| Natural humus, as an organic substance, is the most influence factor for the quality of soil and water, whose molecular weight in300to30,000, even more. In the structure of humus, fat structure and aromatic structure account for37%and21%, respectively. It also contains a variety of active hydroxyl, carboxyl group, which contribute to the hydrophilicity, the ability of ion exchange, binding and adsorption ability of humic acid. Humus exists alone in natural environment (soil and water), but it is often closely associated with minerals (especially with clay minerals). Although in usual pH conditions, humus and clay minerals were negatively charged, but they can through the ligand exchange, cationic bridge and hydrophobic to combined organic-inorganic complexes. After clay minerals adsorpted humus, the physical and chemical properties of the surface of clay minerals will change significantly, becoming stable clay mineral-humus complexes. The clay minerals-humus complex’s adsorption to heavy metals on is different from that to clay minerals or humus. In order to learn the humic acid’s adsorption of heavy metal attached to clay minerals, we did intensive research on the adsorption relationship among clay minerals, humus and heavy metal.Two kinds of clay minerals—kaolinite (1:1type) and montmorillonite (2:1type) were selected as sorbent for this study, and humic acid as the enhancement medium, through two-phase experimental (clay minerals-humic acid, clay minerals-heavy metals) and three-phase experiment(clay minerals-humic acid, the adsorption objects with cadmium and lead), studied the adsorption effect and influence factors (pH value, ionic strength) among clay mineral, humic acid and heavy metal ions, and has obtained the following five aspect conclusions. These research conclusions will contribute to the adsorption behavior of heavy metal ions on clay minerals-humic acid complexes and seletivity adsorption characteristic in the environment, this provides the theory basis for the migration of heavy metal ions into the environment.(1) Whatever concentration and ionic strength of humic acid, in pH5.0conditions, the adsorption quantity of humic acid on kaolinite is greater than montmorillonite. According to the results obtained, pH>2.43, electrostatic repulsion between the surface of kaolinite and humic acid gradually strengthened, the main mechanisms of humic acid adsorption on kaolinite were mainly ligand exchange, hydrogen and hydrophobic. The main mechanisms of humic acid adsorption on montmorillonite were cationic bridge, hydrophobic and hydrogen. In the experimental system of Ca-montmorillonite, calcium bridge may play an important role.(2) The sorption of Cd2+on montmorillonite has not only in the surface but also occurred exchange interaction with cation of interlayers. Kaolinite has less exchangeability cations, Cd2+was difficult into the crystal layer, which mainly by surface adsorption. Therefore, the effect of montmorillonite to remove heavy metals is preferably.(3) As a result of large quantities of humic acid adsorption on the surface of kaolinite, so the adsorption capacity of Cd2+on kaolinite-humic acid complex increased to a larger extent. That montmorillonite has great adsorption ability, so the role of humic acid in the enhancement to be covered. The adsorption mechanism of Cd2+on kaolinite-humic acid complex is mainly by ligand exchange, while the montmorillonite-humic acid complex is mainly on ion exchange adsorption.(4) Selective adsorption experimental results showed that, in the solution containing Cd2+and Pb2+at1:1molar ratio, while initial concentration lower, pure kaolinite and kaolinite-humic acid complex displayed higher selective to Pb2+compared with Cd2+, as the initial concentration gradually increased, the selective adsorption of Cd2+gradually dominated. While initial concentration higher, pure kaolinite and its complex displayed higher selective to Cd2+. Pure montmorillonite and montmorillonite-humic acid complex in the whole range of concentration always displayed higher selective to Pb2+.(5) Experimental results of single and binary systems showed that, the adsorption amounts of Pb2+on pure kaolinite and its complex in the binary system were obviously lower than those in the single system, on the contrary, compared with single system, the adsorption amounts of Cd2+did not reduce, but promote, the reason for the differences adsorption characteristics may be Cd2+and Pb2+in the differences of the hydrolysis. For pure montmorillonite and montmorillonite-humic acid complex, due to the presence of Cd2+, the adsorption amounts of Pb2+in the binary system have a certain degree of decrease, while the presence of Pb2+, promoted the adsorption of Cd2+, this appeared similar to kaolinite and kaolinite-humic acid complex of "promoting" phenomenon. |
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