A publication is born !

Yes ! We did it ! After a lot of concerted effort, working with a team of scientists from all over Europe, each specialised in different aspects of enhanced weathering, we published a scientific paper on the effects of enhanced weathering of olivine in seawater. The paper was published in the journal Environmental Science & Technology (ES&T, in short), and can be found on their website. I think it is safe to say that we managed to publish the most complete work on this subject up till now. Of course, at the end of the day, we are still left with numerous questions, but we’ve experimentally proven that it works ! Olivine dissolution increases the pH (lowers the acidity) and increases CO2 uptake by the seawater. The article is Open Access and can be obtained here.

For those readers who are not scientists (marine, climate, geo-engineering or otherwise) and who are wondering what the paper actually is about, I will try to explain our research in a more accessible way (please do let me know whether I succeeded in doing so…).

There are numerous claims that if olivine is ground to the size of fine sand grains and dumped in the ocean, it would actually enable the sea to take up more carbon dioxide from the air, while at the same time combating ocean acidification. However, this process has only been proven in model simulations or under ideal -and thus unrealistic- conditions. What we have done is taken certain quantities of olivine sand grains of ca. 150 micrometer (15% of a millimeter, see picture at the end of this post) and let those dissolve in bottles of seawater, while the bottles were constantly shaken on a rotational shaker table. A rotational shaker table is a piece of laboratory equipment (picture below), on which you can place bottles or jars or other containers in a fixed manner. The table part with the bottle holders can then be set to freely turn in circles (rotate) with a given number of rotations per minute (rpm) so that the contents of the bottles are constantly mixed.

The rotational shaker table used in our experiments. The aluminium plate on which the bottle holders are fixed, can spin freely, with a pre-set number rotation per minute (rpm).

Then, we opened the bottles at regular times (every 2 to 7 days) and took a bit of seawater out, so we could analyse its composition and chemistry. Our first and most important finding is that we actually could measure the fact that over time a) the seawater became less acidic and b) the CO2 buffer capacity also increased, leading to more CO2 being captured by the seawater from the air.

Of course, we made sure that we used a proper control group. In the context of a properly conducted experiment, this means that you introduce a group to control for the effect of the olivine. In other words: how do you know the effect you measure comes from the substance/treatment you introduced, if you do not have a control group ? So, in order to make sure that it was the olivine that was causing the seawater de-acidification and increase of its buffer capacity, we also dissolved another mineral in seawater at the same time. For this, we used pure quartz minerals. Quartz is a highly inert mineral, meaning that it hardly reacts or dissolves in (sea)water. Ordinary beach sand is basically quartz, and does not have the properties that olivine has, in that it does not (should not) influence the seawater chemistry. When we analysed the seawater of the control (quartz) group, we found just that: no changes in either acid-level or buffer capacity. Conclusion: olivine has indeed the capacity to make our seas less sour.

Our cooking ingredients from left to right: seawater, olivine sand and quartz powder

The second part of our study, was essentially a repetition of the first part. Only now, alongside bottles of natural seawater, we used a series of artificially made seawater mixtures, in which we dissolved the olivine sand. The artificial seawater mixtures had a different composition, meaning that we replaced certain compounds for others. The reason seawater is so salty, is that it has (surprise !) a whole lot of different salts dissolved in it. Rain (fresh water) falls on land, and dissolves a tiny amount of the rocks and earth minerals it runs past. The ultimate fate of all fresh water is to arrive in the world’s oceans, where it eventually evaporates as fresh water, while the dissolved compounds stay behind in the ocean. After millions of years of dissolving rocks and evaporation-rain cycles, the concentration of dissolved compounds can even be tasted as different types of salts. By far the most common salt in seawater is sodium chloride, yes table salt ! But there are also several other salts in seawater, that contribute to the salty-ness, mostly magnesium and calcium salts. So, what we did in the second part of our experiments, is replace the calcium and the magnesium salts for sodium salts. In that way, the artificial seawater would be as salty as before, but made with different kind of salts. What we wanted to know, is if olivine would dissolve in a “different type of seawater”, would it also display a different dissolution “behaviour”

And what we observed was indeed different. We first replaced only calcium and saw that the dissolution went faster, which translated into faster de-acidification and more CO2 taken up from the air. When we also replaced magnesium, the dissolution went about 2 to 4 times faster ! The response of the seawater was off the charts: the pH increased with more than 0.1. The pH scale is a logarithmic scale, meaning that from pH = 6 to pH = 7 is a ten times increase, while from pH= 6 to pH = 8 is a hundred times increase. To place it in perspective: the ocean’s acidity has decreased by about 0.1 in recent years, which gave rise to the concern of many marine and climate scientists. In our experiment, we managed to actually counteract part of that pH change. Also, as a consequence of us replacing magnesium in the artificial seawater mixture, the CO2 buffer capacity increased much more than in natural seawater. Of course, we cannot take out all the magnesium from seawater, nor do we need to, but it shows us that magnesium in seawater seems to put a brake on the effects of olivine in seawater.

