Efficient continuous removal of nitrates from water with cationic cellulose nanopaper membranes

Nitrates constitute a severe problem for the quality of potable water. The removal of nitrates from water can be performed utilizing continuously operating cellulose nanopaper ion-exchangers, which so far are unfortunately of only moderate efficiency. Here we demonstrate cationic cellulose nanopapers comprising cellulose nanofibrils carrying a high amount of ammonium groups (1.6 g mmol−1, i.e. 0.62 mmol g−1), which are anticipated to enable efficient removal of nitrate ions from aqueous solutions. Thin nanopapers were shown to have high adsorption capacities. Therefore we prepared low grammage nanopapers using a papermaking process from cellulose nanofibrils prepared from paper mill sludge. The performance of these cationic nanopapers was characterized by their permeance, with these new cationic nanopapers having a permeance of more than 100 L m−2 h−1 MPa−1, which is far greater than the permeance of conventional nanopapers. Furthermore, nitrate ions were successfully removed from water by capturing them through adsorption onto the cationic nanopaper by primarily an ion-exchange mechanism. These cationic nanopapers possessed adsorption capacities of almost 300 mg g−1, which is superior to commonly used nanopaper ion-exchangers and batch-wise applied adsorbents. Utilization of an industrial side-stream in combination with very good membrane performance demonstrates the use of resource efficient technologies in an important sector.