The nanoparticles were releasing silver ions through dissolution, but these toxic ions were likely binding with natural organic matter in the lake water. Silver nanoparticles AgNPs may be released into surface waters, where they can affect aquatic organisms. However, agglomeration, dissolution, surface modifications and chemical speciation are important processes that control the toxicity of AgNPs. The purpose of the study was to apply various methods for monitoring the persistence and transformation of AgNPs added to littoral lake mesocosms.
Silver Nanoparticles in the Environment
The stability of AgNPs was likely due to the low ionic strength and high concentrations of humic-rich dissolved organic carbon DOC in the lake water. It is likely that the majority of free ions released by dissolution were complexing with natural organic material, such as DOC. Overall, we were able to characterise AgNP transformations in natural waters at toxicologically relevant concentrations through the use of multiple analytical techniques that compensate for the limitations of the individual methods.
Environmental problems - Chemical approaches. C E-AgNPs. D C-AgNPs. In the ASTM medium, the percentage change of dissolution after 24 h in comparison to the dissolution after first 5 min was 0. A T-AgNPs. B E-AgNPs.
C C-AgNPs. A broader absorbance peak, a large background signal and increased HDD indicate aggregation of NPs [ 59 , 70 ].
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Aggregation of NPs depends on particle concentration, pH, ionic strength, ionic composition, concentration and composition of natural organic matter, and other characteristics of the aqueous media [ 71 , 72 ]. Also, all NPs showed no signs of aggregation in MilliQ water within the same time duration. Decreased zeta potential values in ASTM medium with high ionic strength compared to MilliQ water with relatively low ionic strength are in accordance with the classical colloid theory [ 75 , 76 ].
A reduction of the thickness of the diffuse double layer with increased ionic strength allows for the attractive van der Waals interactions to dominate and increases the particle-particle interaction resulting in increased aggregation [ 77 , 78 ]. This may be due to the precipitation of Ag ions by halides Cl - in the medium [ 81 ] which is not expected in the MilliQ water.
Halides may reduce the exposure of organisms to any free ions released from NPs. Small size particles have high surface to volume ratios and therefore, more atoms on the surface come into contact with oxidants in comparison to larger particles [ 83 ].
Nanoparticles in the environment: where do we come from, where do we go to?
Aggregation of NPs results in larger particles with increased HDD reducing the surface area of particles available to release free Ag ions [ 34 , 84 ]. Aggregation, dissolution and change in NP characteristics such as HDD as observed in this study may influence the bioavailability of NPs and hence, play an important role in determining toxicity [ 59 , 86 — 88 ].
Also, culture medium impacted the behaviour and properties of AgNPs which may ultimately lead to various toxicological responses [ 89 ]. This study shows that the type of NP coating and medium significantly influence the degree of aggregation and the behaviour of AgNPs, which are required to consider in environmental risk assessment [ 90 ]. Evaluation of acute toxicity is crucial in environmental risk assessment of NPs in protecting the organisms and setting up water quality guidelines. NP size, type of coating, shape, charge are some major factors that influence toxicity [ 26 , 33 — 35 , 84 , 91 ].
Therefore, the difference in toxicity is presumed to be the effects of different coating materials which reaffirm that different types of coating of NPs do have different effects on toxicity of NPs. Thus, the reduced toxicity of T-AgNPs compared to the other two types could be due to the lower bioavailability of NPs [ 71 , 92 ]. Several previous studies have assessed the acute toxicity of AgNPs to Daphnia sp. The toxicity of coated AgNPs in this study is comparatively less than the values reported by majority of studies for other coated AgNPs.
The broad range of toxicity values among published studies can be explained by different test scenarios and particle characteristics [ 84 , 97 ]. Coatings control the release of ions from AgNPs to the surrounding medium [ 34 , 98 ]. Daphnid feeding rate significantly increased from day 1 to 5 for all treated algae and in controls due to higher food consumption with ageing Fig 3.
However, a consistent difference was not observed and therefore, the data obtained is not sufficient to prove any significant variations of feeding rates among daphnids based on algae treated with AgNPs with different coatings. Besides, less favoritism for untreated algae compared to treated algae shows that AgNPs have some effect on feeding behaviour.
