SPS-Modified Biochar Adsorption for Rare Earth Element Recovery from Acid Mine Drainage

The transition to clean energy technologies, expansion of digital infrastructure, and growing reliance on advanced defense systems have substantially increased demand for rare earth elements (REEs). Sustainable REE recovery from secondary resources is vital for strengthening global supply chains. Acid mine drainage (AMD) has emerged as a promising secondary source for REEs, but its low-pH chemistry and high concentration of competing metal ions (Al³⁺, Fe²⁺) pose major selectivity challenges for conventional sorbents.,In this study, hardwood biochar was functionalized with sulfonated polystyrene (SPS) to introduce negatively charged binding sites for REE adsorption at AMD native pH (2.8), thus eliminating the need for any pH adjustment. Surface characterization confirmed successful functionalization, with zeta potential shifting from +11 mV (pristine biochar) to -30 mV (SPS-functionalized biochar).,Competitive adsorption experiments revealed that Fe²⁺ significantly suppresses REE uptake, while Al³⁺ showed minimal interference. To mitigate this, a staged treatment strategy was developed in which pristine biochar first removes Fe²⁺, enabling SPS-modified biochar to recover REEs more selectively in a second stage. The first step removed 81% of Fe²⁺, while REE uptake increased from about 20% in Fe²⁺ containing systems to nearly 80% in Fe²⁺ free systems, highlighting the critical role of Fe control in selective REE recovery.,Kinetic modeling confirmed pseudo-second-order behavior, consistent with chemisorption at functionalized surface groups, with most uptake occurring within the first hour. These findings demonstrate that ligand-functionalized biochar combined with staged Fe²⁺ management enables selective REE recovery from AMD at native pH, offering a low-cost, sustainable pathway for integrating mine water treatment with critical mineral recovery.