Iva
Talaber
a,
Cornelis A. M.
Van Gestel
b,
Anita
Jemec Kokalj
*a,
Gregor
Marolt
c,
Sara
Novak
a,
Primož
Zidar
a and
Damjana
Drobne
a
aBiotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, Ljubljana, Slovenia. E-mail: anita.jemec@bf.uni-lj.si
bDepartment of Ecological Science, Faculty of Science, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
cFaculty of Chemistry and Chemical technology, University of Ljubljana, Večna pot 113, Ljubljana, Slovenia
First published on 13th August 2020
Silver nanoparticles (Ag NPs), incidentally or intentionally released to soil ecosystems, have the potential to accumulate in organisms. Due to the great variety of natural (field) soil and its interactions with Ag NPs, the few available biokinetic studies performed on standard soils are not sufficient for an environmentally relevant assessment of Ag NP bioaccumulation. In this study, we analyzed the biokinetics of pristine and sulfidized Ag NPs (Ag2S NPs) in two arthropod species with different routes of exposure to soil contaminants (isopod Porcellio scaber and springtail Folsomia candida) exposed to 4 different soils, widely differing in chemical and physical properties. Using the biokinetic constant of uptake as proxy for bioavailability, we found soil pH and soil texture to be the strongest predictors of Ag bioavailability to isopods and springtails, respectively. Both organisms accumulated Ag when exposed to pristine Ag NPs, which suggests a risk for food-chain accumulation of Ag. In contrast, no Ag bioaccumulation was detected in the case of the poorly soluble Ag2S NPs, which is the more environmentally relevant form of Ag NPs. Our study corroborates the dominant role of dissolution in Ag NP bioavailability and draws attention to the biological differences between different soil dwelling organisms modulating their Ag NP exposure.
Environmental significanceThe majority of silver nanoparticle (Ag NPs) biokinetic studies have tested environmentally less relevant pristine nanoparticles in standard soils, which does not reflect the situation in the field. In line with this, our key finding is that Ag NP bioavailability to isopods and springtails depends on the soil properties, mainly soil pH and texture, respectively, and is negligible for the environmentally relevant sulfidized Ag form. These findings are applicable to other soil dwelling organisms with similar routes of exposure and physiology. Our study provides the much-needed empirical data and a set of biokinetic constants for the validation of Ag NP bioavailability models based on specific soil properties and the parameterization of Ag NP uptake models. |
Metal bioaccumulation is assessed with biokinetic studies of uptake and elimination that relate metal body concentrations to time of exposure to a certain metal NP concentration. To derive kinetic rate constants, biokinetic models of varying complexity can be applied, where different transformation processes and NP species can be included in the model descriptions.7,8 Due to analytical challenges in discriminating between ionic and particulate metal forms associated with NP exposure in soil and organisms, models based on total metal concentrations are still frequently applied. The simplest model considers the organism as a single compartment, but more compartments can be assumed, depending on the physiological traits of the test organism.7 It has been argued that the data obtained for soil invertebrates are frequently not precise enough to discriminate more than one compartment9 and that parameterization of simple models should be pursued.8
Studies of Ag NP uptake and elimination kinetics in soil organisms are scarce, and only a few of these considered the effect of soil properties.10,11 It is clear that differences in chemical and physical properties of natural soils have an effect on Ag NP bioavailability, especially their organic matter content, soil texture, ionic composition and pH (ref. 6) and that Ag NPs show a different fate in different environments.12
We performed biokinetic experiments on four different soils spiked with Ag NPs, in two model terrestrial arthropods susceptible to soil contamination: the springtail Folsomia candida (Collembola, Entognatha) and the woodlouse Porcellio scaber (Isopoda, Crustacea), inhabiting soils throughout the world. These animals were chosen to address differences in organism physiology and feeding behavior manifesting in different routes of exposure to Ag NPs in soil. Isopods are mainly exposed to soil contaminants through oral uptake of soil particles,13 and accumulate metals in proportion to their concentration in the soil.14,15 Springtails, however, are mainly exposed to contaminants in the pore water through the ventral tube,16 an organ on the first abdominal segment, with an important role in fluid and electrolyte balance capable of direct uptake of water.17 In some studies, metal uptake in springtails indeed was best explained with soil porewater concentrations,18,19 but other studies did not confirm this, suggesting that ingestion of soil particles and/or other food items (fungi, yeast, litter) may also play a role.20
The aim of this study was to investigate (i) bioaccumulation of pristine and sulfidized Ag NPs in two arthropod species, and (ii) differences in Ag NP bioavailability between different soils. We discuss the role of the ionic Ag form in the bioaccumulation of Ag NPs as well as how differences in feeding strategies and organism physiology influence their biokinetics.
