Adsorption of methylene blue on chemically modified lychee seed biochar: Dynamic, equilibrium, and thermodynamic study

Abstract

Contamination of drinking water due to methylene blue (MB) dye is a severe health hazard. In the present work, biochar from lychee seed was synthesized by chemical activation of KOH and used for the abatement of MB dye from aqueous solution. The chemical, morphological, and textural properties of the biochar were characterized using FTIR, RAMAN, Zeta potential, XRD, SEM, FESEM, and N2 sorption isotherm. The SEM and FESEM images clearly showed the increase in pore density after activating with KOH, further supported by the N2 sorption isotherm analysis. The batch adsorption experiment was carried out to determine the optimum conditions for MB dye adsorption. The adsorption process followed Langmuir isotherm and Pseudo second order kinetic model well with maximum Langmuir adsorption capacity of 124.5 mg g−1 at room temperature. The detailed kinetic study proposed that intraparticle diffusion is not only the rate determining step, but the chemical interaction or external mass transfer also influences the adsorption process. XPS spectra, before and after adsorption confirmed the definite interaction of MB dye onto the biochar. The mechanism for adsorption on MLS was evaluated and ascribed to electrostatic interaction, pi-pi interaction, and hydrogen bonding interaction. The thermodynamic study showed that the adsorption was endothermic and spontaneous. The above results showed that the biochar could be effectively used as a low-cost adsorbent for the removal of MB dyes from aqueous solutions.