Aggregation and dissolution of ZnO nanoparticles synthesized by different methods: Influence of ionic strength and humic acid

•Synthesis procedure can substantially influence surface properties of ZnO-NPs.•Ionic strength enhances ZnO-NPs aggregation regardless of synthesis methods.•Humic acid largely reduces ZnO-NPs aggregation regardless of ionic strength.•Humic acid increases ZnO-NPs dissolution regardless of pH and synthesis methods.•Physicochemical property of ZnO-NPs plays important roles in ZnO-NPs dissolution. Zinc oxide (ZnO) nanoparticles (NPs) are widely used in various industries, and studies on the aggregation and dissolution of this material in an aqueous environment are being actively conducted these days. In this study, ZnO-NPs (i.e., nZnO-1 versus nZnO-2) with two different particle sizes, particle shapes, specific surface areas, and functional groups (OH group and carboxylic acid) were prepared through two different synthesis methods. Aggregation tests at circumneutral pH (∼7.5) confirmed that both ZnO-NPs, which have different zeta potentials, were largely affected by the ionic strength and the amounts of humic acid. In the absence of humic acid, their sedimentation behavior was observed to differ from each other due to the different surface charges of the ZnO-NPs and the aggregation behavior of ZnO-NPs according to the ionic strength corresponded to the classic Derjaguin–Landau–Verway–Overbeek (DLVO) theory. The presence of humic acid significantly enhanced the dispersion stability of both ZnO-NPs, but the stability was not explained by the classic DLVO theory due to steric repulsion. In both ZnO-NPs, the Zn2+ dissolution level increased in the presence of humic acid, and the level increased with the increasing humic acid amount. In particular, in the presence of humic acid at pH 9.0, the dissolution level of nZnO-1 was confirmed to be significantly higher than that of nZnO-2. Although nZnO-2 had a smaller particle size and a greater specific surface area than nZnO-1, the humic acid adsorption amount of nZnO-1 was confirmed to be greater than that of nZnO-2 due to the lower hydrophilicity and the stronger electrostatic attraction between humic acid and nZnO-1 caused by less water content and positive zeta potential of nZnO-1.