Duc Myriam, Gaboriaud Fabien, Thomas Fabien
Laboratoire de Chimie Physique et Microbiologie pour l'Environnement (UMR 7564), CNRS-UHP Nancy 1, 405 rue de Vandoeuvre, F-54600 Villers-lès-Nancy, France.
J Colloid Interface Sci. 2005 Sep 1;289(1):139-47. doi: 10.1016/j.jcis.2005.03.060.
Measuring and modeling the surface charge of clays, and more especially smectites, has become an important issue in the use of bentonites as a waste confinement material aimed at retarding migration of water and solutes. Therefore, many studies of the acid-base properties of montmorillonite have appeared recently in the literature, following older studies principally devoted to cation exchange. It is striking that beyond the consensus about the complex nature of the surface charge of clays, there are many discrepancies, especially concerning the dissociable charge, that prevents intercomparison among the published data. However, a general trend is observed regarding the absence of common intersection point on raw titration curves at different ionic strengths. Analysis of the literature shows that these discrepancies originate from the experimental procedures for the preparation of the clays and for the quantification of their surface charge. The present work is an attempt to understand how these procedures can impact the final results. Three critical operations can be identified as having significant effects on the surface properties of the studied clays. The first one is the preparation of purified clay from the raw material: the use of acid or chelation treatments, and the repeated washings in deionized water result in partial dissolution of the clays. Then storage of the purified clay in dry or wet conditions strongly influences the equilibria in the subsequent experiments respectively by precipitation or enhanced dissolution. The third critical operation is the quantification of the surface charge by potentiometric titration, which requires the use of strong acids and bases. As a consequence, besides dissociation of surface sites, many secondary titrant consuming reactions were described in the literature, such as cation exchange, dissolution, hydrolysis, or precipitation. The cumulated effects make it difficult to derive proper dissociation constants, and to build adequate models. The inadequation of the classical surface complexation models to describe the acid-base behavior of clays is illustrated by the electrokinetic behavior of smectites, which is independent from the pH and the ionic strength. Therefore, there is still a need on one hand for accurate data recorded in controlled conditions, and on the other hand for new models taking into account the complex nature of the charge of clays.
测量和模拟黏土尤其是蒙脱石的表面电荷，已成为将膨润土用作废物封闭材料以减缓水和溶质迁移时的一个重要问题。因此，近期文献中出现了许多关于蒙脱石酸碱性质的研究，在此之前的研究主要致力于阳离子交换。令人惊讶的是，尽管人们对黏土表面电荷的复杂性质已达成共识，但仍存在许多差异，尤其是在可解离电荷方面，这使得已发表的数据难以相互比较。然而，在不同离子强度下的原始滴定曲线上不存在共同交点这一点上，观察到了一个普遍趋势。文献分析表明，这些差异源于黏土制备以及表面电荷定量的实验程序。本研究旨在了解这些程序如何影响最终结果。可以确定有三个关键操作对所研究黏土的表面性质有显著影响。第一个是从原材料制备纯化黏土：使用酸或螯合处理以及在去离子水中反复洗涤会导致黏土部分溶解。然后，将纯化黏土在干燥或潮湿条件下储存，分别通过沉淀或增强溶解对后续实验中的平衡产生强烈影响。第三个关键操作是通过电位滴定法对表面电荷进行定量，这需要使用强酸和强碱。因此，除了表面位点的解离外。文献中还描述了许多消耗滴定剂的二级反应，如阳离子交换、溶解、水解或沉淀。这些累积效应使得难以得出合适的解离常数并建立适当的模型。蒙脱石的电动行为与pH值和离子强度无关，这说明了经典表面络合模型在描述黏土酸碱行为方面的不足。因此，一方面仍然需要在受控条件下记录的准确数据，另一方面需要考虑黏土电荷复杂性质的新模型。
