Lützenkirchen Johannes
Institut für Nukleare Entsorgung, Forschungszentrum Karlsruhe, Postfach 3640, D-76021 Karlsruhe, Germany.
J Colloid Interface Sci. 2006 Nov 1;303(1):214-23. doi: 10.1016/j.jcis.2006.07.037. Epub 2006 Aug 24.
Parsons-Zobel plots can in principle be used to estimate inner Helmholtz-layer capacitance values and electrochemical surface areas for mineral particles. Their application to aqueous suspensions of various minerals has been documented in the literature. For the experimental data used so far, the expected linear relationship between the overall and the diffuse-layer capacitances has been reported. The extracted values either have not been used at all subsequently in a surface complexation model to describe the mineral surface charge versus pH curves, or were found not to be suitable entirely for such purposes. In the latter case, the reported failure was not explained. In one part of the present paper, the Parsons-Zobel plot concept is tested with data generated from a surface complexation model, for which the interfacial structure closely corresponds to that assumed in the application of the Parsons-Zobel plot. From the analysis of the results it turns out that electrolyte binding and non-Nernstian surface potential-pH curves more or less strongly affect the outcome of Parsons-Zobel plots. Despite the fact that the analysis in this paper is restricted to iron(III) minerals only, it is concluded, in general, that the use of Parsons-Zobel plots with aqueous mineral suspensions to determine inner Helmholtz-layer capacitances for subsequent application to surface complexation models cannot be recommended, since the reasons for failure can be traced very nicely with applications to model-generated data. Such application requires the determination of further parameters, and it was found that low electrolyte binding and Nernstian slopes should be imposed. Of these two issues, the more important is electrolyte binding. For the surface complexation models, an inner Helmholtz-layer capacitance and weak electrolyte binding were required for a good fit to experimental data. The values of the electrolyte binding constants required to achieve this end are in conflict with the assumptions of the Parsons-Zobel plot (absence of specific adsorption). However, these parameters would not necessarily cause specific adsorption in terms of a classical colloid chemistry definition (i.e., would not shift isoelectric points). The electrochemical surface areas were found to be in good agreement with the value used to generate the data. Based on this, there is a potential for using the approach to determine surface areas in situ from titration curves. Consequently, in a second part of the paper, Parsons-Zobel plots are applied to experimental data with the objective of determining electrochemical surface areas in situ. Application to various sets of published experimental titration data for hydrous ferric oxide yielded consistently very large electrochemical surface areas for fresh samples. This can be explained by very small particles and/or inclusion of substantial amounts of water in the suspended particles. As would be expected, the electrochemical surface area for aged ferrihydrite was found to be substantially lower.
原则上，帕森斯 - 佐贝尔图可用于估算矿物颗粒的内亥姆霍兹层电容值和电化学表面积。其在各种矿物水悬浮液中的应用已有文献记载。对于迄今为止所使用的实验数据，已报道了总电容与扩散层电容之间预期的线性关系。提取的值要么在随后的表面络合模型中根本未被使用来描述矿物表面电荷与pH曲线，要么被发现完全不适用于此目的。在后一种情况下，所报道的失败原因未得到解释。在本文的一部分中，用从表面络合模型生成的数据对帕森斯 - 佐贝尔图概念进行了测试，该模型的界面结构与帕森斯 - 佐贝尔图应用中所假设的结构密切对应。从结果分析可知，电解质结合和非能斯特表面电位 - pH曲线或多或少会强烈影响帕森斯 - 佐贝尔图的结果。尽管本文的分析仅局限于铁（III）矿物，但总体而言，可以得出结论，不建议使用含矿物水悬浮液的帕森斯 - 佐贝尔图来确定内亥姆霍兹层电容以便随后应用于表面络合模型，因为通过应用于模型生成的数据可以很好地追溯失败原因。这种应用需要确定更多参数，并且发现应施加低电解质结合和能斯特斜率。在这两个问题中，更重要的是电解质结合。对于表面络合模型，需要一个内亥姆霍兹层电容和弱电解质结合才能很好地拟合实验数据。实现此目的所需的电解质结合常数的值与帕森斯 - 佐贝尔图的假设（不存在特异性吸附）相冲突。然而，就经典胶体化学定义而言，这些参数不一定会导致特异性吸附（即不会移动等电点）。发现电化学表面积与用于生成数据的值非常一致。基于此，存在从滴定曲线原位确定表面积的方法的潜力。因此，在本文的第二部分中，将帕森斯 - 佐贝尔图应用于实验数据，目的是原位确定电化学表面积。将其应用于各种已发表的水合氧化铁实验滴定数据集，新鲜样品始终产生非常大的电化学表面积。这可以用非常小的颗粒和/或悬浮颗粒中包含大量水来解释。正如所预期的，发现老化的水铁矿的电化学表面积要低得多。
