The Influence Of Vermiculite On The Uptake Of Silver Nanoparticles In A Terrestrial System
Abstract
The uptake of silver from silver nanoparticles in soil was investigated in the presence of increasing concentrations of Vermiculite, typical 2:1 clay. Two insect species, Acheta domesticus and Tenebrio molitor, and two plant species , Helianthus annuus and Sorghum vulgare, were exposed to silver nanoparticles in the presence of increasing concentrations of Vermiculite in soil. Silver nanoparticles were characterized using techniques including transmission electron microscopy, dynamic light scattering, and powder X -ray diffraction. The levels of silver in test species exposed to silver nanoparticles were measured using an inductively coupled plasma -optical emission spectrometer. An increase in the cation exchange capacity of soil was observed with the increase in the concentration of vermiculite in soil. The results suggested a decrease in the uptake of silver from silver nanoparticles in soil by Acheta domesticus as a function of increasing concentrations of Vermiculite in soil. No apparent trend was observed in the remaining species. Both plant species were found to accumulate silver in their roots. The translocation of silver to stems and leaves was observed in the case of Helianthus annuus. Results from this study suggest that the presence of Vermiculite in soil could possibly decrease the uptake of silver from silver nanoparticles.
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Introduction
The widespread use of silver nanoparticles (Ag NPs) for a variety of applications has resulted in an increase in the concentrations of Ag NPs in terrestrial ecosystems. Typically, Ag NPs find their way into terrestrial ecosystems through the application of sewage sludge as fertilizer to land [1 -3]. The environmental behavior, fate, and ecotoxicity of metal -based nanoparticles in a terrestrial ecosystem are known to be influenced by the physicochemical characteristics of both the nanoparticles themselves and the soil. Physical properties include size and shape of nanoparticles while the chemical characteristics include acid -base character, aqueous solubility and surface coatings, if any [4]. The physicochemical properties of soil can influence the mobility, bioavailability and toxicity of pollutants in a terrestrial ecosystem. These properties include pH, soil texture, organic matter, and cation exchange capacity (CEC), etc [5,6]. The texture of a soil is comprised of sand, silt, and clay. The particle size of sand, silt, and clay are > 50 μm, 2 -50 μm, and < 2 μm respectively [7,8]. Clay particles belong to a group of minerals described as hydrous silicates [9]. Clay particles typically have a negative charge [10]. Additionally, clay particles considerably contribute to the CEC of soil. CEC of soil is the quantity of positively charged ions that could be held by the negatively charged surface of clay minerals [9]. The CEC of a soil provides electrostatic binding site s for cations like silver ions (Ag+) thus rendering them electrostatically immobilized [11]. The negative charge of clay minerals, the high surface area of clay due to small particle size [12] and the contribution of clay minerals to the CEC of soil [9] pl ay a key role in determining the fate of metal contaminants in soil. The four major groups of clay minerals include: 1) the kaolinite group; 2) the mont-morillonite/smectite group; 3) the illite group and 4) the chlorite group [9]. The mont-morillonite/smectite group of clay minerals are known to possess high surface area and CEC compared to the remaining three groups of clay minerals. The general formula of mont-morillonite/smectite group of clay minerals is (Ca,Na,H)(Al,Mg,Fe,Zn)2(Si,Al)4O10(OH)2-XH2O [9].
A member of the mont-morillonite/smectite group of clay minerals, Vermiculite, is used in the present study. The effect of increasing concentrations of Vermiculite in soil on the uptake of silver from Ag NPs in a terrestrial system was investigated. Terrestrial isopods have been used as model organisms to understand the uptake, kinetics and transformation of metal nanoparticles [13-15]. The uptake of nanoparticles in terrestrial isopods occurs exclusively through the oral route. Negligible surface uptake of nanoparticles is observed in terrestrial isopods facilitating the uptake and transformation studies of metal nanoparticles [16]. However, it is equally important to investigate the uptake of metals from metal nanoparticles by other components of terrestrial ecosystems that serve a crucial link in the metal transport chains between trophic levels in the food web.
Two species of insects, Acheta domesticus and Tenebrio molitor, and two species of plants, Helianthus annuus (a dicot plant) and Sorghum vulgare (a monocot plant) were used. The insect and plant species were exposed to Ag NPs in soil containing a range of concentrations of Vermiculite. Insects serve an important role in the metal-transport chains between trophic levels in food webs [17]. For instance, during the breeding season, insects and larvae constitute an important food source for insectivorous birds [18]. Therefore, it is important to determine if insects are able to bioaccumulate Ag from Ag NPs in soil. As seeds are another important food source for granivorous birds [19], the possibility of translocation of Ag to plant tissues was investigated. Results from this study would help understand the role of plants in bioaccumulation of metal nanoparticles.
Conclusion
The influence of a clay mineral, Vermiculite, on the uptake of Ag from Ag NPs in soil by insect and plant species was investigated. The presence of Vermiculite resulted in an increase in the cation exchange capacity of soil. The presence of Vermiculite has also potentially resulted in a decrease in the uptake of Ag from Ag NPs in soil by insect and plant species. The increase in CEC of soil due to the presence of clay minerals, the increased surface area o f clay minerals due to their small particle size, and the negative charge of clay minerals may cumulatively result in a decrease in the uptake of Ag from Ag NPs in soil. Consequently, clay minerals may be used as sorbents for heavy metals in soil thereby decreasing the possibility of their entrance into food webs.