Low Cost Adsorption of Phosphate on Vigna mungo Husk Surface, Kinetics, Equilibrium and Thermodynamics

The adsorption potential of Vigna mungo husk was evaluated for phosphate uptake from aqueous solutions. The vanadate–molybdate and liquid batch methods were used to investigate the effect of various physico-chemical factors such as adsorbent dose, initial phosphate concentration, temperature, pH, and contact time on the adsorption efficiency of Vigna mungo husk particle sizes 1, 2, and 3. Results on Temkin, Freundlich equilibrium, and Langmuir adsorption isotherms confirmed that the data fitted well to the Langmuir isotherm but was unfit to the Freundlich isotherm and the Temkin isotherm. The adsorption capacities (q_max) on monolayer coverage were found to be 10.4263, 10.3854, and 10.3608 mg/g for particle sizes 1, 2, and 3 at 301 K temperature. At 311 K temperature, the monolayer adsorption capacities were found to be 10.4345, 10.3936, and 10.3854 mg/g. At 321 K temperature, the monolayer adsorption capacities were found to be 10.1524, 10.410, and 10.4018 mg/g. At 331 K temperature the monolayer adsorption capacities were found to be 10.4837, 10.4182, and 10.410 mg/g. Similarly, at 341 K temperature, the monolayer adsorption capacities were found to be 10.5493, 10.4345, and 10.4263 mg/g. Pseudo-first-order and pseudo-second-order kinetics were analyzed at different time intervals and concentrations of phosphate. The kinetic data fitted well with the pseudo-second-order kinetics, as the calculated value of q_e in this model was observed very close to the experimental value of q_e. Outcomes on the thermodynamic factors (ΔG°, ΔH°, ΔS°) showed the spontaneous, feasible, and endothermic nature of adsorption.