Chemically modified Azadirachta indica sawdust for adsorption of methylene blue from aqueous solutions

This study explores the competency of Azadirachta indica sawdust (A.I.S.D.) and NaOH-modified Azadirachta indica sawdust (N.A.I.S.D.) as the prospective adsorbents for the removal of methylene blue (M.B.) from aqueous solution. Prepared adsorbents were characterized by performing S.E.M./E.D.X.S., T.E.M./S.A.E.D., B.E.T., F.T.I.R., T.G.A.–D.T.G., X.R.D., pH pzc (point of zero charge), proximate, and component analyses. The proximate analysis indicates that N.A.I.S.D. has higher fixed carbon and cellulose, and lower ash, volatile matter, moisture, hemicellulose, extractive, and lignin than A.I.S.D. The operational parameters (contact time, pH, temperature, amount of adsorbent, and initial dye concentration) were varied. It was found that the equilibrium for the optimum dye removal was achieved in 120 and 75 min for A.I.S.D. and N.A.I.S.D., respectively. Numerous isotherm and kinetic frameworks were used to assess the experimental results. The adsorption of M.B. followed a Langmuir adsorption isotherm and pseudo-second-order kinetics. The thermodynamic variables, such as changes in free energy (ΔG°), entropy (ΔS°), and enthalpy (ΔH°) of adsorption, were assessed. Adsorption by both adsorbents was feasible, endothermic, and associated with an increase in entropy. Four adsorption–desorption cycles of M.B.-loaded A.I.S.D. and N.A.I.S.D. were successfully performed using HCl as a desorbing agent. The finding shows that N.A.I.S.D. is a cost-effective and efficient adsorbent for eliminating M.B. from an aqueous medium. Graphical Abstract