UV-Visible measurements provided important information on
kinetic processes of AgNPs aggregation. UV-Visible spectrophotometry
allowed us to obtain unique and valuable information during the first
seconds of the AgNP aggregation process, which provided evidence on
the first stages of the AgNP aggregation process, of new NP formation
from silver ions. Moreover, it also allowed us to distinguish between
different stages of the kinetic aggregation process that corresponded
to different aggregation rates. The decrease in the SPRB area (A) also
provided confirmation of aggregation as the dominant mechanism,
while the analysis of the Wh/2 provided a good assessment of CCC
values. A relatively high AgNP concentration (μM) was used in this
study due to the detection limits of the spectrophotometric technique
used. As a consequence, the extrapolation of the obtained results to realistic
environmental concentrations of AgNPs is not straightforward.
The aggregation and dissolution of NPs is influenced by their concentration
in solution. Therefore, differences in the aggregation/dissolution
rates are expected upon realistic and lower AgNPs discharges in the
environment.
Alginate coating showed moderate electrostatic stabilization with a
CCC of around 82mMin NaCl solutions at pH=8.0. GumArabic coating
provided AgNP with steric stabilization at low silver ion/silver species
concentrations. In contrast, low electrostatic stabilization was obtained
with a CCC value of around 41mMin NaCl solutions containing b60% of
silver as AgNPs probably because of changes in the coating layer structure
and/or adsorption due to interactions with different silverchloride
species present in excess interactingwith the AgNPs. Oxidation
only played a significant role for AgNP-GAH as a consequence of the
instability of the aggregates,which are oxidised in presence of high concentrations
of Ag+ species. The introduction of ALG and GAL-coated NPs
to natural fjord waters indicated a strong stabilization against the
aggregation of the NPs by natural organic matter, evidencing the
influence of the complex environment in the fjord waters compared
to the synthetic NaCl solutions. Terrestrial fulvic/humic-like compounds,
observed in the natural fjord waters, are likely responsible for
the enhanced stabilization in terms of aggregation rate. Much faster
aggregation kinetics were observed for AgNP-GAH in natural fjord waters
compared to synthetic NaCl solutions. This was attributed to the
high amount of charged silver-chloride species present in solution.
Moreover, the effect over aggregation/oxidation kinetics of multivalent
ions, e.g. calcium, present in the fjordwaters should be also considered.
Divalent ions have a greater influence than the monovalent ions in
screening and neutralizing the surface charge of NPs. Even if calcium
is present in much smaller quantities than sodium (around 50 fold) in
seawaters, it presents a higher electrostatic accumulation and a greater
affinity for the coating binding sites. For example, the cross-linking reaction
that takes place between calciumand the alginatemolecules results
in a cooperative association of long regions of polymer chains, generating
gel-like structures thatmay reduce the stability of theAgNPs present
in the fjord waters due to the modification of the coating structure. This
effect, if present, was not as strong as the stabilization against aggregation
produced by the organic matter observed in this work.
The type of coating layer has been revealed as a key factor in the
aggregation/oxidation behaviour of AgNP in solution. As a consequence,
AgNPs behaving like AgNP-ALG or AgNP-GAH readily aggregate
in waters with salinities higher than 4.5–9, and their likely fate
is sedimentation and kinetic deposition if no additional stabilization
from organic matter is considered. In contrast, the behaviour of
steric-stabilized AgNPs allows them to remain as non-aggregated
NPs for a longer duration in solution, even at salinities of seawater
(~35), and consequently they are available for uptake by pelagicmarine
organism. Therefore, the behaviour of the steric-stabilized
AgNPs is much less affected by ionic strength changes than the
electrostatic-stabilized AgNPs.
Nevertheless, the ionic strength is not the only factor that influenced
the aggregation rate of the polymer-coated AgNPs. The introduction of
AgNPs into fjord waters resulted in additional stabilization from organic
matter independently of the steric/electrostatic behaviour provided by
the polymer coating. The presence of different organic compounds in
natural fjordwaters appears to modify the expected aggregation behaviour
of polymer-coated AgNPs, based on the contribution of NaCl at an
equivalent ionic strength. Therefore, even if AgNP coated with alginate
and gum Arabic could be considered as appropriate models to simulate
the aggregation behaviour of AgNPs, the extrapolation of the results
obtained in studies using NaCl solutions tomore complex matrices containing
organic compounds is not straightforward.
Summary of the synthesis of AgNPs, details about experimental
techniques and equipment, AgNPs characterization and natural fjord
water characterization. Tables S1–S3, Figs. S1–S8 and Videos S1–S8.