Accelerated carbonate biomineralisation of Venetia diamond mine coarse residue deposit (CRD) material-A field trial study

Field trials combining mined kimberlite material (Coarse Residue Deposit; CRD) and mine derived microbes show ac-celerated kimberlite weathering at surface conditions - a potential method for accelerated carbon sequestration via mineral bio-carbonation. A photosynthetic biofilm suspension (20L), sourced from the Venetia diamond mine (Limpopo, South Africa) pit wall, was cultured in 3 x 1000 L bioreactors using BG-11 medium. Bioreactors supple-mented with Fine Residue Deposit (FRD) kimberlite material enhanced microbial growth and kimberlite weathering. This (ca. 1.44 kg) wet weight bio-amendment corresponded to ca. 1.5 x 109 Acidithiobacillus spp. sized bacteria/g CRD (20 kg FRD growth supplement +60 kg FRD used for harvesting biomass +850 kg CRD used in the field trial exper-iment). This bio-amendment promoted carbonate precipitation and subsequent cementation under surface conditions (0-20 cm). Microbial inoculation accelerated pedogenesis of CRD materials. A soil-like substrate resulted from weathering under environmental conditions in Johannesburg from January 2020 to April 2021. Over this 15-month experiment, the biodiversity found in the inoculum shifted due to the selective pressure of the kimberlite. The natural, endogenous biosphere, when combined with the inoculum, accelerated carbonate precipitation in the upper 20 cm of the bioreactor by between +1 wt% and + 2 wt%. Conversely, carbonation of the bioreactor at depth (20-40 cm) de-creased by ca. 1 wt%. All the secondary carbonate observed in the bioreactors was biogenic in nature, i.e., possessing microbial fossils. This secondary carbonate took the form of both radiating acicular crystals as well as colloform inter-granular cements. This microbial inoculum and resulting geochemical changes promoted the transformation of kim-berlite into a Technosol, capable of supporting the germination and growth of self-seeding, windblown grasses, which enhanced weathering in the rhizosphere. The maximum secondary carbonate production is consistent with a ca. 20 % mine site CO2e offset.

Automated summary

Field trials combining mined kimberlite material and mine derived microbes showed accelerated kimberlite weathering, potentially contributing to carbon sequestration. The addition of Fine Residue Deposit (FRD) kimberlite material enhanced microbial growth and weathering. The microbial inoculum and resulting geochemical changes promoted the transformation of kimberlite into a soil-like substrate capable of supporting plant growth.