Yasadi Kamuran, Pinheiro Jose Paulo, Zielińska Katarzyna, Town Raewyn M, van Leeuwen Herman P
Laboratory of Physical Chemistry and Colloid Science, Wageningen University , Dreijenplein 6, 6703 HB Wageningen, The Netherlands.
Langmuir. 2015 Feb 10;31(5):1737-45. doi: 10.1021/la504885v. Epub 2015 Jan 29.
The hydrogel/water partitioning of the various species in the cadmium(II)/soil humic acid (HA) system is studied for two types of gel, using in situ microelectrodic voltammetry. Under the conditions of this work, with HA particles of ca. 25 and 125 nm radius, the CdHA complex is shown to be close to nonlabile toward a 12.5 μm radius microelectrode. This implies that its kinetic contribution to Cd(2+) reduction at the medium/microelectrode interface is practically negligible. The polyacrylamide (PAAm) gels equilibrate with the aqueous medium under significant sorption of HA at the gel backbone/gel medium interface, which in turn leads to induced sorption of Cd(II) in the form of immobilized gel-bound CdHA. The rather high total Cd content of the PAAm gel suggests that the binding of Cd(2+) by the hydrophobically gel-bound HA is stronger than that for dispersed HA particles. Still, the intraparticulate speciation of Cd(II) over Cd(2+) and CdHA corresponds to an intrinsic stability constant similar to that for simple monocarboxylate ligands such as acetate. Alginate gels are negatively charged, and their free [Cd(aq)(2+)] is higher than that in the medium by the corresponding Donnan coefficient. On top of that, Cd(2+) is specifically sorbed by the gel backbone/gel medium interface to reach accumulation factors as high as a few tens. HA and CdHA accumulate in the outer 20 μm film of gel at the gel/water interface of both gels, but they do not penetrate into the bulk of the alginate gel. Overall, the gel/water interface dictates drastic changes in the speciation of Cd/HA as compared to the aqueous medium, with distinct features for each individual type of gel. The results have broad significance, for example, for predictions of reactivity and bioavailability of metal species which inherently involve partitioning and diffusion into diverse gel layers such as biointerfacial cell walls, biofilm matrices, and mucous membranes.
使用原位微电极伏安法，针对两种类型的水凝胶，研究了镉（II）/土壤腐殖酸（HA）体系中各种物质在水凝胶/水之间的分配情况。在本研究条件下，对于半径约为25纳米和125纳米的HA颗粒，CdHA络合物对于半径为12.5微米的微电极而言接近非活性状态。这意味着其在介质/微电极界面处对Cd(2+)还原的动力学贡献实际上可忽略不计。聚丙烯酰胺（PAAm）水凝胶在凝胶主链/凝胶介质界面处大量吸附HA的情况下与水相介质达到平衡，这反过来导致以固定化的凝胶结合CdHA形式诱导吸附Cd（II）。PAAm凝胶中相当高的总Cd含量表明，疏水凝胶结合的HA对Cd(2+)的结合力强于分散的HA颗粒。然而，Cd（II）在Cd(2+)和CdHA之间的颗粒内物种形成对应于一个与简单单羧酸盐配体（如乙酸盐）相似的固有稳定常数。藻酸盐凝胶带负电荷，其游离的[Cd(aq)(2+)]比介质中的相应值高相应的唐南系数。除此之外，Cd(2+)被凝胶主链/凝胶介质界面特异性吸附，积累因子高达几十。HA和CdHA在两种凝胶的凝胶/水界面处积聚在凝胶外层20微米的薄膜中，但它们不会渗透到藻酸盐凝胶的主体中。总体而言，与水相介质相比，凝胶/水界面决定了Cd/HA物种形成的剧烈变化，每种单独类型的凝胶都有明显特征。这些结果具有广泛的意义，例如，对于预测金属物种反应性和生物有效性而言，这涉及到向不同凝胶层（如生物界面细胞壁、生物膜基质和粘膜）的分配和扩散。
