Highly Mesoporous Hybrid Transition Metal Oxide Nanowires for Enhanced Adsorption of Rare Earth Elements from Wastewater

Efficient Capture and Release of the Rare-Earth Element Neodymium in Aqueous Solution by Recyclable Covalent Organic Frameworks

Rare-earth elements (REEs) are present in a broad range of critical materials. The development of solid adsorbents for REE capture could enable the cost-effective recycling of REE-containing magnets and electronics. In this context, covalent organic frameworks (COFs) are promising candidates for REE adsorption due to their exceptionally high surface area. Despite having attractive physical properties, COFs are heavily underutilized for REE capture applications due to their limited lifecycle in aqueous acidic environments, as well as synthetic challenges associated with the incorporation of ligands suitable for REE capture. Here, we show how the Ugi multicomponent reaction can be leveraged to postsynthetically modify imine-based COFs for the introduction of a diglycolic acid (DGA) moiety, an efficient scaffold for REE capture. The adsorption capacity of the DGA-functionalized COF was found to be more than 40 times higher than that of the pristine imine COF precursor and more than four times higher than that of the next-best reported DGA-functionalized solid support. This rationally designed COF has appealing characteristics of high adsorption capacity, fast and efficient capture and release of the REE ions, and reliable recyclability, making it one of the most promising adsorbents for solid-liquid REE ion extractions reported to date.

Efficient recovery and enrichment of rare earth elements by a continuous flow micro-extraction system

The excessive exploitation of rare earth elements (REEs) has caused major losses of non-renewable resources and damage to the ecosystem. The processes of mining and smelting produce massive amounts of wastewater with low concentrations of REEs. Consequently, the enrichment and recovery of low-concentration REEs from wastewater has significant economic and environmental value. For this purpose, operation under large phase ratios (the flow rate ratio between the aqueous phase and extractant) is more desirable and economically viable. However, the traditional REE extraction process suffers from the uneven dispersion of the extractant and the difficulty of phase separation, which leads to long extraction times and large consumption of extractants. Hence, there is an urgent need to develop a green and efficient technique to extract low concentrations of REEs from wastewater. In this work, a droplet-based microfluidic technique was used to continuously extract and recover low-concentration REEs at large phase ratios. Snowman-shaped magnetic Janus nanoparticles were added to the continuous phase as emulsifiers to facilitate uniform extractant dispersion and rapid phase separation. Several key factors affecting the extraction efficiency, including pH, residence time, and the amount of added Janus nanoparticles, were systematically investigated. Compared to batch extraction, droplet-based microfluidic extraction with the addition of Janus nanoparticles showed the advantages of a large specific surface area and fast phase separation during extraction. Meanwhile, the Janus nanoparticles exhibited good emulsification performance after three extraction cycles. In summary, the Janus nanoparticle-stabilized droplet generated by microfluidic methods provides a feasible path for the efficient enrichment and recovery of low-concentration REEs.