Isotherm, kinetics, process optimization and thermodynamics studies for removal of Congo red dye from aqueous solutions using Nelumbo nucifera (lotus) leaf adsorbent

The present study is concerned with the batch adsorption of Congo red (CR) from aqueous solutions using Nelumbo nucifera (lotus) leaf powder (LLP) adsorbent. Decolorization experiments were conducted in batch mode by varying experimental factors such as initial pH, adsorbate concentration, adsorbent dosage, particle size and agitation speed. The experiments are designed to attain the most optimized system using response surface methodology. The maximum adsorption efficiency is obtained at an initial pH: 6, adsorbent dosage: 6 g L-1, adsorbent particle size: 42 mu m and agitation speed: 150 rpm. The prepared adsorbent was characterized by proximate, particle size, Brunauer-Emmett-Teller surface area and pore volume, attenuated transmission reflector, field-emission scanning electron microscopy/energy-dispersive X-ray spectroscopy, and thermogravimetric analysis. The zero-point charge of the adsorbent was found to be at pH 6.2. The isotherm and kinetic rate constants are found using several isotherm and kinetic models, respectively. The dye adsorption rate follows a pseudo-second-order kinetic model, and the equilibrium data fit the Langmuir isotherm model with a maximum monolayer adsorption capacity (q(max)) of 45.89 mg g(-1) at 303 K. The adsorption mechanisms were described by pore diffusion, Bangham and Boyd plots. The overall rate of adsorption is controlled by both film and pore diffusion of dye molecules. Based on the thermodynamic analysis, the adsorption was found to be endothermic in nature, and the process was chemisorption, spontaneous and favored at high temperatures. Desorption studies were conducted with various desorbing reagents and the maximum desorption efficiency was determined using the solvent methanol. The adsorption of solute from real textile industrial CR dye effluent was carried out in batch studies using LLP adsorbent. The chemical oxygen demand removal efficiency of industrial dye effluent was 67.35%.