Optimization, kinetics, equilibrium isotherms, and thermodynamics studies of Coomassie violet dye adsorption using Azadirachta indica (neem) leaf adsorbent

Abstract

Removal of color from dye wastewater is of primary importance because they are highly toxic and carcinogenic to many life forms. To protect the environment, studies need to be conducted to make the use of inexpensive adsorbent for the removal of color from dye wastewater. To the best of our knowledge, there has practically been no work reported for describing the potential of using neem leaf fine powder (NLP) as adsorbent for the decolorization of Coomassie violet (CV) from wastewater either in batch or continuous mode. The present batch study is concerned with the removal of CV dye from synthetic aqueous solutions using NLP as an adsorbent. Decolorization experiments were conducted by varying experimental factors such as initial pH, initial dye concentration, adsorbent dosage, particle size, and agitation speed. The process parameters were optimized using response surface methodology to attain the maximum % decolorization. Further, the batch experiments were conducted to study the effect of a mixture of dyes, and temperature on dye decolorization. The adsorbent was characterized by Brunauer–Emmett–Teller surface area, pore-volume, particle size, attenuated transmission reflector, field emission scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray diffraction, and thermogravimetric analysis. The experimental equilibrium data were analyzed with various isotherm models. The results showed that the best fit was achieved with the Langmuir isotherm model with the maximum monolayer adsorption capacity (qmax) was 39.64 mg g–1 at 303 K. Thermodynamic studies were performed to determine the change in Gibbs free energy (ΔG), change in enthalpy (ΔH), and change in entropy (ΔS) of the adsorption process. Thermodynamic parameters were evaluated using qmax at different temperatures. From the results, the adsorption was found to be spontaneous, endothermic in nature and favored at high temperatures. The value of ΔH and activation energy confirmed that the studied adsorption process was chemisorption. Kinetic rate constants were found using different kinetic models. The adsorption kinetics for CV dye removal by NLP adsorbent follows a pseudo-second-order kinetic model. The adsorption mechanisms were described by pore diffusion, Bangham and Boyd plots. The overall rate of adsorption was controlled by both film diffusion and pore diffusion of dye molecules. It was found that external film diffusion controlled the dye uptake in the earlier stages, followed by pore diffusion, which controlled the rate at later stages. A number of various desorbing reagents were tested to explore the possibility of regenerating the NLP adsorbent and ethanol had the maximum desorption efficiency. The reusability studies of NLP adsorbent for the adsorption of CV was carried out in three runs. The adsorption of solute from textile industrial Congo red dye effluent was carried out in batch studies using NLP adsorbent. The chemical oxygen demand removal efficiency of industrial dye effluent was 69.18%.