Surface characterization and comparative adsorption properties of Cr(VI) on pyrolysed adsorbents of Acacia mangium wood and Phoenix dactylifera L. stone carbon

The adsorbents were prepared from wood (Acacia mangium) and date (Phoenix dactylifera L.) stones by pyrolysis in the presence of purified nitrogen gas. The obtained adsorbents [physically activated wood carbon (WC) and date stone carbon (DC)] were characterized for their surface area through Brunauer–Emmet–Teller (BET) isotherms. The BET surface areas of WC and DC were 377.18 ± 2.48 and 59.42 ± 0.80 m2/g respectively. The surface morphology and functional groups of the adsorbents were respectively determined from the field emission scanning electron microscopy (FESEM) and Fourier transform infrared (FT-IR) analysis. Microcrystalline size of the pyrolysed adsorbents were also studied with the help of powder X-ray diffraction technique, and the average crystallite sizes were found to be 49.9 nm and 32.9 nm for WC and DC, respectively. Thermal stability of raw as well as pyrolysed material was analyzed by thermal gravimetric analysis (TGA) and differential thermograph (DTG) plots. The WC and DC have around 60% and 90%, respectively of thermally non-degradable constituents at a temperature up to 800 °C. Adsorption properties of Cr(VI) on WC and DC were studied by batch method. The effects of pH on the adsorption process studied showed that removal was favored at pH 2.0 for both adsorbents. The results showed that Cr(VI) removal followed the pseudo-second order model for both the adsorbents. Intraparticle diffusion kinetic model was also tested and it was found that WC followed the two stage diffusion whereas, DC followed single stage diffusion. Experimentally, the adsorption capacities of WC and DC were found to be 37.16 and 32.76 mg/g respectively. The applicability of the Freundlich isotherm for WC and Langmuir isotherm for DC adsorption behavior on Cr(VI) have been verified. The thermodynamic parameters (ΔG°, ΔH°, ΔS° and KC) values indicate the spontaneous and endothermic nature of Cr(VI) adsorption on WC and DC.