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Critical Review
In the search for nano-specific effects of dissolution of metallic nanoparticles at freshwater-like conditions – a critical review Jonas Hedberg, Eva Blomberg, and Inger Odnevall Wallinder Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019
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Environmental Science & Technology
In the search for nano-specific effects of dissolution of metallic nanoparticles at freshwater-like conditions – a critical review
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Jonas Hedberg,*,a Eva Blomberg,a,b Inger Odnevall Wallindera
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a
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and Health, Department of Chemistry, Division of Surface and Corrosion Science, Stockholm, Sweden
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b
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* corresponding author, e-mail:
[email protected] KTH Royal Institute of Technology, School of Engineering Sciences in Chemistry, Biotechnology
RISE Research Institutes of Sweden, Division Bioscience and Materials, Stockholm, Sweden
Abstract
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Knowledge on relations between particle properties and dissolution/transformation characteristics of
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metal and metal oxide nanoparticles (NPs) in freshwater is important for risk assessment and product
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development. This critical review aims to elucidate nano-specific effects on dissolution of metallic
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NPs in freshwater and similar media. Dissolution rate constants are compiled and analyzed for NPs of
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silver (Ag), copper (Cu), copper oxide/hydroxide (CuO, Cu(OH)2), zinc oxide (ZnO), manganese
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(Mn), and aluminum (Al), showing largely varying (orders of magnitude) constants when modelled
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using first order kinetics. An effect of small primary sizes (>100 mg/L). For the
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yellow and orange boxes, the ionic metal solubility is intermediate (0.01-5 mg/L) and the solubility
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can also be sensitive to changes in experimental conditions (e.g. DOM and Cl- content).
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Table 1 clearly shows that dissolution experiments of Al and Cu NPs as a function of DOM
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concentration will be influenced by the solubility of the metal ions if the concentrations are close to
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their respective saturation level. Several investigations on Cu metal and Cu oxide NPs have been
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performed at oversaturated conditions at different NOM concentrations.69, 78 Such conditions can result
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in largely lower predicted dissolution rate constants and half-lives compared with unsaturated
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conditions.49 Very slow release (months) of Cu ions from CuO NPs has been observed at
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undersaturated conditions.49 Some dissolution rate constants in the literature may also not be
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representative for realistic environmental conditions (NP concentrations in the order of µg/L or
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lower1) due to too high NP concentrations investigated at laboratory conditions. Environmental
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settings further likely act as sinks at which dissolved metal ions from NPs become immobilized via
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different metal complexation and settling processes.76
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As illustrated in Table 1, the presence of Cl- in solution influences the solubility of Ag+. The release of
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Ag in Cl--containing solutions may hence, depending on the Ag+/Cl- ratio, form insoluble AgCl-
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complexes.79 This formation has been thoroughly discussed in terms of fate and dissolution of Ag
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NPs.53, 80-82
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A schematic depiction of the connection between AgCl speciation and the dissolution rate of Ag NPs
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is shown in Figure 2 based on data from Levard et al.79 The figure shows that the formation of soluble
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AgCl species increases the dissolution rate, while the formation of insoluble AgCl-species results in
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decreased dissolution rates (oversaturated conditions with respect to soluble Ag+ species).
Soluble Ag+ species (%)
100 Formation of soluble AgCl: Higher dissolution rate of Ag NPs compared with pure water conditions
80 60 40 Formation of insoluble AgCl: lower dissolution rate of Ag NPs compared with pure water conditions
20 0 1
10
100 1000 + Cl /Ag ratio
10000
100000
251 252
Figure 2. Fraction of soluble Ag+ species as a function of the Cl-/Ag+ ratio in pure water
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complemented with information on the connection between the Cl-/Ag+ ratio and the dissolution rate
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of Ag NPs. Data based on the work of Levard et al.79
255 256
The results presented in Figure 2 show that it is very likely that investigations of Ag NPs at different
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Cl-/Ag+ ratios result in largely different dissolution rates as the formation of soluble or insoluble AgCl
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species may differ.79 As a consequence, the toxic response will be non-linear with respect to added Ag
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NP concentrations since ionic Ag in general has a large impact on toxicity.83, 84 Knowledge on solution
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speciation of released Ag is thus essential when interpreting toxicity and dissolution results of Ag NPs
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in chloride-containing solutions.79, 85
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The role of Cl- is moreover manifold as it also interacts with the particle surface.84 This interaction
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often reduces the protective ability of the surface oxide, sometimes seen as the occurrence of local
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corrosion events on for example stainless steel and Al metal.86 The presence of Cl- has furthermore
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shown to enhance the dissolution of Ag NPs at undersaturated Ag+ conditions.87 This has been
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deduced using an experimental design using nanosphere lithography where Ag NPs were attached to a
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substrate and thereby hindered to agglomerate.87 This approach is appealing as an increased Cl-
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concentration will increase the extent of particle agglomeration and further confound any comparison
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between different Cl- concentrations. These aspects will be discussed below.