Silver nanoparticles in the environment: Sources, detection and ecotoxicology.
Previous studies also have shown that the feeding rate of daphnids became depressed when contaminants were associated with algae [ — ]. McTeer, Dean [ 42 ] observed a reduction in feeding rates when daphnids were fed with algae treated with polymer-coated AgNPs. Nutritional characteristics of all algae samples were similar and therefore, they emphasized that the feeding reduction was due to Ag toxicity but not due to the nutritional quality of the algae diet.
Zhao and Wang [ 93 ] speculated that the reduced feeding rate was due to accumulation of NPs in the gut or due to higher sedimentation of contaminated algae to the bottom of the vessel causing less availability for filter feeding. Inputs of energy from food is critically important for population growth and survival and therefore, the feeding rate in primary consumers like Daphnia sp.
Silver nanoparticles in the environment
Feeding inhibition may cause reduction of growth, targeted inhibition of internal organs and reproduction of daphnids [ 93 , — ]. It may also have effects on water clarity, altered nutrient regeneration rates, population size of predators [ ] and elevated phytoplankton biomass due to reduced grazing [ ]. Therefore, the observed feeding inhibition of algae associated with NPs by daphnids in this study is a cause of concern and should be considered in assessing aquatic NP pollution. However, the Ag accumulation profile in algae does not mirror the Ag accumulation profiles in daphnids. In contrast, McTeer, Dean [ 42 ] found metal accumulation profiles in D.
The Ag bioaccumulation in daphnids did not correlate with the daphnid survival percentage Fig 4D where no significant difference was observed between each treatment group. A Elemental Ag content per 10 5 R. B Elemental Ag content per daphnid. C Percentage Ag retained per daphnid.
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D Percentage survival of algae-fed D. Treatments that do not share lowercase letters are significantly different. Any remaining treated algae in the digestive tract may lead to overestimation of bioaccumulation. Gillis, Chow-Fraser [ ] determined the length of time required to completely depurate metal contaminated sediments from the digestive track of D. Petersen, Akkanen [ ] observed a limited depuration of carbon nanotubes from D.
Following exposure to the treated algae, daphnids were allowed to feed on fresh algae for 72 h in fresh medium and it was assumed all NPs in the digestive track were removed. Several other studies have also shown the potential transfer of different types of NPs along the food chain [ 41 , 47 , ].
However, NPs less than 20 nm may pass through the algal cell walls since the cell walls are porous 5—20 nm in size and their permeability changes during mitosis. Also, high concentrations of AgNPs may increase the permeability of algae cell wall resulting in more internalization [ 39 ]. Data generated from this study clearly show that the type of coating affects the NP accumulation in algae and trophic transfer from algae to daphnids.
However, it is not possible to predict the toxicity to daphnids based on bioaccumulation through trophic transfer from this study, and hence, further studies are recommended. The transfer of AgNPs along the aquatic food chain could have adverse implications and therefore there is a need to take this into consideration in protecting aquatic organisms. In doing so, great caution must be taken when assessing the risk of differently coated NPs.
Based on the 48 h EC 50 values of D. Since other characteristics such as shape, size and charge are quite similar, the difference in toxicity could be attributed to the effect of different coatings. Feeding experiments revealed that the ingestion rates of NP treated algae were significantly lower than untreated algae revealing associated AgNPs with algae change daphnid feeding behaviour which could have longer term negative effects on D. However, findings from this study are not sufficient to conclude the cause of changed behaviour.
Ingestion rates of algae treated with differently coated NPs were not markedly different showing that different types of coatings had little effect on D. However, bioaccumulation of Ag in daphnids through trophic transfer did not correlate with the accumulation profiles of Ag in algal cells. These results demonstrate that type of coating may have effects on AgNP accumulation profiles at different trophic levels. The behaviour of differently coated NPs in medium, their toxicity profile and trophic transfer data generated in this study demonstrate the importance of considering type of coating in environmental risk assessment.
Fig A: Chemical formulae.