The concentration of free Ag+ in working solutions of Ag NPs and Ag2S NPs (1 g L−1) was analyzed according to previously published protocol.22 Briefly, test dispersions were ultra-centrifuged in duplicate at 100000g for 30 min at 20 °C (Beckman Coulter L8-70M class H preparative ultracentrifuge with Type 70.1 Ti rotor and 10 mL thick wall polyallomer tubes). The supernatants were acidified with 65% HNO3 (analytical grade purity; Fischer Scientific, Leicester, UK) and the total Ag content was quantified by flame AAS (AAS, Perkin Elmer Analyst 100, Waltham, Massachusetts, USA). Negligible concentrations of free Ag+ were measured, representing 0.008, 0.066 and 0.120% of the total Ag working concentration (1 g L−1) in case of 3–8 nm Ag NPs, 50 nm Ag NPs and Ag2S NPs, respectively. We thus consider that none of the working Ag solutions had evident excess of Ag+ that might originate from the synthesis.
Springtails of the species Folsomia candida (Entognatha: Isotomidae) have been cultured at the Department of Ecological Science of the Vrije Universiteit Amsterdam for many years. Cultures were kept in a climate room at 16 ± 1 °C, 75% relative humidity (RH) and a 16/8 h light/dark period. To obtain age-synchronized animals, adult springtails were placed in boxes with a moistened layer of plaster of Paris for 2 days. After removal of the adults and hatching of the eggs produced, the juveniles were allowed to grow until an age of 20–22 d before use in the tests. Age synchronizations were performed at 20 ± 1 °C, 75% RH and a 16/8 h light/dark period. The animals were fed dry baker's yeast. Unfortunately, age synchronizations of F. candida were unsuccessful in some cases and adults taken directly from the culture were used for tests on all three Ag forms in North Wales soil, on Ag2S NPs in Lufa 2.2 soil and on the 50 nm Ag NPs in Woburn soil.
Soil type | pH-CaCl2 | WHC (%) | Organic matter content (%) | Clay (%) | Silt (%) | Sand (%) | CEC (cmolc kg−1) |
---|---|---|---|---|---|---|---|
North Wales | 5.5 | 76.5–80.0 | 16.7 | 12.6 | 29.7 | 57.7 | 33.3 |
Woburn | 6.9 | 31.9–32.1 | 1.8 | 12.6 | 11.8 | 75.6 | 13.3 |
Lufa 2.2 | 6.0 | 43.3–45.0 | 4.2 | 15.3 | 17.4 | 67.3 | 9.7 |
Dorset | 4.2 | 34.5–38.0 | 8.4 | 3.5 | 4.7 | 91.8 | 7.8 |
Among the four soils tested, Dorset soil was most acidic, followed by North Wales, Lufa 2.2 and Woburn. North Wales soil had a much higher organic matter content compared to the other soils, with Woburn having the lowest. North Wales had highest content of clay and Woburn had a highest proportion of bigger grains (sand) (Table 1).