All in all, it seems that the olivine dissolution we measured in our experiments lines up pretty well with what had been predicted in all sorts of model simulations in the literature, and even is a tad faster. But we are the first ones to prove it experimentally !

And now for the remaining questions and challenges. What about the secondary (or side) effects of olivine dissolution on the marine ecosystem ? The main reaction products of olivine dissolution are increases in pH, buffer capacity (alkalinity), dissolved CO2, dissolved magnesium, dissolved silica and dissolved nickel. Now the first three are actually the desired effects, in terms of climate change mitigation. Even number four, an increase in dissolved magnesium in seawater, is not expected to cause any negative effects, because the natural concentration of magnesium in seawater is already much higher than what olivine would add.

Dissolved silica is used by certain groups of microscopic algae in seawater. In turn, these algae would benefit from this “fertilisation” effect, grow faster and would then suck up more CO2, right ? Hmm, yes… But, imagine a sudden (much) higher silica concentration in the seawater. This may (not necessarily, but possibly) cause more intense growth of these groups algae. Algae do not have eternal life and such sudden bursts (also called “algal blooms”), have a tendency for massive die-offs. If such amounts of algae suddenly die, it means a lot of food for bacteria, who will use a lot of oxygen from the seawater to eat up all that dead organic matter. In some areas in the world’s oceans, this already happens in a more or less “natural” way, and really lowers the oxygen concentration in the sea. This leads to the development of so-called “dead zones”, because you can imagine that not a lot of sea organisms (fish, shrimps and crabs, clams, worms etc.) are able to live in under such conditions. I am not saying that this will happen, but it is definitely something we need to find out, before thinking about applying olivine in natural systems.

The sixth and perhaps most pressing, consequence of olivine dissolution is a marked increase in nickel. Nickel is officially counted as a heavy, and potentially toxic, metal. Although there is some research on whether and how toxic nickel is to marine organisms, the overall effect is not very clear. Nonetheless, the potential impact of nickel needs to be clarified as soon as possible. The last thing one wants to do is to try and solve a climate problem, only to find that another aspect of that solution is just as damaging for the ecosystem one is trying to protect.

Olivine grains of different sizes. The olivine on the left is that used in our experiments, while that on the right comes from the lava rocks around Papakolea beach in Hawai’i.

In the last part of our study, we investigate how well our results would do in a real-life situation. We take the example The Netherlands, a country famously known for the fact almost half of it is below sea level and protected from the sea by a large system of dunes. To maintain the coastline, and prevent the hinterland from being exposed to the wrath of Neptune’s, the Dutch government is required by law to perform yearly supplements of sand along the coastal zone. In the last decade, the yearly volume of sand used to maintain the coastline was 12 million cubic meter (424 million cubic feet). That volume is already becoming more because of sea level rise due to climate change. We made a calculation, using the values on how fast the olivine dissolves in seawater, and how much carbon dioxide (CO2) it captures as a consequence. We then imagined that those 12 million cubic meter sand actually consisted of the same olivine sand we used in our experiments. Using the calculation mentioned before, we found that the yearly “olivine sand supplements” along the Dutch coastal zone could capture about 5 % of the yearly CO2 emissions of The Netherlands. This may seem a bit low at first sight, but there are many natural processes in that sandy sea bottom that would considerably speed up the olivine dissolution. It is thus very likely that those 5 % would turn out higher. In any case, we think it would be very important to have a look at those naturally occurring coastal processes, and investigate how they influence the olivine dissolution when applied to a truly natural situation. But that is a story for another (upcoming) publication !

Our take home message ? Dissolving olivine in seawater indeed counteracts ocean acidification, by increasing the alkalinity, and consequently sucks up CO2 from the atmosphere. It sounds like the perfect medicine against climate change, but it is very important to realise that there are secondary effects, which need to be investigated in detail. It is also very important to answer the question whether olivine dissolution would be feasible to apply at a (very) large scale.

For more information on how olivine dissolution may be used in seawater, we expect a review article to come out quite soon. Keep an eye on this website for the latest news and research outcomes. If you have any questions, or want thing clarified, please drop us a line via the contact form !

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