The Cadmium Adsorption on Goethite and Humic Acid-Coated Goethite Complexes Under Phosphate Application

The aim of this study is to investigate the adsorption characteristics of cadmium on goethite (G)and humic acid-coated goethite complexes (GH)under the application of phosphate by the isotherm and kinetic adsorption experiments. The effects of pH and ionic strength on cadmium adsorption were also determined. The results show that both the specific surface area and the isoelectric point of humic acid-coated goethite complexes were lower than those of goethite. However, the surface sites density of the complexes coated by humic acid was higher than that of goethite. The Langmuir equation and pseudo-second-order model were applied to describe the isotherm and kinetic adsorption of cadmium on goethite and humic acid-coated goethite complexes, respectively, and the better fitting effects were achieved. The presence of humic acid coated on goethite enhanced the maximum adsorption (X_m, q_e)and the adsorption affinity (K_L)of cadmium. With the increase of ionic strength, the reduction rate of cadmium adsorption on goethite was lower than that on complexes at pH 4.5, while the reduction rate of cadmium adsorption on complexes was lower than that on goethite at pH 6.5. The cadmium adsorption and its maximum theoretical adsorption on both goethite and complexes were increased at the presence of phosphate. The coexistence of phosphate improved the adsorption affinity, the average adsorption rate constant (k)and the initial adsorption rate (h)at pH 6.5. The inhibitive effects of increase of ionic strength on cadmium adsorption on goethite and humic acid-coated goethite complexes were also weakened by the presence of phosphate.