The effect of pH on rates of reaction and hydrogen generation during serpentinization

Our plain-English summary of The effect of pH on rates of reaction and hydrogen generation during serpentinization, by T.M. McCollom, F. Klein, P. Solheid, et al (Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences, 2020).

The study

• Authors: T.M. McCollom, F. Klein, P. Solheid, B. Moskowitz
• Journal: Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences
• Published: 2020;378(2165):20180428
• Cited by: 10 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

A series of three laboratory experiments were conducted to investigate how pH affects reaction pathways and rates during serpentinization. Two experiments were conducted under strongly alkaline conditions using olivine as reactant at 200 and 230°C, and the results were compared with previous studies performed using the same reactants and methods at more neutral pH. For both experiments, higher pH resulted in more rapid serpentinization of the olivine and generation of larger amounts of H2 for comparable reaction times. Proportionally greater amounts of Fe were partitioned into brucite and chrysotile and less into magnetite in the experiments conducted at higher pH. In a third experiment, alkaline fluids were injected into an ongoing experiment containing olivine and orthopyroxene to raise the pH from circumneutral to strongly alkaline conditions. Increasing the pH of the olivine-orthopyroxene experiment resulted in an immediate and steep increase in H2 production, and led to far more extensive reaction of the primary minerals compared to a similar experiment conducted under more neutral conditions. The results suggest that the development of strongly alkaline conditions in actively serpentinizing systems promotes increased rates of reaction and H2 production, enhancing the flux of H2 available to support biological activity in these environments. This article is part of a discussion meeting issue 'Serpentinite in the Earth System'.

In plain English

This laboratory study did not involve people or health. The authors heated powdered minerals in salty water to study serpentinization, which forms serpentine minerals, including chrysotile, and releases hydrogen. Two experiments reacted olivine with strongly alkaline (high-pH) solutions at 200°C and 230°C, compared with earlier lower-pH experiments. At 230°C, about 70 wt% of the olivine reacted, compared with about 25 wt% at lower pH, and 105 micromoles of hydrogen were produced per gram of original olivine. At 200°C, 41 wt% reacted compared with 5.9 wt%, and hydrogen reached 20.9 compared with 5.9 micromoles per gram of original olivine. The main products were chrysotile, which formed thick fibrous accumulations, and brucite. At higher pH, more iron went into these two minerals and proportionally less into magnetite. A third experiment reacted olivine and orthopyroxene together, raising the pH partway through. Hydrogen production then rose steeply, and the estimated overall reaction rate increased more than fivefold. The authors concluded that higher-pH fluids are likely to speed serpentinization and hydrogen release. They said the mechanisms need further study.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory hydrothermal experiments (three new experiments compared with earlier, lower-pH experiments that used the same reactants and methods)
• Who was studied: No people or animals. The materials studied were powdered olivine from mantle xenoliths from San Carlos, Arizona, and orthopyroxene (Opx) from Bamble, Norway. The experiments were Oliv230pH, Oliv200pH and OlivOpx230pH, compared with earlier experiments Oliv230fine, Oliv200fine, OlivOpx230med and OlivOpx230.
• Exposure: The variable studied was the pH (alkalinity) of the reacting fluid. Strongly alkaline salt solutions made with NaOH were used at 200 and 230°C and a pressure of 35 MPa. In the third experiment, alkaline fluid was injected after about 4500 h at circumneutral pH. No human exposure was studied.
• Main results:
  • Olivine at 230°C, high pH (Oliv230pH): a total of 105 µmol of hydrogen per gram of olivine was produced, counted against the original mass of mineral reactant. Hydrogen generation rates were about three times those of the lower-pH comparison experiment (Oliv230fine).
  • Hydrogen amounts throughout the article are expressed per gram of the original mineral reactant, not per gram of mineral that reacted.
  • Olivine at 230°C: about 70 wt% of the olivine reacted in the high-pH experiment, compared with about 25 wt% in the lower-pH experiment (Oliv230fine), even though the lower-pH experiment ran longer.
  • Olivine at 200°C, high pH (Oliv200pH): hydrogen reached 20.9 µmol per gram of olivine at 7677 h. The lower-pH experiment (Oliv200fine) reached 5.9 µmol per gram at 9219 h, less than 30% of the high-pH amount.
  • Olivine at 200°C: about 41 wt% of the olivine reacted at high pH, compared with 5.9 wt% in the lower-pH experiment (Oliv200fine).
  • In the high-pH olivine experiment at 230°C, chrysotile formed thick fibrous accumulations, with brucite present as platy crystals embedded in the chrysotile.
  • In the higher-pH olivine experiments, more iron went into brucite and chrysotile and less into magnetite for each amount of olivine reacted. Magnetite was 1.6 wt% in Oliv230pH compared with 1.75 wt% in Oliv230fine, and 1.0 wt% in Oliv200pH compared with 0.25 wt% in Oliv200fine. Relative to the amount of olivine reacted, Oliv200pH produced substantially less magnetite.
  • Olivine–orthopyroxene mixture at 230°C (OlivOpx230pH): very little hydrogen was produced before the pH was raised and a steep increase followed. By the end, 13.5 µmol of hydrogen had been generated per gram of reactant minerals.
  • In OlivOpx230pH, an estimated 4 wt% of the minerals had reacted before the pH increase and 19 wt% had reacted by 7715 h. The authors read this as a greater than fivefold increase in overall reaction rate.
• The authors' conclusion: The authors concluded that higher-pH fluids circulating through ultramafic rocks are likely to cause more rapid serpentinization and higher hydrogen fluxes than lower-pH fluids. They also concluded that pH affects how iron is distributed among the reaction products.
• Limitations the authors state:
  • The mechanisms that caused the changes in how iron was distributed at higher pH were not determined and require further study.
  • The data were not sufficient to tell whether the pH increase sped up the reaction of olivine and orthopyroxene equally.
  • The trends in iron distribution need further study to be confirmed.

Topics

Medical terms in this study, explained in our Lexicon: Serpentinization, Olivine, Brucite, Chrysotile, Serpentinite.

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