Moringa oleifera Alginate-Based Beads for Adsorption of Selected Heavy Metals: Preparation, Kinetics, Isotherms, and Reusability Studies

Abstract

Moringa oleifera (MO) leaf powder, a low-cost natural resource, has been extensively used for the removal of heavy metals from water. To overcome challenges associated with powdered adsorbents, including difficult separation and low recovery, this study details the synthesis and evaluation of MO-alginate (MO-Alg) bead adsorbents for the adsorption of arsenic, cadmium, iron, chromium, zinc, and lead from oil-polluted aqueous samples. The beads were prepared through ionic gelation using sodium alginate (SA) as an encapsulation matrix with varying MO loadings. SEM analysis revealed that MO-Alg beads were spherical with a rough and cracked surface texture compared to smooth pristine sodium alginate (SA) beads, while XRD analysis confirmed the amorphous nature of both materials. At pH 5, MO (4 g) beads achieved the highest adsorption efficiencies. Kinetic studies followed the pseudo-second-order model, while equilibrium data fitted the Langmuir model for As(III) and Cr(VI) and the Freundlich isotherm model for Cd(II), Pb(II), Zn(II), and Fe(III). The Langmuir isotherm model estimated maximum adsorption capacities of 44.22, 61.11, 41.16, 59.14, 55.22, and 76.69 mg/g for As(III), Cd(II), Cr(VI), Fe(III), Pb(II), and Zn(II), respectively, at 25 °C. Negative ΔG° and positive ΔH° values over 25–45 °C confirmed that the adsorption process was spontaneous and endothermic. Reusability tests demonstrated that MO-Alg (1:1) beads maintained strong adsorption performance after seven regeneration cycles with removal efficiencies above 70% for Pb(II), Cd(II), and Zn(II), while As(III), Cr(VI), and Fe(III) remained effective up to six cycles with efficiencies above 60%. Overall, MO-Alg beads have great potential as an inexpensive sorbent for the removal of toxic heavy metals.