Liquid-phase adsorption of methylene blue on functionalized Solanum macrocarpon calyx activated carbon: kinetics, equilibrium, and thermodynamic studies

In this study, the adsorption of methylene blue by orthophosphoric acid-modified Solanum macrocarpon calyx activated carbon was investigated. This adsorbent was characterized by scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The impacts of pH, point of zero charge, initial dye concentration, the mass of biomass, contact time, and temperature on the adsorption of methylene blue by modified Solanum macrocarpon calyx activated carbon were determined. Analyses of the equilibrium data were done using Langmuir, Freundlich, Temkin, and Dubinin–Raduskevich models. The Langmuir model gave the best fit for describing the adsorption process. The maximum adsorption capacity obtained was 53.4 mg/g. Pseudo-second-order kinetic model was preferred to the pseudo-first-order model in explaining the adsorption kinetic process. Calculation of thermodynamic parameters such as enthalpy change Δ H , free energy change Δ G , and entropy change Δ S from experimental data showed that the adsorption process was exothermic, spontaneous, and also resulted in an increase in the distribution of the adsorption energy of the system at the adsorbate–adsorbent interface. The cost analysis showed that Solanum macrocarpon activated carbon ($37.73/kg) is eight times cheaper than commercial activated carbon ($259.4/kg). The modified Solanum macrocarpon is thus a recommendable adsorbent for removing methylene blue dye from aqueous solutions.