Biokinetic experiments with P. scaber were carried out for 6 weeks. For the first 3 weeks animals were exposed to spiked soil and afterwards transferred to clean soil for another 3 weeks, representing the uptake and elimination phase, respectively. For each soil type a control without NPs was included, and sampled in the same way as the other exposure groups. Each experimental unit consisted of a glass jar with a lid, containing 30 g moist soil from the same batch of spiked soil, a piece of dried hazel leaf (∼100 mg) and 5 randomly selected P. scaber specimens. For each exposure and the control, 8 experimental units were prepared for 8 sampling days. Samples were collected at days 1, 7, 14, 21 (uptake phase) and 22, 29, 35 and 42 (elimination phase) of experiment. An additional experimental unit was prepared for the control, and sampled at day 0. Collectively, we prepared 148 experimental units and used 740 isopods. On each sampling day all surviving animals from one jar were sampled and weighted. The gut was removed and the rest of the specimens (samples) were individually frozen at −20 °C for further analysis. During the experiment, additional soil samples were collected in duplicate (100 ± 5 mg) at days 7, 14, and 21, and processed in the same way as other soil samples.
Biokinetic experiment with F. candida followed OECD guideline 317 (OECD, 2010)27 and lasted 28 days, including a 14 day uptake phase in spiked soil followed by a 14 day elimination phase in clean soil. For each sampling time, three replicate test jars were prepared. Each test jar contained approximately 25 g dry-weight equivalent of moist soil, which moisture level was maintained by replenishing water loss on a weekly basis. Eight F. candida adult specimens (20–22 d old) were introduced into each test jar, and a few grains of dry baker's yeast were added for food. If necessary, additional yeast grains for food were added on a weekly basis. Test jars were kept in a climate room at 20 °C and a 12 h light/12 h dark cycle. During both the uptake and elimination phase, animals were sampled at 24 and 48 hours as well as 3 (or 4), 7, 10 and 14 days. Approximately 100 ml of tap water was added to a test jar causing the animals to be released from soil and float atop the water surface. All surviving animals from three replicate jars were transferred to plastic boxes containing a thin layer of plaster of Paris and allowed to empty their gut for one hour prior to storage. At each sampling time point, three animals were sampled from each of the three replicate jars, giving a total of nine animals, frozen at −20 °C, freeze dried for 24 hours and then weighed using a micro balance (Mettler Toledo GmbH).
The extraction of total Ag from isopods and soil was performed by microwave assisted acid digestion in aqua regia (1:4 = 65% HNO3:36% HCl; analytical grade purity, Fisher Scientific, Loughborough, UK). Digestion was conducted in quartz inserts at 200 °C and 800 W power, with step 1 (heating) lasting 25 min, step 2 (constant temperature) lasting 15 min, and 45 min cooling to 60 °C (Milestone Ethos E microwave lab station; Milestone, Bergamo, Italy). Springtails were digested in a block heater using a 7:1 mixture of concentrated HNO3 (65%; Mallbaker Ultrex Ultra-Pure) and HClO4 (70%; Mallbaker Ultrex Ultra-Pure). The temperature was raised step-wise up to 180 °C and maintained until complete digestion and evaporation of all acid. If digestion was not complete, the procedure was repeated. Resulting sample residues were dissolved in 300 μL of 1 M HCl. Before analysis, the content of each tube was thoroughly mixed using a vortex mixer.
The total Ag concentration in soil and P. scaber samples was determined by flame atomic absorption spectroscopy (AAS) (Perkin-Elmer AAnalyst100, Massachusetts, USA). Springtail samples were analyzed by graphite furnace AAS (PinAAcle 900Z, Perkin Elmer, Germany) calibrated using Ag standards prior to analysis as well as after every 15 samples to ensure the accuracy of measurements. The LOD was 1.62 μg Ag per g animal dry weight.
Uptake (t < te)
Both equations were fitted to the data of the uptake and elimination phases simultaneously, using the user-defined non-linear fitting OriginC type function in the OriginPro 8.5 software, which estimated the parameters k1 and k2 and corresponding standard errors with the iteration procedure.
