重金属-柠檬酸-针铁矿三元体系的表面络合模型研究

Development of surface complexation model of heavy metal-citric acid-goethite ternary system

根际环境产生的柠檬酸等小分子有机酸可能影响重金属的溶出效应.本文研究了4种重金属Cd、Pb、Cu、Ni及柠檬酸在针铁矿表面的三元体系吸附行为,结果发现柠檬酸促进了4种重金属酸性条件下在针铁矿表面的吸附量,而Cu、Ni促进了柠檬酸碱性条件下的吸附.结合红外光谱图发现,重金属-柠檬酸-针铁矿主要存在以下2种三元体系形态,即以柠檬酸为"桥键"的≡Fe2CitMe形态和以重金属为"桥键"的(≡FeOH)2MeCit形态.采用电荷分配-多位点表面配合(CD-MUSIC)模型成功预测了的三元体系重金属及柠檬酸在针铁矿表面的吸附行为,模型结果发现柠檬酸的存在显著改变了重金属的吸附形态,其中≡Fe2CitMe为Cd、Pb、Ni三元体系中酸性条件下的主要形态,(≡FeOH)2CuCit为Cu的主要形态.研究补充完善了根际环境的土壤形态模型数据库,为预测重金属的溶出及生物有效性的模型研究提供了基础数据.

Low-molecular-weight organic acids such as citric acid produced in rhizosphere may affect the dissolution of heavy metals in soil.In this study,the adsorption behavior of four heavy metals(Cd,Pb,Cu,Ni)and citric acid on the surface of goethite was investigated.Results showed that citric acid enhanced the adsorption of four metals under acidic conditions,while Cu and Ni slightly promoted the adsorption of citric acid under alkaline condition.Combined with the ATR-FTIR spectral evidence,two ternary surfaces species were proposed:i.e.the species≡Fe2CitMe with citric acid acting as a"bridge"ion and the species(≡FeOH)2MeCit with metal cations as a"bridge"ion.A charge distribution multi-site surface complexation model(CD-MUSIC)was proposed to describe the ternary adsorption.Results showed that the addition of citric acid significantly altered the surface species of the four heavy metals.The main surface species of Cd,Pb,and Ni was≡Fe2CitMe under acidic conditions,while Cu was(≡FeOH)2CuCit.The established model successfully predicted the adsorption of metals and citric acid on goethite surfaces,which replenished the database of surface complexation model for soil rhizosphere.