Two-step modification towards enhancing the adsorption capacity of fly ash for both inorganic Cu(II) and organic methylene blue from aqueous solution.

A new adsorption material from fly ash (FA) was prepared by a two-step surface modification process, which showed higher ability for the removal of both inorganic and organic cationic pollutants from aqueous solution, i.e., Cu and methylene blue (MB). Firstly, FA was modified by hydrothermal method in alkaline solution at 80 °C (FA80) to have a larger BET surface area. Afterwards, FA80 was further modified by sodium dodecyl benzene sulfonate (SDBS), of which a layer of anionic functional groups were grafted on the surface. The adsorption performance of SDBS@FA80 for removal of Cu and MB were detailedly investigated. The results showed that SDBS@FA80 presented the optimal adsorption capacity at pH 7.0. Additionally, the maximum adsorption capacities of SDBS@FA80 for the removal Cu and MB were up to 227.3 and 50.76 mg g at 70 °C, respectively, as well as being about three times higher than that of FA. When the initial concentrations of Cu and MB were lower than those of 20 and 10 ppm, their removal efficiencies were as high as 99.75 and 96.4%, respectively. The pseudo-second-order model was well applied to describe the adsorption kinetics, indicating that chemisorption was taking place. Furthermore, a plausible mechanism is proposed by XPS studies, where the high adsorption capacity is mainly contributed to the electrostatic attraction and π-π stacking interaction between the cationic Cu/MB and anionic functional groups of SDBS. Due to the low-cost and high adsorption capacity, SDBS@FA80 is regarded as a promising adsorbent for the removal of cationic pollutants.