Chemical weathering is the process by which various types of rock is broken down by exposure to water, oxygen, and/or carbon dioxide. For our purposes, the most relevant forms of weathering involve uptake of carbon dioxide. CO2 dissolved in rainwater forms carbonic acid, which is quite mild as acids go but sufficient over time to dissolve minerals from rock. Calcium and silicon exposed to carbonic acid will form HCO3 bicarbonate, and release calcium and silicates.
Occurring naturally, this chemical reaction takes place gradually over millions of years. Most of the bicarbonate thus produced eventually washes out to the ocean, where various organisms like coral pull carbon and dissolved calcium out of the water to make shells. The rest of the bicarbonate gradually settles into the deep ocean and eventually adds to the limestone at the ocean floor.
Enhanced weathering is a plan by which humans can accelerate this process, by grinding the appropriate types of rock into particles to maximize surface area and spreading them over an area to take up CO2. There are a number of options.
- Bicarbonate, calcium, and magnesium at appropriate concentrations are beneficial to soil health, especially tropical soils which tend to be depleted in these minerals. Spreading powdered olivine over one third of tropical agricultural land could pull between 30 and 300 parts per million of carbon dioxide out of the atmosphere.
There is a large range in that number because we just don't know enough about how these processes work at scale. Perhaps fortunately, we also don't have the capacity to quickly seed such a large fraction of the planet's land area. Over time, the results of the earliest years of effort can be measured to guide future plans. - Though tropical land is ideal, using olivine as a soil additive in agricultural land elsewhere would still have an effect.
The term "electrogeochemistry" has been coined to refer to enhanced weathering done at large scale. - Mine tailings are the heaps of excess rock discarded from mining operations after the valuable minerals have been extracted. The tailings generally contain large amounts of the types of rock which will absorb CO2 as they weather, and in fact do rapidly form a shell of carbonate at the surface of the pile. If mining regulations are made to require the tailings be ground more finely and appropriately distributed, they can be effective in pulling carbon dioxide from the atmosphere.
Mine tailings also tend to contain trace amounts of substances which can be harmful, like mercury. Processes such as those developed by Advanced Materials Processing, Inc to remove harmful substances from tailings would be necessary.