Preparation and Characterization of Microporous Activated Carbon from Biomass and its Application in the Removal of Chromium(VI) from Aqueous Phase

Water pollution has been a major challenge to environmental engineers today due to the release of toxic heavy metals from various industries. Among various heavy metals, hexavalent chromium [Cr(VI)] is considered as highly toxic due to its carcinogenicity and various health disorders. Different sources of Cr(VI) pollution include effluents from mining, electroplating, leather tanning and electroplating industries. Among various technologies, adsorptive removal of Cr(VI) by using different adsorbents is more promising and economical. Among various adsorbents used, activated carbon (AC) is well known for its high adsorption capacity due to large surface area and pore volume. In recent years, immense research has been focused towards converting the agricultural or lignocellulosic wastes into activated carbon, since this technology not only solves the problem of waste disposal but also converts a potential waste into a valuable product that can be used as an adsorbent for effluent treatment. Bael fruit (Aegle Marmelos) shell, a lignocellulosic material was selected as the precursor for the preparation of microporous (< 2 nm) activated carbon in the present investigation. Activated carbon was prepared through chemical activation using phosphoric acid (AC-PA), zinc chloride (AC-ZC), and potassium hydroxide (AC-PH). The effect of various process parameters such as impregnation, carbonization temperature, and holding time on porous characteristics of the activated carbon was investigated. Characterization of AC by N2 adsorption-desorption isotherms at 77 K was carried out. Total surface area, micropore and mesopore surface area, total pore volume, micropore and mesopore volume, and pore size distribution of the samples were determined. The porous characteristics of prepared ACs were analyzed by applying various isotherm equations like Brunauer-Emmett-Teller (BET), Dubinin-Radushkevich (DR), and Dubinin-Astakhov (DA). The AC-PA prepared at optimum conditions (30% impregnation, 400oC carbonization temperature and 60 min holding time) constitutes highly microporous structure with micropore surface area(1625 m2/g) and micropore volume (0.56 cc/g). The pore size distribution of the prepared samples at optimized conditions shows that samples are comprised greatly with micropores(average pore diameter of 1.68 nm, 1.69 nm, and 1.54 nm for AC-PA, AC-ZC, and AC-PH, XVIII respectively). The Scanning Electron Microscope (SEM) analysis revealed the heterogeneous surface structure of the samples and clearly depicts the presence of macropores which acts as channels to the microporous network while TEM analysis visualized the presence of micropores. The surface groups present on the AC surface were determined by the Fourier Transform Infrared Spectroscopy (FTIR) analysis. The proximate and ultimate analyses were carried out by using CHNS analyzer and by standard methods, respectively. The batch adsorption studies of Cr(VI) on ACs were carried out at optimum conditions by H3PO4, ZnCl2 and KOH activations. The effect of various process parameters like pH, initial metal concentration, adsorbent dose, contact time, and temperature on the efficiency of Cr(VI) removal was investigated. Maximum adsorption of Cr(VI) on AC-PA (98.74 %) was observed at pH 2 . No significant change in efficiency was observed for AC-PA in pH range 2.0 – 5.0 and the 76 % removal was observed at neutral pH. The optimum conditions for adsorbent dose and temperature were determined as 3.0 g/L and 30 oC, respectively. Initial Cr(VI) concentration has no effect on AC-PA in the studied range (2 – 10 mg/L) and in case of AC- ZC and AC-PH, 8.0 mg/L and 2.0 mg/L were determined as optimum concentrations. The time required to reach equilibrium is different for different ACs (90, 150, and 240 min for AC- PA, AC-ZC and ACPH, respectively). The adsorption equilibrium data was well explained by Freundlich isotherm and DR isotherm parameters suggested that the adsorption of Cr(VI) on prepared ACs is physical adsorption. The kinetic data were better followed the pseudo-second order kinetics and both film and pore diffusion mechanisms played important role. The exothermic nature and the randomness of the process were estimated from thermodynamic parameters. The porous characteristics and Cr(VI) removal efficiencies of prepared AC were very high compared to the commercial AC. The spent activated carbon was regenerated by using hot water (80 oC) and mild acid (0.1 M H2SO4) and the adsorption capacities and porous characteristics were compared with the mother sample. Chromium(VI) adsorption process was modeled through Designing of Experiments(DoE) by using Full Factorial Design (FFD). Factorial design was used to reduce the number of experiments in order to achieve the best overall optimization of the process. Two-level and fourfactor full factorial design was used to develop model equations for Cr(VI) removal by using Design Expert 7.1.6 software. The interpretation of effect of main factors and their interactions was carried out and the developed models were validated by conducting experiments at the predicted conditions. For AC-PA, along with main factors interactions such as pH * concentration of Cr(VI), concentration of Cr(VI) * adsorbent dose, and pH * concentration of Cr(VI) * adsorbent dose * temperature were found to be significant on the response