Rejection of inorganic and organic nitrogen species by nanofiltration: Opportunities for nitrogen circular economy

Nanofiltration was used to reject various forms of nitrogen in water. Inorganic nitrogen (IN) species, including ammonium (NH 4 + ), nitrates (NO 3 − ), and nitrites (NO 2 − ), as well as organic nitrogen (ON) compounds, such as urea, and organic matter (OM) with ON content were considered. The rejection of the different nitrogen compounds by nanofiltration was largely governed by the interplay between size exclusion and charge exclusion mechanisms. The characteristics of nitrogen species, including hydrated radius and charge valency, as well as membrane surface characteristics such as pore diameter and surface charge, were the contributing parameters. Varying pH and ionic strength contributed to the permeation of the charged nitrogen species (NH 4 + , NO 3 − , NO 2 − ). The difference in rejection observed when varying pH was attributed to the speciation of some nitrogen species (NH 4 + , NO 2 − ) in addition to pH dependent changes in the surface charge of the membrane. Increasing the ionic strength resulted in charge screening on the membrane surface, reducing the charge exclusion and thus enhancing the solute permeability. This was not the case for neutral species such as urea. The presence of different OM types with different molecular sizes and anionic charges enhances the rejection of the charged nitrogen species, such as NH 4 + , NO 3 − . This is potentially due to an increase in the membrane surface charge due to OM adsorption. ON, found mainly in biopolymers, humic substances, and low molecular weight fractions of the different OM types, was rejected by NF. The results demonstrate the potential of nanofiltration for rejecting diverse nitrogen compounds, thus providing a suitable technology for recycling nitrogen in water treatment processes. The findings contribute valuable insights for nitrogen separation and advancing sustainable nutrient management in the circular economy.