For each parameter estimated by the model, the respective 95% confidence interval (95% CI) was calculated. Often, the lower limit of the 95% CI was negative. This means, that there was insufficient evidence to conclude that a parameter is significantly different from zero at the 95% level of confidence (p < 0.05). In these cases, it was assumed that the parameter value was zero.
95% confidence intervals for parameters were calculated as:
P ± t(0.05, n − df) × SE |
Statistically significant difference between parameters (P1, P2) was evaluated with the standard method:28
When the estimate of k2 (elimination rate constants) was not significantly different from zero, we calculated k1 again, because k1 and k2 are dependent. Hence, to derive a better estimate of the uptake rate constant (k1), we applied the model again by fixing k2 close to zero (assigning a value of 0.00001). We chose this approach to be able to apply the same model to all datasets.
Uptake rate constants (k1) were related to soil properties by Pearson correlation and multidimensional scaling (MDS) analysis, performed with the XLSTAT software in Microsoft Excel.
Fig. 1 Uptake and elimination kinetics of Ag in Porcellio scaber exposed to different forms of Ag (ionic Ag, 3–8 nm Ag NPs, 50 nm Ag NPs and 20 nm Ag2S NPs) in four different test soils (Dorset, Lufa 2.2, North Wales, Woburn). Uptake and elimination period lasted 21 days each. Estimated kinetic parameters are shown in Table 2. |
Soil type | Ionic Ag | 50 nm Ag NPs | 3–8 nm Ag NPs | 20 nm Ag2S NPs | |||||
---|---|---|---|---|---|---|---|---|---|
k 1 | k 2 | k 1 | k 2 | BAF | k 1 | k 2 | k 1 | k 2 | |
Lufa 2.2 | 0.057 (0.043–0.071) | 0 | 0.051 (0.031–0.071) | 0.049 (0.017–0.081) | 1.1 | 0.021 (0.015–0.027) | 0 | 0 | 0 |
Woburn | 0.028 (0.022–0.031) | 0 | — | 0.017 (0.011–0.023) | 0 | 0 | 0 | ||
North Wales | 0.064 (0.056–0.076) | 0 | 0.141 (0.089–0.193) | 0.031 (0.009–0.053) | 4.5 | 0.036 (0.030–0.042) | 0 | 0 | 0 |
Dorset | — | 0.051 (0.032–0.070) | 0 | — | 0.086 (0.075–0.097) | 0 | 0 | 0 |
Except for a few cases (Table 3), k1 and k2 values were significantly different from zero for F. candida exposed to all tested Ag forms, and Ag body concentrations showed an increase, mostly followed by a decrease after transfer to clean soil (Fig. 2).
Soil type | Ionic Ag | 50 nm Ag NPs | 20 nm Ag2S NPs | ||||||
---|---|---|---|---|---|---|---|---|---|
k 1 | k 2 | BAF | k 1 | k 2 | BAF | k 1 | k 2 | BAF | |
Lufa 2.2 | 0.063 (0.042–0.085) | 0.100 (0.050–0.150) | 0.63 | 0 | 0 | 0 | 0.016 (0.004–0.026) | 0.676 (0.163–1.189) | 0.02 |
Woburn | 0.152 (0.111–0.193) | 0.047 (0.023–0.071) | 3.23 | 0.203 (0.065–0.341) | 0.263 (0.066–0.460) | 0.77 | 0.016 (0.009–0.023) | 0.148 (0.077–0.218) | 0.11 |
North Wales | 0 | 0 | — | 0.010 (0.007–0.013) | 0 | — | 0 | 0 | — |
Dorset | 0.276 (0.182–0.370) | 0.097 (0.051–0.143) | 2.85 | 0.381 (0.264–0.499) | 0.033 (0.005–0.061) | 11 | 0.164 (0.018–0.309) | 1.514 ± (0.065–2.963) | 0.11 |
Fig. 2 Uptake and elimination kinetics of Ag in Folsomia candida exposed to different forms of Ag (ionic Ag, 50 nm Ag NPs, and 20 nm Ag2S NPs) in four different test soils (Dorset, Lufa 2.2, North Wales, Woburn). Uptake and elimination period lasted 14 days each. Estimated kinetic parameters are shown in Table 3. |
The highest k1 for ionic Ag in P. scaber was found in North Wales and Lufa 2.2 followed by Woburn. The Dorset soil was not tested. In F. candida, the highest k1 for ionic Ag was estimated in Dorset, followed by Woburn and Lufa 2.2. No significant Ag uptake was detected in North Wales (Table 3).
