Activated Carbon-Modified Iron Oxide Nanoparticles for Cr(VI) Removal: Optimization, Kinetics, Isotherms, Thermodynamics, Regeneration, and Mechanism Study

In this study, the waste neem leaves (Azadirachta indica) gained visible attraction for neem leaf-activated carbon (NLAC) preparation in the thermal process by using ammonium carbonate as an activating agent. Using NLAC, a nanocomposite of NLAC/Fe2O3 was synthesized by hydrothermal techniques for Cr(VI) elimination from the aqueous solution. The formation of NLAC/Fe2O3 composite was additionally confirmed by the use of various powerful investigative techniques such as FE-SEM, VSM, TEM, EDX, FT-IR, XRD, and BET. The NLAC/Fe2O3 has a saturation magnetization of 1.18 emu/g and surface area and pore volume of 245.2 m(2)/g and 0.038 cm(3)/g, respectively. The composite has crystalline structure and the average particle size of Fe2O3 present on the activated carbon surface was about 14-17 nm. The BBD (Box-Behnken design) model was used for Cr(VI) removal and the obtained R-2 and adj-R-2 values for the model were very high about 0.9805 and 0.9555. The ANOVA data indicated that the Cr(VI) model was a statistically significant one with the F and p values of 39.17 and 0.0001. About 91.4% of Cr(VI) was removed by NLAC/Fe2O3 composite in the optimized condition of 0.05 g of adsorbent dose, solution pH 3.58, and initial Cr(VI) solution of 10.42 mg/L respectively. The adsorption of Cr(VI) on NLAC/Fe2O3 composite was imitated by the Langmuir isotherm model (R-2 = 0.98), proceeded with the pseudo-second-order kinetics model (R-2 = 0.99), and the maximum uptake capacity as calculated from the isotherm was 188.71 mg/g. The adsorption process was endothermic and adsorbent can be used for up to four cycles. This study showed that NLAC/Fe2O3 composite could be employed as an effective, affordable cost adsorbent to remove pollutants from wastewater.