Cadmium immobilization in aqueous solution by Aspergillus niger and geological fluorapatite

This study investigated the application of fungus Aspergillus niger and geological fluorapatite (FAp) to cadmium (Cd) immobilization in aqueous solution. The initial Cd concentrations were set at 100, 50, 25, and 10 mg L^-1. The mineralogy of the products was investigated by using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and attenuated total reflection-infrared spectroscopy (ATR-IR). In both A. niger + FAp + Cd and A. niger + Cd treatments, A. niger secreted abundant oxalic acid, then dissolved the FAp, and reacted with Cd²⁺ cations to produce relatively insoluble Cd oxalate. Meanwhile, FAp can provide P source to improve microbial growth. The fungal tolerance to Cd²⁺ was identified at around 100 mg L^-1. The final Cd concentrations of 13.7, 3.2, and 0.2 mg L^-1 were recorded for A. niger + FAp + Cd treatments with initial Cd concentrations of 50, 25, and 10 mg L^-1 respectively. Meanwhile, it was observed that the Cd concentration at 25 mg L^-1 stimulated higher bioactivities of A. niger, which further enhanced Cd bioremediation. The immobilization efficiency (%) of the treatments at low to medium Cd concentrations was in the order: Asp + FAp > Asp > FAp, while FAp alone was most efficient at the high Cd concentration of 100 mg L^-1. This research provides insights into the mechanisms of combining fungus and FAp as a composite to Cd contamination at various Cd levels.