Experimental Study On Cadmium Adsorption And Desorption From Coastal Zone Sediments Of Tianjin

The concentration of heavy metals in coastal sediments reflected the complex geochemical characteristics and migration processes.In this thesis,the adsorption and desorption behaviors of Cd in coastal surface sediments from Tianjin Dashentang were studied by static adsorption batch experiments.Meanwhile the influence of initial concentration of metals,pH,salinity,and organic matter on the adsorption and desorption has been discussed.And In addition,the competitive adsorption of Cd and Zn were also studied.The conclusions were as follows:By using pseudo-second-order kinetic equation and Elovich equation to fit the data,the adsorption and desorption of Cd and its competitive adsorption with Zn could be well described by pseudo-second-order kinetic model.The comparison of the parameters showed that the adsorption capacity and the adsorption rate of Zn on sediments were both greater than those of Cd.The concentration of Cd in the equilibrium solution and adsorption capacity of Cd increased with the initial concentration of Cd.The rate of increase of adsorption decreased.The adsorption of Cd and competitive adsorption of Zn and Cd were accord with the fitting result of the Freundlich model rather than Langmuir model.The Langmuir model is suitable for the desorption of Cd.By the change of the solid-to-liquid ratio,it was concluded that there was a maximum adsorbing capacity at a higher concentration.The adsorption capacity of Cd increased with the pH value,and reached a maximum when pH>8.The desorption capacity of Cd decreased obviously with pH.At a medium salinity between 5 and 10,adsorption capacity would reach its maximum.After removal of organic matter,the adsorption rate decreased,but contribution of organic matter to the adsorption was not obvious.The increase of the desorption capacity indicated that organic matter might play an inhibitory role in desorption of Cd.Competitive adsorption phenomenon became apparent with the increase of concentrations of Zn and Cd.The decreasing rate of the adsorption capacity of Cd was greater than that of Zn.It showed that competitive adsorption capacity of Cd was weaker than that of Zn.The amount of Cd decreased with the increasing concentration of Zn,and therefore,it was inferred,that the amount finally reached a constant.