Phosphorylated chitosan-lignin composites for efficient removal of Pb(II) and Cu(II) from aqueous environments and sustainable upcycling of spent adsorbents.

Efficient removal of Pb(II) and Cu(II) from wastewater is crucial for safeguarding environmental safety and public health. Biomass-based adsorbents with surface-specific functionality hold great promise for selective adsorption of metal cations. In this study, a novel phosphorylated chitosan-lignin (PCSL) composite is successfully synthesized via Mannich reaction. The PCSL exhibits remarkable selectivity in the adsorption of Pb(II) and Cu(II), as evidenced by Density Functional Theory (DFT) calculations. Furthermore, DFT analysis reveals that the incorporation of phosphate groups significantly enhances the chelation capacity of the adsorbent towards heavy metals. The PCSL demonstrates ultrafast adsorption capabilities for Pb(II) and Cu(II). Specifically, the adsorption processes reach equilibrium within 7 min and 5 min, respectively, with maximum adsorption capacities of 207.9 mg·g for Pb(II) and 100.0 mg·g for Cu(II). X-ray photoelectron spectroscopy analysis indicates that the adsorption mechanisms involve both chemical complexation and electrostatic attraction. Notably, the adsorbent can be recycled many times, and the spent Cu-PCSL, upon pyrolysis treatment, demonstrate remarkable catalytic activity in nitrate reduction reactions, with Faradaic efficiencies as high as 98.3 % and NH yield of 4.3 mg·h·mg. This work not only advances the progression of biomass adsorbents but also demonstrates considerable industrial potential in mitigating water pollution and promoting sustainable development.