Effect Of Formation Sequence Of Ternary Mineral-Bacteria-Humic Acid Composites On Cadmium Adsorption

Clay mineral, bacteria and humic acid are the main components of soil matrix, and they are commonly found to associate together and produce various mineral-organic composites in natural conditions. These processes give rise to different physical and biochemical properties of the composites which are different from their end-members, influencing the mobility, speciation, transport and bioavailability of toxic metallic cations. In this study, two kinds of typical soil minerals(goethite(G), montmorillonite(M)), Bacillus subtilis(B), and humic acid(H) were selected as the experimental materials which were prepared for the ternary composites with different formation Sequence(G/M-B+H, G/M-H+B, B-H+G/M and G/M-B-H). The physical and biochemical properties of the ternary composites formed by different sequence were investigated. Atomic force microscope(AFM), laser particle sizer, automatic potentiometric titration, attenuated fourier transform infrared spectroscopy(ATR-FTIR) and isothermal titration microcalorimetriy(ITC) were applied to explore the adsorption mechanism. The main results were listed as follows:1. Ternary composites formed by different sequence showed differences in the degree of binding and the distribution of particle size. For the goethite system, the outline and the edge of the bacterial cells were more clear for G-H+B complex, resulting in the exposure of more naked surface. G-B-H and G-H+B had the greatest average particle size and they were loosely combined. For the montmorillonite system, M-B+H and H-B+M combined more tightly with few free minerals. M-H+B had a poor reunion and its average particle size was minimal.2. The surface site concentration of the ternary composites formed by different sequence exerts significant difference at a specific pH range(p<0.05). For the goethite system, G-B+H had higher buffer capacity at p H 3~4, indicating the occurance of more phosphoryls and carboxyls. For the montmorillonite system, M-H+B and M-B-H possessed more phosphate groups in the pH range of 7~10.5.3. The ternary composites formed by different sequence showed significant differences(p<0.05) in Cd(II) adsorption. The order of the Cd(II) adsorption capacity of the ternary composites was H-B+G > G-B-H > G-H+B > G-B+H; H-B+M > M-B-H > MB+H > M-H+B. Chemical adsorption was the main adsorption mechanism involved in the adsorption process. G-H+B, G-B-H, M-H+B and M-B-H showed a greater initial reaction rate at pH 7 which illustrated more adsorption sites for Cd(II). In addition, more phosphate groups were exposed for the four composites, which played an important role in Cd(II) adsorption.5. For the goethite system, Cd(II) adsorption on ternary composites was a weakly endothermic reaction. Specifically, G-H+B exhibited the highest heat effect. For the montmorillonite system, endothermic reaction was mainly respobsible for Cd(II) adsorption on ternary composites, with M-B-H showing the highest heat. Cd(II) mainly formed outer-sphere complex on the all kinds of ternary composites.