Investigation of the adsorption characteristics of Cr(VI) onto fly ash, pine nut shells, and modified bentonite

Industrial waste or natural materials used as adsorbents for wastewater treatment have the advantage of cost-effective, simple operation, and high efficiency. In this work, batch tests were performed to investigate the adsorption characteristics of Cr(VI) onto fly ash, pine nut shells, and acid-modified bentonite and explore the influence of the solid-to-liquid ratio, pH, ionic strength, and reaction time on adsorption. The results show that the removal rate of the three adsorbents increases with increasing solid-to-liquid ratio, while the unit adsorption capacity decreases. The unit adsorption capacity is maximized when pH = 1-3 and decreases with increasing Cl(- )concentration. The adsorption of fly ash, pine nut shells, and acid-modified bentonite is a rapid reaction process, of which reaches equilibrium at 180, 120, and 90 min, respectively. The maximum adsorption capacities of fly ash, pine nut shells, and acid-modified bentonite is 0.57, 6.06, and 10.55 mg/g, respectively. The adsorption of Cr(VI) onto fly ash is an exothermic reaction, and the adsorption onto either pine nut shells or acid-modified bentonite is an endothermic reaction. The adsorption of Cr(VI) onto fly ash can be described using the Langmuir, Freundlich, and Redlich-Peterson models, and the pine nut shells can be described by the Langmuir and Redlich-Peterson models, whereas the acid-modified bentonite can be described by the Langmuir, Temkin, and Redlich-Peterson models. The pseudo-second-order kinetic model can accurately describe the kinetic processes of Cr(VI) adsorption onto these adsorbents.