Bioextraction of residual phosphorus from phosphogypsum by phosphate-solubilizing fungus Aspergillus niger

Phosphogypsum (PG) is a typical solid waste formed during wet-process phosphoric acid production, and it retains a significant amount of residual phosphorus (P). Bioextraction of phosphorus (BEP) applies microorganisms to dissolve P from phosphate minerals. This study aimed to evaluate the influence of gypsum (CaSO4) on the BEP efficiency of PG by Aspergillus niger. After 6 d incubation with the addition of HPG (high dose of PG, 1.0 g), the biomass and respiration of A. niger reached 0.85 g and 8,937 μg C/kg/h, respectively. Meanwhile, the BEP efficiency of A. niger reached ~40% after 15 d of incubation, compared with ~10% in the treatment without the fungus. Given the amount consumed by A. niger, the efficiency should be significantly higher. Moreover, nanoscale secondary ion mass spectrometry (NanoSIMS) imaging showed that P was absorbed by A. niger cells to meet their nutritional requirements. Simulation from Geochemist's® Workbench (GWB) revealed that PO43− tended to combine with Ca2+ and mineralize into hydroxylapatite as pH increased. However, oxalic acid secreted by A. niger would combine with the abundant Ca2+ in PG to produce CaC2O4, thereby reducing the fixation of P by free Ca2+. Furthermore, the SO42− from PG participated in the biosynthesis of sulfur-containing amino acids within the fungal cells. This study revealed the potential of BEP by A. niger for the treatment of solid waste PG.