable.

(1)   https://spec2000.net/17-specshgas.htm [nofollow]

[Daumic (OP)]

I suspect that this will be the subject of protests, in the same way that coal fracking is today.
- Access roads to injection/extraction points
- increase in minor earthquakes, due to lubrication of pre-existing fault lines
- pollution of underground and surface waters
- injection pressures will be much higher, as basalt is a much tougher rock than coal
- but the biggest hurdle in both cases is providing reasonable remuneration to those affected - in my country, landowners have no rights to the resources under their feet, and the government can award these resources to mining companies with little payment to the current landowners.

The fracturing for the production of hydrogen should have the same disadvantages that the fracturing already used for the exploitation of shale gas: small earthquakes, risk of water pollution.

But the produced hydrogen in this manner could be used to produce electricity and thus to replace coal for this use. The pollution generated by the extraction and the combustion of coal is much larger than those induced by the production of hydrogen by hydraulic fracturing of basalt. The replacement of coal by hydrogen for the electric production represents a considerable environmental benefit.

[Bored chemist]

How much uranium/ thorium would you end up extracting along with the frack water?

[Daumic (OP)]

How much uranium/ thorium would you end up extracting along with the frack water?

I propose to produce hydrogen to put in contact of basalt with limewater in wells dug by hydraulic fracturing. The limewater makes the environment basic. The production of hydrogen makes the environment reducing. The chemical conditions in the well are thus reducing and basic. The Pourbaix diagrams of Uranium and Thorium (1) show that under these conditions the oxides of Uranium and Thorium are very few soluble. 

(1)http://www.eosremediation.com/download/Chemistry/Chemical%20Properties/Eh_pH_Diagrams.pdf