270 271
First-order rate dissolution constants at freshwater-like conditions and principal component
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analysis to visualize experimental conditions
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Dissolution rates constants obtained using first-order rate dissolution kinetics (eq. 2) are presented in
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Figure 3A for NPs of Cu, CuO/Cu(OH)2, Ag, ZnO, Mn, and Al.19, 35, 36, 51, 54, 58, 59, 70, 71, 78, 88-90 Complete
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data and references are given in supporting information (Table S2). The dataset comprises freshwater-
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relevant media with a pH varying between 4.5 and 8.6 (mean value=6.9), an ionic strength varying
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from 0 to 100 mM (mean value=49 mM), and a NOM concentration between 0 and 100 mg/L (Table
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2). Some NPs were coated with capping agents such as citrate or PVP (Table S2). All results reflect a
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test temperature varying between 20 and 25 °C. The dissolution rate constants have been normalized
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to the specific geometrical surface area, which is based on the primary particle size assuming spherical
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particles. The applicability of such normalization depends on several approximations including the
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effect of agglomeration that will be discussed below.
283 284
Table 2. Dissolution rate constants of metal and metal oxide NPs in freshwater-like media calculated
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based on first-order kinetics (eq. 2) and normalized to the specific geometrical area (more details given
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in supporting information in Table S2).
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NPs
Mean
Range of dissolution rate,
Number of
dissolution rate
10-90% range (mole m-2 h-1)
unique data
pH range
k (mole m-2 h-1) CuO/Cu(OH)2
1.9·10-3
4.4·10-6- 7.6·10-4
10
5.8-7.7
Cu
4.8·10-3
4.8·10-4- 1.0·10-2
25
6.2-7
Ag
9.0·10-6
1.3·10-6- 2.9·10-5
43
4.5-7.7
ZnO
1.4·10-4
8.1·10-5- 2.6·10-4
36
6.4-8.6
Mn
4.5·10-6
4.5·10-6
4
6.2
Al
4.9·10-6
3.9 - 7.0·10-6
4
6.2
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Figure 3. A: First-order dissolution rates for NPs of CuO, Cu, Ag, ZnO, and Al at freshwater-like
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conditions. B: PCA plot of observed dissolution rates shown in A, highlighting differences in
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temperature, NOM concentration, loading (NP concentration), primary particle size, and pH.
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The PCA plot for all reported dissolution rates are displayed in Figure 3B and describes differences in
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experimental settings including NOM concentration, primary particle size, temperature, loading (NP
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concentration), and pH. The dotted lines in Figure 3B represent the experimental parameters in terms
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of the principal components. For example, the pH of the majority of investigations on ZnO dissolution
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is higher than in the other investigations and line hence up far out on the “pH” dotted line. The first
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two principal components in this analysis account for 55% of the variance of the dataset. This means
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that there are variations between the parameters that are not included in Figure 3B.
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In line with previous analysis,9, 23 the dataset shows Ag and Al NPs to dissolve among the slowest
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compared to more rapid kinetics reported for Cu and ZnO NPs. However, the scattering in dissolution
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rate constants is large, and largely connected to varying exposure settings, seen in Figure 3B. For
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example, ZnO NPs were predominantly investigated at different experimental conditions compared
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with the other NPs including differences in primary size, pH, and NP concentration. These differences
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in conditions may influence dissolution and disable direct comparisons,91 aspects discussed below.
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Metal oxide NPs such as Fe-oxides, CeO2, and Al2O3 are not included in Figure 3 due to lack of
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detailed dissolution rate constants in the literature. Fe-oxides and CeO2 show slow dissolution rates at
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freshwater-like conditions with half-lives in the order of months, while corresponding rates for Al2O3
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are in the range of weeks.9 Such slow rates would position these NPs in the lower region of Figure 3A.
