| Heavy metals are released into aquatic and soil environments largely from various anthropogenic activities and pose a serious threat to plants, animals and human beings because of their bioaccumulation, non-biodegradable properties and toxicity even at low concentrations. Adsorption of heavy metals to aquatic solid phases is considered very important in controlling heavy metal behaviors in natural waters. Solid phases as biofilm (BF), suspended solids (SS) and deposited sediment (DS), together with water phases, constitute the multi-phases system deciding the activities of heavy metals in the aquatic environment. Cadmium is a typical heavy metal. It is well known by people because of"Itai-itai disease". Many studies have shown that main components as Fe oxides, Mn oxides and organic cause the adsorption of heavy metals onto solid matters. In this paper, Cd adsorption onto solid materials and their main components in aquatic multi-solid coexisting system was studied, in order to understand the distribution of Cd among solid phases and among their chemical components.In this paper, the methods of field sampling - the main chemical component extraction separation - Laboratory simulation of adsorption - statistical analysis of experimental is used. Hydroxyl ammonium hydrochloride is used to extract Mn oxides. Sodium dithionite is used to extract Mn oxides and Fe oxides. Organic matters, Mn oxides and Fe oxides are extracted by hydrogen peroxide. Three kinds of solid matters extracted by the same extractant are placed in the same aquatic system to adsorb Cd. The difference of adsorption amounts before and after extraction is used to determine the adsorption of the components extracted. Then statistical analysis is employed to analyze the experimental results. The results shows that the points of adsorption onto BF, SS and DS in single and multi-phase system are well agree with Langmuir adsorption isotherms. In multi-phase system, adsorption amounts is followed by the order: SS>DS>BF. In the single system, the case is DS>SS>BF. Adoption amounts of Cd onto BF and DS in multi-system are both smaller than their single system cases under higher Cd concentration (about more than 3μmol / L) condition. While in the case of SS, it is bigger. That is , three kinds of solid matters compete with each other when they are in the same system, and SS obtain the advantage from the competition. When Cd concentration is lower (about less than 3μmol / L), adsorption in multi-phases system is more than in the single system for three kinds of solid respectively. This may be caused by the bigger Langmuir parameters K of Fe oxides, Mn oxides and organic matter, which results in greater adsorption amounts in lower Cd concentration.With the use of thermodynamic parameters of the solid matters before and after extraction, along with the contents of each component, thermodynamic parameters of each component can be work out. Then the adsorption contribution and adsorption capacity of each component can also be calculated. In multi-phase system, the contributions of Fe Fe oxides, Mn oxides and organic matter are approximately the same. The contribution of Mn oxides in SS is much more than other two components, organic matter contributes little. Their respective contributions in DS follow the order as: Fe oxides> organic matter>Mn oxides. Compared with single system, the small increase adsorption amounts of organic in multi-phase system does not make up the decrease of Fe oxides and Mn oxides in BF, so the adsorption of BF decreases. The big contribution of Mn oxides in SS in multi-solid coexisting system leads to the larger adsorption amount of SS. As for DS, the small increase adsorption amount of Fe oxides multi-phase system does not make up the decrease of organic and Mn oxides, so the adsorption of DS decreases. There is a little difference in adsorption capacities of Fe oxides and organic matter in solid materials, which are both less than the adsorption capacity of Mn oxides.
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