红壤有机矿物复合体吸附Cu(Ⅱ)的分子机制

Molecular Mechanism of Cu(Ⅱ)Adsorption by Organo-Mineral Complexes of Red Soil

采用元素分析、X射线衍射分析(XRD)、同步辐射扫描透射显微术(STXM)等手段,表征湖南祁阳红壤中颗粒态有机质(Particle organic matter,POM)和有机矿物复合体(Organo-mineral complexes,OMC)的元素组成、矿物组成,以及Cu(Ⅱ)在土体土壤(Bulksoil,BS)和OMC表面的吸附行为、元素微区分布和分子固定机制,以明确实际土壤系统有机矿物复合体对铜吸附的能力及其固定机制。元素分析、XRD结果表明,土体土壤主要由OMC组成,富含黏土矿物,且所含有机质矿化度较BS和POM组分高。等温吸附实验结果表明,BS和OMC等温吸附曲线符合Langmuir和Freundlich方程,BS及OMC对Cu(Ⅱ)吸附等温线类似,且理论最大吸附量相当,说明供试土壤对Cu(Ⅱ)的吸附主要由OMC决定。STXM在亚微米尺度上表征了BS和OMC吸附样品中Cu与C、Fe、Al、Si的微区分布特征,发现Cu在红壤中分布具有空间异质性,Cu主要与羧基碳与铁氧化物形成的有机矿物复合体相结合,控制着红壤中Cu(Ⅱ)的形态分布与有效性。

【Objective】Organo-mineral complexes(OMC)in red soil vary significantly in mineral composition and surface properties,and have a profound impact on the environmental chemical behavior of heavy metals(HMs)in the soil.Though a large volume of researches have been done using models to predict pollutant behaviors in actual soil systems,it is still difficult to make accurate prediction due to the complexity of OMC.In this study,efforts were made to investigate elemental composition and mineral composition of the particulate organic matter(POM)and organo-mineral complexes(OMC)fractions,obtained from an actual red soil from Qiyang of Hunan,and furthermore,Cu(Ⅱ)adsorption behaviors,element micro-distribution and molecular binding mechanism of the Cu(Ⅱ)on the bulk soil(BS)and OMC,so as to specify Cu(Ⅱ)adsorption capacity of and molecular binding mechanism of Cu(Ⅱ)to the OMC in the actual soil system.【Method】POM and OMC were obtained from the bulk soil through density fractionation.X-ray diffraction analysis was performed to determine mineral compositions of the BS and OMC.Free Fe oxides were extracted with the DCB method and amorphous Fe oxides with the acidic ammonium oxalate method,and then determined with ICP-OES.C and N contents were measured with the Elementar Vario MACRO.Moreover,theoretical maximum Cu(Ⅱ)adsorption capacity of the samples were investigated through batch adsorption experiments.Then Cu(Ⅱ)adsorption in the samples were measured with the Scanning transmission X-ray microscopy(STXM)to specify micro-distribution of target elements(C,Fe,Cu,Al and Si)and molecular mechanism of Cu(Ⅱ)immobilization.【Result】Element analysis shows that the OMC is the main component of soil,and has organic matter with lower C/N ratio than the BS.XRD shows that the OMC consists mainly of clay minerals.The Cu isothermal adsorption curves of the OMC and BS could be well fitted with the Langmuir and Freundlich equations.Both the OMC and BS exhibit similar Cu adsorption curves,and equivalent theoretical maximum Cu(Ⅱ)adsorption capacities,which indicated that Cu(Ⅱ)adsorption is determined mainly by OMC in the red soil.With the aid of STXM,micro-distributions of Cu,C,Fe,Al and Si in BS and OMC of the adsorption samples are characterized on the submicron scale.The results indicated that the distribution of Cu in red soil is spatially heterogeneous,and Cu is mainly combined with the OMC formed by organic faction with carboxylic carbon and iron oxide.【Conclusion】Red soil is mainly composed of the OMC,which mainly determine the distribution and adsorption behavior of Cu(Ⅱ)in the red soil.The carboxylic carbon and iron oxides of the OMC serves as the binding sites of Cu(Ⅱ).Therefore,the OMC of the red soil predominantly controls Cu(Ⅱ)adsorption,and thus determines the availability and mobility of Cu(Ⅱ)in the red soil.