Rare Earth Elements (REEs) Recovery and Hydrochar Production from Hyperaccumulators

Phytomining is a promising method for metal recovery, but rare studies have been devoted to metal recovery from hyperaccumulator biomass. The objective of this study was to propose efficient and sustainable methods for treating REE hyperaccumulators, aimed at enhancing REE recovery and obtaining value-added byproducts. Firstly, grass seeds fed with a solution containing Y, La, Ce, and Dy, were found to have the capacity to accumulate around 510 mg/kg (dry basis) of total rare earth elements (TREEs) in grass leaves. With the use of conventional hydrometallurgy, around 95% of Y, La, Ce, and Dy were extracted from the GL using 0.5 mol/L H2SO4 at a solid concentration of 5 wt.%. Subsequently, microwave-assisted hydrothermal carbonization (MHTC) was used to convert the leaching residue into hydrochar to achieve a comprehensive utilization of GL biomass. Scanning electron microscopy (SEM) analysis revealed that the original structure of GL was destructed at 180 °C during MHTC, producing numerous microspheres and pores. As the reaction temperature increased, there was a concurrent increase in carbon content, HHV, and energy densification, coupled with a decrease in hydrogen and oxygen contents of hydrochar. The results showed that the waste biomass of the GL after REE extraction can be effectively converted into energy-rich solid fuel and low-cost adsorbent via MHTC. In addition to utilizing conventional hydrometallurgy for REE recovery and employing MHTC to convert leaching residue into hydrochar, MHTC was also applied to directly recover REEs and produce hydrochar from the GL as a more efficient approach. The effects of acid type and acid concentration on REE extraction from GL using MHTC were investigated. The utilization of 0.2 mol/L H2SO4 led to the extraction of nearly 100% of REEs from the GL into the resulting biocrudes. Concurrently, the acid-mediated MHTC system also caused the degradation of amorphous hemicellulose and crystalline cellulose present in the GL, thereby enhancing the thermal stability of the resulting hydrochar. The physiochemical properties of the hydrochar were also influenced by acid type and acid concentration. Using 0.2 mol/L H2SO4 as the reaction medium, MHTC resulted in a yield of 28% hydrochar with enhanced high heating value and energy densification. These results suggest that MHTC in the presence of an appropriate concentration of H2SO4 is an effective way to extract REEs and produce hydrochar from the GL. A process that combines solvent extraction and struvite precipitation was developed for the treatment of biocrudes containing REEs and other elements. In the extraction step, 95.6% of REEs were extracted using 0.05 mol/L di(2-ethylhexyl)phosphoric acid (D2EHPA) with an aqueous to organic (A/O) ratio of 1:1 at pH 3.0. However, other impurity metals were co-extracted into the organic phase with the REEs. To solve this issue, a subsequent scrubbing step using deionized water was applied, with the removal of over 98% of these impurities, while incurring negligible loss of REEs. After the scrubbing step, over 97% of REEs were ultimately stripped out from the organic phase as REE oxalates using 0.01 mol/L oxalic acid. Furthermore, phosphorous (P) was found to be retained in the raffinate after the solvent extraction process. 94.4% of the P was recovered by forming struvite precipitate at pH 9.0 and a Mg/P molar ratio of 1.5. In general, high purity and value-added REE products and struvite precipitate were eventually achieved from biocrudes in environmentally friendly and economically viable ways. In summary, this study contributes a sustainable and efficient framework for REE hyperaccumulator treatment that integrates acid leaching, MHTC, solvent extraction, and struvite precipitation. This work supports a circular economy, minimizing waste and promoting resource reuse.