Behaviour of silver and silver sulfide nanoparticles in the environment: effects on wastewater treatment processes and soil organisms

Abstract

Manufactured silver nanoparticles (AgNPs) are the most commonly used manufactured nanomaterial in consumer products. They are incorporated into a vast array of products due to their strong, broad-spectrum anti-microbial activity. However, the anti-bacterial properties that render AgNPs desirable may also lead to increased environmental risks. This thesis examines that impact in wastewater treatment plants (WWTP) and terrestrial environments – the key risk pathways for AgNPs. This thesis studied the life-cycle of released AgNPs, from their release to potential plant uptake, with a focus on their effects and fate in the environment. Four main experiments were undertaken to investigate 1) the effects of AgNPs on WWT processes, 2) the bioavailability of AgNPs and transformed AgNPs (Ag₂S-NPs) in soils to lettuce, 3) the effect of fertiliser addition on the bioavailability of AgNPs and Ag₂S-NP and 4) the effects of AgNPs and Ag₂S-NPs on soil microbial communities. The experiments were carried out to model realistic exposure concentrations and pathways (e.g. wastewater and soil cf. synthetic wastewater and hydroponic conditions, and Ag₂S-NPs cf. ‘pristine’ AgNPs). The results from this thesis demonstrate that sub-dominant wastewater microbial communities can be affected by AgNPs. However, dominant microbial communities and key WWT processes, such as nitrification and methanogenesis, are unlikely to be affected by AgNPs at realistic exposure concentrations. During wastewater treatment it was found that AgNPs were almost completely transformed (> 95%) to sulfidised Ag species, predominantly as Ag-sulfide (Ag₂S-NPs). The bioavailability of sludge-borne Ag₂S-NPs in soil was found to be very low. However, when thiosulfate fertiliser was added to soil, significantly more Ag was taken up by plants. Despite this increased uptake, the overall plant concentrations of Ag remained low; the Ag concentrations in edible plant parts (shoots) increased from 0.02% to 0.06% of the total amount of added Ag. Finally, to assess the degree of risk that AgNPs and Ag₂S-NPs pose to soil microorganisms, a new molecular based approach was developed to determine the effect on whole soil microbial communities. This new approach was used to calculate toxicity values for individual soil microbial populations following their exposure to Ag⁺, AgNPs and Ag₂S-NPs. A combination of quantitative PCR (qPCR) and pyrosequencing-based analysis of the 16S rRNA gene region was used to develop dose-response curves for sensitive microbial populations. Based on pyrosequencing results, similar sequences were assigned to operational taxonomic units (OTUs); the abundances of which were then converted to absolute values. Toxicity values (EC₂₀) for sensitive soil OTUs were then plotted on a sensitivity distribution in order to calculate the Ag concentration that would theoretically protect a specified percentage of soil microorganism gene sequences (HCₓ values). At the HC5 and HC10 values (95% and 90% of soil OTUs protected, respectively), there were no significant differences between Ag treatments, while at the HC20 (80% of OTUs protected), Ag₂S-NPs were significantly less toxic than AgNPs and Ag⁺. The most sensitive OTUs (EC₂₀ < HC5) were predominantly from the Bacillaceae family, with lower abundances of other families including Frankiaceae, Comamonadaceae and Pseudonocardiaceae. In all experiments described in this thesis, the negative impacts of AgNPs and Ag₂S-NPs were less than or equal to the effects observed in ionic Ag (Ag⁺) treatments. Overall, results from this thesis show that the risks associated with AgNPs and Ag₂S-NPs are overestimated (and conservatively covered) by the risk of ionic Ag⁺ in terrestrial environments.