Dissolution Kinetics and Solubility of ZnO Nanoparticles Followed by AGNES

Abstract

There is a current debate on whether the toxicity of engineered ZnO nanoparticles (NPs) can be traced back to their nanoscale properties or rather to the simple fact of their relatively high solubility and consequent release of Zn2+ ions. In this work, the emerging electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping), which is specially designed to determine free metal ion concentration, is shown to be able to measure the Zn2+ concentration resulting from dissolution of ZnO nanoparticles dispersed in aqueous salt solutions. Three NP samples from different sources (having average primary particle diameters of 6, 20, and 71 nm) were tested and compared with bulk ZnO material. The enhanced solubility of the nanoparticles with decreasing primary radius allows for an estimation of the surface energy of 0.32 J/m2. AGNES also allows the study of the kinetics of Zn2+ release as a response to a change in the solution parameters (e.g., pH, ZnO concentration). A physicochemical model has been developed to account for the observed kinetic behavior. With this model, only one kinetic parameter is required to describe the time dependence of the free Zn2+ concentration in solution. Good agreement with this prediction is obtained when, starting from an equilibrated NP dispersion, the pH of the medium is lowered. Also, the independence of this parameter from pH, as expected from the model, is obtained at least in the pH range 7–9. When dissolution is studied by dispersing ZnO nanoparticles in the medium, the kinetic parameter initially decreases with time. This decrease can be interpreted as resulting from the increase of the radius of the clusters due to the agglomeration/aggregation phenomena (independently confirmed). For the larger assayed NPs (i.e., 20 and 71 nm), a sufficiently large pH increase leads to a metastable solubility state, suggesting formation of a hydroxide interfacial layer.

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.…”

“…In contrast to SiO 2 which is amorphous, ZnO is crystalline and polar [18]. ZnO shows a tendency to dissolve in aqueous dispersion and release free Zn 2+ ion [19]. Finally ZnO nanoparticles have a strong tendency to aggregate [20,21].…”

Assessing the Risk of Engineered Nanomaterials in the Environment: Development and Application of the nanoFate Model

“…Experimental dissolution rate data in the specific water chemistry was obtained for each ENM and modeled using an assumed first-order rate process that proceeds until the equilibrium ion concentration is reached (Table S4). 105 Since there is no experimental data for dissolution of ENMs in freshwater and marine sediments, the rate of dissolution was assumed to be 1/10th the rate in the corresponding water column, considering mass transfer is slower in porous media. 66,68,106 Transfer of ENMs and suspended sediment from freshwater to coastal waters and the transfer from coastal waters to the marine compartment (out of the modeled system) is considered an advective flux, estimated using regional flow data (SIUG, Section 2.3.6).…”

Aquatic toxicity of manufactured nanomaterials: challenges and recommendations for future toxicity testing

“…[157] Uncoated ZnO NMs are also soluble in aqueous media (unlike TiO 2 NMs, and more so than Ag NMs) releasing Zn 2þ from the NM surface. [158,159] Although this offers further commercial applications of ZnO NMs, including better delivery of Zn 2þ as a nutritional supplement, [160] it can also be detrimental in aquatic environments because Zn 2þ is a well-characterised toxicant at higher concentrations in aquatic organisms, e.g. fish (see recent review by Hogstrand [161] ).…”

“…In suspensions, uncoated ZnO NMs aggregate, sediment and solubilise, and equilibrium Zn 2þ concentrations are affected by pH, particle size (especially where ZnO NM , 6 nm) and ionic strength of the media. [158,159] Toxicity in marine organisms has often been closely correlated with the concentration of released Zn 2þ . For example, cell growth inhibition (measured as decreased cell numbers and chlorophyll concentrations) of the marine phytoplankton Thalassiosira pseudonana exposed to ZnO NMs was closely correlated with the free Zn 2þ concentration in suspension.…”