Co-effects of silver nanoparticles and microplastics on nitrifying microorganisms from wastewater treatment plants and their activities

Abstract

Silver nanoparticles (AgNPs) and microplastics are emerging water contaminants of the decade. They share a similar fate and transport in wastewater treatment plants as they tend to accumulate in sludge of aeration tanks. Since both contaminants have negative effects on microbial growth, the ammonia-oxidizing microorganisms in the aeration tanks are at risk for inhibition and consequently nitrification process fails. This study investigated the effects of AgNPs and microplastics on ammonia-oxidizing activity and community. No inhibition of ammonia oxidation rate was observed at 0.1 mg/L AgNPs. Partial inhibition was found at 0.5 and 1 mg/L AgNPs, while complete inhibition occurred at higher concentrations of 2.5, 5, and 10 mg/L AgNPs. qPCR targeting AOA, AOB, and comammox amoA genes indicated that the numbers of the AOB amoA genes decreased when AgNPs were ≥2.5 mg/L while the comammox amoA genes dropped at ≥0.5 mg/L of AgNPs. Inhibition of AOA was found at AgNP concentrations above 0.5 mg/L but in substantially less compared to AOB and comammox. This study suggests that the three ammonia-oxidizing microorganisms have different responses to AgNP. The co-effect of AgNPs and PVC was studied in microcosms at concentrations of 0.5 mg/L and 500 mg/L, respectively. The results showed that the PVC microplastics had no inhibitory effects on the ammonia oxidation rate. Interestingly, the microcosms, in which the PVC was pre-shaken for 7 days before adding the sludge and AgNPs showed, a faster ammonia oxidation rate than the microcosms containing the sludge and AgNPs with and without fresh (non-shaken) PVC. This suggests that the pre-shaken PVC microplastics may reduce the toxicity of AgNPs. qPCR results indicated that PVC microplastics did not suppress AOA, AOB, and comammox.