Application of Thermally Modified Fly Ash for Adsorption of Ni(II) and Cr(III) from Aqueous Solution: Equilibrium,Kinetic,and Thermodynamic Studies

Abstract

Abstract

Urbanization results in an excessive release of wastewater and solid wastes into the environment. Reuse of fly ash, a coal combustion residue, in wastewater treatment is an example of using waste for waste treatment. In this study, thermally modified fly ash (TFA) was prepared by modifying raw coal fly ash using an environmentally friendly method. TFA was characterized via scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). Performance of TFA in the adsorption of Ni(II) and Cr(III) from an aqueous solution was also investigated. Results show that the specific surface area of TFA is 29.61 m²/g, which is nearly 30 times that of raw fly ash. New chemical groups, such as sodium silicate, sodium aluminum silicate, and calcium iron oxide, were generated on TFA compared with raw fly ash. Satisfactory correlation coefficients and relatively low chi-square analysis parameters between the experimental and predicted values of the Freundlich isotherm demonstrated that TFA adsorption of Ni(II) and Cr(II) was a multilayer chemical adsorption. Compared with the pseudo first-order model, the pseudo second-order kinetic model had higher correlation coefficients for both Ni(II) and Cr(II) adsorption by TFA. Equilibrium adsorption amounts of TFA for Ni(II) and Cr(II) were found to be 1.25 and 2.50 mg/g, respectively. Experimental values examined with intra-particle diffusion models show that there are two and three steps during the adsorption processes for Ni(II) and Cr(II), respectively. Thermodynamic results showed that adsorption processes were spontaneous for both Ni(II) and Cr(II) but were endothermic and exothermic, respectively. Desorption of TFA could be ignored. This article provides a basis for developing a new approach using an industrial solid waste for heavy metal removal from wastewater.