Harnessing Nature to cure its (man-made) ills
When I think about harnessing the power of water, I think of the Hoover Dam in Nevada (where I’m from) or the awesome tide-harnessing turbines that are being installed along coastlines as we speak. As scientists concerned about the future of our planet, we are always looking for ways to co-opt natural processes to greenify (buzz-word!) our energy-producing endeavors. Recently, Greg H. Rau of UC Santa Cruz and Lawrence Livermore National Laboratory described how we could appropriate and intensify the natural carbonate mineral cycle. This innovative work builds on existing proposals to use limestone and seawater to mimic erosion, which has the dual potential for cleaning up power plant exhaust and mitigating ocean acidity.
Every time you see rocks eroded by passing water, you are witnessing one of Nature’s very own carbon dioxide sequestering processes at work. As it washes over rocks like limestone, water reacts with calcium carbonate in the rock and ambient carbon dioxide to create a solution of calcium bicarbonate and calcium di- and tri-oxide. The problem, from a CO2 mitigation standpoint, is that this process works on natural (i.e., geologic) timescales. But we need it working now. Well, it turns out the process can in fact be sped up in the presence of excess CO2, that harbinger of the greenhouse effect. The ever-increasing levels of CO2 in the air all around are not quite enough, but there is a more concentrated source that would work just fine. To the power plants!
To simulate the conditions of a typical power plant exhaust vent (big white tower thingy exhaling smoke), Rau employed a gas source with a mixture of air and 10% CO2 by volume. He expelled this gas through a tube along with a slush of crushed limestone and seawater moving at various relative speeds. Though he initially found this simulated erosion removed 97% of the CO2, it seems that not much of that CO2 was successfully bound up in the calcium bicarbonate. Instead, he’d made carbonated seawater! Yum!
The purpose of co-opting the carbonate weathering process is to bind up the constituents of CO2 in other molecules to prevent their eventual escape as a greenhouse gas. Just like your soda that goes flat after a few days in the fridge, carbonated sea water will not stay carbonated for the millenia over which calcium bicarbonate can effectively drown CO2. By increasing the contact time between the wet limestone and CO2, Rau found that he was able to sequester upwards of 85% of the simulated flue CO2 in a calcium bicarbonate and seawater cocktail. He accomplished this by either introducing more limestone particles with larger surface area per volume (more reaction sites) or simply giving the reactants more time to hang out together (longer incubation time).
This anthropogenically enhanced erosion has great potential for mitigating CO2. And in the spirit of further greenfication, adding the alkaline calcium-bicarbonate-laden seawater back to the ocean may also help reduce the excess acidity that currently plagues ocean wildlife. And this all can be done quite cheaply, since seawater is already used at many power plants for cooling. Of course, this would require hauling in many tons of limestone for its supporting role. The environmental cost of mining and transporting all that necessary rock certainly requires further analysis. And, as highlighted in some discussions of this exciting result, there is a concern that other toxic elements could get bound up in the seawater and actually do more harm than good. On this subject Rau’s work is silent, but he appeals to the need for “[f]urther research … in more realistic settings.” Until then, I think the jury will be out on the real-world utility of this super-charged erosion process, but I expect that a grand canyon of research will be carved out in the exploration of this potential game-changer.
Rau, G. (2011). CO2 Mitigation via Capture and Chemical Conversion in Seawater
Environmental Science & Technology, 45 (3), 1088-1092 DOI: 10.1021/es102671x