Prous Carbon as Sequestrant of Dyes and Heavy Metals: Theoritical and Experimental Study

Abstract

In this study coal and starch-based activated carbons (AC) with surface area of 3452.8 and 1600 m2 g -1 are synthesized and used for entrapping of azo heavy metals and dyes from spiked water. The synthesized adsorbents are characterized through different characterization techniques like FT-IR, SEM, TEM, AFM, BET/BJH surface area, zeta potential, zeta sizer, XRD, EDS and TGA before and after loading of adsorbates. The coal and starch-based ACs exhibited -18.7 and -16.9 mV, surface charges respectively, which are enough for making suspensions. Their average particle sizes and polydispersity indexes, representing sufficient variation in sizes. The coal and starch-based AC are used as potential adsorbents for the entrapping of azo dyes, Acid Black 234 (AB-234), Acid Black 210 (AB-210), Acid Brown 98 (AB-98) and Brown 704 (B-704), heavy metals i.e. HCrO4 - ions and Cd(II) ions and mixtures of dyes and heavy metals and mixture of dyes from wastewater. This study involves the combined experimental and computational study for the better understanding of the adsorption mechanism involved. The maximum uptake (333 mg g -1 ) of Azo dye B-704 by sorbent is observed in acidic medium. The thermodynamics parameters like ∆G, ∆H, ΔS were found to be -12.40 kJ mol-1 , 39.66 kJ mol-1 and 174.55 J mol-1 K -1 at 293 K respectively, reveals that adsorption is spontaneous, endothermic and feasible. The experimental data follows the Langmuir and D-R models and pseudo 2nd order kinetics. DFT investigation shows that the dye sorption onto AC in configuration No. 4 (CFG-4) is more effective, as this configuration has high ∆H (enthalpy change) and adsorption energy (Eads). This is confirmed by Mullikan atomic charge transfer phenomenon. The 99% of AB-234 is adsorbed within 35 minutes equilibration time and 50mg of the adsorbent. Followed Langmuir, D-R and Freundlich isotherms. The thermodynamic computations showed feasible, endothermic and spontaneous adsorption confirmed by DFT calculations. The simultaneous removal of mixture of dyes AB-210 and AB-98 is also explored, ~ 99% of both the dyes are removed at pH 3 within 40 minutes. The adsorption process was spontaneous and endothermic following Freundlich and Langmuir isotherms. HCrO4 - ions are effectively removed from water by using coal derived AC. The maximum of 194 mg g-1 is observed in acidic medium. The ∆G, ∆H, ΔS are found to be -15.54 xvi kJmol-1 , 32.74 kJ mol-1 and 161.95 Jmol-1K -1 at 293 K, revealing that the spontaneous, endothermic and feasible nature of adsorption. The experimental data follows the Langmuir, Freundlich and D-R models and pseudo 2nd kinetics. DFT investigations confirmed the process and demonstrates that the AC is a potential sorbent for HCrO4 - ion adsorption. The competitive entrapping of HCrO4 - ions and AB-98 onto coal based AC is explored. The adsorption obeyed Freundlich and Langmuir isotherms. The maximum adsorption (~99%) was obtained under acidic pH only in 90 minutes. The thermodynamic parameters like ΔG values are -61.68 and - 58.68 kJ mol−1 and ΔH is 50. 158 and 45.16 kJ mol−1 for HCrO4 - ion and AB-98 respectively showing the spontaneous and endothermic nature of adsorption. The ΔS (197.95 J mol-1 K -1 for HCrO4 - ion and 187.64 J mol-1 K -1 for AB-98) showed enhanced entropy reflecting sorbate/sorbent complexation stability. The coal-based AC is effectively regenerated and was stable after using for 4 adsorption desorption cycles. The starch derived AC is used to entrap Cd(II) ions form water. Maximum adsorption (~98 %) was achieved within 40 minutes from pH 7 solution. The adsorption results obeyed Langmuir and Freundlich isotherms and the sorption capacity of the material is 151.5 mgg-1 . The magnitude of ΔG is -15.75 kJ mole-1 , showing that phenomenon of the Cd(II) ion adsorption on to AC is spontaneous. The value of ΔH is 6.419 kJ mole −1 , revealing that the adsorption phenomenon is endothermic. The ΔS was 55.66 Jmole -1 -K -1 , reflecting the sorbent/sorbate interaction.