The "gotcha" is that at 83 microns, this study showed only 0.15% of the theoretical CO2 capacity absorbed in the first year. Worse, the authors expect this rate to taper significantly in the second and subsequent years.
This is why most EW proposals specify a smaller grain size, but that also has the effect of dramatically increasing the grinding energy required. See [1] (included again below) for a more complete and nuanced discussion of these tradeoffs.
I think the second "gotcha" is that even at 83 microns, that particle size is likely unsafe. That's about the size of gypsum dust used in drywall and you can't cut or sand or do anything without PPE for long without serious health consequences. Make it any smaller and it'll be well into the asbestos size range, which would probably be carcinogenic based on the mechanical damage it causes alone (a lot of nanoparticles are toxic for this reason). That's safe in a controlled industrial environment with everyone using fitted respirators but spread out on fields exposed to wind, near residential areas? Dumping that much fine dust everywhere would probably create a similar hazard to the dried out beds of the Great Salt Lake or the Salton Sea or Blackrock Desert* except everywhere there is farmland.
It'd have to be combined with a binder to form much larger particle sizes so that the natural weathering safely exposes the olivine slowly over decades. I suspect that once one does the math on the whole lifecycle, it's net carbon positive only after decades if it is to be done safely.
* People who have been to Burning Man can testify to just how much trouble dust that small causes, not to mention the frequent dust storms with zero visibility.
If you execute all the staff at every power plant in the world and then keep executing the subsequent replacement cohorts, every power plant would have to shut down and we would dramatically reduce global CO2 production. Similarly, if you position machine guns at major roads and keep shooting every car that comes through, you will eventually reduce the environmental toll of driving cars.
> The "gotcha" is that at 83 microns, this study showed only 0.15% of the theoretical CO2 capacity absorbed in the first year
According to the study [1] (and the article), that's because the experiment was purposely conducted in a dry climate (inland northern CA) and during a drought to determine a lower bound on the CO2 absorption capacity. More well suited (wetter) climates are expected to result in more CO2 absorption.
"Given that climate change impacts such as heatwaves and droughts are already widespread, knowledge of the robustness of enhanced weathering under extreme conditions is essential to understanding its future efficacy. Here we show that enhanced weathering maintains modest carbon dioxide (CO2) removal in a multi-acre field trial under an extreme drought in California, one of the largest agricultural producers globally."
This is why most EW proposals specify a smaller grain size, but that also has the effect of dramatically increasing the grinding energy required. See [1] (included again below) for a more complete and nuanced discussion of these tradeoffs.
[1] https://worksinprogress.co/issue/olivine-weathering/