Green solvents for the extraction and bioutilisation of metals from coal fly ash by Magnetospirillum gryphiswaldense MSR1

Abstract

Coal fly ash (CFA), a metal-rich byproduct of coal combustion is produced in vast quantities and poses significant ecological risks. CFA also contains abundant technologically relevant metal oxides and trace metals, including rare earth elements (REE), often at higher concentrations than in primary ores. This makes sustainable recovery strategies a major industrial opportunity. Here, green solvent systems were applied to leach metals from CFA, and the resulting leachates were added to cultures of Magnetospirillum gryphiswaldense (MSR1), a model magnetotactic bacterium that biomineralizes iron into membrane-bound magnetic nanoparticles (magnetosomes) and is capable of interacting with non-iron metals through adsorption and biomineralization. Eleven green solvents, including deep eutectic solvents (DES), were tested for extraction efficiency, with six showing performance comparable to a mineral acid control. Copper (Cu) emerged as the primary toxicant to MSR1, prompting selective precipitation with potassium ferrocyanide trihydrate (PFCT) to reduce its concentration. Cu-depleted lactic acid-based leachates supported MSR1 growth and magnetosome formation even without supplemented iron. Nano-XRF and ICP-MS analysis revealed MSR1 interacts with CFA-derived metals, most significantly showing that produced CFA magnetosomes contained a 5.3–6.1-fold increase in Cu compared to controls. As Cu is both a growth inhibitor and a target pollutant, these findings suggest MSR1 may bioaccumulate Cu within magnetosomes as a detoxification strategy. Overall, this study demonstrates a combined chemical–biological route for CFA valorisation, enabling recovery of diverse metals from waste while producing magnetosomes with distinct compositions.