Bipolar membrane crystallization enables near zero-waste production of high-purity oxalic acid crystals

The sodium formate method is currently the predominant route for oxalic acid production but is environmentally harmful. Water splitting in bipolar membranes provides the possibility for clean production of oxalic acid. However, bipolar membrane electrodialysis(BMED) also suffers from the major drawbacks of low concentrations and low purity products. Herein, we propose the use of bipolar membrane crystallization (BMC) to achieve synergistic conversion and crystallization of oxalic acid dihydrate. The dissolution of oxalate in aqueous solution and the electromigration of oxalate through the membrane were precisely tuned under different current densities. The consecutive electromigration of oxalate could boost the dissolution of oxalate. The morphology of the H2C2O4·H2O crystals is related to the current density: a plate-like shape is obtained at 5 mA/cm2, while a needle-shaped crystal is obtained at 10 mA/cm2. The intermittent crystallization of five batches of BMC demonstrated the robustness of bipolar membrane crystallization for the near zero-waste production of oxalic acid.