Antacid for our Oceans

An electrolysis method that removes CO2 from seawater could be affordably scaled up for commercial carbon sequestration.

WHO: Greg H. Rau, Institute of Marine Sciences, University of California, Santa Cruz, Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, CA
Susan A. Carroll, William L. Bourcier, Michael J. Singleton, Megan M. Smith, and Roger D. Aines, Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, CA

WHAT: An electrochemical method of sequestering CO2 from sea water using silicate rocks.

WHEN: June 18, 2013

WHERE: Proceedings of the National Academy of Sciences (USA), PNAS vol. 110, no. 25

TITLE: Direct electrolytic dissolution of silicate minerals for air CO2 mitigation and carbon-negative H2 production (open access)

This paper was fun – I got to get my chemistry nerd back on thinking in moles per litre and chemical equations! It almost made me miss university chemistry lectures.

No, not those moles per litre! (IFLS facebook page)

No, not those moles per litre! (IFLS facebook page)

So what does chemistry jokes have to do with carbon sequestration? It’s looking increasingly like humanity is going to have to figure out ways to draw carbon out of the atmosphere or the oceans because we’ve procrastinated on reducing our carbon emissions for so long.

There’s two options for this – you can either create a chemical reaction that will draw CO2 out of the air, or you can create a chemical reaction that will draw CO2 out of a solution, and given how quickly the oceans are acidifying, using sea water would be a good idea. The good news is; that’s exactly what these researchers did!

Silicate rock (which is mostly basalt rock) is the most common rock type in the Earth’s crust. It also reacts with CO2 to form stable carbonate and bicarbonate solids (like the bicarbonate soda you bake with). Normally this takes place very slowly through rock weathering, but what if you used it as a process to sequester CO2?

The researchers created a saline water electrolytic cell to test it out. An electrolytic cell is the one you made in school where you had an anode and a cathode and two different solutions (generally) and when you put an electric current through it you created a chemical reaction. What these researchers did was put silicate minerals, saline water and CO2 in on one side, and when they added electricity got bicarbonates, hydrogen, chlorine or oxygen, silicates and salts.

A basic schematic of the experiment (from paper)

A basic schematic of the experiment (from paper)

The researchers used an acid/base reaction (remember those from school?!) to speed up the silicate and CO2 reaction, which also works well in an ocean because large differences in pH are produced in saline electrolysis. Are you ready to get really nerdy with me? The chemical equation is this:

Chemical equation for the experiment (from paper)

Chemical equation for the experiment (from paper)

So how did the experiment go? It worked! They got successfully sequestered carbon dioxide with an efficiency of 23-32% that sequestered 0.147g of CO2 per kilojoule (kJ) of electricity used.

There are issues around the scaling up of the reaction of course – once the bicarbonate has been created, where do you store it? The paper suggested ocean storage as the bicarbonate solids would be inert (un-reactive). I would hope that a re-use option could be found – has anyone looked into using bicarbonate solids as an eco-building material?

There’s also the issue of needing to power the reaction with electricity. If scaled up, this process would have to make sure it was powered by renewable energy, because burning carbon to sequester carbon gives you zero.

Also, if sea water is used, the main by-product is Cl2 so the researchers point out that while it would be feasible to do this process directly in the ocean, the issue of what to do with all that chlorine would need to be dealt with. The paper suggests using oxygen selective anodes in the electrolysis, or ion-selective membranes around the reaction to keep the chlorine separate from the ocean.

That being said, there are some exciting upsides to this process. The paper points out that the amount of silicate rock in the world ‘dwarf[s] that needed for conversion of all present and future anthropogenic CO2.’ Also, using sea water is an easier way to sequester CO2 rather than air-based methods.

Scaling the method up is economically feasible too. The researchers estimated that 1.9 MWh (megawatt hours) of power would be needed per metric tonne of CO2 sequestered. If the waste hydrogen from the process were sold as hydrogen fuel for fuel cells, the price of CO2 sequestered would be $86/tonne. If the hydrogen fuel wasn’t feasible, it would still only be $154/tonne, which compares very favourably to most current carbon capture and storage feasibility estimates of $600-$1000/tonne of CO2.

