Synthesis of Activated Carbon from Lignocellulosic Biomass for Iron Removal from Aqueous Phase

Abstract

The preparation of activated carbon from an economically sustainable precursor Limonia acidissima shell (an agricultural waste) by chemical activation method has been explored in the present study. This investigation emphasized the application of a statistical tool, RSM, coupled with Box-Behnken design (BBD), to optimize the experimental conditions for the preparation of activated carbon from lignocellulosic biomass activated with H3PO4 and ZnCl2. The chemical activation was conducted at a different combination of impregnation ratios (IR), activation time, and carbonization temperatures, as suggested by the Design-Expert software version 7 (Stat-Ease, Minneapolis, U.S.A). The influences of these parameters on the responses, i.e., yield% and iodine no. of activated carbon were investigated. Simultaneous optimization was performed using the desirability approach in the multi-response optimization technique. The desirability value reported for AC-H3PO4 (0.762) was higher than AC-ZnCl2 (0.653). The carbon yield and iodine adsorption value of AC-H3PO4 approached 42.63% and 951.9 mg/g under the optimal conditions of IR (24.21%), activation time (39.83 min), and carbonization temperature (432.4 oC). N2 adsorption (77K) was carried out to determine the pore characteristics of the optimized AC-H3PO4 and AC-ZnCl2 that showed phosphoric acid produced activated carbon with a higher BET surface area (1863.49 m2/g) compared to zinc chloride activation. Important physicochemical properties of both optimized activated carbons were further confirmed by zero-point charge, FTIR, XRD, and TEM Analysis, etc. The removal efficacy of the adsorbent for total Fe ion was studied in batch mode as a function of pH, contact time, initial concentration, adsorbent dosage, and temperature. The findings indicated that the adsorption procedure could be well-defined by Langmuir isotherm and pseudo-second-order kinetic model because of the high coefficient of determination (R2) value is 0.99. The maximum adsorption capacity of total Fe ion by optimized AC-H3PO4 was determined as 48.5 mg/g. The iron ions adsorbed on the surface of the carbon studied by XPS analysis gives the main band positioned at B.E. of 711.8 eV accompanied by secondary one displaced by 13.2 eV to higher B.E. (724.98 eV) with an area ratio of 1:0.5 which bore a resemblance to the characteristic values of Fe3+ in addition to associated satellite peak at around 715.99~716.0 eV approving the presence of Fe3+ as well as a small fraction of Fe2+ present on the carbon surface. The pilot-scale column was fabricated for fixed-bed adsorption x iron ions from aqueous solution in an up-flow mode. The effect of essential factors such as bed height, inlet concentration, and flow rate on the performance of the column bed was investigated. The adsorption capacity augmented with an increase in bed height and initial adsorbate concentration but declined with an increase in flow rate. The maximum uptake capacity of 209.6 mg/g was achieved at 5 cm bed height, 3.32 mL/min, and 50 mg/L initial concentration. The bed depth service time (BDST) model was used to determine the characteristic parameters of the reactor suitable for designing large-scale column studies. The Adams-Bohart, Thomas, and Yoon-Nelson models were applied to the experimental information to predict breakthrough curves using non-linear regression. The ANN-based model was able to efficaciously predict the column performance using the Levenberg-Marquardt (LM) algorithm. A comparison between the preliminary data and model results contributed to a high degree of correlation.