Kinetic, equilibrium and thermodynamic investigations of Ni(II), Cd(II), Cu(II) and Co(II) adsorption on barley straw ash

This work reports the application of a straw ash from barley as a novel bioadsorbent for the removal of several heavy metals: Ni(II), Cd(II), Cu(II), and Co(II). Equilibrium and kinetic models for heavy metals sorption were developed by considering the effect of the contact time, initial heavy metal ion concentrations, effect of temperature, and initial pH. The adsorption of heavy metal ions have been studied in terms of pseudo-first- and -second-order kinetics, and the Freundlich, Langmuir and Langmuir–Freundlich isotherms models have also been used to the equilibrium adsorption data. The equilibrium data fitted well with the Langmuir–Freundlich model and showed the following affinity order of the material: Ni(II)>Cu(II)>Co(II)>Cd(II). The adsorption kinetics followed the mechanism of the pseudo-second-order equation for all systems studied, confirming chemical sorption as the rate-limiting step of adsorption mechanisms. The thermodynamic parameters (ΔG°, ΔH° and ΔS°) indicated that the adsorption of heavy metals ions were feasible, spontaneous and endothermic at 15–80 °C.