Recovery of NH4+‒N and PO43-‒P is Promoted by competitive interactions between M. aeruginosa and MgCO3

Abstract

Here we report that the presence of MgCO₃ stimulates the extracellular polymeric substance (EPS) secretion of Microcystis Aeruginosa (M. Aeruginosa). This stimulation led to a significant reduction in the total concentration of NH₄⁺‒N by more than 86％, and effective recovery of PO₄^3-‒P within three days from concentrated wastewater (WW), although the secreted EPS inhibited the conversion of MgCO₃ to specific crystal forms (MgNH₄PO₄.6H₂O or MgHPO₄.7H₂O). Moreover, with an increase in PO₄³⁻ concentration in WW, these crystals appeared, thus the removal of NH₄⁺‒N and PO₄^3-‒P nutrients can be attributed to the combined effect of M. Aeruginosa and MgCO₃. We used Surface-Enhanced Raman Spectroscopy (SERS) combined with X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (FESEM-EDS), and X-ray Photoelectron Spectroscopy (XPS) to investigate the mechanism for competitive interactions between M. Aeruginosa and MgCO₃ in removing NH₄⁺‒N and PO₄^3-‒P. We identified that the bound EPS accumulated amorphous Mg–P–O dense particles on M. Aeruginosa, while soluble EPS, containing –COOH groups of humic-like substances decreased the pH of the solution and coordinated with Mg²⁺ ions. Therefore, both secreted bound and soluble EPS play a vital role in hindering the transformation of Mg²⁺ ions or MgCO₃ to MgNH₄PO₄.6H₂O or MgHPO₄.7H₂O crystals within WW, and they enhanced M. Aeruginosa 's ability in absorbing nutrients of NH₄⁺‒N and PO₄^3-‒P. This mechanism plays a crucial role in the efficient recovery of NH₄⁺‒N and PO₄^3-‒P from concentrated wastewater sources such as aerobically or anaerobically digested effluent from various sources like agriculture, livestock, and domestic wastewaters.