Na+、Ca2+及Na+-Ca2+混合离子对胡敏酸胶体稳定性的影响

Effects of Na+, Ca2+ and Na+-Ca2+ Mixture on the Stability of Humic Acid Colloids

摘要:

以胡敏酸为研究对象，通过光散射技术比较土壤中常见盐基离子（Na⁺、Ca²⁺和Na⁺-Ca²⁺混合三种电解质）不同浓度条件下胡敏酸胶体凝聚动力学过程，明确不同价态离子界面行为和陪补离子效应对胡敏酸分散稳定性和凝聚体结构的影响。结果发现，Na⁺和Ca²⁺对胡敏酸胶体聚沉能力的差异远远大于舒尔策-哈迪（Schulze-Hardy）规则中因其化合价不同所引起的差异；Na⁺-Ca²⁺混合体系中Ca²⁺主导凝聚过程，且Na⁺-Ca²⁺混合与Ca²⁺两种离子体系中凝聚现象的差异随离子浓度的降低而增大；混合离子体系中Na⁺作为陪补离子，其陪补离子效应对临界聚沉浓度、颗粒间活化能和胡敏酸凝聚体结构均有一定影响，尤以对凝聚体的结构特征影响显著；Na⁺-Ca²⁺混合体系中胡敏酸凝聚体的结构紧实程度介于单纯离子体系之间，因此可通过调节溶液中的离子组成实现对凝聚体结构紧实程度的调控。上述结果表明，Ca²⁺对胡敏酸的聚沉不仅依赖于静电作用，还有Ca²⁺在强电场中的极化诱导其与胡敏酸表面含氧官能团之间发生的共价键和桥键的贡献；此外，陪补离子Na⁺与Ca²⁺在胡敏酸胶体表面的竞争吸附抑制了Ca²⁺对胡敏酸的聚沉作用，从而形成紧实程度适中的结构体。研究结果为探究胶体界面反应及土壤胶体凝聚机制提供新的思路和理论。

Abstract:

【Objective】In this study on humic acid in soil, the technique of light scattering was adopted to compare effects of three different electrolytes common in soil, i.e. Na⁺, Ca²⁺ and Na⁺-Ca²⁺mixture, on agglomeration kinetics of humic acid colloids relative to concentration of the electrolyte and elucidate effects of interface behaviors of ions different in valence and complementary ions on humic acid dispersion stability and agglomerate structure. 【Method】The technique of dynamic light scattering was used to monitor effective hydrodynamic diameters of humic acid agglomerates, which were found increasing with time in all the three electrolytes, regardless of concentration. Based on the time-dependent changes in agglomerate diameter, total mean agglomeration rate, critical agglomeration concentration and activation energy were calcuated, and agglomeration mechanism analyzed. Fractal dimension of humic acid agglomerates 60 d after agglomaeration was determined. 【Result】Resuls show: (1) agglomeration of humic acid colloids was quite similar in all the electrolytes: effective hydrodynamic diameter of humic acid agglomerates linearly increased with time in electrolytes low in concentrations and followed a power law in those high in concentration. The effects of Na⁺ and Ca²⁺ on agglomeration of humic acid colloids in the experiment were much higher than those of the ions different in valence as is described in the Schulze-Hardy law; (2) in the electrolyte of Na⁺-Ca²⁺ mixture, Ca²⁺ ions dominated agglomeration process, and agglomeration process in the electrolyte of Ca²⁺ differed from that in the electrolyte of Na⁺-Ca²⁺ and the sharper the lower the concentration; (3) in the electrolyte of Na⁺-Ca²⁺ mixture, Na⁺, as complementary ion in the electrolyte of Na⁺-Ca²⁺ mixture, had certain complementary effects on critical coagulation concentration, inter-particle activation energy and agglomerate structure, especially the last; and (4) the humic acid agglomerates in the electrolyte of Na⁺-Ca²⁺ mixture was quite compact in structure, varying between those in the electrolytes of Na⁺ and Ca²⁺. It is, therefore, feasible to control compactness of the agglomerates in structure in the electrolyte by adjusting ion composition of the electrolyte. 【Conclusion】 The agglomeration of humic acid colloids induced by Ca²⁺ depends not only on compression of the surface electric double layer on the counter ions, which reduces electrostatic repulsion and causes agglomeration, but also on polarization of Ca²⁺ in the strong electric field that induces and contributes to formation of covalent and bridge bonds between Ca²⁺ and oxygen-containing functional groups on the surface of humic acid. In addition, the competitive adsorptions of Na⁺ and Ca²⁺ on the surface of humic acid inhibits agglomeration of Ca²⁺, thus forming agglomerate moderate in compactness. All these findings may provide some new ideas and theories for exploring colloidal interface reaction and mechanism for agglomeration of soil colloids.