Isotherms, Kinetics and Thermodynamics of Removal of Pb (II), Cd (II) and Cr (III) Ions in Wastewater onto Green Silver Nanoparticle Adsorbent

Moringa oleifera leaf extract mediated silver nanoparticles (MOR-AgNPs) were synthesized from a water-soluble leaf using a green technique. The plant's secondary metabolites served as capping and reducing agent. The spectroscopic characterization of the material was carried out by means of Scanning Electron Microscope (SEM), Ultraviolet-Visible Spectroscopy (Uv-vis), X-ray Diffraction Spectroscopy (XRD), Fourier Transform Infrared (FTIR) spectroscopy, Transition Electron Microscope (TEM), Energy Dispersion Spectroscopy (EDS), and Dynamic Light Scattering (DLS). The UV–Vis spectrum showed absorption peak between 400 – 500 nm, while TEM revealed spherical images of sizes in the nano region. The MOR-AgNPs adsorbent uptake of the metal ions was optimized by varying concentration, shaking time, adsorbent mass, pH, and temperature conditions. The isotherm models studies revealed that the type-1 Langmuir fitted well to the adsorption showing monolayer/homogeneous adsorption. The second order kinetics fitted well to the adsorption study, while positive values of ΔH° and ΔS° indicated endothermic and increased randomness at the MOR-AgNPs-aqueous interface. The ΔG° showed that the sorption of the metal ions onto the adsorbent was spontaneous, making the adsorption process feasible. The Ea values for the metal ion adsorption onto the adsorbent used in this study suggest a physical adsorption process. The adsorption capacity of 294.15 mg/g (40 °C, and pH of 6) of Cd(II), 123.63 mg/g (40 °C, and pH 10) of Pb(II), and 122.93 mg/g (40 °C, and pH 6) of Cr (III) ions on MOR-AgNPs adsorbent were obtained.