科學家發現了一種低成本的方法來捕捉碳——利用常見的岩石

 原文網址：https://news.stanford.edu/stories/2025/02/new-process-gets-common-rocks-to-trap-carbon-rapidly-cheaply

大氣中的二氧化碳是造成全球暖化與氣候變遷的主要推手。最近史丹佛大學的化學家開發出一種經濟實用的方式來永遠移除大氣中的二氧化碳。

史丹佛大學的博士後研究員陳宇軒(左)與教授Matt Kanan，前者拿著他們實驗室製作出來可以捕捉二氧化碳的物質。圖片來源：Bill Rivard，Precourt能源研究所

這種新的製程是透過加熱常見的礦物，使其轉變成可以自行吸收大氣中的碳，並將它們永久封存起來的物質。而且只要用水泥窯這類相當普遍的窯爐，便可以生產這些易於反應的物質。

「地球上可以吸收大氣二氧化碳的礦物可謂取之不竭，但是它們本身的反應速度並不夠快，因此無法抵銷人類排放的溫室氣體，」史丹佛大學人文與科學學院的化學教授Matthew Kanan表示。他也是2月19日發表於期刊《自然》(Nature)的論文資深作者。「我們認為我們的成果解決這項問題的方式，其特點為可以大規模地運用。」

提升風化作用

自然界有一類常見的礦物稱為矽酸鹽，它們可以跟水與大氣二氧化碳反應，形成穩定的碳酸氫根離子與固體的碳酸鹽礦物，此過程也稱為風化作用。然而，這種化學反應可能得花上數百年到數千年才能完成。從1990年代開始，科學家便開始尋找方法來提升風化作用，使岩石以更快的速度吸收二氧化碳。

Kanan與史丹佛大學的博士後研究員陳宇軒證實他們在實驗室開發的新製程，可以將緩慢風化的矽酸鹽轉變成反應能力高很多的礦物，進而快速捕捉大氣二氧化碳並儲存起來。目前史丹福杜爾永續學院的永續加速基金，也提供經費來推動此研究進入實際運用的階段。

「我們猜想有一種新的化學作用，可以透過簡單的離子交換反應活化惰性的矽酸鹽礦物，」論文主要作者陳宇軒表示。他在Kanan的實驗室攻讀化學博士時研發了此技術。「我們當時並沒有預料到它會有這麼好的表現。」

許多專家表示要防止全球暖化變得更加強烈，我們需要在減少使用化石燃料的同時從大氣中永久移除數十噸的二氧化碳。但是目前碳移除的技術不是相當昂貴就是需要大量能源，或是兩者兼具，而且也還沒證明可以大規模使用。其中備受矚目，甚至近期獲得早期投資的技術為「直接空氣補捉」——利用大型扇葉將周圍的空氣引入化學或其他類型的作用中來移除二氧化碳。

Kanan表示：「跟目前最先進的直接空氣捕捉技術相比，我們的製程所需的能量還不到一半。從成本的觀點來看，我們認為我們具有相當強的競爭力。」Kanan也是史丹福杜爾永續學院Precourt能源研究所的資深研究員。

自發碳酸化

啟發這種新方法的是一種有數百年歷史的水泥製造技術。

製造水泥的時候要先把石灰岩在窯爐中加熱到1400℃左右，使其轉變成氧化鈣。接著再把氧化鈣和沙子混和來製造水泥的關鍵原料。

史丹佛大學的研究團隊在他們實驗室的高溫爐中運用了類似的作用，但是跟氧化鈣混和的材料並不是沙子，而是另一種含有鎂和矽酸根離子的礦物。這兩種礦物一起加熱之後會交換離子而轉變成氧化鎂和矽酸鈣，都是可以跟空氣中的酸性二氧化碳快速反應的鹼性礦物。

「這道過程就像是種增殖裝置，」Kanan表示。「你把容易反應的礦物氧化鈣和基本上為惰性的矽酸鎂放在一起，然後就能得到兩種易於反應的新礦物。」

為了快速測試矽酸鈣和氧化鎂在室溫下的反應性，研究人員將它們放置在含有水與純二氧化碳的空間當中。不到兩個小時，這兩種礦物便捉住二氧化碳裡的碳，並完全轉變成新的碳酸鹽礦物。

在更貼近現實條件的實驗當中，研究人員將潮濕的矽酸鈣和氧化鎂樣品直接跟空氣接觸。相較於只裝有二氧化碳的箱子，空氣裡的二氧化碳濃度低了非常多。此實驗的碳酸化作用需要幾個禮拜到幾個月來發生，但是跟自然界的風化作用相比仍快了數千倍。