The highest k1 for 3–8 nm Ag NPs in P. scaber was estimated in Dorset, followed by North Wales and lastly Lufa 2.2 and Woburn (Table 2). The uptake of 3–8 nm AgNPs in F. candida was not tested.
The highest k1 for 50 nm Ag NP uptake in P. scaber was found in North Wales, followed by Dorset and Lufa 2.2. Woburn soil was not tested (Table 2). The highest k1 for 50 nm Ag NPs in F. candida was estimated in Dorset, followed by Woburn and lastly North Wales. No uptake was detected in Lufa 2.2 (Table 3).
In P. scaber, no uptake was detected in case of Ag2S NP exposure in any of the soils, as k1 was 0 in all soils (Table 2). In F. candida, the highest k1 for Ag2S NP uptake was estimated in Dorset, much lower k1 was estimated in Woburn and Lufa 2.2. No uptake was detected in North Wales (Table 3).
In F. candida, k2 for ionic Ag was highest in Dorset and Lufa 2.2, followed by Woburn. No elimination was detected in North Wales (Table 3). The highest k2 for 50 nm Ag NPs from F. candida was estimated in Woburn, and much lower in Dorset. No elimination was detected in Lufa 2.2 and North Wales (Table 3).
In F. candida, the highest k2 for Ag2S NP elimination was found in Dorset and Lufa 2.2, while a much lower k2 was estimated for Woburn. No elimination was detected in North Wales (Table 3).
P.s ionic Ag | P.s.50nm Ag NPs | P.s.3–8nm Ag NP | F.c. ionic Ag | F.c.50nm Ag NPs | F.c. Ag2S NPs | pH | OM | CEC | Clay | Silt | Sand | |
---|---|---|---|---|---|---|---|---|---|---|---|---|
P.s. ionic Ag | 1.00 | |||||||||||
P.s.50nm Ag NPs | 0.06 | 1.00 | ||||||||||
P.s.3–8nm Ag NPs | 0.97 | 0.00 | 1.00 | |||||||||
F.c. ionic Ag | 0.58 | −0.66 | 0.70 | 1.00 | ||||||||
F.c.50nm Ag NPs | 0.58 | −0.51 | 0.74 | 0.97 | 1.00 | |||||||
F.c. Ag2S NPs | 0.88 | −0.35 | 0.94 | 0.89 | 0.88 | 1.00 | ||||||
pH | −0.99 | −0.21 | −0.95 | −0.46 | −0.49 | −0.81 | 1.00 | |||||
OM | 0.36 | 0.95 | 0.32 | −0.41 | −0.25 | −0.03 | −0.50 | 1.00 | ||||
CEC | −0.23 | 0.94 | −0.24 | −0.73 | −0.55 | −0.55 | 0.09 | 0.81 | 1.00 | |||
Clay | −0.85 | 0.16 | −0.96 | −0.84 | −0.90 | −0.95 | 0.81 | −0.15 | 0.32 | 1.00 | ||
Silt | −0.40 | 0.85 | −0.50 | −0.95 | −0.86 | −0.77 | 0.27 | 0.65 | 0.89 | 0.65 | 1.00 | |
Sand | 0.60 | −0.68 | 0.71 | −0.99 | 0.95 | 0.90 | −0.49 | −0.42 | −0.77 | −0.83 | −0.96 | 1.00 |
Fig. 3 Multidimensional scaling analysis (MDS plot) of the Pearson correlation matrix presented in Table 4. The proximity of elements in the plot indicates the strength of Pearson correlation, being either positive or negative. P.s. – Porcellio scaber, F.c. – Folsomia candida, OM – organic matter content and CEC – cation exchange capacity. |
Uptake rate constants varied both between the Ag forms and the soil types for each test species. Each of these results will be discussed below. Significant variation in metal uptake rate constants is frequently reported especially for soil invertebrates,9 because total metal concentrations in the bulk soil or soil solution do not accurately reflect the bioavailable metal fraction. In our study, total Ag concentration in soil was used as the measure of exposure in the biokinetic model, which means that different estimates of the uptake rate constant reflect differences in Ag bioavailability between different test soils.29 Bioavailable Ag in case of exposure to Ag NPs can include both Ag ions and NPs, where NPs can also be bioavailable in terms of attaching to body surfaces of the test organisms.
