Surface complexation modeling of adsorption of Cd(II) on graphene oxides

Graphene oxides (GOs) were synthesized by modified Hummers method and were characterized by scanning electron microscopy, Fourier transform infrared spectra, X-ray photoelectron spectra and Raman spectra. The characteristic results indicated that GOs presented a variety of oxygen-containing functional groups. Effect of solution chemistry (i.e., contact time, pH, ionic strength, temperature and initial concentration) on the adsorption of Cd(II) onto GOs was investigated by batch techniques. The adsorption of Cd(II) on GOs significantly increased with increasing pH from 4.0 to 6.0, and then high-level adsorption was observed at pH > 6.0. The adsorption of Cd(II) on GOs was independent of ionic strength over a wide range of pH, indicating that inner-sphere surface complexation dominated the Cd(II) adsorption. The maximum adsorption capacity of Cd(II) on GOs calculated from Langmuir model at pH 3.5 and 293 K was 111.11 mg/g. The thermodynamic parameters calculated from temperature-dependent isotherms indicated that the adsorption of Cd(II) on GOs was an exothermal and spontaneous processes. The result from surface complexation modeling indicated that the pH-edge adsorption of Cd(II) on GOs can be fitted by ionic exchange and surface complexation sites very well. The results indicated that the GOs can potentially be served as a promising adsorbent for the immobilization of heavy metals in environmental cleanup due to its high adsorption capacity.