Pb(II) Selectivity of Oxygen-Functionalized Lignite-Based Carbon Material in the Presence of Competitive Cu(II)/Cd(II) Interference

Abstract A series of lignite-derived activated carbons (CK-400-CK-700) was prepared by KOH activation to clarify the effects of pore structure and surface oxygen functionality on Pb(II) adsorption. Although the BET surface area increased from 456.60 to 1300.70 m2·g–1 with activation temperature, CK-400 showed the highest Pb(II) uptake at pH 5. Its surface-area and pore-volume-normalized capacities were 0.1910 mg·m–2 and 379.22 mg·cm–3, approximately three times those of CK-700, indicating that maximizing porosity alone did not improve Pb(II) adsorption. CK-400 exhibited an apparent Langmuir capacity of 139.02 mg·g–1 at 298 K. The adsorption process was spontaneous and exothermic, with film diffusion and intraparticle diffusion jointly controlling mass transfer. FTIR and XPS analyses supported the participation of oxygen-containing sites through surface complexation, electrostatic attraction, and possible ion exchange. In the equal-mass Pb(II)/Cu(II)/Cd(II) system, αPb/Cu and αPb/Cd were 0.402 and 4.75, respectively, indicating preferential adsorption of Pb(II) over Cd(II), but not over Cu(II). CK-400 retained 84.16% of its initial capacity after three cycles. These results highlight the importance of balancing accessible porosity and oxygen-containing surface sites.