A biosorption-based approach for selective extraction of rare earth elements from coal byproducts

Abstract

Coal byproducts represent an abundant and untapped potential source of critical rare earth elements (REEs). In this study, we tested the efficacy of a biosorption-based approach to recover REEs from leachate solutions derived from North Dakota lignite coal and Powder River Basin (PRB) coal fly ash. A pairwise comparison was performed using a range of cell densities of two distinct bacteria, including an Escherichia coli strain previously genetically engineered for cell surface display of lanthanide binding tags, and Arthrobacter nicotianae, a native bacterium that exhibits high REE adsorption capacity. At optimal cell densities, we observed a recovery efficiency of 80% for total REEs and > 90% for middle and heavy REEs from the lignite leachate. Higher cell densities were required to achieve a similar total REE recovery efficiency in the PRB leachate due to its higher non-REE content, which negatively impacted light REE recovery. Despite the chemical complexity of both feedstocks, separation factors ≥ 30 were observed for Nd relative to nearly all non-REE metals after a single adsorption/desorption cycle, highlighting the high adsorption selectivity for REEs. Quantification of impurities in the extracted metal solutions revealed that Ca and Mg content in the leachate solutions play a dominant role in both the REE recovery yield and purity. The Mg/Ca impurities can be further reduced by incorporation of a low pH wash step between the adsorption and desorption steps, yielding extracted concentrates with a REE purity (mass fraction REE of total metals) of 80% and 50% for lignite and PRB, respectively. Thermodynamic speciation analysis of the REE-enriched eluents from both feedstocks suggests that at a post-biosorption hydroxide precipitation step can be employed to yield high purity total REE precipitates. Collectively, these results highlight the utility of biosorption for selective REE recovery from coal byproducts and pave the way for a sustainable route to diversify the REE supply chain.