Dissolution of ZnO Nanoparticles at Circumneutral pH: A Study of Size Effects in the Presence and Absence of Citric Acid

Abstract

Understanding size-dependent processes, including dissolution, of engineered nanoparticles is essential in addressing the potential environmental and health impacts of these materials as well as their long-term stability. In this study, experimental measurements of size-dependent dissolution of well-characterized zinc oxide (ZnO) nanoparticles with particle diameters in the range of 4 to 130 nm have been measured at circumneutral pH (pH 7.5) and compared. Dissolution was found to be enhanced with smaller ZnO nanoparticles compared to larger-sized particles, even though the nanoparticles were present in solution as aggregates with hydrodynamic diameters on the order of 1-3 μm in size. The presence of citric acid significantly enhanced the extent of ZnO dissolution for all sizes, and the greatest enhancement was observed for the 4 nm particles. Although these results are found to be in qualitative agreement with theoretical predictions, a linearized form of the Kelvin equation to calculate a surface free energy yielded quantities inconsistent with expected values from the literature. Reasons for this inconsistency are discussed and include potential deviations of solubility behavior from classical thermodynamics as a result of a lack of detailed knowledge of surface structure and surface properties, including the presence of different surface crystal facets, and the aggregation state.

ZnO nanoparticle interactions with phospholipid monolayers

“…The release of Zn 2+ by Nanosun and in-house synthesised ZnO particles has been studied in a previous paper. It has been shown that the extent of Zn 2+ release is dependent on the ZnO primary particle size [19,48]. This is commensurate with the classical predictions from the Kelvin equation relating solubility to the curvature of the particle.…”

The influence of natural organic matter and aging on suspension stability in guideline toxicity testing of silver, zinc oxide, and titanium dioxide nanoparticles with Daphnia magna

“…Kinetics studies have shown that ZnO ENPs (at pH 7.5) can reach dissolution steady state in 24 hours [49]. However, in our study (pH 8.2), stable testing conditions were not reached until 48 hour aging, revealing clear dose-response relationships ( Figure 3) which corresponded to the increase in dissolution (Supplemental Data, Table S3).…”

Dissolution and Microstructural Transformation of ZnO Nanoparticles under the Influence of Phosphate

“…The amorphous ZnO was considered to be an amorphous Zn(OH) 2 coating formed via hydrolysis of ZnO. 15 This amorphous Zn(OH) 2 was slightly soluble and Zn 2+ ions were released into solution. 12 Aqueous PO 4 3− can be adsorbed on the hydroxylation surface via exchange with OH − , forming an amorphous Zn phosphate layer.…”

“…9−12 Some recent studies have also investigated interactions between NPs and natural organic matter (NOM), taking into consideration their ubiquity in the environment together with their high reactivity, 13 and the results have shown that dissolved NOM can enhance the stability and dissolution of NPs in water. 14,15 Nevertheless, we are still far from understanding the influence of environmental factors on the behavior of NPs in the natural environment. Many other important components such as phosphates are also present in addition to NOM, and these may interact with NPs and affect their behavior and fate.…”