The Mechanisms Of Cadmium Adsorption On Montmorillonite-and Goethite-Bacteria-Humic Acid Composites

Clay mineral, bacteria and humic acids in soil were combined to form organo-mineral composites, which affected the mobility, speciation, transport and bioavailability of toxic metallic cations. The studies on the behaviors of heavy metal on soil components can provide scientific basis for in-situ remediation of heavy metal contaminated soil. In our studies, soil minerals, Pseudomonas putida and humic acid were used as model materials. The interface interactions between Cd(II) and soil components were investigated using a combination of batch adsorption experiments, isothermal titration calorimetry (ITC), Fourier transform infrared spectroscopy (FTIR) and potentiometric titration.1. Minerals-HA composites adsorpted more Cd(II) than single minerals. Cd(II) adsorption on bacteria-HA decreased compared with single bacteria. Minerals-bacteria-HA composites had a larger adsorption capacity than bacteria-mineral, indicating that HA enhanced the Cd(II) adsorption on ternary composites.2. in the pH range of3to7, Cd(II) adsorption capacity of montmorillonite, bacteria, humic acid and their complexes increased with the increment of pH from3to5and then keep steady. The adsorption on goethite and its complexes were affected by pH more significantly and the adsorption capacity increased distinctly in the entire pH range from2to7.3. Preudo-second-order model fit well the experimental data of Cd(II) adsorption on montmorillonite, bacteria, humic acid and their complexes, which indicated that a chemical reaction mechanism play an important role. The adsorption rate of montmorillonite-P. putida-HA is faster than other sorbents.4. FTIR results show that carboxyl of bacteria and hydroxyl on montmorillonite play a role in the Cd(II) adsorption on the ternary composites. Phosphate on bacteria take part in the form of goethite-bacteria-HA composites. 5. Cd(Ⅱ) adsorption on montmorillonite, bacteria, HA and their composites was exothermic reaction, and the△Hads values were in the range -20.80～-1.04kJ mol－1. Montmorillonite-bacteria-HA had the largest△Hads (-20.80kJ mol-1) and△Sads (-91.77J mol-1K-1), which demonstrated that montmorillonite-bacteria-HA had a higher affinity for Cd(II) and outer-sphere complexes were formed between Cd(II) and montmorillonite-bacteria-HA. While for goethite-bacteria-HA composites, the Cd(II) adsorption was a slightly endothermic process and ASads was positive indicating that inner-sphere complexes were formed.