In Chapter 7[1], "Geochemical Negative Emission Technologies", the Oxford author details mineralization methods of CO2 sequestration that can last for millions of years when considering storage. The Oxford author also contradicts the MIT author of Chapter 6, DAC[1], when discussing the potential for cost decrease of DAC processes. In his opinion, the Oxford author, low quality (~10% CO2 purity stream) and in-situ mineralization, can decrease DAC costs by > 60%. If the MIT author's "true cost" quote of 1200 dollars t CO2 is used, the Oxford author is suggesting the potential for a cost of equal to or less than 480 dollars t CO2. 

Chapter 7 is 56 pages in length and quite detailed as related to chemistry, processing, etc. Thermodynamics is favorable, but kinetics of mineralization processes are slow. The author discusses how anthropogenic alkaline sources from steel slag, concrete kiln, mining, etc. can be used for mineralization. The crucial components are CaO and MgO. In the case of steel slag, it dissolves at several orders (4-5) of magnitude faster than natural alkaline. The CO2 sequester potential of anthropogenic sources could be 2.98-8.5 Gt CO2 per year by 2100. 

In contrast to ex-situ carbon mineralization, in-situ is direct injection into the ground. In nature, this reaction precipitates to olivine ((Mg, Fe)2SiO4, Clinopyroxene (CaMgSi2)6-CaFeSi2O6) and serpentine ((Mg, Fe)3Si2O5(OH4)). In-situ is less labor intensive than ex-situ. Theoretically, there is > 10^5 Gt C storage potential across the globe. In general, it is difficult to mine the natural minerals so in-situ injection is more practical. It is estimated that it will take as many bore holes as generated by Oil and Gas to be effective for the future sequestration of CO2--100,000 to 1e6 boreholes. 

If 1 mm layer of basalt powder were spread over 2/3 of the global cropland, 0.5-4 Gt per year by 2100 could be sequestered. The higher the content level of Ca, Mg, K, or Na in the rock, the better the carbon capture. Igenous and metamorfic rocks are rich in alkaline Earth metals and are a great source of divalent metal cations for carbon mineralization. Ultramafic rock are estimated to be distributed over 736, 000 km^2 globally, and flood basalt is distributed over ~ 100,000 to 1e6 km^2. 

Most geochemical mineralization is aqueous: 1) mineral dissolution; 2) CO2 dissolution; and 3) carbonate mineral precipitation. Dissolved carbon content survives up to 100,000 years in the ocean and is considered permanent storage. It is calculated and estimated that the ocean sequesters ~ 38,000 Gt C. Still, ocean alkalinity is considered controversial. 

The chapter gives detailed descriptions of the effect of pH, alkalinity, temperature, concentration, etc on CO2 chemistry. 



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References: 

[1] Greenhouse Gas Removal Technologies. (2022). United Kingdom: Royal Society of Chemistry.