Transformation of four silver/silver chloride nanoparticles during anaerobic treatment of wastewater and post-processing of sewage sludge

142

123

The use of antimicrobial silver nanoparticles (AgNPs) in consumer-products is rising. Much of these AgNPs are expected to enter the wastewater stream, with up to 10% of that eventually released as effluent into aquatic ecosystems with unknown ecological consequences. We examined AgNP impacts on aquatic ecosystems by comparing the effects of two AgNP sizes (12 and 49 nm) to ionic silver (Ag + ; added as AgNO 3 ), a historically problematic contaminant with known impacts. Using 19 wetland mesocosms, we added Ag to the 360 L aquatic compartment to reach 2.5 mg Ag L −1 . Silver treatments and two coating controls were done in triplicate, and compared to four replicate controls. All three silver treatments were toxic to aquatic plants, leading to a significant release of dissolved organic carbon and chloride following exposure. Simultaneously, dissolved methane concentrations increased forty-fold relative to controls in all three Ag treatments. Despite dramatic toxicity differences observed in lab studies for these three forms of Ag, our results show surprising convergence in the direction, magnitude, and duration of ecosystem-scale impacts for all Ag treatments. Our results suggest that all forms of Ag changed solute chemistry driving transformations of Ag which then altered Ag impacts.

Section: ■ Materials and Methodsmentioning

confidence: 99%

Emerging Contaminant or an Old Toxin in Disguise? Silver Nanoparticle Impacts on Ecosystems

Colman

Espinasse

Richardson

et al. 2014

142

123

“…18 In brief, at each time interval (0, 4,8,18,28,48,72, and 96 h), 4 mL of aqueous sample was taken from each quartz glass bottle. Every 4 mL of solution was then divided into two subsamples: one group of subsamples (1 mL) was characterized by DLS and TEM or HRTEM−EDX.…”

Section: ■ Materials and Methodsmentioning

confidence: 99%

“…Besides soils, Ag 2 S-NPs in sewage sludge were also shown to be stable over a 6 month period simulating composting/stockpiling. 8 However, previous studies have also repeatedly shown that sunlight has potential to influence the stability and transformation of metal (e.g., silver and gold) NPs in a natural aquatic environment. 20−22 Yu et al 23 presented that sunlight induced aggregation of Ag-NPs, causing particle fusion or self-assembly to form larger structures and aggregates.…”

Section: ■ Introductionmentioning

confidence: 99%

Rethinking Stability of Silver Sulfide Nanoparticles (Ag₂S-NPs) in the Aquatic Environment: Photoinduced Transformation of Ag₂S-NPs in the Presence of Fe(III)

Wang

Liu

et al. 2015

ABSTRACT:The stability of engineered nanomaterials in a natural aquatic environment has drawn much attention over the past few years. Silver sulfide nanoparticles (Ag 2 S-NPs) are generally assumed to be stable in a natural environment as a result of their physicochemical property; however, it may vary depending upon environmental conditions. Here, we investigated whether and how the environmentally relevant factors including light irradiation, solution pH, inorganic salts, dissolved organic matter (DOM), and dissolved oxygen (DO) individually and in combination influenced the stability of Ag 2 S-NPs in an aquatic environment. We presented for the first time that transformation of Ag 2 S-NPs can indeed occur in the aqueous system with an environmentally relevant concentration of Fe 3+ under simulated solar irradiation and natural sunlight within a short time (96 h), along with significant changes in morphology and dissolution. The photoinduced transformation of Ag 2 S-NPs in the presence of Fe 3+ can be dramatically influenced by solution pH, Ca 2+ /Na + , Cl − /SO 4 2− , DOM, and DO. Moreover, Ag 2 S-NP dissolution increased within 28 h, followed rapid decline in the next 68 h, which may be a result of the reconstitution of small Ag 2 S-NPs. Taken together, this work is of importance to comprehensively evaluate the stability of Ag 2 S-NPs in an aquatic environment, improving our understanding of their potential risks to human and environmental health.

“…[10][11][12][13]16 However, a recent study showed that rapid oxidation of Ag 2 SNP can occur in wastewater effluent treated by ozone, along with the release of silver ions and particularly a change in species, resulting in an increase in their acute toxicity to green algae. 20 Our study has shown that light irradiation also influences the stability of Ag 2 SNPs in water containing Fe(III), 21 indicating that light and Fe(III) probably have the potential to drive Ag 2 SNPs transformations.…”

Section: ■ Introductionmentioning

confidence: 99%

Formation of Nanosilver from Silver Sulfide Nanoparticles in Natural Waters by Photoinduced Fe(II, III) Redox Cycling

Zhou

Geng

et al. 2016

Nanosilver (nAg) has been repeatedly demonstrated to end up as silver sulfide nanoparticles (Ag2SNPs), but little is known about the potential transformations of Ag2SNPs in natural environments that are very important for comprehensive assessments of nAg risks to human and environmental health. Here we show that Ag2SNPs can release tiny amounts of silver ion via cation exchange reactions between Ag(I) and Fe(III) in the dark, while in the light dramatic dissolution of Ag2SNP occurs, which is mainly attributed to the Ag2SNP oxidation by the hydroxyl radical formed during the reduction of Fe(III) to Fe(II) in water under sunlit conditions. However, silver ions are subsequently reduced to nAg in the light due to the strong reducing power of Fe(II). Thus, the formation of nAg from Ag2SNPs in the presence of Fe(III) under light conditions proceeds through a two-step reaction mechanism, the photoinduced and Fe(III)-dependent dissolution of Ag2SNPs, followed by the reduction of silver ions to nAg by Fe(II). The formation of nAg from Ag2SNPs is also validated in environmental waters under light conditions. It is thus concluded that photoinduced Fe(III)/Fe(II) redox cycling can drive the formation of nAg from Ag2SNPs in natural waters. These findings suggest that the previous consensus about the stability of Ag2SNPs in aquatic environments should be reconsidered.