Impact of cyanobacteria on zinc and carbon sequestration in hydrous magnesium carbonates

Mineral carbonation has the potential to permanently sequester CO₂ in stable carbonate minerals, mimicking the natural precipitation of Ca- or Mg-rich carbonates. These minerals are found in ultramafic rocks and mine wastes, which also contain metals that are crucial resources for renewable energy technologies. This study aims to better characterize the mechanisms and quantify the potential of cyanobacterium (Gloeocapsa sp.) for hydrous Mg carbonate (HMC) precipitation sequestering both CO₂ and heavy metals such as Zn. The results show that Zn removal mechanisms depend on solution pH and bacterial biomass, primarily involving the co-precipitation of Zn with newly formed HMC. Under abiotic conditions, up to 100 % of the initial dissolved Zn (at 1 mg L^-1) can be co-precipitated with HMC after 7 days at pH 9.5 to 11. During cyanobacterial growth, up to 87 % of the initial Zn (at 5 mg L^-1) can be removed from the aqueous solution through incorporation into live cells and co-precipitation with biotically induced HMC over 8 days of growth. However, at higher concentrations (>5 mg L^-1), Zn becomes toxic to cyanobacteria, inhibiting their development. Additionally, sizable fraction of aqueous Zn (up to 40 %) was removed through adsorption onto cell surfaces and incorporation in cyanobacterial biomass. Overall, this study demonstrates that cyanobacterially-induced HMC precipitation can efficiently sequester both Zn and inorganic carbon, and thus atmospheric CO₂, from aqueous solutions, in the context of ultramafic waste remediation.