酸性紫色水稻土颗粒有机质对镉的吸附特性

采集典型的酸性紫色水稻土（APPS），从中分离出颗粒有机质（POM），通过批量试验研究POM及其来源土壤Cd²⁺的吸附动力学、等温吸附和热力学特征，通过扫描电镜-能谱仪、傅里叶红外光谱仪等手段及吸附前后镉的形态变化的测定，研究了POM对Cd²⁺的吸附机制.结果表明：POM对Cd²⁺的亲和力远高于其来源土壤.POM及土壤对Cd²⁺的吸附动力学最优模型均为准二级动力学.Langmuir、Freundlich方程均能较好地描述其等温吸附特征，其中对POM，以Freundlich方程更优，表明POM对Cd²⁺的吸附属于多分子层的非均质吸附.吸附热力学参数△G^θ均小于0、△H^θ和△S^θ均大于0，表明吸附属于自发吸热过程.根据△H^θ值及解吸试验判定POM对Cd²⁺的吸附以化学吸附为主，土壤对Cd²⁺的吸附过程以物理吸附为主.吸附平衡后，土壤中可交换态镉比例提高，而POM中交换态和络合态镉比例增加.综上及吸附前后POM的表征结果说明，POM对Cd²⁺的吸附机制包括含氧官能团的络合、离子交换、阳离子-π键、沉淀作用和静电吸附.

Characteristics of cadmium adsorption on particulate organic matter isolated from an acidic purple paddy soil

Characteristics of cadmium ion (Cd²⁺) adsorption on particulate organic matter (POM) isolated from a typical acid purple paddy soil were investigated by means of scanning electron microscopy-energy dispersive spectrometer (EDS), fourier transform infrared spectroscopy (FTIR), and the fractionation of Cd²⁺ in POM before and after adsorption. The affinity of Cd²⁺ to POM was much higher than that to the original soil. The adsorption kinetic data could be well described by the pseudo-secondary order equation. The Freundlich equation was better than Langmuir model in describing Cd²⁺ adsorption isotherm of POM, indicating that the adsorption of Cd²⁺ on POM was a heterogeneous multi-molecular layer adsorption. The negative Gibbs free energy (DG^θ), and the positive Entropy (DH^θ) and Enthalpy (DS^θ) illustrated that the adsorption was a spontaneous endothermic process. The value of DH^θand the desorption rate of adsorbed Cd²⁺ indicated that Cd²⁺ was bond to POM primarily through chemical reactions, while to the original soil it did mainly through physical forces. The fractionation showed that the adsorption process increased the percentage of exchangeable Cd²⁺ in the original soil and the percentage of exchangeable and complex forms in POM. The above results together with the analysis of EDS and the FTIR spectra revealed that the mechanisms of Cd²⁺adsorption on POM included complexation of oxygen-containing functional groups, ion exchange, cation-π bonding, precipitation and electrostatic adsorption.