Enhancing the Sustainability of Phosphogypsum Recycling by Integrating Electrodialysis with Bipolar Membranes

Phosphogypsum-a byproduct of phosphoric acid production-piled near eco-sensitive regions, poses both environmental and health risks. By reacting phosphogypsum with NaOH, Ca(OH)₂ is produced, which can be utilized for CO₂ sequestration. The cost of NaOH, however, and the low market value of the byproduct, a concentrated Na₂SO₄ stream, hinder the economic viability of this approach. Here, we propose to increase the cost-efficiency of phosphogypsum processing by converting the Na₂SO₄ to NaOH and H₂SO₄ using bipolar membrane electrodialysis (BMED). The result is a cleaner and more circular process, as NaOH can be recycled for dissolving phosphogypsum, while H₂SO₄ can be reintroduced into the industrial phosphoric acid production or used to leach valuable metals out of the phosphogypsum. To assess the feasibility of BMED integration, we used a bench-scale system to probe the trade-off between obtaining high base-product concentration and obtaining high specific energy consumption. This was done for synthetic Na₂SO₄ solutions at various concentrations, applied voltages, and types of anion-exchange membranes. Economic analysis based on the results showed that a two-step batch BMED process is able to reduce the chemical cost by 50%, as compared to purchasing NaOH. Integrating BMED with existing approaches can, therefore, improve the sustainability and cost-efficiency of phosphogypsum valorization.