Kinetics, thermodynamics, and isotherm studies for the adsorption of BR2 dye onto avocado integument

Ability of avocado integuments (AI), an agricultural waste, to adsorb Basic Red 2 (BR2) was studied by batch process. Effect of parameters such as initial adsorbate solution pH, adsorbent dosage, initial adsorbate concentration, contact time, temperature, and background electrolyte on BR2 adsorption was studied. The BR2 adsorption was optimum (15.6 mg/g) at pH: 7. Acid-base titration studies showed dominance of acidic sites on AI surface. Two and three parameter isotherm models such as Langmuir, Freundlich, Toth, Redlich–Peterson, and Sips were applied to isotherm data. Studies showed Sips model applicability as revealed by linear, nonlinear isotherms, and statistical error functions. Contact times studies showed that 70% of BR2 adsorption at various concentrations on AI was achieved in 180 min. Pseudo-first-order, pseudo-second-order, and intra-particle diffusion models were applied to kinetics data. Results revealed applicability of pseudo-second-order kinetics model. Intra-particle diffusion plots at various BR2 concentrations were nonlinear over the entire contact time range indicating involvement of two or three steps during adsorption. Thermodynamic studies revealed endothermic and spontaneous process.