In this study, a comparison of walnut-based carbon materials (CM) obtained by hydrothermal carbonization (HTC) and HTC in combination with steam gas activation (SGA) was carried out. In order to study the effect of steam activation on the functional layer formation, the obtained materials were studied by SEM, nitrogen adsorption/desorption by Brunauer-Emmett-Teller (BET) method, Raman spectroscopy, X-ray diffraction spectroscopy (XRD), X-ray fluorescence elemental analysis (XRF). Functional groups (FG) were evaluated qualitatively and quantitatively by Fourier transform infrared spectroscopy (FTIR), acetone extract analysis of CM by gas chromatography-mass spectrometry (GCMS), and potentiometrically Boehm titration. The described mechanism of the influence of the base nature on the surface functionality correlates well with the results of the powder addition method. The HTC or HTC+ SGA treatment provides a wide range of possibilities for further controlled modification of the carbon sorbent surface for specific adsorption purposes.