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TiO2 and Au NPs are not included in the compiled dataset due to their very low solubility at near
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neutral pH conditions such as freshwater, which makes any determination of dissolution rate constants
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very difficult.92-94 Literature findings show however that Au NPs can dissolve even at non-acid
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conditions as a result of interaction with macrophages and in cell medium.95, 96
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Other equations apart from the first-order equation (eq. 2) have been proposed to describe NP
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dissolution, for instance taking into account effects of particle size, pH, and oxygen partial pressure.51,
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63, 81, 90, 97
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phenomena clustered in Figure 1.
This will be discussed next in connection with the different dissolution processes and
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Influence of particle size on dissolution rate constants and solubility
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A reduction in particle size will in general result in an increased dissolution rate due to increased
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surface area.25, 98, 99 An increased surface area with decreased NP size promotes in turn dissolution due
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to an increased number of surface sites that can take part in the dissolution process.25 A reduced
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particle size can also result in a surface oxide with more defects and edges that further can promote
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dissolution, especially for nano-sized particles ( 20 nm (kbulk, Cbulk) are collected from the studies depicted in Table 4. The
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dashed line represents bulk-like conditions in terms of solubility or dissolution rate constant.
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Table 4. Compilation of reported investigations used to compare NP solubility and dissolution rate
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constants. Nanoparticle ZnO
Primary particle size (nm) 50, 100
Hydrodynamic radius (nm) N/A
Medium
Observation
Reference
pH 7, 0.01 M ammonium
Higher dissolution rate constant for bulk Zn
Avramescu et al.110
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acetate ZnO
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>178
0.01 M Ca(NO3)2 solution in Milli-Q water, pH 7.5 HEPES buffer, pH 7.5 1 mM NaNO3, pH 7
ZnO
4-130
2000-3000
Ag
5-20
13-24
Ag
6-70
10-72
Ultrapure water, pH 7
Ag
5-38
61-156
1 mM NaHCO3, pH 8
when normalized to specific geometric surface area Similar dissolution rate constants (not surface area normalized) and solubility for bulk ZnO and ZnO NPs Higher solubility for smaller-sized NPs Higher dissolution rate constant for 5 nm compared with 20 nmsized particles, normalized to specific geometric surface area Higher dissolution rate constant, normalized to specific geometric surface area, for smallsized NPs Higher solubility for smaller sized NPs
Franklin et al.37
Mudunkotowa et al.104 Mollerman et al.34
Peretyazko et al. 35
Ma et al.46
358 359
Figure 4 shows nano-specific effects of small primary NPs with significant changes taking place for
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particles sizes less than approximately 15 nm.103 The trend qualitatively follows the Gibbs free energy
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and Kelvin equation (eq. 4). The Kelvin equation assumes that the surface free energy is independent
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on particle size. However, large variations in surface energy values have been reported for ZnO (ca.
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0.06-1.31 J/m2).104 These differences in surface energy may indicate nano-specific effects as may
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reflect a size-dependence.28
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Despite forming agglomerates, small primary NPs can still show nano-specific size effects. This can
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be seen from increased hydrodynamic sizes compared with primary particle sizes, Table 4. Nano-
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specific effects of the curvature (eq. 4) or increased amount of defects can hence be maintained also
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upon agglomeration in the sense that the smaller sized NPs retain at least some of their more reactive
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dissolution behavior also when present in an agglomerated state. Even though the normalization of
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dissolution rate constants to specific geometric surface areas in Figure 4 is a crude approximation, the
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results indicate nano-specific effects for NPs sized less than 15 nm with higher dissolution rate
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constants for smaller NPs when normalizing to the surface area.
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There are also examples of a lack of effect on dissolution rate constants for primary NPs sized less
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than 15 nm,38, 80 but the reason is unresolved. One explanation could be the use of far higher ionic 17 ACS Paragon Plus Environment
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strength (seawater38) than in the studies compiled in Figure 4. This could imply a stronger binding
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between the particles, which would reduce any nano-specific effect. This explanation is supported by
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the work of Allen et al. whom showed higher oxidation potentials of monodisperse and agglomerated
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Au NPs sized 50 nm compared with smaller sized Au NPs (4, 15 nm), and a shift in potential for the
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smaller NPs when agglomerating to similar values as the 50 nm-sized NPs.
