ADSORPTION OF CADMIUM (II) AND ZINC (II) IONS BY BOTTOM ASH IN SINGLE AND BINARY SOLUTIONS

Abstract

Bottom ash is a waste material obtained from electricity generation through combustion of coal at the Mae Moh power plant in Lampang province. In this research studied, bottom ash was used as a low cost adsorbent for the removal of Cd (II) and Zn (II) ions from single and binary aqueous solutions. The chemical and physical properties of bottom ash such as pH, density, moisture content, cation exchange capacity (CEC), particle size distribution and specific surface area were investigated. It was found that bottom ash had a slightly alkaline pH value, light weight and low moisture content. The CEC value of bottom ash was 10.6 cmol/kg. The particle size distribution and the specific surface area were 24.6 µm and 5.14 m2/g, respectively. The major chemical compositions of bottom ash as characterized by X-ray fluorescence spectrometry (XRF) were SiO2, CaO, Al2O3 and Fe2O3. The single and binary adsorption of Cd (II) and Zn (II) ions from aqueous solutions onto bottom ash was studied using batch experiments. The effect of pH (4-6), contact time (10-300 min) and temperature (10-40 ºC) were investigated. The optimum pH for metal ions adsorption by bottom ash was pH 6. The equilibrium adsorption was achieved within 60 min. The kinetics of adsorption process was tested by pseudo-first- order and pseudo-second-order models. The results revealed that the adsorption kinetics followed pseudo-second-order model. In the single system, Langmuir and Freundlich isotherms were applied to evaluate adsorption equilibrium data at different temperatures. Based on the correlation coefficient (R2) values, both isotherms suitably described the adsorption equilibrium data. Furthermore, the adsorption capacity of bottom ash for Zn (II) ion was greater than that of Cd (II) ion. The thermodynamic parameters such as enthalpy change (∆H°), entropy change (∆S°) and Gibbs free energy change (∆G°) were calculated from the experimental data. The positive value of ∆H° corroborated that the adsorption process was endothermic, which was supported by the increase of adsorption capacity when the temperature increased. In binary system, the multicomponent isotherms including the extended Langmuir and IAST-Freundlich isotherms were used to predict the adsorption of Cd (II) and Zn (II) ions. The appropriate multicomponent isotherm was investigated by minimizing the average relative error (ARE) function. The results confirmed that the extended Langmuir isotherm fitted adsorption equilibrium data better than the IAST-Freundlich isotherm for the binary adsorption of Cd (II) and Zn (II) ions onto bottom ash. In order to increase the adsorption capacity of bottom ash, it was treated by the alkaline hydrothermal method with 2 M NaOH at 100 ºC. The physical properties of bottom ash were improved. Scanning electron microscope (SEM) images presented that the treated bottom ash had high porous surface as compared to the untreated bottom ash. The single adsorption of Cd (II) and Zn (II) ions onto treated bottom ash was also studied and the adsorption equilibrium data were fitted to both Langmuir and Freundlich isotherms. The results of adsorption studies showed that the adsorption capacity of treated bottom ash for both metal ions was higher than that of untreated bottom ash.