Residue alkalinity limits microbial leaching of rare earth elements from industrial wastes

Microorganisms that make acids can recover rare earth elements from industrial residues such as red mud, coal fly ash, phosphogypsum and electronic waste. Most published studies test one organism on one residue, so their results are hard to transfer. Here we compiled 17 bioleaching studies, covering 10 residues, 5 groups of organisms and 217 runs. We described them all with one physical model, BioLixa. The model links acid production by the organism, the charge balance of the solution and the alkalinity of the residue. From the resulting pH it computes release of the metal from three hosts in the solid and its loss to oxalate precipitation and hydrolysis. A single parameter set, shared by organism group and by residue, was fitted to 13 studies. It reproduced 552 recovery measurements with a mean absolute error of 2.3 percentage points (R² = 0.97) and 811 pH measurements with a mean error of 0.54 pH units. The fitted model shows that the alkalinity of the residue often decides whether bioleaching works. Leaching proceeds only when the acid made by the organism exceeds the base capacity of the residue. For red mud, a fungus succeeds only below about 3 % solids. The model also gives, for each residue, the threshold acid dose and the share of rare earths that any acid can reach. For forecasting, BioLixa was the most accurate of eight methods when only the conditions of a run were known, in a development rehearsal and in four held-out studies. Once early recovery points were known, assimilating the early pH as well cut its rehearsal error by 30 % and placed it ahead of all data-driven regressors, although simply holding the last measured value remained hard to beat and the success criterion of the held-out test was not met. BioLixa makes published bioleaching results comparable, explains why runs fail, and gives parameters for designing microbial recovery of rare earths from residues.