RGO-CBPAC versus CBPAC for diluted leachate treatment: adsorption isotherms, kinetics and thermodynamics

This study compares carbon-based activated carbon (CBPAC) and reduced graphene oxide-modified CBPAC (RGO-CBPAC) for removing COD, NH3–N, and turbidity from 1:4 diluted leachate from the Bingöl landfill site. Brunauer Emmett Teller (BET) analysis indicated severe micropore blockage in CBPAC after adsorption (887 → 0.76 m2/g), whereas RGO-CBPAC retained a higher surface area (908 → 23 m2/g) and mesoporous structure, demonstrating enhanced structural stability due to graphene incorporation. Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDX) confirmed the presence of surface functional groups and effective pollutant uptake, with RGO-CBPAC showing stronger surface–pollutant interactions. Zeta potential analysis showed pH-dependent surface charge behaviour, with CBPAC exhibiting amphoteric characteristics and RGO-CBPAC maintaining a more negative surface. After adsorption, both became more negatively charged, indicating the accumulation of negatively charged species. Kinetic analysis showed that COD adsorption on CBPAC followed pseudo-first-order and intraparticle diffusion models, whereas NH3–N and turbidity followed pseudo-second-order kinetics. In contrast, RGO-CBPAC exhibited pseudo-second-order and intraparticle diffusion behaviour for all pollutants, suggesting chemisorption with diffusion-controlled contributions. Adsorption equilibrium was best described by the Langmuir isotherm, yielding maximum capacities of 500 and 588 mg O2/g for COD, 17.5 and 36.5 mg/g for NH3–N, and 18.1 and 58.5 mg/g ((NH2)2H2SO4)/g for turbidity on CBPAC and RGO-CBPAC, respectively. Thermodynamic analysis demonstrated that COD and turbidity removal using RGO-CBPAC were spontaneous (∆G° < 0) and endothermic (∆H° > 0), whereas NH3–N removal remained non-spontaneous for both adsorbents. RGO incorporation significantly enhanced adsorption performance.