4.2. Vadose conditions
While the buried, vadose experiment wasn’t particularly successful from a mineral carbonation perspective, it demonstrated the critical importance of biology in mineral carbonation, i.e., secondary carbonate was only formed in association with bacteria. The small pockets of secondary carbonate observed within the vadose burial system all possessed fossilised bacteria (Fig. 6D). The formation of secondary calcium sulphate minerals suggesting that these systems possessed little water. While the likely occurrence of gypsum indicates that primary mineral dissolution is occurring, releasing calcium and sulphur into the environment, the system appears to be carbon starved preventing calcium from forming carbonates even though the samples were ‘directly’ exposed to atmospheric CO2 under vadose conditions.
In contrast, the mixed photosynthetic experiment was also incubated under vadose conditions, but the precipitation of sulphates was not observed. This was likely due, in part, to the increased biomass and exopolymeric substances formed under surface conditions allowing for higher water retention, limiting evaporite formation, and the dominance of cyanobacteria, generating alkalinity, which would preferentially form carbonate minerals (Eq. 3).
4.3 Microbial role in weathering and precipitation
Weathering of primary, blue ground mineralogy is a prerequisite for mineral carbonation and, given the enhanced biogeochemical weathering observed in this study, the activity of the microorganisms within the system must play a role in the breakdown of the kimberlite and geochemical cycling. While kimberlite can be extremely heterogenous across a single pipe (Field et al., 2008; Mervine et al., 2018), the weathered consistency of yellow ground, relative to the background soil matrix, in near-surface kimberlites suggest that weathering of these materials just like soil formation must provide some nutritional benefit to promote microbially enhance weathering, e.g., the TOC at T = 0. This microbial factor (bacterial metabolism) facilitated by their need for nutrients accelerated weathering of primary minerals releasing inorganic and presumably organic materials into solution, forming secondary mineral products.
Microbial carbonate precipitation occurred in both the photic columns and those deprived of light, indicating that reactions other than those driven by photosynthesis may also contribute to weathering and mineral carbonation. For example, the microenvironments dominated by Ca-sulphate mineralogy suggests the importance of sulphate reducing bacteria (though none observed in the most abundant OTU’s presented in Fig. 4) in producing carbonate mineralogy via the reduction of sulphate to hydrogen sulphide:
\({{C}_{2}{H}_{4}O}_{2}+ {SO}_{4}^{2-} \to {H}_{2}S+{2HCO}_{3}^{-}\) | Eq. 6 |
Microbial sulphate reduction in anaerobic environments produces chemistry that can trend toward carbonate precipitation. This has been observed in modern marine stromatolites and cyanobacterial mats, where carbonate laminae precipitate in zones of active sulphate reduction (Krumbein and Cohen, 1977; Lyons et al., 1984; Visscher et al., 2000). The level of saturation within the environment evidently plays a large role in in the carbonation efficiency of CRD once it becomes buried with subsequent material – with the higher saturation leading to an increase in secondary carbonate precipitation. The saturated environment (Fig. 7) did not result in the occurrence of gypsum evaporites at 1 year incubation, possibly supporting carbon sequestration / mineral carbonation by dissimilatory sulphate reduction preventing gypsum formation.
The growth of the Venetia Consortia in the lab significantly shifted the population away from the originally sampled microbes, and towards that seen in the T = 0 CRD material that was used in the bioreactor to apply environmental pressure and aid in biomass growth. The resulting Lab. Consortia that was used in the column experiments contained microbes present in both the Venetia consortia and the CRD material, along with microbes not prominent in either.
The bacterial enrichment culture and the corresponding biogeochemical weathering has initiated an early stage in yellow ground formation, beginning the transformation of the unweathered CRD blue ground kimberlite into a soil.
4.4 Biotechnology: the Venetia CRD deposits as a microbially driven carbon sink.
The Venetia mine produces 4.74 Mt of residue (treated ore) per year, a combination of both 40% CRD and 60% Fine Residue Deposit (FRD) (Mervine., et al, 2018). The biogeochemical conditions occurring in CRD, i.e., crushed kimberlite, will vary macroscopically across layers and microscopically based on water saturation and oxygen ‘penetration’ - affecting biogeochemical weathering. The surficial and burial phases of the CRD depositional cycle differ in environmental conditions, though remarkably, each process resulted a 34% and a 27% annual offset, respectively (based on total processed kimberlite).
The mixed inoculum photosynthetic treatment of the CRD produced the highest autotrophic CO2 fixation and carbonate precipitation, totalling 34% of annual CO2e, which corresponds to the precipitation of 178,145 tons of calcium carbonate per year, if the total processed kimberlite could achieve the same level of mineral carbonation. Similarly, if the measured burial reactivity, corresponding to 142,081 tons calcium carbonate in one year, can continue beyond the 1-year incubation employed in this study, then much higher mineral carbonation levels can ultimately be reached.
The Venetia CRD is a vast deposit of processed material, which if treated under conditions for biologically accelerated weathering, i.e., mineral dissolution and mineral carbonation, can act as a large-scale carbon sink requiring very little intervention, i.e., simply requiring the growth of photosynthetic biofilm in this ambient, sunny climate, followed by inoculation of CRD tailings. The inoculation of kimberlite via mixing is recommended, which would see the bacteria dispersed through the material, exposing the highest amount of materials to the microbes. This higher degree of microbe-mineral interaction increases the potential for biogeochemical weathering and enhanced carbonation (Table 1). A suggested site for inoculation of crushed kimberlite material would be on the conveyor belt whilst being transported for deposition.
While photosynthetically-driven near surface biogeochemical carbonation reactions were optimal in the laboratory-scale experiment (Fig. 2), it is impractical to expose this degree of mine waste at a decimetre depth. Given the massive amount of waste materials produced by diamond mines (Mervine et a., 2018), burial in CRD piles will represent the more common occurrence of these materials. Under burial conditions, the saturated environment proved to be far more conducive to the precipitation of secondary carbonate material than the vadose environment. However, the reduced biogenic carbonation observed in the dark, vadose condition may simply require more time to achieve optimal (complete) mineral carbonation.