Long-Term Exposure and Effects of rGO-nZVI Nanohybrids and Their Parent Nanomaterials on Wastewater-Nitrifying Microbial Communities

Single nanomaterials and nanohybrids (NHs) can inhibit microbial processes in wastewater treatment, especially nitrification. While existing studies focus on short-term and acute exposures of single nanomaterials on wastewater microbial community growth and function, long-term, low-exposure, and emerging NHs need to be examined. These NHs have distinctly different physicochemical properties than their parent nanomaterials and, therefore, may exert previously unknown effects onto wastewater microbial communities. This study systematically investigated long-term [similar to 6 solid residence time [(SRT)] exposure effects of a widely used carbon-metal NH (rGO-nZVI = 1:2 and 1:0.2, mass ratio) and compared these effects to their single-parent nanomaterials (i.e., rGO and nZVI) in nitrifying sequencing batch reactors. nZVI and NH-dosed reactors showed relatively unaffected microbial communities compared to control, whereas rGO showed a significantly different (p = 0.022) and less diverse community. nZVI promoted a diverse community and significantly higher (p < 0.05) biomass growth under steady-state conditions. While long-term chronic exposure (10 mg.L-1) of single nanomaterials and NHs had limited impact on long-term nutrient recovery, functionally, the reactors dosed with higher iron content, that is, nZVI and rGO-nZVI (1:2), promoted faster NH4+-N removal due to higher biomass growth and upregulation of amoA genes at the transcript level, respectively. The transmission electron microscopy images and scanning electron microscopy-energy-dispersive X-ray spectroscopy analysis revealed high incorporation of iron in nZVI-dosed biomass, which promoted higher cellular growth and a diverse community. Overall, this study shows that NHs have unique effects on microbial community growth and function that cannot be predicted from parent materials alone.