Practical and thermodynamic constraints on electromicrobially accelerated CO2 mineralization

Abstract

By the end of the century, tens of gigatonnes of CO₂ will need to be removed from the atmosphere every year to maintain global temperatures. Natural weathering of ultramafic rocks and subsequent mineralization reactions can convert CO₂ into ultra-stable carbonates. Although this will draw down all excess CO₂, it will take thousands of years. CO₂ mineralization could be accelerated by weathering ultramafic rocks with biodegradable lixiviants. We show that if these lixiviants come from cellulosic biomass, this demand could monopolize the world's biomass supply. We demonstrate that electromicrobial production technologies (EMP) that combine renewable electricity and microbial metabolism could produce lixiviants for as little as $200 to $400 per tonne at solar electricity prices achievable within the decade. We demonstrate that EMP could make enough lixiviants to sequester a tonne of CO₂ for less than $100. This work highlights the potential of this approach and the need for extensive R&D.

Keywords: Biotechnology; Energy sustainability; Engineering; Microbiology.

Graphical abstract

Figure 1

Overview of electromicrobially accelerated CO₂ mineralization process Key parameters in this article are highlighted in this figure, Figure 2, and Tables 1 and 2.

Figure 2

Schematic of the electromicrobial production of lixiviants for CO₂ mineralization (A) Single bio-electrochemical cell system where electricity is used to power in vivo CO₂ - and subsequent lixiviant synthesis. (B) Dual electrochemical cell system where CO₂ is reduced in the first cell, and then assimilated in the second cell to produce lixiviant molecules. (C) Long-range e⁻ transfer mechanisms considered in this article. In the first, H₂ is electrochemically reduced on a cathode, transferred to the microbe by diffusion or stirring, and enzymatically oxidized. In the second mechanism, extracellular electron uptake (EEU), e⁻ are transferred along a microbial nanowire (part of a conductive biofilm), or by a reduced medium potential redox shuttle such as a quinone or flavin, and are then oxidized at the cell surface by the extracellular electron transfer (EET) complex. From the thermodynamic perspective considered in this article, these mechanisms are equivalent. Electrons are then transported to the inner membrane where reverse electron transport is used to regenerate NAD(P)H, reduced Ferredoxin (not shown), and ATP (Rowe et al., 2018, 2021). Parameters for these systems are shown in Table 2.

Figure 3

Accelerated mineralization could require hundreds of millions to tens of billions of tonnes of lixiviants per year If these lixiviants were produced from cellulosic biomass, this could put a significant strain on the world agricultural system. We calculated the mass of lixiviant (M_lix) needed to accelerate the forsterite dissolution step of the mineralization of 20 GtCO₂ per year using Equation 10 as a function of the inverse CO₂ mineralization performance, ζ, the combination of the most uncertain parameters in our estimate of lixiviant mass. We chose to display results for gluconic acid as it has the highest molecular weight and provides an upper bound on the lixiviant mass requirement. Our most optimistic estimate for ζ (ζ₁) is shown as the left most vertical line on the plot. The second marked value of ζ (ζ₂) corresponds to a mass of lixiviant equal to all of the cellulosic biomass produced in the United States in a year. The third, fourth, and fifth lines (ζ₃ to ζ₅) correspond to increasing biomass withdrawals from the biosphere that come with increasingly severe consequences for agriculture and human society including the adoption of vegetarian diets, population control and widespread managed agriculture and forestry (Slade et al., 2014). The sixth (ζ₆) and final line corresponds to the biomass production of the entire world in a year (net primary productivity). This plot can be reproduced with the nlixiviant.py code in the ElectroCO2 repository (Barstow, 2021).

Figure 4

Electromicrobial production technology could reduce the electrical energy costs of lixiviant production to a few tens of kilojoules per gram (A–D) Energy and financial costs for producing four lixiviant molecules are shown in each panel: (A) acetic acid, (B) citric acid, (C) 2,5-diketo-gluconic acid (DKG), and (D) gluconic acid. The electrical energy cost of producing a gram of each lixiviant is shown on left-hand side y axis for each sub-plot. The dollar cost of producing a tonne of the lixiviant using electricity supplied by solar photovoltaics at a cost of 3¢ per kWh (the US Department of Energy’s cost target for solar electricity for 2030 (SunShot 2030, 2016)). This plot can be reproduced using the efficiency.py code in the ElectroCO2 repository (Barstow, 2021). The upper error bars correspond to ΔU_membrane = 240 mV, lower bars to 80 mV, and the center to 140 mV.

Figure 5

Electromicrobial production technology could enable the production of enough lixiviants to sequester 1 tonne of CO₂ for less than $100 We combined our lixiviant mass requirements from Figure 3, with our estimates for the energy and financial cost of producing a tonne of each lixiviant compound with H₂-mediated EMP using CO₂-fixation with the Calvin cycle (basically the Bionic Leaf configuration (Liu et al., 2016; Torella et al., 2015)) from Figure 4. For illustrative purposes, we have marked the values of the inverse CO₂ mineralization performance (ζ₁ to ζ₆) highlighted in Figure 3, and the corresponding cost to sequester a tonne of CO₂ as an intersecting horizontal line. However, it is important to note that in this case, no cellulosic biomass is produced. This plot can be reproduced using the clixiviant.py code in the electroCO2 repository (Barstow, 2021).