Effect of Concentration of Silver Nanoparticles on the Uptake of Silver from Silver Nanoparticles in Soil

The antimicrobial properties exhibited by silver and silver nanomaterials have propelled their widespread use in many consumer products that include detergents, textiles, home appliances, nutritional supplements, etc. In fact, silver based nanomaterials are one of the most common and most used among all nanomaterials [1]. According to a study by the Grand View Research, Inc. the global market for silver nanoparticles (Ag NPs) is projected to reach USD 2.45 billion by 2022 [2]. The widespread use of Ag NPs invariably raises questions and concerns about the risks and consequences resulting from their release into the environment. 

Ag NPs are introduced into the terrestrial systems primarily through applications of sewage sludge to land [3-17]. Once present in an ecosystem, the environmental behavior, fate and ecotoxicity of metal-based nanoparticles are known to be influenced by their physical and chemical characteristics. Physical characteristics include size and shape of nanoparticles and chemical characteristics include acid-base character of the surface and aqueous solubility of the metal. The physical and chemical characteristics of nanoparticles influence their transformation phenomena such as aggregation, sorption to surfaces, and dissolution to metal ions. Additionally, surface coatings on metal based nanoparticles also influence their environmental behavior, fate and ecotoxicity [10]. For instance, the properties of Ag NPs that influence their uptake and toxicity to the earthworm Lumbricus rubellus in soil have been investigated by Makama et al. 2016 [18]. 

The physicochemical characteristics of soil influence the chemical form, mobility, bioavailability and toxicity of pollutants in terrestrial ecosystems. These physicochemical characteristics include pH, ionic composition, grain size, soil texture, organic matter content, temperature, solar radiation exposure, hydrostatic pressure, and cation exchange capacity. Thus, it is important to understand that the environmental behavior, fate, bioavailability and ecotoxicity of nanomaterials and other pollutants are influenced and determined by a combination of the physicochemical characteristics of the soil, the physicochemical characteristics of nanomaterials, and the physiological status of biota [19, 20]. 

Once present in a terrestrial environment, it has been shown that certain types of nanoparticles possess the ability to be taken up by insects [ 21 ] and plants [22-27], thereby entering the food web. The uptake of nanoparticles from soil depends on a large number of factors. One such factor is the concentration of nanoparticles present in soil. Larger amounts of nanoparticles may be prone to a phenomenon called aggregation, which causes the final size of the nanoparticles to be larger than they were originally. This could potentially affect the uptake of nanoparticles as the uptake of nanoparticles is inversely proportional to their size. The small size of nanoparticles enables their easy uptake, dissolution and release of ions with increased surface area [28]. Conversely, the presence of a large amount of nanoparticles in a system may result in an increased uptake. 

In the present study, the uptake of silver by insects and plants as a function of concentration of Ag NPs in soil was investigated. Studies to understand the uptake, kinetics and transformatoin of metal nanoparticles in terrestrial ecosystems usually include the terrestrial isopods [29-31]. Terrestrial ispods enable the study of uptake and transformation of metal nanoparticles because of their ability to uptake nanopartiles extensively through the oral route. Surface uptake of nanoparticles in terrestrial isopods was found to be negligible [32]. However, to comprehensively understand the effect of nanoparticles on terresrtial ecosystems, it is important to investigate their uptake in other components of terrestrial ecosystems that play a keyrole in terresrtial food webs. 

Two species of insects, Acheta domesticus and Tenebrio molitor, and two species of plants, Helianthus annuus (a dicot) and Sorghum vulgare (a monocot) were used in this study. Insects constitute an important source of food to insectivorous birds. They serve as a crucual link in the metal-transport chains between trophic levels in food webs [33]. As they are a very good source of protein and other nutrients, insects and larvae serve as an important food source to birds especially during the breeding season [34]. Considering the signficance of insects in the food web of all insectivorous birds, it is important to understand if insects are able to uptake abd accumulate silver from Ag NPs in soil. Two different species of plants, a monocot and a dicot, were chosen for this study to understand if there is any difference in the uptake of silver from Ag NPs in soil by these plants. The objective was to examine if either of the monocot or dicot plant species uptake silver from Ag NPs in soil. Additionally, the possibility of translocation of silver to other tisseus of plant such as stem, seeds, etc was also evaluated. Considering seeds of plants also serve as an important food source for granivorous birds [35], such a study would help understand if these plants are playing a role in the metal-transport chain in the food web of granivorous birds. 

The test species were exposed to concentrations of Ag NPs ranging from 0-625 mg/kg (ppm) in soil. Studies have suggested that sewage sludge can contain a wide range of concentrations of silver. Concentrations of silver as high as 960 ppm were reported in some sludge samples [12]. Thus, the concentration of Ag NPs used in this study is not outside the realms of possibility.