Adsorption of methylene blue dye from aqueous solutions onto walnut shells powder: Equilibrium and kinetic studies

The efficiency of walnut shells powder as a low-cost adsorbent for removing a cationic dye methylene blue (MB) from aqueous solution has been investigated by using batch mode experiments. The effects of different parameters namely amount of adsorbent, dye concentration, contact time, temperature, pH solution and ionic strength of dye solution have been studied to understand the adsorption behavior of the adsorbent under various conditions. Various techniques for characterizing the adsorbent have been applied including, zero point charge measurement (pHzpc), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) coupled by energy dispersive X-ray spectroscopy (EDX). The experimental isotherm data have been analyzed using Langmuir and Freundlich isotherm equation. The maximum monolayer adsorption capacity was 178.9 mg/g for 100 mg/L of MB concentration, pH = 6.8 at a temperature 20 °C and 0.5 g/L of adsorbent. The kinetic results of adsorption obeyed a pseudo-second order model. Methylene blue adsorption fitted the Langmuir isotherm. Thermodynamic parameters including the Gibbs free energy ΔG°, enthalpy ΔH° and entropy ΔS° have revealed that the adsorption of methylene blue on the walnut shells powder is feasible, spontaneous and exothermic. In addition, the economical and environmental aspects of used walnut shells powder adsorbent underlines the re-use significance taking into account the material capacity to regenerate.