Preparation of Starch-Hard Carbon Spherules from Ginkgo Seeds and Their Phenol-Adsorption Characteristics

Carbon spherules from ginkgo seed starch were prepared through stabilization and carbonization processes. The ginkgo seed starch was first stabilized at 195 °C for 18 h, then carbonized at 500 °C for 2 h under an N₂ atmosphere. The characterization results confirmed that carbon spherules were in the size range of 10-20 μm. Experimental data were also evaluated to find out the kinetic characteristics of phenols on the carbon spherules during the adsorption process. Adsorption processes for phenol, p-nitrophenol and p-chlorophenol were found to follow the pseudo-first order kinetic model with R² values of 0.995, 0.997 and 0.998, while the rate constants k₁ = 0.014, 0.009 and 0.011 min^-1 showed that the adsorption is mainly controlled by adsorbate diffusion. The equilibrium data were analyzed with the Langmuir, Freundlich and Temkin-Pyzhev models and the best fit was observed with the Freundlich isotherm, suggesting the physical adsorption of phenols. From the thermodynamic functions, ∆G, ∆H, and ∆S were calculated, which showed that adsorption is more favorable at low temperature and is an exothermic process, and the adsorption of p-nitrophenol and p-chlorophenol were more advantageous than that of phenol.