用EXAFS研究Zn在水锰矿上的吸附-解吸机理

用延展X光吸收精细结构光谱(EXAFS)研究了重金属Zn(Ⅱ)在水锰矿(γ-MnOOH)上吸附产物的微观结构及其吸附机理.结果表明,Zn(Ⅱ)-水锰矿体系中(pH 7.5,0.1mol/LNaNO₃介质,25℃),Zn²⁺主要是通过共用水合Zn²⁺的O原子及水锰矿表面上的O原子形成Zn-O键,从而结合到水锰矿固体表面上的.平均Zn-O原子间距为1.998±0.010 A(n=3).这个Zn-O键键长是六配位的Zn(H₂O)²⁺₆及其水解产物四配位的Zn(OH)₂或Zn(OH)₄^2-各以一定比例混合吸附于水锰矿表面而形成的.同时,对第二配位层(Zn-Mn相互作用)的EXAFS图谱分析证明存在2个典型的Zn-Mn原子间距,即R₁=3.08±0.024A(n=3)和R2=3.54±0.018 A(n=3).这2个Zn-Mn原子距分别对应于水锰矿结构单元MnO6八面体与Zn水合离子ZnO多面体结合的2种方式,即共用2个O原子的边-边结合与共用1个O原子的角-角结合.边-边结合是较强的吸附位,Zn-Mn原子距较短(R₁=3.08A),吸附较不可逆.角-角结合是较弱的吸附位,Zn-Mn原子距较长(R₂=3.54A),吸附较为可逆.这一结果从微观上证明了亚稳平衡态吸附理论(MEA理论)的基本假设,即具有相同吸附密度的同一吸附质由于吸附力强弱以及微观构型的不同可具有不同的化学位,因而证明了修正传统吸附热力学的基本假设(吸附密度为热力学状态函数)的必要性.宏观的吸附-解吸热力学实验表明Zn(Ⅱ)在水锰矿上的吸附是不可逆的,EXAFS结果指出这种不可逆性主要是由Zn水合离子中ZnO多面体与水锰矿结构单元MnO₆八面体之间的边-边结合所导致的.

EXAFS Studies on Adsorption-Desorption Mechanism of Zn at the Manganite-Water Interface

Microscopic structures of Zn(II) surface complexes adsorbed at the manganite(gamma-MnOOH)-water interface were studied using extended X-ray absorption fine structure(EXAFS) spectroscopy. Quantitative analysis of the EXAFS spectra showed that, in a 0.1 mol/L NaNO3 solution of pH 7.5, Zn(II) was adsorbed onto the solid surface by sharing the oxygen atom in the hydrous Zn2+ ions and in the structural unit MnO6 on the manganite surface. Most of the adsorbed Zn(II) was in the form of octahedral Zn(H2O2)(6)2+, but part of Zn(II) was adsorbed as tetrahedral Zn(OH)2 or Zn(OH)(4)2-. The average Zn-O bond length was 1.998 +/- 0.010 A (n = 3). The ZnO polyhedron of the hydrous Zn2+ ions was linked to the octahedron MnO6 of the manganite in two modes by sharing two O atoms on the edges of the polyhedral and/or one O atom on the corners of the polyhedral. EXAFS analysis of the second sphere indicated that Zn (II) adsorbed on the manganite resulted in two Zn-Mn atomic distances of 3.08 +/- 0.024A (n = 3) (R1) and 3.54 +/- 0.018 A (n = 3) (R2), corresponding to the edge-linkage (stronger adsorption site) and corner-linkage (weaker adsorption site), respectively. This result confirmed the basic hypothesis of Metastable Equilibrium Adsorption (MEA) theory from a molecular level that adsorption density gamma (mol/m2) is not a state variable, because adsorption density for the same adsorbate may have different chemical potential values due to different microscopic structure and adsorbate energy status. Macroscopic adsorption-desorption experiments showed that adsorption of Zn(II) onto manganite was largely irreversible (big adsorption hysteresis). EXAFS results indicated that the microscopic mechanism for the adsorption irreversibility was corresponded to the strong adsorption sites of edge sharing linkage between the adsorbate and adsorbent polyhedra.