磷对镉离子在针铁矿及针铁矿-胡敏酸复合体表面吸附的影响

The Cadmium Adsorption on Goethite and Humic Acid-Coated Goethite Complexes Under Phosphate Application

为了探讨磷镉共存条件下镉离子在针铁矿和针铁矿-胡敏酸复合体表面的吸附特征,通过室内合成方法制备针铁矿和针铁矿-胡敏酸复合体,利用等温吸附试验和动力学吸附试验来获取磷酸盐共存时镉离子在针铁矿和复合体表面的吸附规律,并分析pH和离子强度对镉离子吸附的影响。结果表明,胡敏酸在针铁矿表面的吸附包被降低了针铁矿的比表面积和电荷零点,但提高了矿物表面的位点浓度。用Langmuir方程和二级动力学方程描述镉离子在针铁矿和复合体表面的等温吸附和动力学吸附行为,均获得较好的拟合结果。胡敏酸吸附包被形成的复合体对镉离子具有较高的吸附量(Xm,qe)和吸附强度(KL);在pH 4. 5条件下随着离子强度的增加针铁矿对镉离子的吸附下降幅度小于复合体,而在pH 6. 5条件下镉离子在复合体表面的吸附随离子强度增加而降低的幅度小于在针铁矿表面。磷酸盐共存和pH的增加均提高镉离子的吸附量和最大理论吸附量;在pH 6. 5条件下,磷酸根的共存增强了镉离子在针铁矿和复合体表面的KL、吸附速率常数(k)和初始吸附速率(h);同时磷酸根的存在减弱了离子强度增加对镉离子吸附的抑制效应。

The aim of this study is to investigate the adsorption characteristics of cadmium on goethite(G)and humic acid-coated goethite complexes(GH)under the application of phosphate by the isotherm and kinetic adsorption experiments. The effects of pH and ionic strength on cadmium adsorption were also determined. The results show that both the specific surface area and the isoelectric point of humic acid-coated goethite complexes were lower than those of goethite.However,the surface sites density of the complexes coated by humic acid was higher than that of goethite. The Langmuir equation and pseudo-second-order model were applied to describe the isotherm and kinetic adsorption of cadmium on goethite and humic acid-coated goethite complexes,respectively,and the better fitting effects were achieved. The presence of humic acid coated on goethite enhanced the maximum adsorption(Xm,qe) and the adsorption affinity(KL) of cadmium.With the increase of ionic strength,the reduction rate of cadmium adsorption on goethite was lower than that on complexes at pH 4. 5,while the reduction rate of cadmium adsorption on complexes was lower than that on goethite at pH 6. 5. The cadmium adsorption and its maximum theoretical adsorption on both goethite and complexes were increased at the presence of phosphate. The coexistence of phosphate improved the adsorption affinity,the average adsorption rate constant(k)and the initial adsorption rate(h)at pH 6. 5. The inhibitive effects of increase of ionic strength on cadmium adsorption on goethite and humic acid-coated goethite complexes were also weakened by the presence of phosphate.