Department of Civil & Environmental Engineering, University of Houston, Houston, TX 77004, United States; College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, China.
Department of Civil & Environmental Engineering, University of Houston, Houston, TX 77004, United States.
Environ Res. 2018 Feb;161:49-60. doi: 10.1016/j.envres.2017.10.045. Epub 2017 Nov 2.
Improving the colloidal stability of magnetite nanoparticles (MNPs) is essential for their successful applications. In this study, the surface zeta potential and particle size evolutions of citric acid coated magnetite nanoparticles (CA-MNPs) were measured under varied aqueous conditions using dynamic light scattering (DLS). The effects of pH (5.0-9.0), ionic strength (IS), cations (Na and Ca), anions (phosphate, sulfate, and chloride) and humic acid on the aggregation behaviors of CA-MNPs were explored. Compared with bare MNPs, the stability of CA-MNPs were greatly improved over the typical pH range of natural aquatic environments (pH = 5.0-9.0), as the coated CA-MNPs were highly negatively charged over the pH range due to the low pK value (3.13) of citrate acid. CA-MNPs were more stable in the presence of monovalent cation (Na) compared with divalent cation (Ca), as Ca could neutralize the surface charge of MNPs more significantly than Na. In the presence of anions, the surface charges of CA-MNPs became more negative, and the stability of CA-MNPs followed the order: in phosphate > sulfate > chloride. The observed aggregation trend could be explained by the differences in the valences of the anions and their adsorption behaviors onto CA-MNPs, which altered the surface charges of CA-MNPs. The measured critical coagulation concentrations (CCC) values of CA-MNPs in these electrolyte solutions agreed well with Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations. With the addition of Humic acid (HA), the aggregation of CA-MNPs was inhibited in all electrolyte solutions even with the critical coagulation concentrations. This is due to the adsorption of HA onto CA-MNPs, which enhanced the electrostatic and steric repulsive forces between CA-MNPs. Considering the good stability of CA-MNPs in solutions with varied pH and electrolyte compositions, as well as with the easy synthesis of CA-MNPs and their non-toxicity, this study suggested CA coating as a good strategy to increase the stability of MNPs.
提高磁铁矿纳米粒子（MNPs）的胶体稳定性对于它们的成功应用至关重要。在这项研究中，使用动态光散射（DLS）测量了柠檬酸包覆的磁铁矿纳米粒子（CA-MNPs）在不同水相条件下的表面zeta 电位和粒径演变。研究了 pH（5.0-9.0）、离子强度（IS）、阳离子（Na 和 Ca）、阴离子（磷酸盐、硫酸盐和氯化物）和腐殖酸对 CA-MNPs 聚集行为的影响。与裸 MNPs 相比，由于柠檬酸的低 pK 值（3.13），在典型的天然水相环境 pH 范围内（pH=5.0-9.0），包覆 CA-MNPs 具有很高的负电荷，因此 CA-MNPs 的稳定性得到了极大提高。与二价阳离子（Ca）相比，一价阳离子（Na）的存在使 CA-MNPs 更稳定，因为 Ca 比 Na 更能显著中和 MNPs 的表面电荷。在阴离子存在的情况下，CA-MNPs 的表面电荷变得更负，CA-MNPs 的稳定性顺序为：磷酸盐>硫酸盐>氯化物。观察到的聚集趋势可以通过阴离子的价态及其在 CA-MNPs 上的吸附行为的差异来解释，这改变了 CA-MNPs 的表面电荷。在这些电解质溶液中测量的 CA-MNPs 的临界聚沉浓度（CCC）值与德加古林-兰德沃维-奥弗贝克（DLVO）计算吻合良好。随着腐殖酸（HA）的加入，即使在电解质溶液的临界聚沉浓度下，CA-MNPs 的聚集也被抑制。这是由于 HA 在 CA-MNPs 上的吸附，增强了 CA-MNPs 之间的静电和空间排斥力。考虑到 CA-MNPs 在不同 pH 和电解质组成的溶液中具有良好的稳定性，以及 CA-MNPs 易于合成且无毒性，本研究表明 CA 涂层是提高 MNPs 稳定性的一种很好的策略。
