Stolte Nore, Pan Ding
Department of Physics , Hong Kong University of Science and Technology , Hong Kong , China.
Department of Chemistry , Hong Kong University of Science and Technology , Hong Kong , China.
J Phys Chem Lett. 2019 Sep 5;10(17):5135-5141. doi: 10.1021/acs.jpclett.9b01919. Epub 2019 Aug 21.
The chemistry of carbon in aqueous fluids at extreme pressure and temperature conditions is of great importance to Earth's deep carbon cycle, which substantially affects the carbon budget at Earth's surface and global climate change. At ambient conditions, the concentration of carbonic acid in water is negligible; therefore, aqueous carbonic acid was simply ignored in previous geochemical models. However, by applying extensive ab initio molecular dynamics simulations at pressure and temperature conditions similar to those in Earth's upper mantle, we found that carbonic acid can be the most abundant carbon species in aqueous CO solutions at ∼10 GPa and 1000 K. The mole percent of carbonic acid in total dissolved carbon species increases with increasing pressure along an isotherm, while its mole percent decreases with increasing temperature along an isobar. In CO-rich solutions, we found significant proton transfer between carbonic acid molecules and bicarbonate ions, which may enhance the conductivity of the solutions. The effects of pH buffering by carbonic acid may play an important role in water-rock interactions in Earth's interior. Our findings suggest that carbonic acid is an important carbon carrier in the deep carbon cycle.
在极端压力和温度条件下，水溶液中碳的化学性质对于地球深部碳循环至关重要，而地球深部碳循环对地球表面的碳收支和全球气候变化有着重大影响。在环境条件下，水中碳酸的浓度可忽略不计；因此，在先前的地球化学模型中，碳酸水溶液被简单地忽略了。然而，通过在与地球上地幔相似的压力和温度条件下进行广泛的从头算分子动力学模拟，我们发现，在约10吉帕斯卡和1000开尔文的条件下，碳酸可能是CO水溶液中最丰富的碳物种。在等温线上，碳酸在总溶解碳物种中的摩尔百分比随压力增加而增加，而在等压线上，其摩尔百分比随温度升高而降低。在富含CO 的溶液中，我们发现碳酸分子和碳酸氢根离子之间存在显著的质子转移，这可能会提高溶液的电导率。碳酸的pH缓冲作用可能在地球内部的水岩相互作用中发挥重要作用。我们的研究结果表明，碳酸是深部碳循环中的重要碳载体。
