Effect of pre-treatment and direct contact on rare earth element bioleaching from former Pea Ridge mine tailings by phosphate-solubilizing microbes.

Rare earth elements (REEs) are critical materials essential to modern energy, electronic, and defense technologies, yet their supply and extraction is environmentally intensive and geopolitically complex. Biohydrometallurgy using phosphate-solubilizing microorganisms (PSMs) offers a promising low-impact mineral processing scheme, though mechanistic understanding of microbe-ore interactions remains limited. Five PSM strains were used to examine REE bioleaching from monazite-rich mine tailings obtained from the Pea Ridge deposit in Missouri, USA: Gluconobacter oxydans, Paraburkholderia fungorum, Burkholderia vietnamiensis, Paraburkholderia tropica, and Ralstonia picketti. The performance of bioleaching was assessed utilizing both calcined and uncalcined tailings in contact and non-contact (spent-medium) treatment conditions with mine tailings as the sole phosphate source (i.e., there were no soluble phosphate amendments). Contact bioleaching and calcination significantly enhanced REE mobilization relative to non-contact systems and no pretreatment. Surprisingly, calcination resulted in lower leaching efficiency for Nd and Dy, despite solubilizing much higher total amounts of Ce and La. Of the strains tested, G. oxydans exhibited the highest overall leaching efficiency of 15 % of the initial REEs, while P. fungorum showed strong biofilm-mediated dissolution following calcination. Overall, pH trends, scanning electron microscopy (SEM) analysis, and REE fractionation patterns are consistent with a mechanism in which direct microbial attachment, sustained metabolic activity, and phosphate assimilation synergistically enhance REE release, while limiting secondary precipitation. These findings demonstrate that combining calcination pre-treatment with direct microbial contact enhances rare earth element leaching from phosphate-mineral mine tailings.