studiot Posted July 22 Share Posted July 22 https://www.bbc.co.uk/news/articles/c728ven2v9eo Quote Scientists have discovered “dark oxygen” being produced in the deep ocean, apparently by lumps of metal on the seafloor. About half the oxygen we breathe comes from the ocean. But, before this discovery, it was understood that it was made by marine plants photosynthesising - something that requires sunlight. Here, at depths of 5km, where no sunlight can penetrate, the oxygen appears to be produced by naturally occurring metallic “nodules” which split seawater - H2O - into hydrogen and oxygen. Link to comment Share on other sites More sharing options...
exchemist Posted July 22 Share Posted July 22 13 minutes ago, studiot said: https://www.bbc.co.uk/news/articles/c728ven2v9eo Yes I saw this. I have tracked down the paper in Nature Geosciences and have been reading it: https://www.nature.com/articles/s41561-024-01480-8 It's not entirely clear there is sufficient potential difference to electrolyse water, but the researchers do seem to have evidence that oxygen is produced from the nodules rather than via something else. What the bobble hat persuasion has seized on, of course, is another argument for preventing the mining of these nodules to help the green energy transition. But the interesting thing to me is what the mechanism for electrolysis can be. I can't seem to find any mention of detecting hydrogen, which strikes me as suspicious if electrolysis is assumed to be the process. Or can it be some other process involving reduction and abstraction of hydrogen into the structure of these compounds? The paper doesn't really seem to get into the chemistry. Perhaps someone will pick this up and look into it further. Link to comment Share on other sites More sharing options...
studiot Posted July 22 Author Share Posted July 22 26 minutes ago, exchemist said: Yes I saw this. I have tracked down the paper in Nature Geosciences and have been reading it: https://www.nature.com/articles/s41561-024-01480-8 It's not entirely clear there is sufficient potential difference to electrolyse water, but the researchers do seem to have evidence that oxygen is produced from the nodules rather than via something else. What the bobble hat persuasion has seized on, of course, is another argument for preventing the mining of these nodules to help the green energy transition. But the interesting thing to me is what the mechanism for electrolysis can be. I can't seem to find any mention of detecting hydrogen, which strikes me as suspicious if electrolysis is assumed to be the process. Or can it be some other process involving reduction and abstraction of hydrogen into the structure of these compounds? The paper doesn't really seem to get into the chemistry. Perhaps someone will pick this up and look into it further. Well the article does say that they have measured a (contact) voltage difference of the order of a Leclanche battery and there are plenty of possibilities for the impurities to be say 1.1 to 1.6 volts different in work function. If the main metal in the nodule is Iron then Calcium fits the bill. Quote They found it to be almost equal to the voltage in a typical AA-sized battery. There is a convenient table on Wikipedia https://en.wikipedia.org/wiki/Work_function Link to comment Share on other sites More sharing options...
exchemist Posted July 22 Share Posted July 22 (edited) 20 minutes ago, studiot said: Well the article does say that they have measured a (contact) voltage difference of the order of a Leclanche battery and there are plenty of possibilities for the impurities to be say 1.1 to 1.6 volts different in work function. If the main metal in the nodule is Iron then Calcium fits the bill. There is a convenient table on Wikipedia https://en.wikipedia.org/wiki/Work_function OK but work function is not really the right criterion. This is not removing an electron from a metal surface in a vacuum, it is taking water molecules, splitting them and forming new molecules of H2 and O2, in an aqueous environment, and in the process donating 2 electrons per molecule, from a metal oxide or hydroxide. What I took from the paper was that one needs 1.23V + 0.37V overpotential, so 1.6V whereas the potentials they have detected are only up to 0.95V. So I guess they must be assuming there are spots with a potential difference >0.95 that they just have not picked up. I can see that if Mn II goes up to MnIV, say, you have your 2 electrons, but the rest looks handwavy. Edited July 22 by exchemist Link to comment Share on other sites More sharing options...
studiot Posted July 22 Author Share Posted July 22 2 hours ago, exchemist said: OK but work function is not really the right criterion. This is not removing an electron from a metal surface in a vacuum, it is taking water molecules, splitting them and forming new molecules of H2 and O2, in an aqueous environment, and in the process donating 2 electrons per molecule, from a metal oxide or hydroxide. What I took from the paper was that one needs 1.23V + 0.37V overpotential, so 1.6V whereas the potentials they have detected are only up to 0.95V. So I guess they must be assuming there are spots with a potential difference >0.95 that they just have not picked up. I can see that if Mn II goes up to MnIV, say, you have your 2 electrons, but the rest looks handwavy. Does it have to be oxygen and hydrogen ? That is not the way ( biological) photosynthesis works. Link to comment Share on other sites More sharing options...
exchemist Posted July 22 Share Posted July 22 5 minutes ago, studiot said: Does it have to be oxygen and hydrogen ? That is not the way ( biological) photosynthesis works. Well indeed, it’s the paper that is suggesting electrolysis, but the absence of free hydrogen is why I was speculating in my earlier post about some other process trapping hydrogen in some way. Link to comment Share on other sites More sharing options...
