Science of The Total Environment
Volume 575
<>, 1
January 2017, Pages 536-544
Enhanced weathering of olivine in seawater: The efficiency as revealed by
thermodynamic scenario analysis
Author links open overlay panelJasperGriffioen
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Measures may be needed to lower the atmospheric CO2level and combat climate
change as well as ocean acidification

Enhanced weathering of olivine in marine environment is such a measure

The efficiency of olivine weathering depends on the Mg/Fe fractions of
olivine and precipitation of secondary minerals or not

For Fe-rich olivine, CO2 may be released to the atmosphere and ocean pH
might decrease during oxic olivine weathering

Enhanced weathering of olivine has been suggested as a measure to lower the
atmospheric CO2 level and it might also mitigate ocean acidification. This
study aimed to characterise how olivine can weather in seawater, to
elucidate the role of secondary precipitation and to ascertain the
efficiency in terms of molar CO2 removal per mole of olivine dissolution.
Geochemical thermodynamic equilibrium modelling was used, which considered
both the variable mineralogical composition of olivine and the kinds of
secondary precipitates that may be formed. The advantage is that such an
approach is independent from local or regional factors as temperature,
related kinetics, mineralogy, etc. The results show that the efficiency
falls when secondary precipitates are formed. When Fe-bearing olivine
undergoes weathering in an oxic environment, Fe(III) hydroxides will
inevitably be formed, and as a result of this acidifying process, CO2 could
be released to the atmosphere. This might also enhance ocean acidification
when Fe-rich olivine becomes used. Ocean alkalinisation only happens when
more than 1 mol/kgH2O Mg-rich olivine weathers. Maintenance of
supersaturation for calcite or aragonite as holds in seawater reduces the
efficiency by about a factor of two compared to the efficiency without
secondary precipitation. Precipitation of sepiolite as Mg silicate reduces
the efficiency even more. Magnesite precipitation has a similar effect to
Ca carbonate precipitation, but according to the literature magnesite
precipitation is improbable at ambient conditions and relatively low
supersaturation. When less than 0.05 mmol olivine/kg(seawater) weathers the
efficiency is slightly different than at higher intensities, due to strong
buffering by seawater alkalinity.

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