Converting industrial and agricultural waste streams into carbon-storing oxalate minerals

Carbon capture and storage has emerged as a crucial strategy for mitigating global warming. Ex situ mineralization, which converts CO₂ into stable carbonate minerals aboveground, promises to store carbon safely for geological timescales. However, this approach typically requires mining and processing rocks that contain calcium and magnesium, making it both costly and environmentally damaging.

To reduce quarrying demands, industrial waste streams, such as gypsum and coal fly ash, have been proposed as alternative feedstocks. While studies have demonstrated the feasibility of converting these waste materials into carbonate minerals through reaction with CO₂, the process is limited by stoichiometry: each cation can sequester only one carbon atom. However, the efficiency of carbon mineralization could be doubled by instead forming oxalate minerals, such as glushinskite (MgC₂O₄·2H₂O) and whewellite (CaC₂O₄·H₂O).

These minerals form through reactions between Ca and Mg-bearing phases and oxalic acid (H₂C₂O₄). While oxalic acid is typically produced through expensive electrochemical CO₂ reduction, we propose sourcing it directly from plants. Oxalate occurs naturally in nearly 80% of plant families, comprising up to 80% of dry weight in some species. Since plants produce oxalate through photosynthetic CO₂ fixation, this represents a net atmospheric carbon removal pathway.

Our study demonstrates a proof-of-concept that oxalic acid can be extracted from agricultural waste and reacted with industrial waste to mineralize carbon. Through experiments that react gypsum and fly ash with oxalic acid extracted from two common plants, we quantify Ca oxalate formation efficiency and estimate the carbon storage potential. Our findings have significant implications for integrating waste management with carbon removal and storage at a global scale.