Institute of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, DK-5230 Odense, Denmark.
Environ Sci Technol. 2012 Oct 2;46(19):10487-98. doi: 10.1021/es3018013. Epub 2012 Sep 13.
The dynamics of metal complexation by small humic substances (fulvic acid and aquatic humic acid, collectively denoted as “fulvic-like substance”, FS) are explored within the framework of concepts recently developed for soft nanoparticulate complexants. From a comprehensive collection of published equilibrium and dissociation rate constants for CuFS and NiFS complexes, the association rate constant, ka, is determined as a function of the degree of complexing site occupation, θ. From this large data set, it is shown for the first time that ka is independent of θ. This result has important consequences for finding the nature of the rate limiting step in the association process. The influence of electric effects on the rate of the association process is described, namely (i) the accelerating effect of the negatively charged electrostatic field of FS on the diffusion of metal ions toward it, and (ii) the extent to which metal ions electrostatically accumulate in the counterionic atmosphere of FS. These processes are discussed qualitatively in relation to the derived values of ka. For slowly dehydrating metal ions such as Ni(H2O)6 2+ (dehydration rate constant, kw), ka is expected to derive straight from kw. In contrast, for rapidly dehydrating metal ions such as Cu(H2O)6 2+, transport limitations and electric effects involved in the formation of the precursor outer-sphere associate appear to be important overall rate-limiting factors. This is of great significance for understanding the chemodynamics of humic complexes in the sense that inner-sphere complex formation would not always be the (sole) rate limiting step.
小分子腐殖质（富里酸和水生腐殖酸，统称为“富里酸样物质”，FS）与金属配合物的动力学，是在最近为软纳米配合物开发的概念框架内进行探索的。从发表的大量关于 CuFS 和 NiFS 配合物的平衡和解离速率常数中，确定了作为配位位点占有率θ函数的络合速率常数ka。从这个大数据集中，首次表明 ka 与θ无关。这一结果对发现络合过程中速率限制步骤的性质具有重要意义。描述了电效应对络合过程速率的影响，即（i）FS 的带负电荷静电场对金属离子向其扩散的加速作用，以及（ii）金属离子在 FS 的反离子气氛中静电积累的程度。这些过程与得出的 ka 值定性地进行了讨论。对于像 Ni(H2O)6 2+（脱水速率常数 kw）这样缓慢脱水的金属离子，ka 预计直接来自 kw。相比之下，对于像 Cu(H2O)6 2+这样快速脱水的金属离子，在形成前体外壳配合物过程中涉及的输运限制和电效应似乎是整体速率限制因素。这对于理解腐殖质配合物的化学动力学具有重要意义，因为内配合物形成并不总是（唯一的）速率限制步骤。
