A hydroponic experiment was conducted to investigate the cation exchanges during the process of Cd2+ absorption by Alfalfa in aqueous solution. The absorption efficiency of Alfalfa plants with 0-10 mg·L-1 Cd2+ treatments,changes of Na+,K+,Mg2+,Ca2+ and NH+4 concentration, and the variation of pH values at different absorption time(0, 1, 2, 4, 8, 12, 24 and 72 h) were studied separately.The multiple linear regressions between Cd2+absorption and cation variation were analyzed.The results indicated that when Cd2+ concentrations were 0.1, 1, 5, 10 mg·L-1,the absorption efficiencies of Cd2+ by Alfalfa after 72 h were 85.86%, 52.14%, 15.97% and 7.81%.Cation exchange was involved in the removal of Cd2+ by Alfalfa in aqueous solution.Except for NH+4,the concentrations of cationic metals Na+,K+,Mg2+and Ca2+in aqueous solution increased over time,which increased 11.30%-61.72%, 21.44%-98.73%, 24.09%-48.90% and 37.04%-191.96%,respectively.Kinetic studies illuminated that the release of Na+,K+,Mg2+ and Ca2+ by Alfalfa in Cd2+ solution with initial concentrations of 0, 0.1, 1, 5, 10 mg·L-1 best fitted pseudo-second-order equation,while the NH+ 4release fitted this model when Cd2+concentrations were 1, 5, 10 mg·L-1.The gradual decrease of pH during adsorption of Cd2+by Alfalfa was observed.As the competition ion of Cd2+,H+ might affect the capacity of Alfalfa root system to absorb Cd2+.The ternary linear equation results demonstrated that the content of Cd2+absorption by Alfalfa strongly related with the release of Ca2+,Mg2+,Na+.And this exchange mainly occurred among Cd2+and divalent cations. |