Kinetics, equilibrium and thermodynamic study on removal of Cr (VI) from aqueous solutions using low-cost adsorbent Alligator weed

The removal of Cr (VI) from aqueous solutions using Alligator weed, a freshwater macrophyte, was investigated as a function of initial pH, contact time, reaction temperature and adsorbent concentration in batch studies. An initial solution pH of 1.0 was most favorable for Cr (VI) removal. The kinetic experimental data fitted the pseudo-second-order equation, Elovich equation and Langmuir–Hinshelwood equation very well. The adsorption of Cr (VI) onto Alligator weed conformed to the linear forms of the Langmuir, Freundlich and Temkin equations. The removal efficiencies increased with the increased adsorbent dose from 1 to 8 g/L and were 86.6, 97.6 and 99.7% at the adsorbent dose of 8 g/L, solution pH 1.0 and temperatures of 30, 40 and 50 °C, respectively. Thermodynamic parameters (activation enthalpy change, activation entropy change and activation free energy change) revealed that the adsorption of Cr (VI) onto Alligator weed is endothermic, non-spontaneous, with a decreased randomness in nature.