Removal of hazardous azocoumarin dye from aqueous solutions using activated carbon prepared from rice straw

Activated carbon made from rice straw (ACRS) was tested as a low-cost adsorbent for the removal of hazardous azocoumarin dye from aqueous solution in batch adsorption procedure. The effect of different variables in the batch method as a function of solution pH, contact time, concentration of adsorbate, adsorbent dosage, and temperature were investigated and optimal experimental conditions were ascertained. More than 85% removal efficiency was obtained within 65 min at adsorbent dose of 0.5 g for initial dye concentration of 100 mg L⁻¹ at pH 3. The Brunauer–Emmett–Teller surface area and Barrett–Joyner–Halenda pore volume were calculated and found to be 67.4 m² g⁻¹ and 0.134 cm³ g⁻¹, respectively. The point of zero charge (pH_PZC) of ACRS was determined and found to be 7. The experimental equilibrium data were tested by the isotherm models namely, Langmuir and Freundlich adsorption and the isotherm constants were determined. The results revealed that Freundlich isotherm fit the experimental results well. The kinetic data obtained with different initial concentration and temperature were analyzed using a pseudo-first-order and pseudo-second-order equations. The pseudo-second-order sorption kinetics was the best model and indicates that physorption controls the process. Thermodynamic parameters, such as Gibbs free energy, enthalpy, and entropy of adsorption of the dye-rice straw fly ash systems were evaluated and it was found that the reaction was spontaneous and exothermic in nature