The performance of hexavalent chromium (Cr(VI)) adsorption by activated carbon (AC) prepared from coconut shell (AC1) and modified with silver nanoparticles (AC2), titanium oxide (AC3), and magnetic powder (AC4) was evaluated in this study. The interaction between AC surface properties and Cr(VI) was also assessed via elemental distribution and binding energy analyses. More Cr(VI) was adsorbed onto AC1 than onto any other AC, indicating that the specific surface area and surface functional groups are key factors for Cr(VI) adsorption from the aqueous phase (with correlation coefficients of 0.988 and 0.868–0.949, respectively). Activation of the coconut shell with silver nanoparticles, titanium oxide, and magnetic powder did not increase Cr(VI) adsorption. Cr atoms were detected on the AC1 surface by electron probe microanalysis only after adsorption. Moreover, the binding energies of Cr (2s, 2p, 3s, and 3p) and O (1s) were confirmed after absorption. These results indicate that AC surface properties were strongly related to adsorption performance. Finally, this study reveals the optimal pH conditions for the removal of Cr(VI) from the aqueous phase of approximately pH 2–3 (acidic conditions). In conclusion, this study elucidates the Cr(VI) adsorption mechanisms of coconut shell-derived AC.