Application of zincon analysis to investigate the zinc speciation in aqueous media for further understanding of ZnO solubility

Abstract

The environmental accumulation and toxicity potential of metal oxide nanoparticles because of their increasing use in various commercial products have increased the interest in understanding the behavior of these materials in the aquatic environment. For example, zinc oxide (ZnO) is used in various industrial applications but exhibits partial solubility in aqueous media, therefore it has potential aquatoxic action. ZnO dissolves in water by releasing Zn2+ ions; thus, understanding the complexation behavior of these ions with water is crucial to be able to give a complete description of the fate of ZnO in aquatic environment. In this study, we propose application of a reliable and facile route based on the zincon analysis to further understand the Zn2+ complexation when ZnO is placed in aqueous media. To evaluate this, eight Zn2+ standards at a certain concentration were first prepared in the pH range between 7.0 and 10.0. Samples were analyzed via Inductive Coupled Plasma-Mass spectrometry (ICP-MS/MS) and the zincon analysis by monitoring their UV absorbance maximum at 620 nm. Analysis before and after centrifugation aided the investigation of the pH dependent complexation of Zn2+. In the centrifuged samples, the measured zinc ion fraction decreased, suggesting precipitation of zinc as zinc hydroxide (Zn(OH)2(s)) in the solid form from the solution when the pH was increased from 7.0 to 10.0. However, there was no significant decrease in the non-centrifuged samples in the same pH range. These results show that the zincon analysis can be used to quantify both Zn2+(aq) ions and Zn(OH)2. Moreover, the zincon results were consistent with the results of ICP-MS/MS analyses. The centrifugation helps to sediment solid hydroxide particles enabling one to predict fraction of the ions and the hydroxides. Solid hydroxide precipitate was characterized by FTIR, XRD, SEM and EDX; and formation of orthorhombic Zn(OH)2 phase was confirmed.