Anaerobic bioremediation of acid phosphogypsum stacks leachates: Assessment of leachate's biochemical changes and microbial community dynamics

Phosphogypsum (PG) is the largest by-product of the phosphate industry. Nearly 300 M tons of PG are annually discarded, of which 58% is dry stacked. The exposure of PG stacks to rainwater and processed waters moisture creates hydraulic pressure, generating acidic PG leachates into nearby aquifer systems. Because of its elevated levels of sulfates, phosphorus (P) and metals, the PG water leachate can negatively affect the surrounding environments. This work investigates the effect of water activity on the PG leaching, by studying the effect of different PG:Water ratios on the leaching process, followed by the anaerobic bioremediation of PG leachate through biological sulfate removal activity (BSRA). This later involves using sulfate-reducing bacteria consortium within an anaerobic bioreactor, allowing a simultaneous monitoring of the leachate's biochemical changes, impurities removal, and microbial community dynamics. The results determined the highest sulfates and impurities leaching from PG with a PG:Water ratio of 1:200 (w:v). Subsequently, the biological treatment of the leachate exhibited an efficient removal of sulfates (79%), P (99%) and chemical oxygen demand (93%), with a substantial decline in metal concentrations (Cd, As and Al by 99%, and Zn by 70%) from the leachate. Moreover, the acidity of the leachate was also neutralized through the BSRA process by increasing the pH from 4 to 7.52. Furthermore, the microbial community dynamics unveiled a significant correlation between the leachate's biochemical changes and co-existence of specific sulfate-reducing bacteria within various bacterial phyla, including Desulfobacterota, Firmicutes, and Proteobacteria, allowing efficient and eco-friendly bioremediation process of PG leachate.