Adsorption/desorption behavior of ionic dyes on sintered bone char

The aim of this study was to investigate the regularity and differences in the adsorption/desorption behavior of bovine rib char (BC) as an adsorbent material for the cationic Malachite Green (MG) and anionic Sunset Yellow (SY) dyes. The effects of pyrolysis temperature, adsorbent dosage, solution pH, initial concentration, contact time, and adsorption temperature were investigated. The pore characteristics of BC were determined by the Brunauer-Emmett-Teller (BET) method. The changes in functional groups before and after the adsorption of BC were analyzed by Fourier transform infrared spectroscopy. And the chemical composition and surface morphology of BC were characterized by X-ray diffraction and scanning electron microscopy. The experimental data were further analyzed using different isotherm models, kinetic models, and thermodynamic equations, and an understanding of the adsorption studies was carried out. The results show that BC is 90% mesoporous with a BET specific surface area of approximately 150 m2/g. BC is rich in active sites at 700 °C and performs best in the adsorption process. The adsorption of MG increases with increasing pH, while the opposite is true for SY, with optimum pH values of 7 and 3, respectively. Ionic dyes bind to BC easily (the fitted values for the Freundlich model 1/n are all less than 0.5, R2 > 0.748). The maximum adsorption capacity (Qmax) of MG was much higher than that of SY (the Langmuir model indicated Qmax of 1168.4 mg/g and 47.0 mg/g, respectively, R2 > 0.830). The adsorption of ionic dyes on BC was mainly non-homogeneous surface chemisorption, which was spontaneous (ΔG<0, ΔH>0). The desorption of BC after adsorption of MG and SY in HCl and NaOH solutions, respectively, was better. Analysis of the adsorption contribution of the BC fraction showed that the mineral fraction played an important role in the adsorption process (about 94%), while the organic fraction contributed about 6%. The experimental results indicate that the adsorption mechanism of MG and SY on BC can be attributed to the filling of mesopores, electrostatic interaction between the ionic polar sites of BC and the different charged dyes, the formation of chemical bonds between the oxygen-containing functional groups on the surface of BC and the dyes, and the ionic exchange between the polar sites of BC such as Ca2+, PO43−, CO32− and OH−.