Rose Andrew L, Bligh Mark W, Collins Richard N, Waite T David
Southern Cross GeoScience, Southern Cross University , Lismore 2480, Australia.
Langmuir. 2014 Apr 1;30(12):3548-56. doi: 10.1021/la404712r. Epub 2014 Mar 21.
Small angle X-ray scattering (SAXS) measurements coupled to a stopped-flow device has permitted the observation of the kinetics of Fe(III) oxyhydroxide (FeOx) formation and transformation from around 1 s to 30 min after initiation under environmentally relevant conditions at pH 3. The Unified Model approach was used to determine the evolution of multiple key parameters (particle scattering mass, mean particle volume, particle concentration, particle dimensionality, and particle size) for two separate structural levels as a function of time, with the results obtained enabling clarification of the mechanisms underlying FeOx formation and transformation under these conditions. Colloidal primary particles (radius of gyration 2-10 nm) that were observable by SAXS formed within 1 s of stopping the flow and subsequently grew over several minutes, first by cluster-cluster addition and then by a monomer-addition mechanism. Aggregation of these primary particles via a secondary cluster-cluster addition mechanism simultaneously resulted in a distinct population of larger (25-40 nm radius of gyration) secondary particles. The primary particles evolved into compact spheroidal forms with fractally rough surfaces, while the secondary particles were relatively open mass fractal structures. Comparison of the observed rates of these processes with those predicted for Fe polymerization indicates that kinetics of primary particle formation were likely controlled initially by rates of exchange between water molecules coordinated with Fe and those in the bulk solution. These findings provide new insights into the mechanisms underlying FeOx formation and transformation, and the kinetics of these mechanisms, at pH 3.
将小角X射线散射（SAXS）测量与停流装置相结合，能够在环境相关条件下，于pH 3时观察氢氧化铁（FeOx）形成和转化的动力学过程，时间范围从起始后约1秒至30分钟。采用统一模型方法来确定两个不同结构层次的多个关键参数（颗粒散射质量、平均颗粒体积、颗粒浓度、颗粒维度和颗粒大小）随时间的演变情况，所得结果有助于阐明在这些条件下FeOx形成和转化的潜在机制。通过SAXS可观察到的胶体初级颗粒（回转半径为2 - 10纳米）在停止流动后1秒内形成，随后在几分钟内生长，首先通过簇 - 簇添加，然后通过单体添加机制。这些初级颗粒通过二次簇 - 簇添加机制聚集，同时形成了明显的较大（回转半径为25 - 40纳米）次级颗粒群体。初级颗粒演变成具有分形粗糙表面的致密球状形式，而次级颗粒则是相对开放的质量分形结构。将这些过程的观察速率与铁聚合预测速率进行比较表明，初级颗粒形成的动力学最初可能受与铁配位的水分子与本体溶液中水分子交换速率的控制。这些发现为pH 3时FeOx形成和转化的潜在机制以及这些机制的动力学提供了新的见解。
