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Contributors: Paul Reginato1, Pritha Ghosh2, Nathan G. Walworth3, Martin Van Den Berghe4
Lead contact: Paul Reginato ([email protected])
Motivating Factor
Atmospheric carbon dioxide removal and point-source carbon capture technologies are well-accepted as being necessary for meeting climate goals [1]. Weathering of alkaline minerals in the environment naturally generates alkalinity that draws ~0.3 GtCO2/yr from the atmosphere and converts it to solid carbonates or (bi)carbonates which are transported to the ocean and stably stored [2][3]. Enhanced rock weathering (ERW) technology seeks to accelerate alkaline mineral dissolution for carbon storage by grinding minerals to increase reactive surface area and exposing them to weathering conditions [4][5]. A core challenge of ERW is cost-effectively increasing mineral dissolution kinetics to enable scaling [6].
Specific Constraint
Microbes can accelerate mineral weathering [7][8][9][10], for example through chelation by siderophores [11], chelation by organic acids [12][13], oxidoreductive chemolithotrophy [14][15], or prevention of surface passivation [16][17]. ERW in bioreactor systems (reactor-based bio-ERW) has been proposed, wherein microbes or their exudates would accelerate alkaline mineral dissolution to absorb and store concentrated CO2 streams [18][19][20], possibly in concert with valuable metal recovery [18][21]. However, despite these conceptual propositions, we are not aware of specific designs having been proposed in public literature. It is therefore difficult to assess the feasibility of reactor-based bio-ERW, and engineering efforts lack targets.
Actionable Goals
Designs for reactor-based bio-ERW should be proposed. Designs should account for process design and proposed feasible microbial weathering mechanisms, even if specific microbial communities are not proposed. Designs should be accompanied by simple life-cycle analyses (LCA) and techno-economic analyses (TEA) that identify drivers of cost and supply-chain sustainability, such as CapEx; weathering rate; bio-feedstocks of carbon, nutrients, and energy; water; and energy for mineral grinding. As an example energy feedstock, Marecos et. al propose electromicrobial production of mineral-dissolving biomolecules [20].
The most actionable analyses would estimate benchmarks for biological parameters required for scaling, such as the necessary enhancement over abiotic weathering rates and microbial carbon/nutrient/energy-efficiency, and identify scale-dependence of unit economics. Attention should be given to the possibility of fixing nitrogen by biological means and/or deriving energy from photosynthesis or chemolithotrophy, thereby reducing input costs. Such analyses would help assess scaling feasibility, identify optimal scale for unit economics, provide targets for engineers, and guide identification of candidate microbial communities [22].
Open Questions
The goals would be clarified by providing a proposed list of microbial communities or community functions with estimates of feasible weathering efficiencies, which would constrain or parameterize possible designs [22].
Related problem statements
Propose and assess microbial community functions for reactor-based bio-enhanced weathering [22]
Systematically characterize microbially-enhanced mineral weathering [23]
Platforms for high-throughput characterization of biologically-enhanced mineral weathering [24]
Other information and comments
The goals of this problem statement are also relevant to improving foundational understanding for biomining.
Some economic and life-cycle assessment resources that may be useful for reference include refs [25][26][27][28]
Efforts to scale bio-ERW reactors are likely to run into the challenge of heterogeneous microenvironments that form in large bioreactors where homogeneous conditions would be ideal, which is a challenge common to scaling microbial processes that leverage specific functions. This will introduce a design challenge. Solutions may draw inspiration from similar challenges in wastewater treatment or biogas digesters.
Microbially-catalyzed acid generation, such as sulfide oxidation by Acidithiobacillus spp., can enhance dissolution of alkaline minerals [29][30]. However, due to the introduction of acidity from a non-CO2 source, the alkalinity generated for CO2 absorption through such methods is limited.