用密度泛函和XANES计算研究Zn^(2+)在水锰矿表面的吸附和沉淀

Zn(II) Adsorption and Precipitation on γ-MnOOH-Water Interfaces DFT and XANES Calculation

用密度泛函理论(densityfunctiontheory,DFT)和X射线近边结构(X-rayabsorptionnearedgestructure,XANES)模拟计算了不同酸度(pH=7.0,7.5和8.0)下Zn(II)在水锰矿表面的吸附.优化的几何结构表明,只有双边吸附方式的水解簇既能解释H+释放机制,又能与扩展X射线吸收精细结构(extendedX-rayabsorptionfinestructure,EXAFS)实验键长值相吻合.吸附能计算表明,各种吸附方式的稳定性双边(DE)>双角(DC)>B型单边(SE-B)>A型单边(SE-A);水解能计算表明各种吸附态Zn2+均比溶液中水合锌离子易水解.各种吸附簇模型的XANES计算谱未能与实验谱吻合,即,表面发生的并不是简单的吸附.pH=7.5和pH=8.0吸附样品的XANES实验谱与Zn5(OH)6(CO3)2的实验谱非常接近,因此认为pH=7.5和pH=8.0下Zn(II)在水锰矿表面发生沉淀,Zn(II)是Zn—O八面体和Zn—O四面体的混合,它们按类似Zn5(OH)6(CO3)2结构中的八面体和四面体排列方式排列.pH=7.0时,Zn(II)在水锰矿表面发生的主要是边连接方式的吸附.

Mechanisms of adsorption and precipitation of Zn（Ⅱ） on γ-MnOOH-water interfaces were studied using DPT （density functional theory） and XANES （X-ray absorption near edge structure） simulation at different pH conditions. Optimal adsorption structures showed that only hydrolyzed clusters of double-edge-link mode could explain the H^＋ release mechanism and experimental bond-length values. Adsorption energy calculations indicated that the stability sequence of adsorption modes was in the order of double-edge（DE）〉double-corner （DC）〉single-edge-B （SE-B）〉 single-edge-A （SE-A）. Hydrolysis energy calculations showed that the adsorbed Zn^2＋ of all the linkage modes hydrolyzed easier than Zn^2＋ in solution. XANES simulations of optimal adsorption clusters did not accord well with experimental results, indicating that mechanisms other than surface adsorption were involved in the adsorption samples. Surface precipitation species were formed in adsorption samples of pH 〉 7.5, where mixtures of Zn-O tetrahedron and Zn-O octahedron were arranged in the way similar to that of Zn5（OH）6（CO3）2. Simulated and experimental XANES spectrums of Zn5（OH）6（CO3）2 were very close to that of adsorption samples of pH 〉 7.5. At pH 〈 7.0, Zn（Ⅱ） was mainly adsorbed in edge link mode on the surface of γ- MnOOH.