A pilot scale study on the treatment of rare earth tailings (REEs) wastewater with low C/N ratio using microalgae photobioreactor

Metagenomic analysis revealed the evolution of microbial communities, metabolic pathways, and functional genes in the heterotrophic nitrification-aerobic denitrification process under La3+ stress

Trivalent lanthanum (La3+) exists widely in ammonia nitrogen (NH4+-N) tailing water from ionic rare earth mines; however, its effect on heterotrophic nitrification-aerobic denitrification (HN-AD) is unknown, thereby limiting the application of the HN-AD process in this field. In this study, we conducted an HN-AD process using a sequencing batch reactor (5 L) that was continuously operated to directly treat acidic (NH4)2SO4 wastewater (influent NH4+-N concentration of approximately 110 mg/L and influent pH of 5) containing different La3+ concentrations (0-100 mg/L). The NH4+-N removal efficiency of the reactor reached 98.25 % at a La3+ concentration of 100 mg/L. The reactor was in a neutral-to-alkaline environment, which favored La3+ precipitation and complexation. Metagenomic analysis revealed that the relative abundance of Thauera in the reactor remained high (88.62-92.27 %) under La3+ stress. The relative abundances of Pannonobacter and Hyphomonas significantly increased, whereas that of Azoarcus significantly decreased. Metabolic functions in the reactor were mainly contributed by Thauera, and the abundance of metabolic functions under low La3+ stress (≤5 mg/L) significantly differed from that under high La3+ stress (≥10 mg/L). The relative abundance of ammonia assimilation-related genes in the reactor was high and significantly correlated with ammonia removal. However, traditional ammonia oxidation genes were not annotated, and unknown ammonia oxidation pathways may have been present in the reactor. Moreover, La3+ stimulated amino acid biosynthesis and translocation, the citrate cycle, sulfur metabolism, and oxidative phosphorylation and promoted the overproduction of extracellular polymeric substances, which underwent complexation and adsorbed La3+ to reduce its toxicity. Our results showed that the HN-AD process had a strong tolerance to La3+, stable NH4+-N removal efficiency, the potential to recover La3+, and considerable application prospects in treating NH4+-N tailing water from ionic rare earth mines.

Biomining for sustainable recovery of rare earth elements from mining waste: A comprehensive review

Rare earth elements (REEs) are essential for advanced manufacturing (e.g., renewable energy, military equipment, electric vehicles); hence, the recovery of REEs from low-grade resources has become increasingly important to address their growing demand. Depending on specific mining sites, its geological conditions, and sociodemographic backgrounds, mining waste has been identified as a source of REEs in various concentrations and abundance. Yttrium, cerium, and neodymium are the most common REEs in mining waste streams (50 to 300 μg/L). Biomining has emerged as a viable option for REEs recovery due to its reduced environmental impact, along with reduced capital investment compared to traditional recovery methods. This paper aims to review (i) the characteristics of mining waste as a low-grade REEs resource, (ii) the key operating principles of biomining technologies for REEs recovery, (iii) the effects of operating conditions and matrix on REEs recovery, and (iv) the sustainability of REEs recovery through biomining technologies. Six types of biomining will be examined in this review: bioleaching, bioweathering, biosorption, bioaccumulation, bioprecipitation and bioflotation. Based on a SWOT analyses and techno-economic assessments (TEA), biomining technologies have been found to be effective and efficient in recovering REEs from low-grade sources. Through TEA, coal ash has been shown to return the highest profit amongst mining waste streams.