史丹佛大學的研究團隊說他們的方法可以走出實驗室，以工業規模來捕捉二氧化碳。

「你可以想像把氧化鎂和矽酸鈣撒在大範圍的土地上，就可以移除周遭空氣裡的二氧化碳，」Kanan表示。「我們正在測試一項令人十分興奮的應用：將它們加到農田土壤當中。這些礦物風化之後會轉變成碳酸氫根，接著滲入土壤，最後到達海洋並永遠儲存於此。」

Kanan表示這項方法也能讓農夫獲益。他們通常會在土壤pH值太低的時候加入碳酸鈣以提高pH值，這種措施稱為「石灰處理」。

「在土壤加入我們的產品之後就不再需要石灰處理，因為兩者的礦物成分都是鹼性物質，」他解釋。「此外，矽酸鈣風化之後釋放到土壤的矽是植物可吸收的形式，因此能增加農作物的產量與韌性。理想上農夫會願意購買這些礦物，因為它們有利於農田的產量與土壤的健康，而且還有個額外的好處——移除空氣中的碳。」

打造未來

Kanan的實驗室一個禮拜可以製造15公斤左右的氧化鎂和矽酸鈣。但是要讓封存的二氧化碳量對全球氣溫產生實質的影響，每年需要生產數百萬公噸的氧化鎂和矽酸鈣才行。

研究人員表示採用與製造水泥的窯爐相同的設計就可以生產他們需要的礦物，矽酸鎂則來自產量豐富的橄欖石或蛇紋石，它們在加州、巴爾幹半島與其他許多地方都可以採到，而且也是礦業活動常見的廢棄物「尾礦」。

「全世界每年有超過4億公噸的尾礦含有適合這道製程的矽酸鹽，因此可作為原物料的龐大潛在來源，」陳宇軒表示。「據估計地球上蘊藏的橄欖石和蛇紋石超過100兆公噸，可以永久移除的二氧化碳量比人類從古至今排放的還要超出許多。」

這些窯爐需要燃燒天然氣或生質燃料來運作，研究人員把相關的排碳量算進去之後，估計這些易於反應的礦物每公噸可以從大氣移除一公噸的二氧化碳。科學家估計2024年全球燃燒化石燃料產生的二氧化碳超過了33公噸。

Kanan正在跟Jonathan Fan(史丹佛大學工程學院的電機工程副教授)合作開發以電力而非燃燒化石燃料為能量來源的窯爐。

「人類社會早就找到方法每年生產數十億公噸的水泥，而且這些水泥窯可以持續運作數十年，」Kanan表示。「如果我們能夠充分利用現有的知識與設計，便能找到一條清晰的道路來把這項碳移除的技術從實驗室的發現，提升到足以產生實質影響的規模。」

 

Scientists discover low-cost way to trap carbon using common rocks

Stanford University chemists have developed a practical, low-cost way to permanently remove atmospheric carbon dioxide, the main driver of global warming and climate change.

The new process uses heat to transform common minerals into materials that spontaneously pull carbon from the atmosphere and permanently sequester it. These reactive materials can be produced in conventional kilns, like those used to make cement.

“The Earth has an inexhaustible supply of minerals that are capable of removing CO₂ from the atmosphere, but they just don’t react fast enough on their own to counteract human greenhouse gas emissions,” said Matthew Kanan, a professor of chemistry in the Stanford School of Humanities and Sciences and senior author of the Feb.19 study in Nature. “Our work solves this problem in a way that we think is uniquely scalable.”

Enhanced weathering

In nature, common minerals called silicates react with water and atmospheric CO₂ to form stable bicarbonate ions and solid carbonate minerals – a process known as weathering. However, this reaction can take hundreds to thousands of years to complete. Since the 1990s, scientists have been searching for ways to make rocks absorb carbon dioxide more rapidly through enhanced weathering techniques.

Kanan and Stanford postdoctoral scholar Yuxuan Chen developed and demonstrated in their lab a new process for converting slow-weathering silicates into much more reactive minerals that capture and store atmospheric carbon quickly. A grant from the Sustainability Accelerator at the Stanford Doerr School of Sustainability is now supporting efforts to move the research into practical applications.

“We envisioned a new chemistry to activate the inert silicate minerals through a simple ion-exchange reaction,” said Chen, lead author of the study, who developed the technique while earning a chemistry PhD in Kanan’s lab. “We didn't expect that it would work as well as it does.”