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The primary particle size will moreover change over the course of the experiment as the NPs dissolve,
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which in turn will affect the Gibbs free energy and the surface area and hence also the dissolution rate
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constant. Vogelsberger et al. have performed extensive modelling and experimental work on
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dissolution and solubility of SiO2 NPs.44, 45, 111-113 The reduction of particle size due to dissolution was
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taken into account when constructing models able to estimate changes in surface charge and surface
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potential for NPs of different size. The dissolution process was moreover shown to be highly
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dependent on the NP mass to solvent ratio.113 This is not the same effect that was discussed in the
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previous section on the solution complexation capacity, but rather a nano-specific effect that for
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example will influence the nucleation of new NPs (Ostwald ripening).113 Ostwald ripening has also
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been observed for dissolution of Ag NPs sized below 20 nm.35 The reported modelling work of SiO2
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NPs44, 45, 111-113 is very encouraging and the application of these models to more reactive NPs would be
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a very important contribution to an improved understanding of the dissolution of metallic NPs.
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Models for dissolution rates have in some cases been constructed without considering the primary
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particle size.97 The size of agglomerates formed in solution has instead been included as illustrated in a
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model for ZnO NP dissolution.97 The influence of agglomeration and surface area on dissolution of
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metal and metal NPs is discussed next.
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The influence of agglomeration and aggregation on dissolution of metallic NPs
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As already discussed in several reviews,42, 114 NPs occur in freshwater-like media as agglomerates
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(particles held together by relatively weak physical interactions), aggregates (particles fused together,
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a largely irreversible process), heteroagglomerates (NPs attached to other particles such as naturally
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occurring colloids), or as free NPs.17 This means that the description of size and surface area in
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solution is a distinctly complex assembly of different particle states, each with different dynamic 18 ACS Paragon Plus Environment
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behavior.42, 115, 116 Adsorbed NOM plays an important role for the extent of agglomeration of metal and
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metal oxide NPs in natural waters.42, 43 NOM can provide stabilization through steric repulsion or
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electrostatic forces upon its adsorption,117, 118 but there are also examples where NOM induces
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agglomeration due to charge neutralization and bridging.78, 119-121
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Agglomeration of NPs has in several cases been shown to reduce the rate of dissolution due to a
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reduced surface area.25 It has also been shown that agglomeration will reduce diffusion of species
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involved in the dissolution reactions and thereby further reduce the dissolution rate.122, 123 This
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reduction has for example been shown for PbS NPs that were more rapidly dissolved when freely
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exposed in solution compared when exposed in more confined (nm scale) conditions.124
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The effect of agglomeration on the actual surface area of NPs available for dissolution is however
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poorly understood.63 In some cases, agglomeration does not lead to a large reduction of the dissolution
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rate constant,68 also shown in Figure 4 for NPs sized less than 15 nm in media of low ionic strength.
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Since agglomeration processes play an important role for the dissolution process, it is desirable to
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normalize dissolution rate constants to surface areas and also to find relationships between
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agglomerate structures and rate constants to obtain mechanistic insights on NP dissolution.
