Nachtegaal Maarten, Sparks Donald L
Molecular Environmental Soil Chemistry Group, Department of Plant and Soil Sciences, University of Delaware, 152 Townsend Hall, Newark, DE 19717-1303, USA.
J Colloid Interface Sci. 2004 Aug 1;276(1):13-23. doi: 10.1016/j.jcis.2004.03.031.
Oxide surface coatings are ubiquitous in the environment, but their effect on the intrinsic metal uptake mechanism by the underlying mineral surface is poorly understood. In this study, the zinc (Zn) sorption complexes formed at the kaolinite, goethite, and goethite-coated kaolinite surfaces, were systematically studied as a function of pH, aging time, surface loading, and the extent of goethite coating, using extended X-ray absorption fine structure (EXAFS) spectroscopy. At pH 5.0, Zn partitioned to all sorbents by specific chemical binding to hydroxyl surface sites. At pH 7.0, the dominant sorption mechanism changed with reaction time. At the kaolinite surface, Zn was incorporated into a mixed metal Zn-Al layered double hydroxide (LDH). At the goethite surface, Zn initially formed a monodentate inner-sphere adsorption complex, with typical Zn-Fe distances of 3.18 A. However, with increasing reaction time, the major Zn sorption mechanism shifted to the formation of a zinc hydroxide surface precipitate, with characteristic Zn-Zn bond distances of 3.07 A. At the goethite-coated kaolinite surface, Zn initially bonded to FeOH groups of the goethite coating. With increasing aging time however, the inclusion of Zn into a mixed Zn-Al LDH took over as the dominant sorption mechanism. These results suggest that the formation of a precipitate phase at the kaolinite surface is thermodynamically favored over adsorption to the goethite coating. These findings show that the formation of Zn precipitates, similar in structure to brucite, at the pristine kaolinite, goethite, and goethite-coated kaolinite surfaces at near neutral pH and over extended reaction times is an important attenuation mechanism of metal contaminants in the environment.
氧化物表面涂层在环境中普遍存在，但人们对其对下层矿物表面固有金属吸收机制的影响却知之甚少。在本研究中，利用扩展X射线吸收精细结构（EXAFS）光谱，系统研究了在高岭石、针铁矿以及针铁矿包覆的高岭石表面形成的锌（Zn）吸附络合物，该络合物是pH值、老化时间、表面负载量以及针铁矿包覆程度的函数。在pH值为5.0时，锌通过与羟基表面位点的特异性化学结合分配到所有吸附剂上。在pH值为7.0时，主要的吸附机制随反应时间而变化。在高岭石表面，锌被掺入混合金属锌 - 铝层状双氢氧化物（LDH）中。在针铁矿表面，锌最初形成单齿内球吸附络合物，典型的锌 - 铁距离为3.18埃。然而，随着反应时间的增加，主要的锌吸附机制转变为形成氢氧化锌表面沉淀，其特征性锌 - 锌键距离为3.07埃。在针铁矿包覆的高岭石表面，锌最初与针铁矿包覆层的FeOH基团结合。然而，随着老化时间的增加，锌掺入混合锌 - 铝LDH中成为主要的吸附机制。这些结果表明，在高岭石表面形成沉淀相在热力学上比吸附到针铁矿包覆层更有利。这些发现表明，在接近中性的pH值和延长的反应时间下，在原始高岭石、针铁矿以及针铁矿包覆的高岭石表面形成结构类似于水镁石的锌沉淀物是环境中金属污染物的一种重要衰减机制。
