(269d) CO2 Mineralization Via Produced Water: Effect of Mg2+, Sr2+, and Ba2+ on Calcium Carbonate Polymorphs Precipitated Under Alkaline Conditions in a Semi-Batch Process.

CO₂, a major greenhouse gas, significantly impacts climate change through emissions from both natural and anthropogenic sources. Utilizing CO₂ through catalytic, biological, and mineralization processes offers considerable potential for reducing atmospheric CO₂ levels. Among these methods, CO₂ mineralization is particularly advantageous due to its favorable thermodynamics, enabling spontaneous and permanent sequestration with minimal energy input. Utilizing produced water as the alkaline earth metal source further enhances the process by repurposing industrial wastewater, reducing disposal challenges while simultaneously achieving CO₂ capture and storage. Additionally, CO₂ mineralization via produced water presents an attractive alternative to the energy-intensive production of precipitated calcium carbonate (PCC) commonly used in industry.

Mineralization involves converting CO₂ into stable carbonates through reaction with minerals, such as calcium or magnesium-rich silicates and hydroxides, or alkali and alkaline earth metal cations found in brines from natural or industrial processes. A significant advantage of the carbon mineralization approach is the exothermic nature and high negative Gibbs free energy associated with alkaline earth metal carbonation reactions, potentially resulting in permanent CO₂ sequestration with little energy requirements.

CO₂ mineralization can produce CaCO₃ polymorphs (calcite, aragonite, and vaterite), each with industrial value. Calcite, the most stable polymorph, is vital for building materials and pharmaceuticals, while aragonite and vaterite also hold significant applications. However, previous research on CO₂ mineralization using produced water (PW) has faced challenges related to batch processing and variable metal concentrations^{[1][2][3][4][5]}. This study, supported by the U.S. Department of Energy (FE32144), addresses these challenges by investigating continuous CO₂ mineralization using PW, focusing on the formation of CaCO₃ polymorphs and the influence of aqueous alkaline earth metals (Mg, Sr, and Ba) on their crystallization.

A sparged one-liter continuously stirred reactor (CSTR) system was used to investigate the influence of produced water chemistry, CO₂ introduction, operating temperature, and system pH. To scale up the process, a 200-liter sparged CSTR was employed for calcium carbonate production. Post-trial analyses, including particle characterization, were conducted to compare the properties of the produced CaCO₃ with commercial PCC products. ICP-OES was used in the analysis of cation depletion. Also, XRD/SEM was used to characterize precipitated carbonates to evaluate sequestration efficiency and polymorph selectivity.

Results indicate that, irrespective of brine composition, all precipitated CaCO₃ initially transitions from calcite to mixed phases (aragonite, vaterite, and calcite) when formed at 25°C, with Mg²⁺ presence accelerating this transition. In contrast, at 80°C, only calcite forms. Cation-specific effects were noted, with Sr²⁺ favoring aragonite formation and Ba²⁺ promoting vaterite. Further investigations using calcium-rich field-derived brine (Wolf Fish Well brine) showed that brucite precipitation at 80°C facilitated the necessary pH adjustment. At 25°C, only calcite formed, whereas at 80°C, both aragonite and calcite were present, indicating that high supersaturation may influence polymorph formation. Additionally, calcium carbonates generated from produced water demonstrated comparable properties to commercial PCC products, including particle size and color, while exhibiting improved flowability.

Future research will explore the carbon capture potential of different produced water samples in a multi-batch setup and further examine the impact of brine composition on CaCO₃ polymorph formation.

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