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Unfortunately, no analytical method currently exists that can measure the reactive surface area in
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solution. The BET specific surface area is therefore often used for normalization. However, since this
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method measures the surface area at dry conditions, its relevance for solution conditions with
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agglomeration processes taking place is questionable.111 Another approach sometimes used to estimate
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the surface area is the average hydrodynamic particle size in solution, determined by means of laser
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scattering techniques. However, such estimates also have drawbacks as the calculations for example
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assume spherical particles and do not account for agglomerate porosity. Another option is to use the
425
size distribution to estimate the surface area, however still with the uncertainty connected to the
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reduction in area due to possible strong binding between particles and dissolution effects (as described
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above).63
428
Another way to approach the complexity of agglomeration is to consider the fractal dimension (DF) of
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the agglomerate.125 DF is the power that an equivalent radius of a NP agglomerate scales to in order to
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scale to its mass. A DF of 3 hence corresponds to a solid sphere (no porosity) and a value of 1 19 ACS Paragon Plus Environment
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describes a rod. NP agglomerates can be divided into diffusion limited agglomerates (DLCA) with DF
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values of approximately 1.8, and reaction limited cluster agglomerates (RCLA) with a DF value
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between 2.1 and 2.5.115 The DF value can be determined via experimental methods such as small-angle
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x-ray scattering,115 static light scattering,126 or the volumetric centrifugal method (VCM).127 A high
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fractal dimension should result in a lower dissolution rate constant compared with a lower DF value as
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more compact agglomerates have a lower surface area (all else assumed equal). This was indeed
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observed in a study by He et al.,47 whom formulated a model to account for changes in surface area
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due to agglomeration. The model showed that an increased DF and size of the agglomerates resulted in
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lower dissolution rate constants.47 Observed relationships between the normalized apparent dissolution
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rate constant (kd*, see definition in47) of a given metal NP (Ag NPs, primary particle size 50 nm) in
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NaCl and its agglomerate size as a function of DF are displayed in Figure 5, using the model of He et
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al.47
Normalized apparent dissolution rate constant, kd*
1,0 DF 1.7 DF 1.9 DF 2.1 DF 2.3 DF 2.5
0,8 0,6 0,4
More compact agglomerates 0,2 0,0 50
100 150 200 250 Agglomerate size (nm)
300
443 444
Figure 5. Modelling of apparent dissolution rate constants of Ag NPs (primary particle size set to 50
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nm) in NaCl (5-500 mM) as a function of agglomerate size for different DFs. The calculations are
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based on the work of He et al.47 The results are presented as dissolution rate constant normalized (kd*)
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to the dissolution rate of primary 50 nm-sized Ag NPs.
448 449
The model of He et al. introduced the parameter α that reflects the proportion of accessible reactive
450
sites on primary particles compared to the total surface area. This parameter was approximated to 0.1,
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as deduced from curve fitting.47 The impact of different Cl- concentrations on the dissolution rate was
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taken into account since Cl- influences the dissolution rate via surface interactions (as previously
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discussed) and if non-considered, would confound any conclusions based on agglomerate size and
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fractal dimension. DLCA-agglomerates with a DF value of approximately 1.7 were observed in
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solutions of high ionic strengths (500 mM NaCl) and RCLA-agglomerates at lower ionic strength (50
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mM NaCl).47 Other effects can be observed for different Cl-/Ag ratios as Chambers et al. observed that
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the formation of solid AgCl in solution induced RCLA.128 Reported DF values should however be
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used with caution since the multidimensional nature of the agglomerates has not always been taken
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into account in the analysis of DF values.129
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Other modelling methods exist to estimate changes in surface area upon NP agglomeration.63 Jiang et
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al. modelled as an example a reduced surface area of ZnO NPs with time due to dissolution,68 and
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David et al. constructed a model that with information on the radius of the agglomerates and the NP
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concentration could predict the dissolution rate of ZnO NPs.97 The structure of the agglomerates was
464
most probably similar for these exposure conditions as the dissolution rate otherwise would have been
465
influenced by different surface areas of agglomerates of different DFs. Estimates of the surface area
466
from theoretical predictions coupled with dissolution experiments of SiO2 NPs have been done by
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Vogelsberger et al.44
468
Existing models are promising but in need of further experimental data and validation,63 including
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parameters for fractal modelling.130 Second order dissolution rate equations have been employed for
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Ag NPs to capture the initially fast release of Ag ions followed by slower kinetics due to effects such
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as particle agglomeration.50 Such second-order equations have in several cases been shown to fit
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experimental data better compared with first-order equations,50 but need further validation with respect
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to experimental data for different kind of NPs. 21 ACS Paragon Plus Environment
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The surface characteristics of the NPs influence their agglomeration behavior and hence the trend of
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reduced surface area for dissolution reactions. The DLVO theory describes the resulting surface forces
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between particles, classically taking into account electrostatics and van der Waals forces.131 Revised
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versions of DLVO also consider steric interactions and solvation forces.132 Hammes et al. have
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modelled the electrostatic component for different freshwaters representative of Europe,133 showing
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variations due to differences in ionic strength.133 That modelling did however not take into account the
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presence of NOM. The van der Waals forces depend on intrinsic NP properties and are particularly
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large for metal NPs.75 This results in strong attractive inter-particle forces (and hence lower DFs) that
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reduce the surface area compared with metal oxide NPs (assuming that other surface forces are
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equal).134
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Heteroagglomeration of NPs has been described as the most important process for the environmental
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dispersion of NPs as it likely dominates over homoagglomeration.10, 135 Heteroagglomeration can be
486
related to an attachment coefficient of the NPs to other particles or materials.136, 137 However, there are
487
no experimental investigations on relationships between heteroagglomeration parameters such as the
488
attachment efficiency and the dissolution of NPs. These are important aspects to consider in future NP
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dissolution studies.
