Direct Upcycling of Rare Earth Hyperaccumulator Biomass into Luminescent CaS:Ce Phosphors

Abstract

Rare earth elements (REEs) are essential to high-tech industries and sustainable development, but traditional mining poses significant environmental challenges. Phytomining, which uses hyperaccumulators to recover REEs from soils or low-grade mines, has emerged as a sustainable alternative; however, the high cost and low efficiency of extracting and purifying REEs from harvested biomass remain major barriers to its practical application. Here, we propose a novel in situ doping strategy to directly convert cerium (Ce)-enriched hyperaccumulator biomass into color-tunable CaS:Ce phosphors and value-added syngas, bypassing complex separation of Ca and REEs. The results show that Ce, Ca, and S could be selectively enriched from hyperaccumulator biomass to the resulting char through solvothermal pretreatment with sulfuric acid and ethanol. Subsequent steam gasification of the char produces Ce-doped CaS phosphors with broad, green-yellow emission and high photoluminescence quantum yields of 18.5–37.4%. Our approach reduces energy use by 33.6%, carbon emissions by 22.4%, and costs by \$2.62 kg–1 biomass, outperforming conventional REE recovery in both environmental and economic sustainability. This strategy provides an environmentally sustainable and economically viable route for high-value utilization of REE-rich biomass, addressing a critical bottleneck in phytomining and advancing its development as a green REE recovery technology.