StringJunky Posted July 23 Share Posted July 23 (edited) Would the pressure at 5km deep make a difference to the rate of reaction? 1649atm. Edited July 23 by StringJunky Link to comment Share on other sites More sharing options...
studiot Posted July 23 Author Share Posted July 23 12 hours ago, exchemist said: Well indeed, it’s the paper that is suggesting electrolysis, but the absence of free hydrogen is why I was speculating in my earlier post about some other process trapping hydrogen in some way. Isn't the contact potential the potential difference equal to the difference of the work functions ? So this should exist between any impurity and the base metal? Link to comment Share on other sites More sharing options...
exchemist Posted July 23 Share Posted July 23 5 minutes ago, studiot said: Isn't the contact potential the potential difference equal to the difference of the work functions ? So this should exist between any impurity and the base metal? OK sorry, I see what you mean and I'm sure that's right. But we don't have any metals here, so far as I'm aware, just chunks of porous aggregates of metal oxides and mixed oxide/hydroxides, in a saline aqueous environment. So it seems to me it's going to be more like an electrochemical cell than the potential difference between 2 metallic conductors. Link to comment Share on other sites More sharing options...
John Cuthber Posted July 23 Share Posted July 23 8 hours ago, StringJunky said: Would the pressure at 5km deep make a difference to the rate of reaction? 1649atm. Yes, but in the wrong direction. https://en.wikipedia.org/wiki/Le_Chatelier's_principle 1 Link to comment Share on other sites More sharing options...
studiot Posted July 23 Author Share Posted July 23 (edited) 1 hour ago, exchemist said: OK sorry, I see what you mean and I'm sure that's right. But we don't have any metals here, so far as I'm aware, just chunks of porous aggregates of metal oxides and mixed oxide/hydroxides, in a saline aqueous environment. So it seems to me it's going to be more like an electrochemical cell than the potential difference between 2 metallic conductors. Contact potentials apply to all materials in any states, not just solid metals, though some contacts may be unobtainable. For instance Dowling showed the CP between liquid and solid bismuth in 1928. https://journals.aps.org/pr/abstract/10.1103/PhysRev.31.244 Seawater is a soup of ions and those nodules are a mix of many substances. Perhaps hydrogen is adsorbed on the nodule and released over a long time period. Whatever is going on is very complicated. As a student in the 1960s, I remember reading about those nodules in SCIAM. They were all set to mine them in those days, but it has never happened. But as far as I remember they were reported as metallic rather than oxides. One component was manganese. Edited July 23 by studiot Link to comment Share on other sites More sharing options...
exchemist Posted July 23 Share Posted July 23 35 minutes ago, studiot said: Contact potentials apply to all materials in any states, not just solid metals, though some contacts may be unobtainable. For instance Dowling showed the CP between liquid and solid bismuth in 1928. https://journals.aps.org/pr/abstract/10.1103/PhysRev.31.244 Seawater is a soup of ions and those nodules are a mix of many substances. Perhaps hydrogen is adsorbed on the nodule and released over a long time period. Whatever is going on is very complicated. As a student in the 1960s, I remember reading about those nodules in SCIAM. They were all set to mine them in those days, but it has never happened. But as far as I remember they were reported as metallic rather than oxides. One component was manganese. That was one of the things I noted with interest in the paper. I too had got the vague idea, from the newspaper reports of "manganese nodules" etc., that they were metallic. However it seems clear from the paper they are oxides and hydroxy-oxides, rather than native metal. When one thinks of it, given that even the lowest depths of the oceans are far from anoxic, the environment is an oxidising one, so it seems implausible that any mechanism could exist that would reduce metal ions to the metallic state. Link to comment Share on other sites More sharing options...
studiot Posted July 23 Author Share Posted July 23 34 minutes ago, exchemist said: That was one of the things I noted with interest in the paper. I too had got the vague idea, from the newspaper reports of "manganese nodules" etc., that they were metallic. However it seems clear from the paper they are oxides and hydroxy-oxides, rather than native metal. When one thinks of it, given that even the lowest depths of the oceans are far from anoxic, the environment is an oxidising one, so it seems implausible that any mechanism could exist that would reduce metal ions to the metallic state. I agree that journalistic science can be quite misleading, especially when rushed to press. I really hope that a BBC Science Correspondent knows the difference between voltage and current, but I am not sure from this Quote The researchers put this theory to the test in the lab, collecting and studying the potato-sized metal nodules. Their experiments measured the voltages on the surface of each metallic lump - essentially the strength of the electric current. They found it to be almost equal to the voltage in a typical AA-sized battery. And this certainly says metals Quote The nodules form when dissolved metals in seawater collect on fragments of shell - or other debris. It's a process that takes millions of years. But I never thought about it till now, I just took it on trust. Nor had I heard of the nodule forming on a nucleation surface such as a shell fragment, but that makes sense. Link to comment Share on other sites More sharing options...
