Unraveling the capability of graphene nanosheets and γ-Fe2O3 nanoparticles to stimulate anammox granular sludge

In this study, we investigated the potentials of nanomaterials to enhance anaerobic ammonium oxidation (anammox) process, in terms of nitrogen removal, microbial enrichment, and activity of key enzymes. Graphene nanosheets (GNs) and gamma-Fe2O3 nanoparticles (NPs) were selected due to their catalytic functions as conductive material and electron shuttles, respectively. The obtained results revealed that the optimum dosage of GNs (10 mg/L) boosted the nitrogen removal rate (NRR) by 46 +/- 3.1% compared to the control, with maximum NH4+-N and NO2--N removal of 86.5 +/- 2.7% and 97.1 +/- 0.5%, respectively. Moreover, hydrazine dehydrogenase (HDH) enzyme activity was augmented by 1.1-fold when using 10 mg/L GNs. The presence of GNs promoted the anammox granulation via enhancement of hydrophobic interaction of extracellular polymeric substances (EPS). Regarding the use of gamma-Fe2O3 NPs, 100 mg/L dose increased NRR by 55 +/- 3.8%; however, no contribution to HDH enzyme activity and a decrease in EPS compositions were observed. Given that the abiotic use of gamma-Fe2O3 NPs further resulted in high adsorption efficiency (similar to 92%), we conclude that the observed promotion due to gamma-Fe2O3 NPs was mainly abiotic. Moreover, the 16S rRNA analysis revealed that the relative abundance of genus C. Jettenia (anammox related bacteria) increased from 11.9% to 12.3% when using 10 mg/L GNs, while declined to 8.3% at 100 mg/L gamma-Fe2O3 NPs. Eventually, nanomaterials could stimulate the efficiency of anammox process, and this promotion and associated mechanism depend on their dose and composition.

Automated summary

This study investigated the potential of nanomaterials, specifically graphene nanosheets and gamma-Fe2O3 nanoparticles, in enhancing anaerobic ammonium oxidation (anammox) process. Results showed that graphene nanosheets improved nitrogen removal rate and enzyme activity, while gamma-Fe2O3 nanoparticles increased nitrogen removal rate but had no effect on enzyme activity. The use of nanomaterials also influenced the composition of extracellular polymeric substances (EPS) and microbial enrichment in the anammox process.