Formation of Silver Nanoparticles and its Joint Toxicity to Microcystis aeruginosa with Chlorine in Aquatic Environments

Abstract

The increasing use of silver nanoparticles (AgNPs) in consumer products and their probable release into the environment have raised several concerns for the overall health of water ecosystems Understanding their fate in the surface water is crucial for its risk assessment AgNPs can go through several transformations that alter their final fate and toxicity One such transformation is the neo-formation of AgNP in natural waters As a result in recent years several researchers have focused on understanding the natural formation of AgNPs The presence of AgNPs in surface waters can also lead to its exposure to oxidants during water treatment processes Oxidants can readily cause the dissolution of AgNPs in turn influencing their fate and toxicity This work tries to extend the understanding of AgNPs transformation in surface waters by evaluating the natural formation of AgNPs and assessing the collective impact of an oxidant and AgNP on cyanobacteria In aerobic natural surface water silver ion (Ag+) exists in various Ag+-Cl- complexes due to a strong affinity for chloride ion (Cl-); however little information is available about the role of the Ag+-Cl- complex in the formation of AgNPs The first part of this study demonstrates that soluble AgClx(x?1)? species act as a precursor of AgNPs under simulated sunlight irradiation The AgNP photoproduction increases with Cl- levels up to 0 0025 M ([Ag+] = 5 × 10-7 M) and decreases with continued Cl- level increase (0 09 M to 0 5 M) At [Cl-] ? 0 0025 M (freshwater systems) photoproduction of AgNP correlates with the formation of AgCl(aq) suggesting that it is the most photoactive species in those systems Matching the ionic strength of experiments containing various Cl- levels indicate that the trend in AgNP photoproduction correlates with Cl- concentrations rather than ionic strength induced effects The photoproduction of AgNPs is highly pH-dependent especially at pH > 8 3 The UV and visible light portions of the solar light spectrum are equally important in the photoreduction of Ag+ AgNPs have shown to be toxic to freshwater cyanobacterial species and sodium hypochlorite (NaOCl) is a common oxidant for the treatment of cyanobacterial cells AgNPs have a high possibility of co-existing with the cyanobacterial cells in the aqueous environments leading to their exposure to NaOCl during water treatment; however their combined effects on the cyanobacterial cells are largely undocumented The second part of this work compares the individual and combined effect of AgNP and NaOCl on the integrity and toxin (microcystins) release of Microcystis aeruginosa at varying levels The results show that the AgNP (0 2 – 0 6 mg/L) alone has negligible effects on the cell lysis while NaOCl alone shows concentration-dependent (0 2 < 0 4 < 0 6 mg/L) rupturing of cells In contrast the AgNP + NaOCl (0 2 – 0 6 mg/L) samples show increasing loss in cell integrity at higher AgNP (0 4 and 0 6 mg/L) levels than the NaOCl only samples NaOCl exposure results in increasing dissolution of AgNPs with time releasing silver ions (Ag+) affecting its size and morphology The cell-associated total Ag declines over time with an increase in NaOCl levels maybe due to increasing cell-lysis or NaOCl induced oxidative dissolution of AgNPs The cell-associated total Ag and released Ag+ possibly weaken the cellular membrane thus assisting NaOCl in faster cell-lysis The combined exposure of AgNP and NaOCl also results in a higher release of toxin from the cells Overall the present work shows evidence that AgClx(x?1) ? species irradiated under sunlight conditions contribute to nanosized silver (Ag) formation in the environment This signifies that the presence of environmentally relevant levels of Ag in aerobic surface waters will likely form Ag+?Cl? complex that can lead to photoformation of AgNPs The AgNPs combined with NaOCl can also enhance the cell lysis and release of toxins in Microcystis aeruginosa cells It implies that the presence of AgNPs in water bodies could influence cell lysis of toxic cyanobacteria during chlorination The findings of this work can have significant environmental implications for understanding the possible fate and toxic effects of AgNPs in surface waters

Date of Award	2021
Original language	English
Supervisor	Tsair-Fuh Lin (Supervisor)