Towards a better understanding of nickel mobilization and phase composition changes during ex situ mineral carbonation of serpentinized peridotites

Peridotites and serpentinites (ultramafic rocks) serve as natural reservoirs of Mg²⁺, which can react with carbonate ions during mineral carbonation to form Mg-carbonates and permanently sequester atmospheric CO₂. Whole rock analyses of a large number of ultramafic rocks from various environments show that Ni concentrations in these rocks can reach 10.000 mg/kg^[1]. In contrast, mid-ocean ridge basalts (MORB), also recognized as a promising feedstock for mineral carbonation, have an average Ni content of 200 mg/kg^[2,3].

The fate of Ni during ex situ mineral carbonation is still poorly understood. This issue is critical, as the large-scale application of mineral carbonization may pose ecotoxicological risks by mobilizing specific metallic elements naturally occurring in ultramafic rocks. To elucidate possible Ni mobility during ex situ mineral carbonation, 15 grams of powdered serpentinized peridotite was carbonated in a batch-type reactor for 96 hours at 185°C and a PCO₂ of 100 bar. The experiment resulted in the dissolution of the forsterite and the extensive crystallization of magnesite, demonstrating that the serpentinized peridotite is a highly effective natural material for permanent CO₂ storage in ex situ carbonation processes. Ni released during the dissolution of forsterite was mainly incorporated in newly formed Ni-rich phyllosilicates (more than 98%) and a small portion was mobilized into carbonating fluid (less than 2 %), reaching a concentration of ~18 mg/kg after 96 hours.

We thus recommend monitoring the formation of potential Ni-rich phases during carbonation as well as the concentration of Ni in the carbonating fluids, particularly in future large-scale mineral carbonation projects using ultramafic rocks. Experimental results indicate that both CO₂ sequestration and the synthesis of Ni-rich phyllosilicates can be achieved through ex situ mineral carbonation. Further work is needed to evaluate the stability of the newly formed phases and to assess their long-term potential for nickel immobilisation.

^[1]Kierczak, J., Pietranik, A., & Pędziwiatr, A. (2021). Ultramafic geoecosystems as a natural source of Ni, Cr, and Co to the environment: A review. Science of The Total Environment, 755, 142620.

^[2]Snæbjörnsdóttir, S. Ó., Sigfússon, B., Marieni, C., Goldberg, D., Gislason, S. R., & Oelkers, E. H. (2020). Carbon dioxide storage through mineral carbonation. Nature Reviews Earth & Environment, 1(2), 90-102.

^[3]McDonough, W. F., & Sun, S. S. (1995). The composition of the Earth. Chemical geology, 120 (3-4), 223-253.