490
More efforts are needed in order to predict and determine the actual surface area to more accurately
491
quantify dissolution and understand the importance of agglomeration. Information on agglomeration
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sizes was unfortunately missing in several of the datasets of Figure 2, which prohibited the use of this
493
parameter in the PCA plot in Figure 3B. Still, with a single value of hydrodynamic size there is still an
494
uncertainty of several orders of magnitude of connecting this size to a surface area due to effects of
495
particle size distribution and particle fusing or lack of the same.63 A method to standardize
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measurements of NP stability in simulated environmental media has recently been proposed by
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OECD.138 This experimental protocol may in the future be further developed to include estimates of
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fractal dimensions and surface area. The generation of dissolution rate data normalized to surface area
499
would simplify the deduction of possible nano-specific mechanisms and further facilitate comparison
500
with bulk dissolution data.80, 139
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501 502
NP-interactions with ligands such as natural organic matter
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It is expected that dissolution processes can have nano-specific attributes due to changes in NP
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characteristics with size (e.g. defects, surface charge, curvature)43 140 that will influence the adsorption
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of ligands (Figure 1).27, 103 The number of surface kinks and defects increases with decreasing NP size,
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an effect observed to influence the adsorption geometry of oxalic acid on 5 versus 32 nm-sized TiO2
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NPs.27 Investigations of ZnO NPs in the presence of citrate revealed enhanced dissolution (normalized
508
with the BET surface area) only for the smallest sized particles (4 nm) compared with small-sized NPs
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(5-20 nm).104 This was explained by ligand-induced dissolution due to the adsorption of citrate to flat
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terraces that masked any increased dissolution at kinks and defects for the small-sized NPs.104
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NOM is the most important group of ligands in freshwater and is mainly composed of humic and
512
fulvic acids. It is difficult to draw general conclusions on the effect of NOM on the underlying
513
dissolution mechanisms of metal and metal oxide NPs compiled in Figure 3 due to the large variety in
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NPs and NOM characteristics. Humic acid is the most common NOM in the dataset. The NOM was
515
however not characterized in some studies as it, for example, was derived from natural sources such as
516
river water.
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Increased dissolution rates of metallic NPs have been reported for ZnO NPs,90, 141 (only at high pH105),
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Al NPs,78 Cu NPs,41, 69, 78, 91, 142 and CuO (in amino acids) in the presence of NOM.143 This effect is the
519
same as expected for larger sized particles and bulk material as an increased dissolution is connected
520
to a relatively high affinity of the functional groups of NOM to the metallic surface. 144 In general
521
(valid also for bulk material), it has been observed that the adsorption of covalent (inner-sphere)
522
monodentate complexes results in increased dissolution rates.139 Multinuclear complexes are
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conversely less prone to promote the extent of dissolution since their adsorption to multiple surface
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sites makes the detachment of the ligand-metal complex less likely.145
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A change in the interaction between NOM and NPs due to nano-specific attributes mentioned above
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can influence the dissolution process. This depends for example on the binding geometry between
527
functional groups of NOM and the metal (discussed above) in addition on the thickness and surface 23 ACS Paragon Plus Environment
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528
coverage of adsorbed NOM. Observations of such effects are unfortunately so far largely lacking. A
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study on Cu NPs and humic acid suggests interactions between carboxylate groups and the oxidized
530
copper surface followed by a relatively slow build-up and gradual change in the structure of the
531
adsorbed NOM layer (hours). This adsorption process resulted in enhanced dissolution.78 Surface
532
interactions with NOM can hence enhance the dissolution of metallic NPs via destabilization of
533
adsorbed metal-ligand complexes. This process is caused by the breakage of metal-oxygen bonds due
534
to localization of electrons to the adsorbed-ligand metal complex,146 a process that is proportional to
535
the concentration of adsorbed ligands. The stability of metal-oxide bonds can, as mentioned previously
536
(Kelvin effect), be reduced for small NPs (
