Ecotoxicological Effects of Silver Nanoparticles : the Relevance of Test Species and Test Conditions

Abstract

Silver nanoparticles (AgNP) are among the most widely used nanoparticles in various consumer products due to their antibacterial properties. The commercial applications of AgNP in consumer products have dramatically increased in recent decades, which increases the possibilities of their release into the environment via wastewater. As the toxicity of AgNP is not limited to bacteria, other organisms may suffer from them as well. Various studies have shown that AgNP exposure leads to membrane damage, oxidative stress, avoidance and mortality of organisms. According to present knowledge, the toxicity of AgNP is highly influenced by their colloidal stability, dissolution, agglomeration, aggregation and transformation processes, and many environmental factors affect their stability and toxicity. In this thesis, influencing factors such as the concentration of test chemicals, aging of AgNP, soil preparation, the food source provided to test organisms, seasonal variation and the experimental setup under laboratory conditions were studied. In addition, a method was developed using oxygen micro-optical sensors to monitor both respiratory and photosynthesis activity of green algae by measuring oxygen gas concentration in the headspace. This contact-free method provides a good solution for toxicological experiments with nanoparticles in the future. Under field conditions the aging of AgNP and potential amendment effect of soil additives were addressed in this study. Main questions in focus were: i) How do AgNP affect the test organisms? ii) What factors can affect the toxicity of silver? iii) Does the toxicity vary between AgNP and AgNO3? iv) Do AgNP have long-term toxic effects on the test organisms? v) Do compost and biochar as soil amendments help to improve contaminated soil conditions? vi) How to develop a contact-free measurement of green algae activity using microsensors? Green algae and Collembola were selected as test organisms in this thesis. The toxicity of AgNP and AgNO3 to Chlorella vulgaris and Folsomia candida were investigated by using short-term (laboratory experiments) and long-term exposures (semi-field experiments). The toxicity of AgNP to the algal growth rate was much lower than that of AgNO3. For F. candida, two experimental setups were used in the toxicity tests, i.e. the OECD standard reproduction test and a miniaturised design of that. The results showed the miniaturised reproduction test is a valid and efficient alternative comparing with the OECD standard test. The presence of AgNP at a concentration of 30 mg Ag/kg soil resulted in significant inhibition of F. candida reproduction in both experimental setups. Similar toxicity of AgNP and AgNO3 to the reproduction of F. candida was observed. All tested factors significantly affected the toxicity of AgNP to the test organisms except the experimental setup. The semi-field study showed that the presence of AgNP at a concentration of 30 mg Ag/kg soil resulted in significant, but time dependent inhibition on soil respiration and F. candida reproduction. Significant inhibition due to AgNP and AgNO3 was only observed over a period of 400 days but not after 600 days, which indicated that the toxicity of AgNP and AgNO3 to the soil microbial biomass, soil mesofauna abundance and F. candida reproduction reduced over time. Amendment effects of biochar and compost to the soil alleviated the toxicity of AgNP, which was proven over a long period of time.