| Iron oxide coated granular activated carbon (IOCGAC) was synthesized by iron salt solution immersion-heating technique and characterized by SEM/EDAX, XRD, FTIR and surface analyzer. It was found that the particle size of the iron oxide coated on GAC ranged from about 200nm to 2.5μm. The surface elements of IOCGAC were O, Si, Al ,S and Fe, with Cr added after adsorption. The dominate crystalline phases of iron oxide on IOCGAC was hematite (α-Fe2O3) and didn't change after adsorption of Cr(VI). The FTIR study indicated that the peaks positioned at 471 cm?1and 573 cm?1(563 cm?1) corresponded to the characteristic of crystallineα-Fe2O3. The band around 1612cm-1 was assigned to the bending vibration of water molecules . The bands around 1045 cm?1 and 915 cm?1 were attributed to the bending vibration of Fe—OH group and the peaks positioned at 943cm?1and 839cm?1 represented the stretching vibration of Cr–O. Compared to 811.89m2/g of GAC, the special surface area of IOCGAC was 312.24m2/g. In addition, all of the porosity, micropore volume, mescpore volume and macropore volume have decreased as much as 1/2~2/3 after iron oxide coated on the surface of GAC.Adsorption of Cr(VI) on IOCGAC were performed by batch experiments under different conditions. Significantly improving adsorptive removal efficiency of Cr(VI) were observed with the reduction of pH, increase of adsorbent dosage and temperature. When the initial concentration of Cr(VI) rose from 10mg/L to 200mg/L, the removal efficiency decreased from 99.99% to 62.33%. Competitive ions, such as Na+,Ca2+, Mg2+,Cu2+,NO3- and Cl- had little effect on the adsorptive efficiency of Cr(VI), while the existence of SO42- reduced the removal efficiency obviously. The adsorption procedure can be divided into two steps: the first one was the quick adsorption stage in which the removal efficiency increased with adsorption time linearly; the second one belonged to later stage in which the adsorption rate increased slowly. Under three different test temperatures of 283K, 293K and 313K, the data for Cr(VI) were well fitted to the Langmuir adsorption isotherm, Compared to other reagents, 0.2mol/L NaOH was the best desorption reagent and the adsorption capacity of IOCGAC reduced from 18.30mg/g to 17.56mg/g after six cycles of adsorption-desorption experiments.The kinetic data for Cr(VI) could be well described by pseudo-second order rate model. Both the initial adsorptive rate v0 and pseudo-second order rate constant k 2 increased with the increase of temperature. The adsorption procedure included fluid film diffusion and intraparticle diffusion and the former is the controlling step of the overall adsorption rate. In addition, the intraparticle diffusion procedure was composed of pore diffusion and surface diffusion whose intensity was about equivalent to pore diffusion's.Thermodynamic investigations showed that the adsorption of Cr(VI) on IOCGAC was spontaneous and endothermic. The activation energy was 10.03kJ/mol and the average adsorption energy under three test temperatures (283K, 293K, and 313K) was 14.74 kJ/mol,15.81 kJ/mol and 22.36 kJ/mol respectively.The mechanism of removal of Cr(VI) using IOCGAC was adsorption other than redox. The complexation between Cr(VI) and IOCGAC included both outer-sphere and inner-sphere. The differences of FT-IR spectrum before and after adsorption showed that the Fe-OH band was the active adsorption site.
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