Preparation And Adsorption Of A Novel Fly Ash/chitosan/graphene Oxide Composite Adsorbent

Graphene oxide(GO), with abundant oxygen-containing functional groups and a high theoretical surface area, is an excellent adsorbent material so it is expected for the treatment of sewage. However, GO is so difficult to collect and separate from treated water that it is almost impossible to be applied to actual industry, because the hydrophilic oxygenous functional groups of the compound easily disperse in water. In this study, a new fly ash/chitosan/graphene oxide(FCGO) composite adsorbent is prepared by wrapping GO on fly ash with chitosan as combiner and glutaric aldehyde as cross-linking agent. The synthesized FCGO is then used to remove dyes and heavy metals by static adsorption experiments. The main conclusions of this paper are as follows:1. A novel graphene oxide-based adsorbent(FCGO) is synthesized from fly ash cross- linked with chitosan and graphene oxide and the optimal preparation conditions of adsorpting acid red GR, cationic red X-5GN and Hg(II), determined by the orthogonal test, are as follows: 1 g of fly ash, 15 m L of GO solution(5 mg/m L), 25 mg of chitosan, 1 m L of glutaric aldehyde and cross- linked at 80°C. FCGO is characterized through scanning electron microscope, diffuse reflectance infrared Fourier transform spectroscopy, X- ray photoelectron spectroscopy and X-ray diffraction analyses. Fly ash with smooth surface, composed mainly of Si O2 and Al2O3, becomes rough surface with wrinkles after loading GO. The new elements of C, N and extra element of O on the surface of FCGO indicate that GO has been successfully grafted on the fly ash particles.2. The adsorbent effectively removes anionic and cationic dyes, namely, acid red GR and cationic red X-5GN, respectively. With adsorption time and temperature increasing, the adsorption effects of dyes are better. The removal ratio of acid red GR by FCGO decreases with increasing initial p H in acidic solutions but is not affected at p H higher than 6. By contrast, initial p H minimally influences the removal ratio of cationic red X-5GN. The maximum adsorption capacities are 38.87 and 64.50 mg/g for acid red GR and cationic red X-5GN, respectively.3. The adsorption kinetics of acid red GR and cationic red X-5GN follow the pseudo-second-order kinetic model. Adsorption is mainly controlled by boundary diffusion and internal diffusion for acid red GR but only by boundary diffusion for cationic red X-5GN, and the process satisfactorily fits with the Redlich-Peterson model. Negative(35)G0 values and positive(35)H0 values indicate that adsorption is a spontaneous and endothermic process. The adsoption of acid red GR may mainly be electrostatic attraction and cationic red X-5GN may mainly be electrostatic attration and ?-? interaction.4. The FC GO adsorbent also removes Hg(II) effectively. The adsorption capacity of Hg(II) increases with increasing adsorption time and temperature and is maximum when Hg(II) solution is closing to neutral. The vast majority of coexisting anions was benefit to but the coexistence cations inhibited the adsorption of Hg(II). The adsorption kinetics of Hg(II) by FC GO follows the Elovich kinetic model. And the adsorption process fits satisfactorily with Redlich-Peterson model with a maximum adsorption capacity of 42.20 mg/g. Thermodynamic parameters indicate that the adsorption process of Hg(II) is spontaneous and endothermic.5. The adsorption of Hg(II) is analyzed through the response surface method. The quadratic regression equation of removal ratio of Hg(II)(Y) and dosage of FCGO(X1), adsorption temperature(X2) and adsorption time(X3) is: Y=12.243 + 10.656 X1 – 1.356 X2 + 31.288 X3 – 0.029 X1X2 – 0.406 X1X3 + 0.009 X2X3 – 0.603 X12 – 0.013 X22 – 7.126 X32. Interactions of X1X2 and X1X3 for adsorption of Hg(II) are remarkable, but X2X3 is not signification. FCGO is not single molecular layer so the adsorption of Hg(II) is not uniform, which may be electrostatic attraction and complexation mainly.