Toxicity Testing of Silver Nanoparticles in Artificial and Natural Sediments Using the Benthic Organism Lumbriculus variegatus

Abstract

The increased use of silver nanoparticles (AgNP) in industrial and consumer products worldwide has resulted in their release to aquatic environments. Previous studies have mainly focused on the effects of AgNP on pelagic species, whereas few studies have assessed the risks to benthic invertebrates despite the fact that the sediments act as a large potential sink for NPs. In this study, the toxicity of sediment-associated AgNP was evaluated using the standard sediment toxicity test for chemicals provided by the Organization of Economic Cooperation and Development. The freshwater benthic oligochaete worm Lumbriculus variegatus was exposed to sediment-associated AgNP in artificial and natural sediments at concentrations ranging from 91 to 1098 mg Ag/kg sediment dry weight. Silver nitrate (AgNO3) was used as a reference compound for Ag toxicity. The measured end points of toxicity were mortality, reproduction, and total biomass. In addition, the impact of sediment-associated AgNP on the feeding rate of L. variegatus was studied in a similar test set-up as mentioned previously. The addition of AgNP into the sediment significantly affected the feeding rate and reproduction of the test species only at the highest concentration (1098 mg/kg) of Ag in the natural sediment with the lowest pH. In comparison, the addition of AgNO3 resulted in reproductive toxicity in every tested sediment, and Ag was more toxic when spiked as AgNO3 than AgNP. In general, sediments were observed to have a high capacity to eliminate the AgNP-derived toxicity. However, the capacity of sediments to eliminate the toxicity of Ag follows a different pattern when spiked as AgNP than AgNO3. The results of this study emphasize the importance of sediment-toxicity testing and the role of sediment properties when evaluating the environmental effects and behavior of AgNP in sediments.

Silver nanoparticles (AgNP), used, e.g., in healthcare, textiles, paints, cosmetics, and cleaning agents, have the highest degree of commercialization (as a number of products) of all nanoscale materials due to their unique optical and antibacterial properties (Vance et al. 2015). In surface waters, AgNP are mainly released through wastewater-treatment plants and untreated wastewater (Gottschalk et al. 2009). Sediment is the final sink for the AgNP, and the modeled annual increase of sediment concentrations varies between 0.15 and 10.18 µg/kg/y resulting in a possible hazard for aquatic organisms (Gottschalk et al. 2009).

In environmental media, AgNP may be oxidized, which leads to dissolution and release of Ag ions (Ag+) (Loza et al. 2014). Ionic Ag is highly toxic to aquatic organisms, and thus the toxicity of AgNP may be related to the concentration of dissolved Ag+ (Navarro et al. 2008; van Aerle et al. 2013). However, the concentration of freely dissolved Ag+ in environmental media is typically low due to strong complexation with chloride, sulfide, and natural organic matter (Levard et al. 2013; Loza et al. 2014). AgNP also pose NP-specific toxicity (Chan and Chiu 2015; Cozzari et al. 2015; García-Alonso et al. 2014). One of the primary identified toxic mechanisms at the molecular level is the generation of reactive-oxygen species resulting in oxidative stress (Cozzari et al. 2015; Roh et al. 2009).

The behavior and toxicity of AgNP in sediment is still poorly understood, and there is an urgent need for studies and standardized test methods. The biggest challenge in studies with nanomaterials in sediment and other complex environmental media is the lack of proper characterization methods. Because most of the nanomaterials are not stable in water, sediment studies are still considered to be relevant and sometimes even more representative of environmental exposure than aqueous tests (Petersen et al. 2015). In water-only exposure tests of AgNP with varying coatings resulted in LC50 (lethal concentration to kill 50 % of the test organisms in 96 h) values of 0.07–0.33 mg/L to the benthic organism Lumbriculus variegatus (Khan et al. 2015). When the same species was exposed by way of sediment, AgNP showed no mortality on exposure at 367 mg/kg (Coleman et al. 2013). However, in sediment exposure, AgNP induces oxidative stress in Nereis diversicolor at concentrations >10 mg/kg (Cozzari et al. 2015). The results indicate that AgNP-induced toxicity is decreased when particles are introduced into sediment, but the role of sediment properties has not yet been studied.

The aims of this study were (1) to examine how sediment properties influence the toxicity of AgNP; (2) to compare the toxicity of Ag spiked as AgNP with dissolved Ag spiked as silver nitrate (AgNO3); and (3) to evaluate the suitability of the Organisation for Economic Co-operation and Development (OECD) standard test method guideline 225 for use with nanomaterials. Artificial and two natural sediments that differed in their characteristics were selected and spiked with polyvinylpyrrolidone-coated AgNP and AgNO3. OECD standard test guideline 225, Sediment–Water Lumbriculus Toxicity Test Using Spiked Sediment (OECD 2007), was followed, and mortality, reproduction, and changes in biomass were used as indicators of toxicity to the endobenthic aquatic Oligochaeta L. variegatus. In addition, the feeding rate of L. variegatus was used as an end point of toxicity for AgNP.