Efficient recovery of waste hair keratin via L-Cys/MEA for selective separation of Fe3+/Li+

Efficient recovery and high-value utilization of keratin from waste human hair are of significant interest for sustainable materials development. However, the densely cross-linked disulfide bonds and extensive hydrogen-bond network of keratin make its mild and efficient extraction challenging. In this work, a redox-active deep eutectic solvent (DES) composed of L-cysteine (L-Cys) and monoethanolamine (MEA) was developed for keratin recovery from waste human hair. Under optimized conditions of 80 °C, 3.5 h, and 10 wt% solid loading, the keratin recovery yield reached 96.5%, and the solvent maintained stable performance over five recycling cycles. Experimental results, together with DFT and MD simulations, indicate that the dissolution process is driven by synergistic disulfide bond cleavage and disruption of the hydrogen-bond network, while the protein backbone remains largely intact. The recovered keratin was further phosphorylated and co-regenerated with cellulose to fabricate a phosphorylated keratin/cellulose composite membrane (P-KCM). The membrane exhibited efficient and selective Fe 3+ adsorption, with a maximum adsorption capacity of 297.8 mg·g −1 . FT-IR and XPS analyses reveal that phosphate groups serve as the primary coordination sites for Fe 3+ . In tests with real LiFePO₄ leachate, P-KCM removed 97.3% of Fe 3+ while leaving 80.1% of Li + in solution, and it retained 84.7% of its initial adsorption capacity after four adsorption-desorption cycles. This work provides a mild, efficient, and recyclable strategy for keratin valorization and offers a feasible approach for selective metal ion separation and pre-treatment in battery recycling.