Many experts say that preventing additional global warming will require both slashing the use of fossil fuels and permanently removing billions of tons of CO₂ from the atmosphere. But technologies for carbon removal remain costly, energy-intensive, or both – and unproven at large scale. One of the technologies getting much interest and even early-stage investment lately is direct air capture, which uses panels of large fans to drive ambient air through chemical or other processes to remove CO₂.

“Our process would require less than half the energy used by leading direct air capture technologies, and we think we can be very competitive from a cost point of view,” said Kanan, who is also a senior fellow at the Precourt Institute for Energy in the Stanford Doerr School of Sustainability.

Spontaneous carbonation

The new approach was inspired by a centuries-old technique for making cement.

Cement production begins by converting limestone to calcium oxide in a kiln heated to about 1,400 degrees Celsius. The calcium oxide is then mixed with sand to produce a key ingredient in cement.

The Stanford team used a similar process in their laboratory furnace, but instead of sand, they combined calcium oxide with another mineral containing magnesium and silicate ions. When heated, the two minerals swapped ions and transformed into magnesium oxide and calcium silicate – two alkaline minerals that react quickly with acidic CO₂ in the air.

“The process acts as a multiplier,” Kanan said. “You take one reactive mineral, calcium oxide, and a magnesium silicate that is more or less inert, and you generate two reactive minerals.”

As a quick test of reactivity at room temperature, the calcium silicate and magnesium oxide were exposed to water and pure CO₂. Within two hours, both materials completely transformed into new carbonate minerals with carbon from CO₂ trapped inside.

For a more realistic test, wet samples of calcium silicate and magnesium oxide were exposed directly to air, which has a much lower concentration of CO₂ than pure CO₂ from a tank. In this experiment, the carbonation process took weeks to months to occur, still thousands of times faster than natural weathering.

The Stanford team says their approach can be used beyond the laboratory to capture CO₂ at industrial scale.

“You can imagine spreading magnesium oxide and calcium silicate over large land areas to remove CO2 from ambient air,” Kanan said. “One exciting application that we’re testing now is adding them to agricultural soil. As they weather, the minerals transform into bicarbonates that can move through the soil and end up permanently stored in the ocean.”

Kanan said this approach could have co-benefits for farmers, who typically add calcium carbonate to soil to increase the pH if it's too low – a process called liming.

“Adding our product would eliminate the need for liming, since both mineral components are alkaline,” he explained. “In addition, as calcium silicate weathers, it releases silicon to the soil in a form that the plants can take up, which can improve crop yields and resilience. Ideally, farmers would pay for these minerals because they’re beneficial to farm productivity and the health of the soil – and as a bonus, there's the carbon removal.”

Cementing the future

Kanan’s lab can produce about 15 kilograms (about 33 pounds) of material a week. But trapping CO₂ on the scale required to meaningfully affect global temperatures would require annual production of millions of tons of magnesium oxide and calcium silicate.

The researchers say the same kiln designs used to make cement could produce the needed materials using abundant magnesium silicates such as olivine or serpentine, which is found in California, the Balkans, and many other regions. These are also common leftover materials – or tailings – from mining.

“Each year, more than 400 million tons of mine tailings with suitable silicates are generated worldwide, providing a potentially large source of raw material,” Chen said. “It’s estimated that there are more than 100,000 gigatons of olivine and serpentine reserves on Earth, enough to permanently remove far more CO₂ than humans have ever emitted.” (A gigaton equals 1 billion metric tons, or about 1.1 billion tons.)

After accounting for emissions associated with burning natural gas or biofuel to power the kilns, the researchers estimate each ton of reactive material could remove one ton of carbon dioxide from the atmosphere. Scientists estimate global emissions of carbon dioxide from fossil fuels exceeded 37 billion tons in 2024.

Kanan is also collaborating with Jonathan Fan, associate professor of electrical engineering in the School of Engineering, to develop kilns that run on electricity instead of burning fossil fuels.

“Society has already figured out how to produce billions of tons of cement per year, and cement kilns run for decades,” Kanan said. “If we use those learnings and designs, there is a clear path for how to go from lab discovery to carbon removal on a meaningful scale.”

原始論文：Yuxuan Chen, Matthew W. Kanan. Thermal Ca²⁺/Mg²⁺ exchange reactions to synthesize CO₂ removal materials. Nature, 2025; DOI: 10.1038/s41586-024-08499-2

引用自：Stanford University. " Scientists discover low-cost way to trap carbon using common rocks."