A marked distinction between ionic Ag and pristine Ag NPs, as opposed to sulfidized Ag2S NPs was observed. Upon exposure to Ag2S NPs, the Ag uptake (i.e. a temporal increase in Ag body concentration) was much lower in springtails and undetectable in isopods. The absence of Ag uptake upon exposure to the presumably insoluble Ag2S NPs was expected for the isopod, since there is considerable evidence that metal uptake in P. scaber exposed to metal NPs is due to ions released.32,33 However, Kampe et al.34 recently reported Ag2S in sewage sludge was taken up by P. scaber in their study. Nevertheless, they proposed that the Ag2S in their experimental set-up may have been sparingly soluble and implied the role of the ionic form in Ag uptake. Analogously, the same could be argued for the detected Ag uptake in springtails upon exposure to Ag2S NPs, considering that it was highest in the acidic Dorset soil, where a higher dissolution is expected30 and Ag uptake was at least an order of magnitude lower compared to that of pristine Ag NPs and ionic Ag in every tested soil. Clearly, under the assumption that Ag2S NPs are sparingly soluble, the dissolved Ag ions would also be taken up by the isopods, but the amount of assimilated Ag in the isopods' storage tissue (the hepatopancreas) in this case might be too low to be distinguished from the relatively high Ag background level in our test isopods (Fig. S1†). We acknowledge that the average Ag concentration in control isopods (6.6 ± 2.3 μg Ag per g dry body weight (n = 153)) observed in this study is higher than the only other published value (a closely related species Porcellionides pruinosus (2.3 ± 1.3 μg Ag per g dry body weight (n = 3))35), but this most probably represents the natural variability in Ag background concentration in isopods. It must be pointed out however, that if we assume that Ag2S NPs in our study were sparingly soluble, prolonging the time of exposure to Ag2S NPs might result in a detectable Ag uptake in isopods. However, no conclusions can be made without proof of Ag2S NP dissolution. With this, the uptake of particulate Ag in case of springtails cannot be excluded. Nevertheless, based on the estimated uptake rate constants in isopods (Table 2) and springtails (Table 3), it can be concluded that Ag2S NPs are less bioavailable compared to pristine Ag NPs.
Although generally Ag was not eliminated from the isopods, elimination was detected in the case of the 50 nm Ag NPs in Lufa 2.2 and North Wales soils. This suggests some particle-specific interaction with either the organism or the soil, presumably connected to hetero- or homo-aggregation of these particles. It is frequently assumed that the metal elimination rate constant is a property intrinsic to the organism and not influenced by soil properties or the form of Ag. The first was for instance shown for the earthworm Eisenia andrei,36 the second for the isopod P. pruinosus35 and the earthworm Eisenia fetida.21 Hence, the Ag elimination in isopods observed in only two cases might not reflect the elimination of Ag from the storage tissue, but rather the elimination of the material adsorbed/attached to body surface, which could be dependent on the chemical and physical properties of both the soil and the NPs tested. This however, needs further investigation, in particular as to why this was not the case for the smaller sized Ag NPs (3–8 nm). In addition to particle size, also the type of the coating material may affect the aggregation and dissolution behaviour of NPs,37 and therefore these aspects might affect its biokinetics.