exchemist Posted July 23 Share Posted July 23 (edited) 25 minutes ago, studiot said: I agree that journalistic science can be quite misleading, especially when rushed to press. I really hope that a BBC Science Correspondent knows the difference between voltage and current, but I am not sure from this And this certainly says metals But I never thought about it till now, I just took it on trust. Nor had I heard of the nodule forming on a nucleation surface such as a shell fragment, but that makes sense. Well that seems to be wrong, so as usual the journos have screwed up😄. This Wiki article is pretty unambiguous in stating the nodules are concretions composed of silicates, oxides and hydroxides of metals, in which Mn and Fe are major components: https://en.wikipedia.org/wiki/Manganese_nodule. "In both marine and terrestrial environments, ferromanganese nodules are composed primarily of iron and manganese oxide concretions supported by an aluminosilicate matrix and surrounding a nucleus.[2][3]" I expect the confusion arises due to the previous focus on the value of them as ores to be mined. Articles on that topic will talk in terms of the metal resource they represent, rather than the actual chemistry of the ore. Edited July 23 by exchemist 1 Link to comment Share on other sites More sharing options...
studiot Posted July 23 Author Share Posted July 23 24 minutes ago, exchemist said: Well that seems to be wrong, so as usual the journos have screwed up😄. This Wiki article is pretty unambiguous in stating the nodules are concretions composed of silicates, oxides and hydroxides of metals, in which Mn and Fe are major components: https://en.wikipedia.org/wiki/Manganese_nodule. "In both marine and terrestrial environments, ferromanganese nodules are composed primarily of iron and manganese oxide concretions supported by an aluminosilicate matrix and surrounding a nucleus.[2][3]" Thanks +1 Link to comment Share on other sites More sharing options...
studiot Posted July 23 Author Share Posted July 23 I have two good oceanography books. Oceanography Summerhayes and Thorpe Oceanography Thurman and Trujillo Both both have nice pictures, and discuss the nodules at length. I didn't realise that 'manganese nodules' were first reported by the Challenger expedition (1872 to 1876) It is interesting that when introduced they go by the name manganese or ferromanganese and the introductions all seem to refer to them as metals. It is only when you delve deeply into the text that they admit they are a mixture of iron oxide, manganese oxide and other oxides and salts. Link to comment Share on other sites More sharing options...
exchemist Posted July 23 Share Posted July 23 (edited) Just found another synopsis of this, which touches on some of the issues we were discussing: https://www.nature.com/articles/d41586-024-02393-7 So the question of what about the hydrogen is indeed something to resolve, as is the energy source, given that the stored energy implied by the measured potential differences ought, by rights, to have run down long ago. Edited July 23 by exchemist Link to comment Share on other sites More sharing options...
exchemist Posted July 24 Share Posted July 24 21 hours ago, studiot said: I have two good oceanography books. Oceanography Summerhayes and Thorpe Oceanography Thurman and Trujillo Both both have nice pictures, and discuss the nodules at length. I didn't realise that 'manganese nodules' were first reported by the Challenger expedition (1872 to 1876) It is interesting that when introduced they go by the name manganese or ferromanganese and the introductions all seem to refer to them as metals. It is only when you delve deeply into the text that they admit they are a mixture of iron oxide, manganese oxide and other oxides and salts. One thought that occurred to me later about this finding is that the world is badly in need of more efficient electrolysis methods, for green hydrogen production. Research into what is going on here might just yield new insights into options for catalysts. But it's still a mystery where the energy for this comes from. One would expect any potential difference between areas on the nodules to have become discharged long ago, given the whole thing is immersed in seawater. Something doesn't stack up here. I think we need to see the findings replicated. I wonder if they will discover there is some organism living in these nodules that is responsible, or something. I'm a bit sceptical about the battery idea. 1 Link to comment Share on other sites More sharing options...
studiot Posted July 24 Author Share Posted July 24 27 minutes ago, exchemist said: One thought that occurred to me later about this finding is that the world is badly in need of more efficient electrolysis methods, for green hydrogen production. Research into what is going on here might just yield new insights into options for catalysts. But it's still a mystery where the energy for this comes from. One would expect any potential difference between areas on the nodules to have become discharged long ago, given the whole thing is immersed in seawater. Something doesn't stack up here. I think we need to see the findings replicated. I wonder if they will discover there is some organism living in these nodules that is responsible, or something. I'm a bit sceptical about the battery idea. +1 I didn't see any quantitative evaluation of the rate of oxygen production. Only a general statement that the oxygen in the water was higher than expected. The oxides that make the nodules are semiconductors, not conductors so this will lead to a high internal resistance for any 'battery'. We regard the voltage voltage step across a pn junction as a barrier not an EMF. Voltage / current curves of such devices do not start at zero. Your earlier overvoltage comment applies. So the rate of oxygen production will be low but, of course, geological timescales are available at the bottom of the ocean. Link to comment Share on other sites More sharing options...
Recommended Posts