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Environmental pollution has been widely considered by researchers, especially the heavy metals damage to the human and ecological environment is irreversible. Adsorption is an important method to remove heavy metal ions from the environment. In this paper, humic acid (HA) was functionalized by the improved Hummers method, and its adsorption capacity for Pb(II) was studied. The results of scanning electron microscope (SEM), X-ray diffraction (XRD), Roman, and Brunauer-Emmett-Teller (BET) showed that the thickness of irregular particles decreases to a layered structure during the transformation process. In addition, X-ray photoelectron spectroscopic (XPS) and Fourier transform infrared spectra (FT-IR) spectra showed that the surface of oxidized-biochar (OBC) was rich in reactive oxygen species, which was conducive to the formation of coordination bonds with Pb(II). Further adsorption experiments showed that it was a spontaneous monolayer chemisorption. The results of the DFT calculation showed that -COOH had the lowest adsorption energy for Pb(II), and it was easier to form stable chemical bonds than -OH, -C=O, and -C-O-C-. Because those oxygen-containing functional groups not only can promote electrostatic attraction but also are more favorable for forming a covalent bond with Pb(II). This study had guiding significance for the deep modification and application of weathered coal as a heavy metal ion adsorbent or cation exchanger. [ABSTRACT FROM AUTHOR]