Superior Adsorption of Direct Dye from Aqueous Solution by Y(III)-Chitosan-Doped Fly Ash Composite as Low-Cost Adsorbent

A novel adsorbent, Y/CS/MFA composite was successfully prepared by doping a very small amount of chitosan (CS) and Y(III) ions onto the acid-modified fly ash (named as MFA), and characterized by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and N₂ gas adsorption/desorption technique. The adsorption property and mechanism of Y/CS/MFA for Direct green 6 (DG 6) removal from aqueous solution were investigated by carrying out batch adsorption experiments. We found that the adsorption of DG 6 on Y/CS/MFA rapidly reached equilibrium within 60 min and could be efficiently removed in a wide range of pH. The adsorption process followed the pseudo-second-order kinetic model well and was chiefly controlled by intraparticle diffusion. The equilibrium data was analyzed through three isothermal models, and it was shown that the Langmuir model could accurately describe adsorption behavior of DG 6, and the saturated adsorption capacity could reach 627 mg/g. The results of mean sorption energy from Dubinin–Radushkevich (D–R) model and FTIR analysis revealed that the adsorption should be the double properties of physisorption and chemisorption. Thermodynamics research indicated that the adsorption reaction was of feasible, spontaneous and exothermic nature. The dye-adsorbed Y/CS/MFA can be effectively desorbed by sodium hydroxide solution and recycled. The adsorption capacity of DG 6 on Y/CS/MFA was about 12 times higher than that on fly ash (FA). Y/CS/MFA composite based on solid waste fly ash would be a promising adsorbent with low cost and superior adsorption properties for high-concentration direct dye removal from wastewater.