Chitosan-ZnO decorated Moringa oleifera seed biochar for sequestration of methylene blue: Isotherms, kinetics, and response surface analysis

Abstract

The biochar adsorption process is very efficient in the treatment of wastewater treatment. In this study, Moringa oleifera seeds-derived biochar (B) was modified using chitosan-ZnO (CZ), denoted as CZ-B, applied for the removal of methylene blue (MB) dye in an aqueous solution. The biochars were characterized using spectroscopic, Scanning electron microscope (SEM) and X-ray diffraction (XRD) techniques. Response surface methodology (RSM) was applied to explore the impact of time, concentration, and pH on the sorption of MB. The surface functional groups, e.g., hydroxyls, nitriles, carbonyls, carboxylic, ester, and phenol structures were enhanced as observed from spectroscopic results. The irregular porous surface of biochar, the uniform orientation of spherical-shaped CZ particles, and agglomerated chunks of porous rock in CZ-B particles were identified from SEM analysis. XRD patterns indicated a fully amorphous structure of biochar, whereas crystallinity introduced in the CZ-B structure by incorporating CZ was observed. At pH 8, the sorption of MB on all the sorbents was maximum. MB removal efficiency (% R) of biochar, CZ, and CZ-B were 44 %, 47 % and 89.7 %, respectively. The CZ incorporation has increased the MB removal efficiency by ∼104 %. Biochar preferred pseudo-second-order and intraparticle diffusion models, indicating a dual mechanism of chemisorption and pore diffusion, whereas CZ-B followed the pseudo-2nd-order model. Biochar and CZ followed Langmuir isotherm with sorption capacity of 55.10 and 55.85 mg/g, respectively; the Freundlich model explained the sorption isotherm of CZ-B with 135.20 mg/g sorption capacity. RSM analysis for CZ-B revealed that experimental results were best fitted (R2 = 0.9729) into a quadratic 2nd-order polynomial model. The correlation of the actual to the predicted value indicated a reasonable response estimation. Therefore, with the optimization of process parameters, CZ-B can be applied as an efficient sorbent to eliminate organic dyes.Main findings of the work: Modification of M. oleifera seed-originated biochar by chitosan-ZnO composite results in an excellent ternary composite with enhanced sorption properties against organic dye, methylene blue (MB).