Abstract
Mucuna beans (MB) adsorbents were developed for the adsorption of lead Pb(II) and methylene blue from aqueous solutions. The pristine untreated mucuna beans were labelled; (UT-MB), acid treated mucuna beans; (AT-MB), base treated mucuna beans (BT-MB), 200 °C carbonized mucuna beans (UT-MB 200) and 300 °C carbonized mucuna beans (UT-MB 300). The adsorbents were characterized by SEM, FTIR, TGA, BET and XRD. The SEM images of AT-MB and BT-BM show spherical particles with some showing amorphousness, whilst UT-MB, UT-MB 200 and UT-MB 200 images, show irregular morphology. The FTIR results confirmed the presence of –COOH, –OH, –NH2 and –C = O functional groups on all adsorbents. The BET data indicate an increase in surface area and pore width for AT-MB of 8.25 m2/g and 4.65 nm in comparison with BT-MB (4.09 m2/g and 3.20 nm) and UT-MB (3.52 m2/g and 3.79 nm). It is observed that the adsorption data for Pb(II) ions onto UT-MB, AT-MB and BT-MB better fitted Freundlich, whilst onto carbonized adsorbents UT-MB 200 and UT-MB 300 fitted Langmiur model. Methylene blue adsorption onto AT-MB, UT-MB 200 and UT-BM 300 better fitted Freundlich and onto UT-MB and BT-MB fitted Langmiur model. The adsorbents maximum capacity trend for Pb(II) was UT-MB > BT-MB > UT-MB200 > UT-MB300 > AT-MB and for methylene blue was BT-MB > UT-MB > UT-MB200 > AT-MB > UT-MB300. The kinetic model PSO better fitted the adsorption of Pb(II) and methylene blue than PFO model. The thermodynamic parameter (ΔH°) for Pb(II) ions adsorption onto all adsorbents was exothermic, hence insignificant adsorption was observed as temperature increased from 298 to 352 K and methylene blue adsorption onto UT-MB, AT-MB and BT-MB were also exothermic. However UT-MB 200 and UT-MB 300 were endothermic implying that temperature increase was expected to enhance removal uptake as temperature increased from 298 to 352 K. Also the obtained (ΔH°) magnitude for the adsorption of Pb(II) and methylene blue suggested a weak interactive force process whose low energy is associated with electrostatic attraction. The (ΔG°) for both Pb(II) and methylene blue adsorption onto all adsorbents suggested the uptake processes were spontaneous and feasible.
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