So, like an antacid for the oceans, if this process can be scaled up commercially through research and development, we could have an effective way to not only capture and store carbon, but also reduce ocean acidification. A good-news story indeed – now we just need to stop burning carbon.


The Forests of Planet Ocean

Estuaries are the forests of the ocean and they take up carbon 90 times more efficiently than land forests

WHO: Colin Campbell (Science Advisor, Sierra Club BC, PhD Marsupial Evolution, UC Berkley)

WHAT: ‘Blue Carbon’ report looking at how carbon is stored in oceans

WHEN: August 2010

WHERE: Sierra Club BC – in print and online

TITLE: Blue Carbon British Columbia: The Case for the Conservation and Enhancement of Estuarine Processes and Sediments in B.C.

Since I’ve moved to Canada, I’ve learnt a lot about salmon and it’s a pretty tough gig from what I can tell. Everyone wants to eat you along the way (including humans like me – yum!) and evolution decided it would be a good idea to swim up the river – way to make it harder!

Salmon – it’s a hard knock life (photo: Dan Bennett, flickr)

But even harder again – once baby salmon have hatched and are swimming back down the river to eat and grow and become edible for me, they have to also make the switch into salt water which is impressive in any animal.

Changing from fresh water to salt water as a fish takes a whole heap of osmoregulation, which is the transfer of water into and out of your cells to keep an even salt balance on both sides of the cell membrane. When there’s too much salt on one side, the water moves to dilute it, which is the reason we feel dehydrated after eating a lot of salty food. So a salmon changing to salt water would be like you or I changing to only drinking salt water instead of fresh water. Salmon can do this, we would not survive.

How do salmon do this? Well it involves estuaries. Estuaries are the underwater grasslands at river mouths which look pretty gross to walk through in your bare feet when you’re going swimming. But if you’re a salmon, it’s your training ground. Salmon will hang out in estuaries full of eelgrass or salt marshes for a week or two until their bodies switch to salt water. The area is full of microbes dissolved in the water for the salmon to eat and the eelgrass provides a great hiding place for salmon that aren’t yet fast enough to swim away from everyone who wants to eat it. Without estuaries, the salmon race down the river and shoot straight out into the salt water, which is quite a shock to their systems.

Eelgrass – salmon training ground and underwater forest (photo: Eric Heupel, flickr)

This paper from the Sierra Club here in British Columbia talks about estuaries other feature – they’re great carbon sinks, and we need more of them. Carbon moves around our planet in a cycle with sources and sinks. Humans burning fossil fuels are a huge source of it and the sinks can’t keep up the balance (hence ocean acidification and global warming). So the more carbon sinks we can preserve and create the better for our survival.

Because eelgrass, mangroves and salt marshes are in the shallow water, they get lots of sunlight, which allows them to photosynthesise. 1km2 of mangroves can store the same amount of carbon as 50km2 of tropical rainforest and estuaries are constantly sucking carbon dioxide in, and releasing oxygen while pushing carbon further down into the sediment on the ocean floor.

They suck carbon in, send oxygen out and provide a training ground for animals – so far there’s no downside.Except that many estuaries have been or are being reduced by human development. In the Fraser River here in Vancouver alone, 99% of the delta marshlands are gone due to flood prevention dyking, farmland or development. That’s a whole lot of carbon that could be sucked into the estuary that isn’t.

Currently, it’s estimated that BC estuaries sequester 180,200 tonnes of carbon per year, even with their diminished size due to humans. When you realise that Metro Vancouver’s carbon emissions were 10.5 million tonnes in 2010, estuaries take up 1.7% of our emissions each year. If we keep burning carbon, we will need a lot more estuaries. Additional benefit for our own self interest as well of course is that more estuaries means bigger salmon runs and more delicious food for us, the bears, the seals, the whales, the eagles and all the rest.

As Vancouver aims for its 2020 Greenest City targets, one of the most efficient ways of sequestering more carbon while also reducing our emissions might not be our forests, but our underwater forests – the tidal estuaries.