South African Journal of Chemical Engineering (Jul 2020)
Removal of methylene blue dye and lead ions from aqueous solution using activated carbon from black cumin seeds
Abstract
This work reports the preparation of carbon from black cumin seeds (BCC) and then activated with 10 and 20 % sulfuric acid (H2SO4) to obtain new adsorbents designated black cumin activated carbon (BCAC-10) and (BCAC-20) respectively for the adsorption of lead (Pb(II)) ions and methylene blue (MB) dye from aqueous solution. The adsorbents were characterized by scanning electron microscopy (SEM), Fourier transformed infrared (FIR), thermogravimetric analyzer (TGA), X-ray diffractometer (XRD) and Brunauer, Emmett and Teller (BET) analysis. The SEM images show that both activated carbon adsorbents (BCAC-10 and BCAC-20) have rough irregular surfaces with cavities. FTIR results show the (-COO−), (-NH2), (-HSO−4), (-C=O) functional groups were involved in the adsorption processes of Pb(II) ions and methylene blue (MB) dye . Nitrogen adsorption studies show that there is an increase in surface area and pore size for both BCAC-10 and BCAC-20 in comparison with BCC. The operational parameters (concentration, contact time, temperature and pH) of Pb(II) ions and MB dyes adsorption were assessed in batch mode. It was observed that the trends for Pb(II) ions and MB dye adsorption increased with increasing initial concentration of the solution, (i.e.) greater uptake was observed in solutions with higher initial concentration. Therefore the adsorption of Pb(II) ions with initial concentration of 100 mg/L was 17.19, 17.71 and 17.98 mg/g onto BCC, BCAC-10 and BCAC-20 respectively. Whilst for MB dye it was 11.63, 12.71 and 16.85 mg/g onto BCC, BCAC-10 and BCAC-20 respectively. The equilibrium data for Pb(II) ions and MB dye fitted Freundlich isotherm model onto BCC, BCAC-10 and BCAC-20. This suggested that the process involve multi-layer adsorption with interactions between the adsorbate and the adsorbents. The results show that the adsorption processes of Pb(II) ions and MB dye were rapid in the first 20 min. It was observed that the maximum adsorption trend of Pb(II) ions and MB dye was obtained at pH 9 which made the surface of black cumin seeds to be negatively charged and therefore making it easier to be attracted to positively charged Pb (II) ions and MB dye. Kinetic studies showed that the adsorption of Pb(II) ions and MB dye onto BCC best fitted; pseudo-first order model whilst BCAC-10 and BCAC-20 data fits; pseudo-second order. The estimated pore adsorption for Pb(II) ions onto BCC, BCAC-10 and BCAC-20 was 65.85, 94.28 and 78.43 % respectively whilst MB dye was 73.37, 70.82 and 95.74 % respectively. Temperature and thermodynamics data indicated that the adsorption capacity of adsorbents increase with increasing temperature and the enthalpy (ΔH°) values indicated that the adsorption of Pb(II) ions and MB dye on the adsorbents was endothermic reaction. The entropy (ΔS°) gave positive values which indicated the randomness and degree of freedom for Pb(II) ions and MB dye in aqueous solution. The free energy (ΔG°) values for the adsorption of Pb(II) ions onto BCC, BCAC-10 and BCAC-20 show an increase in negative values as the temperature increases.
