[Adsorption of Cd²⁺ from wastewater by modified fly ash.]

The modified fly ash (MFA) was prepared through roasting the mixture of fly ash and NaOH/Ca(OH)₂ at 250 ℃ for 1.5 h. The physicochemical properties of MFA were analyzed by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), surface area analyze (BET), and Fourier transform infrared spectroscopy (FTIR) techniques. Results from BET analysis showed that the BET of MFA was enlarged by 20.6 times compared with FA. Results from SEM analysis showed that the glass phase was dissolved, with a rough surface and porous structure. Results from FTIR analysis demonstrated that -OH played an important role in Cd²⁺ adsorption. Results from the static adsorption experiment revealed that the removal efficiency of Cd²⁺ reached 97.3% when 0.2 g MFA was applied while the concentration of Cd²⁺ was 100 mg·L^-1, the solution pH was 7.0, the adsorption temperature was 25 ℃ and the adsorption time was 90 min. In addition, the coexisting cations (K⁺, Na⁺, Mg²⁺, and Ca²⁺) might inhibit Cd²⁺ adsorption. Among all the cations, Ca²⁺ showed a most significant inhibitory effect on the removal of Cd²⁺. Langmuir isotherm and the pseudo-second-order kinetic models could well describe the adsorption behavior of Cd²⁺ on MFA, with the maximum adsorption capacity of 55.77 mg·g^-1. Meanwhile, thermodynamic studies showed that Cd²⁺ adsorption onto MFA was spontaneous and endothermic process. MFA had better adsorption capacity than FA and had certain application pro-spects in wastewater treatment.