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

Magnetite (FeO) 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 FeO microparticles (MPs) and nanoparticles (NPs) at a dosage of 200 mg FeO/L significantly increased the specific anammox activity (SAA) of AnGS. Additionally, the transcriptional activities of the and genes involved in the anammox process, as well as the heme content in AnGS, were also notably enhanced. Notably, FeO NPs were more effective than MPs in boosting anammox activity within AnGS. Mechanistically, FeO MPs released free iron, which anammox bacteria utilized to promote the synthesis of key enzymes, thereby enhancing their activity. Compared to MPs, FeO 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 FeO NPs enhanced anammox activity within AnGS. These findings provide new insights into the effects of nanoparticles on granular sludge systems, extending beyond AnGS.