Transformation and Effects of Silver and Copper Oxide Nanoparticles during Treatment and after Land Application of Sludge

Abstract

The increasing antimicrobial application of silver nanoparticles (AgNPs) and copper oxide nanoparticles (CuONPs) in industries has led to significant quantities being released into wastewater and sludge. This has raised significant concerns about their impact on soil biological activities after sludge land application. This thesis evaluates the transformation of AgNPs and CuONPs and their impact on the bacterial community during sludge treatment (anaerobic digestion, chemical conditioning, and lime stabilization) and subsequent land application. Land application of sludge was simulated in the laboratory using soil microcosms over 105 days. Bacterial population and diversity were assessed through examining specific phyla or the overall bacterial composition in the sludge and soil. The results showed that AgNPs in sludge went through dissolution, agglomeration, and surface coating. During sludge anaerobic digestion, AgNPs and CuONPs at 2, 10 and 30 mg/g TS sludge had no significant impact on the overall bacterial community structure at phylum and genus levels. AgNPs and CuONPs also did not significantly impact the biogas generation over time, except for sludge reactor that had higher concentration of CuONPs (30 mg NPs/g TS), which showed significant reduction in biogas generation over time. After applying the anaerobically digested sludge on soil, AgNPs at 20 mg/kg soil had no significant impact, while the concentrations of AgNPs at 300 mg/kg soil and CuONPs at 20 and 300 mg/kg soil decreased the average relative abundance of highly abundant phyla and genera, and lowered the population of the plant growth promoting rhizobacteria (PGPR) by 10.5, 18, 42 and 41%, respectively, suggesting that CuONPs pose higher toxicity. Sludge chemical conditioning using ferric chloride, alum, and synthetic polymer efficiently removed AgNPs from the aqueous phase and concentrated them in sludge solid. Lime stabilization also removed AgNPs through association with lime molecules and sludge solids. The presence of AgNPs at 2 mg/g TS sludge showed no significant impact on the population of the phyla Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria in soil that received untreated, conditioned or lime stabilized sludge. This thesis presents important findings that can serve as a foundation for risk assessment of nanoparticles toxicity in soil and potentially plant systems.