Herein, three types of graphene oxides (GOs, GO-M1, GO-M2 and GO-M3) have been successfully prepared from graphite by three different methods and utilized for the removal of Cr(VI) from aqueous solutions. Further, the effects of initial concentration and pH, adsorbent dosage, contact time and temperature on the adsorption performance of GOs were investigated by batch adsorption experiments. Furthermore, the adsorption mechanisms for Cr(VI) adsorption by GOs are mainly the redox reaction and electrostatic attraction, while there are also pore filling, ion exchange and complexation involved in these adsorption processes. The adsorption kinetic and isotherm data indicate that these adsorption processes of GOs on Cr(VI) are dominantly monolayer chemisorption and equilibrium can be reached in 30 min. The saturation adsorption capacities (Q_m, 298.15 K) of GO-M1, GO-M2 and GO-M3 for Cr(VI) are estimated to be 3.5412 mg⋅g⁻¹, 2.3631 mg⋅g⁻¹ and 7.0358 mg⋅g⁻¹, respectively. Moreover, the adsorption thermodynamic study showed that these adsorption processes of Cr(VI) by the three types of GOs at 298.15 K to 323.15 K are endothermic, entropy-driven and thermodynamically spontaneous and feasible. Overall, these findings provided vital insights into the mechanism and application of Cr(VI) removal by GOs.

In this paper, an advanced statistical physics adsorption model (double-layer model with two energies) is successfully established. On the basis of this model, statistical thermodynamic functions (e.g., entropy (S), Gibbs free enthalpy (G), and internal energy (E_int)), pore size distribution (PSD), and site energy distribution (SED) functions were successfully developed and applied to investigate the adsorption mechanisms of nanopore-structured ashitaba waste-based activated carbons (AWAC) and walnut shell-based activated carbons (WSAC) on Congo red (CR) and methylene blue (MB) dyes in aqueous solutions. Statistical thermodynamic results indicated that the adsorption reactions involved in this study are entropy-increasing, endothermic, and spontaneous in nature. Furthermore, PSD and SED described the heterogeneity of these adsorbents in terms of geometry or structure and energy and illustrated that the aforementioned adsorption processes are endothermic physisorption. All in all, this study contributed to broadening the understanding of the adsorption mechanisms of dye molecules onto biomass-based activated carbons.