Lead ions and methylene blue dye removal from aqueous solution by mucuna beans (velvet beans) adsorbents

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.