【Objective】The knowledge about chemical composition and interactions of humic substances with metal ions in the soil is of paramount importance to elucidating behaviors and fates of trace metals in the environments. Adsorption is one of the most important interactions between humic substances and metal ions. Among the three fractions of humic substances, i.e., humic acid, fulvic acid and humin, the last is the least studied because of its hard-to-dissolve nature. So far, it is still unclear how humin adsorbs metal ions and its mechanism. 【Method】In this study, humin was extracted from two zonal soils of East China, i.e., brown soil (Argosol) and lateritic red soil (Ferrosol), with 0.1 mol•L^-1 NaOH+0.1 mol•L^-1 Na₄P₂O₇ extraction, treated with 10% (v/v) HF-HCl and then prepared into samples for analysis. Structure of the humin was characterized with elemental analysis, solid-state carbon-13 cross-polarization magic-angle-spinning nuclear magnetic resonance (¹³C CPMAS NMR) spectroscopy and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) techniques. On such a basis, the batch equilibrium method was used to study adsorption of Cu(II) on humin as a function of exposure duration, adsorbate concentration and reaction temperatures. Local chemical structure of the Cu(II) adsorbed onto the humin was explored with the synchrotron-based X-ray absorption near edge structure (XANES) spectrum and the extended X-ray absorption fine structure (EXAFS) spectrum. Furthermore, comparison was also made between humin and humic acid in chemical compositions and Cu(II) adsorption feature. 【Result】Compared with humic acid, humin is relatively higher in content of C, H, alkyl C, carbohydrate C, di-O-alkyl C, polysaccharides and fatty acids, but lower in content of N, S, O, methoxyl C, aryl C, phenol C, carbonyl C, lignin, phenols and nitrogen-containing compounds. With rising duration of exposure, Cu(II) concentration, and reaction temperature, Cu(II) adsorption on humin increased, too. Kinetics of the adsorption could be best described by pseudo-second order equation with equilibrium achieved in approximately 120 min, and isotherm of the adsorption well fitted the Freundlich and Langmuir equations. The adsorption activation parameters, i.e., activation free energy (ΔG^#), activation enthalpy (ΔH#) and activation entropy (ΔS^#), indicate that the process of adsorption is an association reaction that requires energy and absorbs heat. Moreover, the low activation energy (Ea) (less than 26 kJ•mol^-1) implies that the adsorption reaction is controlled by boundary layer diffusion and Cu(II) was physically adsorbed onto the surface of humin. The adsorption thermodynamic parameters, i.e., standard Gibbs energy (ΔGº), standard enthalpy change (ΔHº) and standard entropy change (ΔSº), indicate that the adsorption reaction is a spontaneous, endothermic process with increasing freedom of motion. Similar to humic acid, humin has Cu(II) adsorbed on the surface in the form of a twisted octahedron, with the first coordination sphere consisting of 4 O atoms 1.91~1.97 Å apart and the second coordination sphere consisting of 2 C atoms 2.80~2.83 Å apart, which demonstrates that Cu(II) is preferentially adsorbed onto the organic functional groups on the surface of humin in the form of inner-sphere compound.【Conclusion】 All the findings demonstrate that humin differs from humic acid in chemical composition. Humin is relatively higher in aliphaticity and polarity. However, the Cu(II)s adsorbed on the surface of the two fractions of humic substances have similar local chemical structure.