Duval Jérôme F L, Wilkinson Kevin I, Van Leeuwen Herman P, Buffle Jacques
CABE (Analytical and Biophysical Environmental Chemistry), University of Geneva, Sciences II, 30 Quai E. Ansermet, Geneva 4, CH-1211 Switzerland.
Environ Sci Technol. 2005 Sep 1;39(17):6435-45. doi: 10.1021/es050082x.
Due to the complexity of the humic substances (HS), mathematical models have often been employed to understand their roles in the environment. Since no consensus exists with respect to the structure and conformation of the HS, models have alternatively given them properties corresponding to impermeable hard spheres or fully permeable polyelectrolytes. In this study, the hydrodynamic permeability of standard HS (Suwannee River fulvic, humic, and peat humic acids) are evaluated as a function of pH and ionic strength. A detailed theoretical model is used to determine the softness parameter (lambda0), which characterizes the degree of flow penetration into the HS on the basis of measured values of electrophoretic mobilities, diffusion coefficients, and electric charge densities. Their motion in an electric field is evaluated by a rigorous numerical evaluation of the governing electrokinetic equations for soft particles. The hydrodynamic impact of the polyelectrolyte chains is accounted for by a distribution of Stokes resistance centers and partial dissociation of the hydrodynamically immobile ionogenic groups distributed throughout the polyelectrolyte. The results demonstrate thatthe studied HS are small (radius ca. 1 nm), highly charged (500-650 C g(-1) when all sites are dissociated), and very permeable (typical flow penetration length of 25-50% of the radius, depending on pH). The HS also coagulate slightly when lowering the pH of the solution. Modeling of the HS as hard spheres with a charge and slip plane located at the surface is thus physically inappropriate, as are a number of analytical theories for soft particles that hold for low to moderate electrostatic potentials and large colloids. The shortcomings of these simpler approaches, when interpreting the electrophoretic mobilities of HS, are highlighted by comparison with rigorous theoretical predictions.
由于腐殖质（HS）的复杂性，人们常常采用数学模型来理解它们在环境中的作用。由于对于HS的结构和构象尚无共识，模型便赋予它们与不可渗透硬球或完全可渗透聚电解质相对应的性质。在本研究中，评估了标准HS（苏万尼河富里酸、腐殖酸和泥炭腐殖酸）的流体动力学渗透率与pH值和离子强度的函数关系。使用一个详细的理论模型来确定软度参数（lambda0），该参数根据电泳迁移率、扩散系数和电荷密度的测量值来表征流体穿透HS的程度。通过对软颗粒的控制电动方程进行严格的数值评估来评估它们在电场中的运动。聚电解质链的流体动力学影响通过斯托克斯阻力中心的分布以及分布在整个聚电解质中的流体动力学固定离子ogenic基团的部分解离来考虑。结果表明，所研究的HS体积小（半径约1纳米）、电荷高（所有位点解离时为500-650 C g(-1)）且渗透性很强（典型的流体穿透长度为半径的25-50%，取决于pH值）。当降低溶液的pH值时，HS也会轻微凝聚。因此，将HS建模为表面带有电荷和滑移面的硬球在物理上是不合适的，许多适用于低至中等静电势和大胶体的软颗粒分析理论也是如此。与严格的理论预测相比，突出了这些更简单方法在解释HS的电泳迁移率时的缺点。
