Department of Soil Quality, Wageningen University, Wageningen, The Netherlands.
Environ Sci Technol. 2010 Apr 1;44(7):2529-34. doi: 10.1021/es903080d.
Very low concentrations of free metal ion in natural samples can be measured using the Donnan membrane technique (DMT) based on ion transport kinetics. In this paper, the possible effects of slow dissociation of metal complexes on the interpretation of kinetic DMT are investigated both theoretically and experimentally. The expressions of the lability parameter, Lgrangian , were derived for DMT. Analysis of new experimental studies using synthetic solution containing NTA as the ligand and Cu(2+) ions shows that when the ionic strength is low (<or=0.2 mM Ca(NO(3))(2)) the dissociation rate of NTACu becomes the limiting step in Cu transport of the DMT measurement. In natural waters, dissolved organic matter (DOM) is the most important source of ligands that complex metals. By comparing the fraction of labile species measured using other dynamic sensors (DGT, GIME) in several freshwaters, it is concluded that in most waters ion transport in DMT is controlled by diffusion in the membrane. Only in very soft waters (<0.7 mM Ca+Mg), the dissociation rate of natural metal complex may influence ion transport in DMT. In this case, neglecting this effect may lead to an underestimation of the free metal ion concentration measured.
采用基于离子输运动力学的唐南膜技术（DMT），可以测量天然样品中极低浓度的游离金属离子。本文从理论和实验两方面研究了金属配合物缓慢离解对动力学 DMT 解释的可能影响。推导出了用于 DMT 的不稳定参数 Lagrangian 的表达式。对含有 NTA 作为配体和 Cu（2+）离子的合成溶液进行的新实验研究的分析表明，当离子强度较低（<或=0.2 mM Ca（NO3）2）时，NTACu 的离解速率成为 DMT 测量中 Cu 输运的限制步骤。在天然水中，溶解有机物（DOM）是与金属形成配合物的最重要配体来源。通过比较几种淡水使用其他动态传感器（DGT、GIME）测量的可利用物种分数，可以得出结论，在大多数水中，DMT 中的离子输运受膜扩散控制。只有在非常软的水中（<0.7 mM Ca+Mg），天然金属配合物的离解速率才可能影响 DMT 中的离子输运。在这种情况下，忽略此影响可能会导致测量的游离金属离子浓度低估。
