Studies On The Removal Efficiency Rule And Mechanism Of Cadmium And Thallium By River Sediments

In this study, we took the surface sediments of Hejiang River in Guangxi province after Cd2+ and Tl+ pollution events as the objects. The single and competitive adsorption-desorption of Cd2+ and Tl+ by surface sediments of different particle size were investigated. Then the effects of organic matter, free iron oxide and amorphous iron oxide on Cd2+ and Tl+ adsorption on sediments were also studied. The adsorption mechanisms were explored through Fourier transform infrared spectroscopy(FT-IR) and X-ray photoelectron spectroscopy(XPS) analysis. The experimental results showed that sediments have certain adsorption capacity to Cd2+ and Tl+. Adsorption amounts of Cd2+ and Tl+ by sediments increased gradually accompanied with the increase of initial concentration while adsorption amounts gained the reverse tendency with the of particle size of sediments increasing. The descending order was L3(DMed=6.43μm)> original sediment(DMed=16.83 μ m) > L1(DMed=71.69 μ m) > L2(DMed=36.70μm). Adsorption isotherms of Cd2+ and Tl+ on sediments agreed well with Freundlich and Langmuir isotherm models, respectively. The maximum adsorption amounts(Qm) of Cd2+ were higher than Tl+, it may ralate to itself properties of heavy metals. Besides, the Qm of Cd2+ and Tl+ were significantly associated with specific surface area and porosity of sediments, also, the Qm have high correlation with free iron oxide and amorphous iron oxide, it indicated that the basic physical and chemical parameters were the main factors influencing Cd2+ and Tl+ adsorption processes. For desorption experiment results, the desorption rate of Cd2+ was higher than Tl+, which demonstrated that Tl+ fixed in sediments was easier to be released into water and taken greater ecological risk to environment.The retention abilities of Cd2+ and Tl+ by sediments of different particle size were decreased in some extent after deducting organic matter, free iron oxide and amorphous iron oxide and the decreases in Qe quantity were different with the differences in contents of chemical constituents. The decrease percentage made by chemical constituents followed the order of free iron oxide > amorphous iron oxide > organic matter. Similarly, after removal of organic matter and free iron oxide, the adsorption abilities of Tl+ on sediments were also decreased, the Tl+ adsorption reduction percentage of organic matter was greater than the amorphous iron oxide, but, the reduces were less than Cd2+. On the contrary, the adsorption amounts of free iron oxide got the opposite tendency after removal of free iron oxide, indicated that free iron oxide played the negative role on Tl+ adsorption.When in coexistence environment, the adsorption quantities of Cd2+ and Tl+ were dcreased in some extent and followed the order of L2(DMed=36.70μm) > L1(DMed=71.69μm) > original sediment(DMed=16.83μm) > L3(DMed=6.43μm), which matched with the reverse order of adsorption capacities for original sediment and their sieved fractions. When in desorption environment, Cd2+ and Tl+ desorption rates were slightly higher than in single system, which suggested that heavy metals absorbed in sediments were easier to be released into water.From FT-IR investigation, involvement of various functional groups such as hydroxyl and carboxyl in the binding of Cd2+ and Tl+ were evident. Furthermore, XPS results indicated that the valence state of Cd2+ and Tl+ did not change in adsorption process.