One-step isolation of L-cysteine functionalized lignin with high adsorption capacity for heavy metal ions

As a sustainable alternative for producing materials that can adsorb heavy metal ions (HMI), chemical or physical modification is usually required to improve the activity of lignin. This is because traditional methods of isolating lignin result in a lack of active sites. Herein, we present a simple and efficient method that uses L-cysteine to simultaneously isolate and functionalize lignin from wheat straw, yielding L-cysteine lignin (LCL) with an isolation yield as high as 75 %. Comprehensive characterization revealed LCL retained comparable β-aryl ether bonds to cellulolytic enzymelignin (CEL), representing native lignin with considerable active sites. Additionally, the extraction process introduced amino and carboxylic acid groups to LCL. Compared to CEL, LCL exhibited significantly improved adsorption capacities: 92 mg/g (Cu²⁺ ions), 66 mg/g (Pb²⁺ ions), 37 mg/g (Cd²⁺ ions), and 32 mg/g (Co²⁺ ions), respectively. The adsorption kinetics and isotherms trend of Cu²⁺ signify the HMI were adsorbed onto LCL through a pseudo-second-order kinetic and Langmuir isotherm adsorption mechanism. LCL also displayed a high surface area (60.03 m²/g) on account of the ordered porous structure. XPS and FTIR results confirmed that the L-cysteine (L-cys) substitution in LCL played a crucial role in the adsorption process, indicating that both chemical and physical adsorption contributed to the enhanced adsorption capacity. In summary, this study presents an efficient one-step method for isolating and functionalizing lignin using L-cysteine-assisted extraction. The results demonstrate the promising performance of LCL for adsorbing HMI and contribute to the development of lignin-based adsorption materials.