Experimental and computational chemistry studies on the removal of methylene blue and malachite green dyes from aqueous solution by neem (<i>Azadirachta indica</i>) leaves

Adsorption of methylene blue and malachite green dyes from aqueous solution by neem leaves was studied using experimental and computational chemistry approaches. The two dyes have N–H stretch, C≡C vibration vibration, C=C stretch and C–Cl stretch as common functional groups. Batch adsorption experiments were concluded by measuring absorbance (at wavelength of maximum absorption, i.e. 660 and 630 nm for methylene blue and malachite green dyes, respectively). The adsorption of the dyes on neem leaves favoured Langmuir and Temkin isotherms and was influenced by concentration, biomass dose, pH and by contact time, and favoured chemisorption mechanism. Although surface adsorption and intra particle diffusion were identified as the rate-limiting steps, a pseudo second-order kinetics was also favoured. Quantum chemical analysis supported better adsorption of methylene blue dye on neem seed surface than malachite green dye and through Fukui function analysis, the site for adsorption of the dyes was proposed.