Comparative adsorption of methylene blue on GO- and rGO-modified natural clinoptilolite: role of carbon oxidation state and pore blocking

Building on our earlier work on graphene oxide (GO)-impregnated clinoptilolite for methylene blue (MB) removal, this study directly compares GO-modified zeolite (GOZ) with an ascorbic-acid-reduced analogue (rGOZ) prepared on the same clinoptilolite matrix by liquid-phase impregnation and evaluated under identical batch conditions. Spectroscopic/microscopic, elemental (CHNS) and textural analyses (UV-Vis, FT-IR, SEM/EDS, N-2 sorptiometry and thermogravimetry) confirm successful attachment of the carbon phases (similar to 9% graphene loading) and reveal distinct surface chemistries: GOZ retains abundant oxygenated groups, whereas rGOZ is more graphitic and both composites exhibit partial pore blocking. Relative to pristine clinoptilolite (q similar to 35 mg g(-1) at 298 K), the maximum MB uptake (Langmuir) increases to similar to 157 mg g(-1) for GOZ and similar to 123 mg g(-1) for rGOZ, demonstrating a substantial enhancement upon carbon-phase incorporation. Adsorption is favored near neutral pH, follows pseudo-second-order kinetics for both composites, and shows endothermic, entropy-driven, spontaneous behavior over 298-333 K. Equilibrium data indicate more heterogeneous/pore-filling contributions for GOZ and a more monolayer-like, pi-pi-assisted uptake for rGOZ, consistent with their different functional-group densities. Regeneration by 0.1 N HCl washing over five cycles shows higher durability of GOZ, with MB removal decreasing from similar to 98% (cycle 1) to similar to 84% (cycle 5), compared with rGOZ decreasing from similar to 93 to similar to 71%. Overall, these results quantify how the oxidation state of the graphene phase governs MB uptake and reuse performance in clinoptilolite-based hybrid adsorbents.