镉在针铁矿、针铁矿-腐植酸复合胶体中吸附解吸行为比较研究

采用批平衡试验法,比较研究了重金属镉在针铁矿和针铁矿-腐植酸复合胶体中的吸附热力学和动力学行为。结果表明,在试验浓度范围内,两种吸附剂对Cd2＋的等温吸附特征均可用Langmuir、Freundlich和Linear方程加以描述。其中以Langmuir方程的拟合效果最佳,线性相关系数为：0.991、0.999,由此推导出的最大吸附量分别为41.667和45.455mg·g^-1,表明腐植酸与针铁矿复合胶体较单一针铁矿的吸附力有所提高,且所吸附的镉均难以解吸,平均解吸率分别为5.871%和1.068%。胶体对Cd^2＋的吸附是一个快速反应过程,4h达到吸附平衡。Elovich方程是拟合吸附动力学过程的最优方程（相关系数分别为0.987和0.997）。通过计算镉在针铁矿和针铁矿-腐植酸复合胶体中的吸附自由能变化量（ΔG的绝对值均〈40kJ·mol^-1）,推测镉在两种胶体中的吸附为物理吸附过程,其吸附机理可能有氢键、偶极作用力和范德华力等作用,而不存在化学键合作用。

Adsorption and Desorption Behaviors of Cadmium on/from Goethite and Its Compound Colloid with Humic Acids

The thermodynamics and kinetics of Cd^2＋ adsorption and desorption on/from goethite and its compound colloid with humie acids were studied by batch experiments. The results showed that within the range of cadmium concentration tested, Cd^2＋ adsorption was increased with the increase of its initial concentration, and the compound colloid had a higher Cd^2＋ adsorption capacity than goethite. The adsorption isotherms of Cd^2＋ by the two samples could significantly confirm to Langmuir, Freundlieh and Linear equation, while Langmuir was the best one. The maximum adsorption of Cd^2＋ by the two tested samples was 41.667 and 45.455 mg·g^-1, respectively, according to Langmuir equation. The compound colloid had a lower Cd^2＋ desorption rates than goethite, which was only 1.068% as compared to 5.871% from goethite, indicating that compound colloid had a higher adsorption strength for cadmium. The Cd^2＋ adsorption reactions by the two tested samples was a rapid process ,which could reach an equilibrium within 4 hours. Elovich equation was the optimal model to describe the kinetics of Cd^2＋ adsorption. The enthalpy changes AH of the adsorption reaction were all positive and the changes of free energy ΔGwere negative,while the changes of adsorption entropy AS were positive, which suggested that the adsorption processes of Cd^2＋ on goethite and its compound colloid with humic acid were both endothermic and belonged to a spontaneous process. And the absolute value Of AG were all less than 40 kJ·mol^-1, much lower than the free energy change caused by chemical bonds（ 〉 60 kJ· mol^-1 ）, indicating that adsorption of Cd^2＋ by two tested samples belonged to physical adsorption processes, with possible mechanisms of H-bonds, dipolar action force and van der Waals force.