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This study investigated heterotrophic bioleaching of rare earth elements (REEs) and base metals from spent nickel-metal-hydride (NiMH) batteries. Furthermore, the impacts of phosphorous source [Ca 3 (PO 4 ) 2 , KH 2 PO 4  and K 2 HPO 4 ] and its concentration on organic acid production by  Gluconobacter oxydans  and  Streptomyces pilosus  were evaluated. Phosphorous source affected microbial acid production and metal leaching. Among the studied phosphorous sources, use of K 2 HPO 4  resulted in highest organic acid production by both bacteria. Increasing K 2 HPO 4  concentration from 2.7 to 27 mM enhanced pyruvic acid production by  S. pilosus  from 2.2 to 10.7 mM. However, no metal was leached from the spent NiMH batteries with  S. pilosus  using either one-step, two-step or spent-medium bioleaching. With  G. oxydans , highest gluconic acid concentration of 45.0 mM was produced at the lowest K 2 HPO 4  concentration of 2.7 mM. When using two-step bioleaching with  G. oxydans , higher leaching efficiencies were obtained for base metals (88.0% vs. 68.0% Fe, 41.5% vs. 35.5% Co, 18.5% vs 16.5% Ni), while more REEs were leached using spent-medium bioleaching (9.0% vs. 6.0% total REEs). With both bioleaching methods, base metals leaching was faster than that of REEs. Surplus of phosphorous should be avoided in bioleaching cultures as precipitation especially with REEs is possible. Graphic Abstract

waste and biomass valorization

Impacts of Phosphorous Source on Organic Acid Production and Heterotrophic Bioleaching of Rare Earth Elements and Base Metals from Spent Nickel-Metal-Hydride Batteries