AAAS:"Farm fertilizer could suck carbon dioxide from the atmosphere." A new study reported in Nature suggests a way to accomplish [in]direct air capture [DAC] of carbon dioxide using natural minerals. Mimics the natural entrapment of CO2 by what is usually the slow but inexoraable process of weathering. "Since the start of the Industrial Revolution in the 1760s, humans have lofted some 2650 gigatonnes [Gt] of CO2 into the atmosphere, raising the concentration of the heat-trapping gas by 50%." Reforestation is discussed, but trees eventually burn or die + decay. Mechanical DAC is energy-intensive but 'consumes some 2 MWh of energy for every ton of CO2 wrung from the air.'  "Another approach is carbon mineralization: spreading vast amounts of crushed alkaline rocks—usually abundant magnesium silicates, such as olivine and serpentine—on soils worldwide." Enhanced weathering by using pulverized rock is still too slow.  Secret may be to switch from magnesium silicates to calcium silicates, which react quickly with CO2. But unlike magnesium-based minerals, calcium silicates aren’t naturally abundant, so the researchers combined calcium oxide (CaO) with magnesium silicates and heated the mix to about 1200°C for 4 hours—Mg swapped places with Ca, producing magnesium oxide and calcium silicates, which bound 'all the CO2 they could possibly hold within months,' thousands of times faster than natural weathering.  CaO or lime can be produced from heating abundant limestone, energy-intensive but with direct CO2 capture from the exhaust, just as with production of Portland cement. "Even [adding] in...energy costs of making CaO + mixing it with magnesium silicates, the total energy needed to capture CO2 is only about half that for DAC. Cost of this carbon removal could be about $100 per ton [a widely held target], plus farmers will pay to decrease soil acidity. Trials underway in Louisiana + New Jersey. Red + blue working together, right?
