Adsorptive removal of catechol and resorcinol by orange flavedo (citrus sinensis): mechanisms based on the flavedo com-ponents d-limonene, carotenoids, ascorbates, flavonoids and hydroxycinnamic acids

Orange flavedo and its adsorption behavior towards catechol (Ctc) and resorcinol (Res) were studied. Adsorption experiments were conducted in batch mode at room temperature. X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), Brunauer？Emmett？Teller (BET) and Fourier transform infrared (FTIR) data were used to characterize the biosorbent. The Effects of various parameters including equilibrium pH, sorbent dosage, initial adsorbate concentration and contact time were investigated. The optimum contact time and pH for the removal of Ctc and Res were 35 min and pH 2 respectively. The adsorption isotherms fitted well with Freundlich model, the adsorption of Ctc and Res being multilayer and the surface of orange flavedo heterogeneous; the pseudo-second order kinetic model better reflects the adsorption phenomena. An adsorption mechanism based on the molecules grafted to the surface of the orange flavedo is proposed in this study. 0.2 gram of the biosorbent was sufficient to completely eliminate 2.2 milligrams of resorcinol and catechol from solution. Therefore, non-modified orange flavedo is a promising candidate, as a low-cost biosorbent, for the removal of Ctc and Res from aqueous solution.