镉在胡敏酸上的吸附动力学和热力学研究

KINETICS AND THERMODYNAMICS OF CADMIUM ADSORPTION ON HUMIC ACID

采用镉离子选择电极研究了镉在胡敏酸上的吸附特征。实验结果表明 ,胡敏酸对镉的等温吸附规律与介质pH有关。当pH为 3 0 0和 3 50时符合Freundlich方程 ,当pH在 4 0 0～ 6 50之间更好地符合Langmuir方程 ;在pH为 3 0 0～ 6 50之间 ,吸附量与pH成显著正相关。温度升高吸附量降低 ,吸附热与反应介质的pH有关 ,pH升高 ,吸附反应放热增加 ;胡敏酸对镉的吸附作用力随介质pH改变发生较大变化 ,当pH为6 50时主要为配位基交换作用。胡敏酸对镉的吸附含有部分不释放氢的静电吸附 。

Kinetic and thermodynamic characteristics of c ad mium adsorption on humic acid were studied using ion selective electrode Resu lts show that the characteristics of cadmium adsorption on humic acid differed w ith the medium pH: the adsorption data were best described by Freundlish equatio n at pH 3 0～3 5 and by Langmuir equation at pH 4 0～6 5, respectively Sig nificant correlations existed between cadmium adsorption amounts and the medium pH at the range of pH 3 00～6 50 under the experimental conditions The incre ase of temperature resulted in a decrease of the cadmium adsorption amount on hu mic acid, suggesting a heat release reaction between humic acid and cadmium ion s The adsorption heat was pH relevant with an increased adsorption heat at hi gher medium pH The adsorption forces of cadmium on humic acid varied substanti ally with the change of medium pH, with ligand exchange being dominating at pH 6 5 and hydrogen bond and dipole force being the main forces at pH 5 5 The ad sorption processes involved partly electrostatic force without the release of hy drogen ions The logarithm of the adsorption distribution coefficient (log K f) positively correlated with the medium pH at low cadmium concentration A n initial fast reaction stage followed by as slow reaction between humic acid an d cadmium ions were observed and it took at least 500 min for the reaction equil ibrium to be established The reaction rates were accelerated with the increase of medium pH and the adsorption kinetics were best described by Elovich equatio n followed by double constant equation