The Clay-SRB (sulfate-reducing bacteria) system: Dissolution and fractionation of REY

Rare earth elements (REYs) originate from the weathering of parent granite, whose clay-sized fractions are pivotal in the regolith-hosted rare earth elements (REEs) deposits. Regarding microbial action on REY mobilization and fractionation, their patterns remain unclear. Chemical extraction and bio-leaching experiments utilizing sulfate-reducing bacteria (SRB) were performed to exemplify the chemical and microbial effects on REY mobilization among the clay-sized phases. Our results indicate that the REYs occur primarily in the three fractions: i.e., amorphous FeMn phase, crystalline Fe phase, and carbonate in chemical reactors wherein the mineral phase was critical to the adsorption of REY. The 30-day SRB-leaching experiments led to an increase in the percentage of REY from 6% to 45% in the residue phase, implying that the residue phase, RAmor iron phase, and ROrg phase hosted the REYs. The disorder of iron-bearing minerals, formation of iron-organic matters (Fe-OM), and secondary iron-bearing minerals represented a significant bio-leaching mechanism. Compared to chemical extraction, relatively higher MREY and HREY release efficiencies were obtained via bio-leaching, with average LREY/HREY ratios of 1.34–5.91 and 0.2–2.24 in chemical and bio-reactors, respectively. Our findings exhibited high potential microbial effects on the mobilization and fractionation of REY among mineral phases, offering real insights into the biogeochemical processes between minerals and bacteria. •With chemical extraction and bio-leaching experiments, REY-bearing clay-sized mineral phases were quantified.•REYs of the amorphous iron phase are easily separated by the chemical procedure.•Clay-sized REYs leached by the SRB are more readily released from the residue phase.•MREYs and HREYs are much more dissolved out than LREYs with the leaching of SRB.