Aggregation kinetics of nanosilver in different water conditions

NaCl and CaCl₂ solutions were prepared by adjusting the electrolyte concentrations from 10 to 4500 mM. Humic acid (HA) was added to the electrolyte solutions to mimic natural organic matter (NOM). Natural water samples were collected from lake water and seawater. The collected water samples were first filtered (0.45 μm) and then autoclaved to remove microorganisms in the water. HA and NOM contents in the synthetic and natural water were measured as total organic carbon (TOC) using a TOC analyzer (Apollo 9000, Tekmar Dohman). Major ions in natural water samples were analyzed using ion chromatography (IC) (DX-120, Dionex).

Nanosilver stabilized with PVP (average molecular weight: 29 000, Sigma-Aldrich) was prepared via a Tollens method. The initial concentrations of the reactants were 1 × 10⁻³ mol l⁻¹ and 1 × 10⁻² mol l⁻¹ for AgNO₃ and maltose, respectively [3]. The concentration of ammonia was 5 × 10⁻³ mol l⁻¹ and PVP concentration was 0.35 w/w%. pH value of the reaction system was adjusted to 11.5 using NaOH. Nanosilver solutions were cleaned with deionized (DI) water using a 10 kDa nominal molecular weight cut-off (NMWCO) ultrafiltration membrane (Millipore, Model 8200; NMWCO: 10 000). UV-Vis absorption spectrum was recorded using a spectrophotometer (Thermo Unicam). Concentration of nAg (C_nAg) was measured by ICP-MS (X series, Thermo Elemental). Cryo-transmission electron microscope (Cryo-TEM) was used to observe the morphology. ξ-potential of nAg was determined using a Zetasizer (Nano ZS, ZEN 3600, Malvern) at 25 °C.

Aggregation kinetics were obtained from the rate of change of hydrodynamic diameter, r, versus time, t. In brief, nAg was injected into testing tubes containing different water solutions. One millimeter of the mixture was introduced into a cuvette, quickly hand-shaken and then transferred into the zetasizer (Nano ZS, ZEN 3600, Malvern). Measurements were started immediately and the changes in radius of nAg were recorded over 500s. The aggregation rate constant, k₁₁, can be expressed as follows [6]

$$\begin{equation} k_{11} \propto \frac{1}{{N_0 }}\frac{{{\rm d}r}}{{{\rm d}t}},\end{equation}\noindent \tag{ 1 }$$

where N₀ is the initial nAg concentration in solution. In this study the rate of change of hydrodynamic diameter (dr/dt) can be calculated using a linear least-square regression analysis of the increase of r measured at 25 °C.

Attachment efficiency α was used to quantify the aggregation kinetics. It can be calculated by normalizing k₁₁ to the rate constant, (k₁₁)_fast, which is acquired in the fast (diffusion-limited) aggregation regime

$$\begin{equation}\alpha = \frac{{k_{11} }}{{(k_{11} )_{{\rm fast}}}} \cdot\end{equation} \tag{ 2 }$$

Nanosilver exhibits irregular round shape (figure 1). Figure 1(b) shows that the surface plasmon resonance band of nAg locates approximately at 400 nm. This observation agrees with previous studies [3, 4, 11].

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Figure 2 shows that the aggregation rate of nAg in electrolyte solutions increases with increasing cationic concentrations. As the ξ-potential of nAg in DI water is negative (−20.8 mV), the increasing concentration of cations elevates the screening of surface charges of nAg. Therefore, the aggregation rates are increased due to the reduction of electric repulsion force and energy barrier between nanoparticles [12].

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Figure 3 shows the attachment efficiency of nAg in different water conditions. As mentioned in the experimental section, at high electrolyte concentrations the surface charges of nAg are completely eliminated and the nanoparticles are thus in the fast aggregation (diffusion-limited) regime where α = 1. In the diffusion-limited regime, the aggregation rates reached a maximum and are independent of the electrolyte concentration. When α < 1 (figure 2), a reaction-limited regime occurs when the aggregation rates increase with increasing cationic concentrations due to the decreasing electrical repulsion force. Critical coagulation concentration (CCC) is used to define the minimum electrolyte concentration to reach the diffusion-limited regimes. It is determined by calculating the intersection of the extrapolations through both diffusion-limited and reaction-limited regimes. Figure 3 and table 1 show that the CCC values of nAg in CaCl₂ and NaCl solutions are in the following order: CCC(CaCl₂) < CCC(NaCl). The obtained result is consistent with the Schulze–Hardy rule, which indicates that the CCC of nanoparticle suspension is sensitive to the valence of the counter-ions. Consequently, divalent cation (Ca²⁺) has a stronger effect to destabilize nano-suspension compared to monovalent cation (Na⁺). Similarly, other studies indicate that CCC of Na⁺ is higher than that of Ca²⁺ for different nanomaterials [13–18]. Figure 3 and table 1 also show that humic acid (HA) could elevate CCC as HA macromolecules could be adsorbed onto nanoparticle surface and create steric repulsion forces preventing nAg from aggregating. This observation is also consistent with previous published data [13–18].

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The aggregation behavior of nAg could be enhanced in water containing high Ca²⁺ with high concentration (figure 4). Figure 3 also shows that the attachment efficiency keeps increasing even to α > 1. Possible explanations have been raised by Chen and Elimelech [9], indicating that HA macromolecules and Ca²⁺ could form complex and the complex could cause bridging effect between nanoparticles.

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A higher aggregation rate of nAg in seawater in comparison with that in lake water is observed (figure 5), which is likely because seawater contains divalent and monovalent cations at higher concentrations compared to lake water (table 2). In addition, concentration of natural organic matter (NOM) measured as TOC in lake water is much higher than in seawater (table 2), which could contribute a greater stabilizing effect according to the above-mentioned discussion. The result suggests that nAg in seawater could settle fast due to the formation of large aggregates. In contrast, nAg in lake water could settle slower because of its better dispersion.

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