The Aggregation and Dissolution of Citrate−Coated AgNPs in High Ammonia Nitrogen Wastewater and Sludge from UASB−Anammox Reactor

Silver nanoparticles (AgNPs) are released into the sewage pipes and ultimately wastewater treatment plants during manufacturing, use, and end–life disposal. AgNPs in wastewater treatment plants aggregate or dissolve, and may affect the microbial community and subsequent pollutant removal efficiency. This study aims to quantitatively investigate the fate of AgNPs in synthetic high ammonia nitrogen wastewater (SW) and sludge from an up–flow anaerobic sludge blanket (UASB) anammox reactor using a nanoparticle tracking analysis (NTA), dynamic light scattering (DLS), transmission electron microscope (TEM), and atomic absorption spectroscopy (AAS). Results showed that 18.1 mM NH 4 + , 2.11 mM Mg 2+ in SW caused less negative zeta potential (ζ−potential, −18.4 vs. −37.4 mV), aggregation (388.8 vs. 21.5 nm), and settlement (80%) of citrate−coated AgNPs (cit−AgNPs) in 220 min. The presence of 18.5 mM Cl − in SW formed AgCl 2 − , AgCl (aq) and eventually promoted the dissolution (9.3%) of cit−AgNPs. Further exposure of SW−diluted AgNPs to sludge (42 mg L −1 humic acid) and induced a more negative ζ−potential (−22.2 vs. −18.4 mV) and smaller aggregates (313.4 vs. 388.8 nm) due to the steric and hindrance effect. The promoted Ag dissolution (34.4% vs. 9.3%) was also observed after the addition of sludge and the possible reason may be the production of Ag(NH 3 ) 2 + by the coexistence of HA from sludge and NH 4 + from SW. These findings on the fate of AgNPs can be used to explain why AgNPs had limited effects on the sludge−retained bacteria which are responsible for the anammox process.