ZIF-8 Used for the Selective Recovery of Heavy Rare Earth Elements from Mining Wastewater.

In this study, a sample of 2-methylimidazole zinc salt (ZIF-8) demonstrated high selectivity for the recovery of heavy rare earth elements (REEs) from real rare earth mining wastewater. Results show that the distribution coefficient values of Y3+ (4.02 × 104 mL·g-1), Gd3+ (7.8 × 104 mL·g-1), and Dy3+ (6.8 × 104 mL·g-1) are orders of magnitude higher than those of K+ (359.51 mL·g-1), Mn2+ (266.67 mL·g-1), Ca2+ (396.42 mL·g-1), and Mg2+ (239.48 mL·g-1). Moreover, the desorption efficiency of heavy REEs exceeded 40%. Advanced characterizations and density functional theory (DFT) calculations were utilized to elucidate that the heavy REEs were more likely to bind to the nitrogen atoms of imidazole groups on ZIF-8 compared to non-REEs. Furthermore, the adsorption and desorption of heavy REEs primarily depend on the chemical interaction confirmed by adsorption kinetics, isotherm model, and thermodynamic analysis, which involves the dissociation of water and the formation of REE-O bonds. Finally, the ZIF-8 exhibits a remarkable recovery efficiency of over 40% for heavy REEs in column tests conducted over 7h. The findings reported here provide new insights into the selective recovery of heavy REEs from real mining wastewater.

Transport and distribution of residual nitrogen in ion-adsorption rare earth tailings.

A large amount of nitrogen remains in ion-absorption rare earth tailings with in-situ leaching technology, and it continually ends up in groundwater sources. However, the distribution and transport of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) across tailings with both depth and hill slopes is still unknown. In this study, the amount of NH4+-N and nitrate nitrogen (NO3--N) was determined in tailings, and a soil column leaching experiment, served to assess the transport and distribution following mine closure. Firstly, a high concentration of NH4+-N in the leachate at the initial leaching stage was detected, up to 2000 mg L-1, and the concentration of NH4+-N clearly diminished as time passed. Meanwhile, the NH4+-N contents remained relatively high in soil. Secondly, both the content of NH4+-N and NO3--N varied greatly according to vertical distribution after leaching lasting several years. The amounts of NH4+-N and NO3--N in surface soil were much smaller than those in deep soil, with 3-4 orders of magnitude variation with depth. Thirdly, when disturbed by NH4+-N, the pH not only diminished but also changed irregularly as depth increased. Fourthly, although the amount of NO3--N was smaller than that of NH4+-N, both their distribution trend was similar with depth. In fact, NH4+-N and NO3--N were significantly correlated but this declined from the knap to the piedmont. Based on these results, it is suggested that mining activity could cause nitrogen to be dominated by NH4+-N and acidification in a tailing even if leaching occurs over several years. NO3--N derived from NH4+-N transports easily and it becomes the main nitrogen pollutant with the potential to be a long-lasting threat to the environment around a mine.

Pilot-scale remediation of rare earth elements ammonium wastewater by Chlamydomonas sp. YC in summer under outdoor conditions.

This work evaluated the performance of real rare earth elements (REEs) wastewater purification and carbon dioxide (CO2) fixation by Chlamydomonas sp. YC with pilot-scale airlift-photobioreactors (AL-PBRs), tubular photobioreactors (TB-PBRs) and raceway ponds (ORWPs) under high-temperature outdoor conditions in summer. The obtained results showed that Chlamydomonas sp. YC at 1 g/L oyster shell piece (OSP) and 3 % CO2 had the highest biomass (1.9 g/L) and NH4+-N removal efficiency (34.0 %) during the REEs wastewater treatment. Among the selected photobioreactors, Chlamydomonas sp. YC to treat real REEs wastewater at 3 % CO2 under high-temperature outdoor conditions attained the highest biomass (2.3 g/L) in the TB-PBRs with the best NH4+-N removal efficiency (43.0 %). Furthermore, the input cost and CO2 net sequestration evaluation revealed that TB-PBRs was more economical photobioreactors to treat REEs wastewater and fix CO2 by Chlamydomonas sp. YC, providing some vital scientific details for REEs wastewater and CO2 fixation by microalgal biotechnology.