Nanoengineering cellulose for the selective removal of neodymium: Towards sustainable rare earth element recovery

Abstract

Rare earth elements (REE) have exceedingly become critical in advanced industries. Despite the high demand for REE, the world is still experiencing a shortage in ready-to-exploit resources, environmentally friendly processing, and reliable recovery strategies, rendering sustainable REE removal an immediate and unmet environmental, industrial, and economical challenge worldwide. We nanoengineered cellulose, the most abundant biopolymer in the world, to develop a sustainable bio-based technology named anionic hairy nanocellulose (AHNC) for the high-capacity and selective removal of neodymium ions (Nd3+), one of the most widely used REE, from aqueous media. AHNC comprises fully solubilized dicarboxylated cellulose (DCC) chains and cellulose nanocrystals (CNC) decorated with DCC (hairs) bearing a charge density that is about one order of magnitude higher than conventional CNC. The unique colloidal properties of AHNC, particularly the polyanionic hairs, enable the removal of ~ 264 ± 14 mg of Nd3+ per gram of the nanoadsorbent within seconds, which, to the best of our knowledge, place this advanced material among the adsorbents with the highest removal capacity at the shortest contact time. We investigated the roles of ionic strength, pH, and competing iron species on the performance of AHNC. Besides Nd3+ removal at high initial Nd3+ concentrations (C0 > 150 ppm) wherein AHNC is fully neutralized and precipitated, we show, for the first time, that at C0 ≤ 100 ppm wherein AHNC maintains its partial colloidal stability, Nd3+ removal can be enhanced via complementary calcium ion-mediated colloidal bridging. Together, our colloidal engineering approach combined with the biorenewability of cellulose and an ambient, low-cost unit operation, renders AHNC a promising sustainable nanotechnology for the removal of Nd3+ from industrial wastewater, mining tails, e-waste, and NdFeB permanent magnet leachates.