The Influence of Soil Organic Matter 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 soil organic matter. Especially, the effect of Humus component of soil organic matter on the uptake of Agfrom silver nanoparticles was studied. Two insect species, Acheta domesticus and Tenebrio molitor, and two plant species, Helianthus annuus and Sorghum vulgare, were exposed to silver nanoparticles(25 ppm in the presence of increasing concentrations of Humus (0, 1, 5, 10, 15, and 20% Humus) in soil (by weight). The techniques of transmission electron microscopy, dynamic light scattering, and powder X-ray diffraction were used to characterize the silver nanoparticles used in the study. An inductively coupled plasma-optical emission spectrometer was used to measure the levels of silver in test samples. Increasing concentrations of Humus in soil has resulted in an increase in the sulfur content and cation exchange capacity of the soil. A general decrease in the concentrations of silver was observed in Acheta domesticus and both the plant species, as a function of increasing concentrations of Humus in soil. In the case of plant species, the accumulation of silver nanoparticles was predominantly observed in the root tissue. Additionally, the translocation of silver from the roots to other plant tissues was observed in the case of Helianthus annuus. Results from this study suggest that the presence of Humus in soil could possibly decrease the uptake of silver from silver nanoparticles by insect and plant species.
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Introduction
Silver nanoparticles (Ag NPs) are among the most commonly used metal nanoparticles today with an estimated global market projected to be worth USD 2.45 billion by 2022 (https://www.grandviewresearch.com/press-release/global-silver-nanoparticles-market) [1]. The range of applications of Ag NPs includes consumer products such as detergents, textiles, home appliances, nutritional supplements, etc (www.nanotechproject.org) [2]. Such widespread use of Ag NPs may inadvertently facilitate their entry into various ecosystems. Predominantly, Ag NPs often find their way in to terrestrial ecosystems through the application of sewage sludge to land [3-5].
Metals in soil are found to be present in eight different fractions: 1) free metal cations; 2) inorganic complexes; 3) organo-metal complexes; 4) organo-complex chelates; 5) in association with high molecular weight organic materials; 6) bound as diverse colloids; 7) adsorbed to colloids and 8) within the soil particles [6]. Spurgeon and Hopkin 1996 [7] observed that metals in the first four fractions remain in soil solution and subsequent uptake [7]. The fate and toxicity of metal nanoparticles in soil is governed by the physicochemical properties of both the metals and soil. The size and shape of nanoparticles, the aqueous solubility and acid-base character of nanoparticles, and the presence of any surface coatings on nanoparticles are some of the factors that influence their fate in a terrestrial system [8]. On the other hand, the many different properties of soil such as pH, texture of the soil, cation exchange capacity and the soil organic matter (SOM) govern the mobility, bioavailability and toxicity of metals in a terrestrial system [9,10].
Decomposing plant material predominantly serves as a source of organic matter in soil [11]. SOM is composed of humic substances (often referred to as humus) and non-humic substances. Humus is comprised of humic and fulvic acid (Foth 1978, cited in [12]). SOM plays a key role in determining the mobility, fate and toxicity of metals in terrestrial systems [11,12]. Complexation and adsorption are the phenomenon involved in the retention of metals by SOM [13]. Additionally, SOM influences many difference functional properties of soil that include: 1) water holding capacity; 2) aggregate stability; 3) compaction characteristics and friability; 4) soil erodibility; 5) nutrient cycling; 6) buffering capacity to acidification and 7) cation exchange capacity[14] . SOM and clay content of soil play a major role in decreasing the availability and subsequent uptake of metals because of the negative charges and these negative charges are observed to be temporary in the case of SOM (Wild 1993 cited in [7]). The negatively charged functional groups in SOM include phenols, carboxyl(-ate), and amino groups. It was also observed that the negative functional groups increase in number during the process of humification of organic matter (Foth 1978, cited in [12]). However, SOM is also known to chelate metals [15], a phenomenon that may enhance their availability and uptake in soil.
This study attempted to measure the effects of Humus on the uptake of Agfrom Ag NPs in soil by insect and plant species in a terrestrial system. The chelating ability of SOM may increase the bioavailability to plants; conversely, the increased S content and CEC could decrease the overall bioavailability.
The interaction between nanoparticles and SOM was thoroughly reviewed by Grillo et al. 2015 [16]. The present study investigated the uptake of Agfrom Ag NPs in a terrestrial system using two insect and two plant species. The insect species used in the study include Acheta domesticus and Tenebrio molitor. The plant species employed in the study include Helianthus annuus (a dicot plant) and Sorghum vulgare (a monocot plant). All insect and plant species employed in the study are native to the region where the soil was collected. Insects serve a crucial role in the food chain of insectivorous birds, especially during the breeding season [17,18]. Similarly, plants serve as an important food source for grainivorous birds [19]. Results from this study would enable the understanding of the uptake of Agfrom Ag NPs in soil by the insect and plant species under consideration and their subsequent role in bioaccumulation and bioconcentration of Ag NPs.
Conclusion
The effect of increasing concentrations of soil organic matter (Humus) on the uptake of Agfrom Ag NPs in soil by two insect and plant species was investigated. Ag NPs were thoroughly characterized using the techniques of transmission electron microscopy, dynamic light scattering and powder X-ray diffraction analyses. In general, it was observed that the presence of Humus decreases the uptake of Agfrom Ag NPs in soil by insect and plant species. Hence, it can be inferred that the presence of increasing concentrations of Humus in soil decreases the bioavailability of Agfrom Ag NPs in a terrestrial system.