The total Ag body concentrations during the uptake phase generally were higher in isopods compared to springtails. This agrees with the fact that among terrestrial invertebrates, isopods are known to be among those with the highest metal-storing capacities.15 Lastly, the most evident difference between the two species is in their route of metal uptake. While isopods are exposed to Ag mainly orally through ingestion of soil particles,13 springtails are mainly exposed to metals in the pore water.16 Accordingly, less Ag is taken up by the springtail, because only a small fraction of the total Ag concentration is present in the pore water and as such bioaccessible, as opposed to the isopod, where total soil Ag is consumed. It has been argued that complexation of Ag ions with soil organic matter might increase Ag bioavailability to soil ingesting organisms, such as isopods. When organic matter is consumed and digested, the presence of surfactant molecules and specific gut lumen conditions and its microbial community might facilitate the release of Ag into solution.36 It has also been demonstrated that metals which are unavailable for chemical extraction are available and are accumulated by P. scaber, meaning that isopods are probably able to acquire metals from several soil fractions, not just the water-soluble one.38 Additionally, as bioavailability of metals from ingested soil is expected to be highly dependent on the pH in the different compartments of the gastrointestinal tract, being generally larger at low pH,39 the slightly acidic conditions in the intestinal tract of P. scaber40 could contribute to the high bioavailability of Ag to isopods in the organic soil observed in this study.
Among the soils tested in our study, Dorset soil was most acidic, had the highest sand content and lowest content of clay. Indeed, in this soil highest Ag uptake was observed in F. candida for all Ag forms tested and in P. scaber for the 3–8 nm Ag NPs. Considering the significant positive correlation between the k1 values for ionic Ag and 3–8 nm Ag NPs, we presume that the uptake of ionic Ag would also be highest in the Dorset soil in case of P. scaber, had it been tested. Moreover, the higher Ag bioavailability in Dorset soil compared to the other soils is most probably due to a higher dissolution of Ag NPs and higher Ag mobility. Ag NPs have been reported to preferentially interact with granulometric clays,42 implying a decreased bioavailability in soils with high clay contents. Accordingly, a negative correlation of k1 values and clay content was observed for all Ag forms in F. candida and for 3–8 nm Ag NPs in P. scaber (Table 4).
The negative relationship of Ag bioavailability and pH was more pronounced in P. scaber, where uptake of ionic Ag and 3–8 nm Ag NPs was high also in the second most acidic soil, the North Wales soil. This further supports the higher mobility of Ag NPs at low pH. On the contrary, in F. candida uptake was lowest in this soil. Incidentally, this soil had the highest organic matter content. Although there was no clear relationship between k1 values and organic matter content in either organism (Table 4 and Fig. S4†), Ag was least bioavailable to F. candida in the soil with the highest organic matter content. This is presumably due to complexation of Ag with organic matter,30,43 and consequent decreased Ag concentration in the pore water. As argued above, the bioavailability of Ag to P. scaber in this situation might be higher compared to F. candida, due to larger contribution of soil constituents in the ingested soil.
In conclusion, we demonstrated that compared to pristine Ag NPs, sulfidized Ag2S NPs are considerably less bioavailable to both isopods and springtails. From an environmental perspective this is a positive finding, since most Ag NPs released to the environment become sulfidized with time and less bioavailable. Isopods and springtails have different Ag-bioaccumulation capacity, depending on their metal metabolism and feeding habits, which calls for integrating data from different test species. We clearly showed that for a single organism, Ag bioaccumulation upon exposure to pristine Ag NPs differs widely between different field soils and is higher in acidic sandy soils. Other soil properties, especially organic matter content, contribute to differences in Ag bioavailability between organisms. Adsorption of Ag NPs to body surfaces presumably contributes to the variation in Ag elimination dynamics observed in both species and should be addressed in future biokinetic studies.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0en00291g |
This journal is © The Royal Society of Chemistry 2020 |