Mechanistic insights into selective removal of Cu(II) and gentian violet by tea residue-modified LDHs

As an eco-friendly bio-adsorbent, tea residue (T) possesses abundant surface active functional groups. Herein, a novel and innovative composite adsorbent, tea residue/layered double hydroxides (T-LDHs), was successfully synthesized for the selective removal of Cu(II) and gentian violet (Gv) from aqueous solutions. Notably, for Cu(II) adsorption, 20%T-LDHs exhibited a remarkably higher capacity (76.8 mg/g) than mono-component T (27.8 mg/g) and LDHs (48.3 mg/g). In contrast, T showed the optimal Gv adsorption (198 mg/g) while LDHs performed poorest (21.6 mg/g), demonstrating that the composite achieves a unique pollutant type-adaptive adsorption performance and a prominent synergistic enhancement effect that individual components lack. Kinetic and isotherm fitting confirmed that the pseudo-second-order model and Langmuir model accurately describe the adsorption processes. Stability tests revealed that after 5 adsorption-desorption cycles, the removal rates of Cu(II) and Gv by 20%T-LDHs remained 85.2% and 96.9%, respectively, ensuring reliable reusability. Density functional theory calculations further elucidated the distinct adsorption mechanisms: Cu(II) adsorption mainly involves surface complexation and isomorphous substitution, while Gv adsorption relies on the synergistic effects of electrostatic attraction, π-π stacking, and hydrogen bonding. Collectively, this innovative 20%T-LDHs composite holds broad application prospects in treating heavy metal-dye composite wastewater, highlighting its novel value in environmental remediation.