Ionic liquid as regenerable media for simultaneous recovery of melanin and amino acid from waste human hair: An elaborate and efficient biorefining approach

Abstract

Waste human hair (WHH) is rich in valuable resources like keratin and melanin. However, current resource recovery methods face challenges such as complex procedures, consumption of acidic and alkaline reagents, unsatisfactory yield, and inferior purity of melanin products, primarily due to the difficulty in avoiding the generation of melanin-keratin blends (MKBs). For the first time, this study established an ionic liquid-targeted depolymerization/isolation (IL-TDI) process for the simultaneous recovery of melanin and monomeric amino acids from WHH with both satisfactory yield and superior purity via four steps, including depolymerization, liquid-solid separation, extraction, and recovery of IL. IL-TDI process yielded 6.02% melanin and 34.58% monomeric amino acid, and the environmentally friendly ionic liquid, 1-Butyl-3-methylimidazolium chloride (BMIMCl), was recyclable. SEM and FTIR analyses revealed that the melanin extracted by IL-TDI process retained its native chemical structure and microstructure. In contrast, the traditional base dissolution and acid isolation (BDAI) process achieved only a 2.2% melanin yield and completely destroyed the natural microstructure of melanin. Various characterizations, including UV–visible spectroscopy, quantitative amino acid analysis, and kinetic rate calculations, were conducted to study the separation mechanism of melanin and keratin. In IL-TDI process, disulfide bonds in keratin were initially cleaved by BMIMCl during dissolution. Subsequently, melanin isolation followed two consecutive first-order reactions, and hydrogen ions in BMIMCl preferentially hydrolyzed dissolved keratin, during which preventing the generation of MKB proved essential for achieving high yield of melanin. Economic and E-factor calculations demonstrated this novel biorefining approach's suitability for sustainable WHH recovery.