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

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.