An overlooked nanofluids effect from Fe3O4 nanoparticles enhances mass transfer in anammox granular sludge

Magnetite (Fe 3 O 4 ) particles have been widely reported to enhance the anammox's activity in anammox granular sludge (AnGS), yet the underlying mechanisms remain unclear. This study demonstrates that both Fe 3 O 4 microparticles (MPs) and nanoparticles (NPs) at a dosage of 200 mg Fe 3 O 4 /L significantly increased the specific anammox activity (SAA) of AnGS. Additionally, the transcriptional activities of the hzs and hdh genes involved in the anammox process, as well as the heme c content in AnGS, were also notably enhanced. Notably, Fe 3 O 4 NPs were more effective than MPs in boosting anammox activity within AnGS. Mechanistically, Fe 3 O 4 MPs released free iron, which anammox bacteria utilized to promote the synthesis of key enzymes, thereby enhancing their activity. Compared to MPs, Fe 3 O 4 NPs not only elevated the synthesis of these key enzymes to a higher level but also induced a nanofluids effect on the surface of AnGS, improving substrate permeability and accessibility to intragranular anammox bacteria. Moreover, the nanofluids effect was identified as the primary mechanism through which Fe 3 O 4 NPs enhanced anammox activity within AnGS. These findings provide new insights into the effects of nanoparticles on granular sludge systems, extending beyond AnGS.