Biogenic pathways for rare-earth recovery from coal fly ash via fungal mineralization

Rare earth elements (REEs) are essential for modern technologies, yet their supply is limited, and current extraction methods rely on strong chemicals, energy-intensive processing, and produce residual chemicals that are difficult to dispose. Large amount of Coal Fly Ash (CFA) are produced annually, which contains small amounts of REEs which are locked within stable glass and mineral phases of CFA that are difficult to dissolve. This study explores an integrated biological process that uses the fungi Aspergillus niger to produce bio-acid to release REEs from fly ash and subsequently uses fibrous fungi mycelium to absorb the REEs from leachate. Experimental results showed that the fungus produced natural organic acids such as citric and oxalic acid, which are effective in breaking down phosphate and oxide phases in the coal fly ash and released REEs into solution. The results showed that high performance of fungi bio-acids in REEs extraction from CFA, achieving the efficiencies of Nd (44.3%), Gd (43.3%), Er (43.4%), and U (40.5%). Compared with conventional inorganic acid, the bio-acids produced by A. niger exhibited substantially higher leaching efficiencies, achieving at least 5–9 times higher in leaching Nd, Gd, Er, and U than industry inorganic acid. Meanwhile, the fungal biomass was found to be very efficient in capturing the released REEs from the leachate solution. These demonstrate that extraction and recovery can be achieved using a single biological system. Compared with chemical leaching, the fungal process required no harsh reagents, operated at low energy input, and produced minimal secondary waste. The results show that biogenic acids and fungal mycelium biomass can effectively mobilize and collect REEs from a CFA, offering a promising pathway for high efficiency and sustainable recovery of critical minerals.